Roadtrek E-Trek Battery Replacement

[avanti]

Originally Posted by markopolo

I'd try the approval route first. They might be very interested in seeing the results. And, they might point out something missed on the forum.

I think your lawyer would advise you to ask for "advice", not "approval".

[markopolo]

Originally Posted by avanti

Why would you need RTs "approval"? There is no way they are going to warrant your work anyway (why should they?). And, at least in the U.S., Magnuson–Moss makes it illegal for them to deny warranty claims based on aftermarket modifications, unless, of course, the modifications actually CAUSE the failure.

I read that too fast & missed some of it. I wasn't thinking DIY. I'm hoping RT would be interested enough to cover the cost of having it done at a dealer. I see it as a warranty fix now that will save money spent replacing more batteries under warranty later.

[avanti]

Originally Posted by markopolo

I'm hoping RT would be interested enough to cover the cost of having it done at a dealer. I see it as a warranty fix now that will save money spent replacing more batteries under warranty later.

Ah. That's different.

[markopolo]

I see this as a continuation of the add-the-equalizer-to-solve-the-problems warranty work.

It appears that a more effective fix would be the simple rewire and use a converter instead of an equalizer.

Adding the equalizer with no other changes does not look to have achieved much.

[Boxster1971]

I would think RT has already looked at the 24-12 VDC converter option. I would think that there 12 V load is under 50 Amps. Any good engineer would consider a converter before the multiple battery mess they have today. Maybe there 12 V load is more than typical in a B-van.

[booster]

Roadtrek may also be suffering from the "never admit you were wrong" syndrome. They have always harshly rebuked anyone that even mentioned that a system might have issues, even as they were failing in the field.

Personally, I don't think Roadtrek will be amenable to any changes they didn't come up with themselves, as it would show they didn't do it as perfectly as they claimed in the first place. IMO, if it came from a discussion on the Class B Forum, which we know they monitor, I would almost guaranty they will not do it. I do hope I am wrong, though, as these systems need to be fixed for the customers sake.

[gsm]

photolimo , gregmchugh & All

I did not intent to cause the ripple in the smooth flow of this thread. It is all too easy to write few words behind which an avalanche other considerations are hidden.

In my Post # 69 I visualized a monitoring-solution that would virtually simultaneously monitor and record all eight battery voltages, currents and temperatures. It would operate full time, while the coach was being used, continuously gather and store information, analyze performance, and warn of impending failures,

Your comments have given me second thoughts about pursuing such an elaborate solution, Much less seems to be sought, something for the individual owner to assess against his situation..


General

I agree that voltage of the battery pairs is an important indicator of battery health. This is helped greatly by having other pairs of the battery-stack for comparison.

I continue to believe that the individual battery-currents are needed to determine health of each member of a battery-pair while charging and discharging the battery-bank. If one member fails to charge properly the other member of the pair will tend to take on some of the load, It will suffer a succession of grief and die. It is important to catch this before it goes too far.

I will reference all the following to my original proposal in which I suggested rewire of batteries A. B, E and F to AB in series with EF. However, to allow separate sensing of the currents of A and B, the two cables running front to rear should be used to separately to bring A+ and B+ to the rear where they are connected together at E-F-. With this small change, all current-sensing can now be done in the rear.

NOTE: -
I noted in the photo of the rear battery compartment previously referred to that charging input is on either battery D or H while the load is taken from the other battery of the pair. Both these cables should go to the same member of the pair, while also ensuring that each member has a jumper in series to the connection with pair CG, and so on down through pairs CG, EF and AB. Only if that is achieved, is the battery system balanced, and only then can the balance of each of the four battery-pairs be monitored on charge and discharge.


The Monitor Points

I like to suggest implementation of a version of Bosster's inexpensive and effective technique of using the voltage-drop along jumper-cables to sample battery-current.

Driving sewing-pins through the insulation into cable a few inches apart creates the needed pairs of monitoring points. The pins are made of stainless steel. However, they can be a bit difficult to solder wires to. A jig should make easier to achieve consistent separation between pins.

NOTE: - If you decide to install such a monitoring points, connect a 1 K-Ohm, half-Watt resistor between the each pin and the wire end. Should you have a momentary short to another potential you will not smoke the wire and your hand. The resistors will not affect your reading as the milli-volt meter has at least one Meg-Ohm input resistance.

Some experimentation may be needed to determine a suitable pin-separation to get enough voltage-drop. Aim for around 50 milli-volt at full charging and load current. The separation will also depend on the cable's gauge. In our case we are more concerned with ensuring that both monitoring points in a battery-pair are as similar as possible. We are not interested in comparing currents between pairs up and down the battery-stack. There is no current entering or leaving along the way.

You may find it acceptable to use some of the current monitoring points for voltage measurements.


Manual Monitoring, Recording, Analysis and Warning System

To create an entirely manual monitoring-system, the wire-pairs from each of the eight pairs of current-sensing-pins can be brought to a two-deck rotary-switch, preferably with “break before make contacts” unless the recommended 1 K-Ohm resistors are installed. The DC milli-volt meter gets connected between the decks at the wiper. One member of each pair of wires will be more negative than the other, that wire should be regarded as the “common” the other wire as bearing the “signal”. Connecting all “common” wires to one deck, and all the “signal” wires to the other deck avoids the nuisance of the meter switching between positive and negative as the switch-rotor is turned.

If the wire pair from each member of a parallel battery-pair, for example C and G, are wired to adjacent switch-positions it becomes easy to quickly determine current-balance of a battery pair.

With the rotary switch and connection-jacks for the meter mounted permanently, it should take only a few minutes to run through a current-balance evaluation, and the voltages and temperatures. As the commonly available 11 or 12 position rotary switches will not have sufficient positions to accommodate all the inputs, I suggest a second switch to carry the voltage and temperature switching duties with its own separate meter jacks. This allows battery-pair currents and voltage to be observe simultaneously if two meters are available.

NOTE: - It is likely much more convenient to work backwards. Wire the switches first with a generous wire-length, enough to reach the farthest point, mark wire pairs and selector position.


Automated Monitoring, Recording, Analysis and Warning System

Given the firm, if not heated comments, I had a brief look at the to me overwhelming flood of modules offered by manufacturers, such as the vast array of function-blocks from Arduino.. As states before I have no experience with any of this. However, it is easy enough to recognize that adding a laptop and appropriate software a capable designer can create powerful tools.

In my original statements I was concerned, among other things, with allowing the analysis tools to operate at chassis/ground potential. Avoiding the potential for incorrect readings and for damage from accidentally shorting equipment, connected to a high-current-capable battery, to chassis via the instrumentation and computer.

In the approach proposed here, the automated setup cannot be allowed to have a stray path to chassis or any other potential. Even a small capacitance part of a AC power supply can as minimum interfere.

In my experience microelectronic multiplexing devices, when connected directly to the “OFF-Board World”, offer too limited common-mode range, and are susceptible to transients and electrostatic discharges even with the best attempts at protection. Of course there may be implementations that are immune to all this, entirely unknown to me. For now I believe that their inputs cannot easily be made robust enough.

I believe a more appropriate solution is a DPDT relay based multiplexer running through 16 inputs in say 16 seconds with about 0.9 second sample time per input pair, and 0.1 second gap in between. The latter avoids any possibility of momentarily shorting two test-points together across a battery. Although this would not be damaging to the relay contacts if the recommended 1 K-Ohm resisters have been installed at the sewing-pins.

I am thinking in terms of a bank eight to sixteen miniature OMRICON relays. With 5 or 12 volt coils. Whatever voltage can be driven by the Arduino board. The bank of relays is wired to create a floating electro-mechanical multiplexer of functionally similar to the rotary switch. The Multiplexer “common” and “signal” of course go to the Arduino board perhaps via an intervening noise and RF filter.

This where my contribution to the project ends. I must say I am keenly interested in the development of this automated system. I am available to help in anyway I can along the way.


Best Regards All
GerryM

[markopolo]

A quick test for high resistance is to check for heat. In the Better wiring, big difference topic I posted that I discovered a fuse was hot after running a 67 amp load. It was hot not warm so easy to notice. Easy fix, just reinstalled it properly.

A temperature probe on a multimeter would let you find lesser temp differences. Or use one of the laser / IR thermometers (suggested to me offline).

[gsm]

Photolimo & All


1 - I have not seen any further development of this subject after my Post # 87. I wonder if the topic has been left to die? I believe death would be too early.

2 - It is no news to anyone that it is common practice, when creating a battery-array by paralleling two or more batteries, to take and feed power via diagonally opposed battery-posts in order that all batteries face the same impedance.

3 - RT has extended the practice of paralleling individual batteries first and connecting these blocks in series. In my review I found that this is not the best practice.

4 - When dealing with 12 volt system, i.e. an array of two or more 12 volt batteries, there is only the above way to connect them for best performance. Essentially all batteries in the array are forced to see the same terminal voltage. The small departure from the idealized terminal voltage that is the individual native preferred operating voltage is overridden.

5 - We are far too quick to apply this parallel and series technique to build higher voltage battery-systems. The approach becomes undesirable when stacking several paralleled battery arrays.

6 - Discussion of paralleling batteries has always forced terminal voltage-characteristics and accepted what ever total capacity results. In series strings matching of capacities becomes the main issue.

7 - Internally 12 volt batteries consist of strings of six cells. And this seems to work adequately. When stacking two six-volt batteries we again end up with a string of six cells, and this works well.

8 - This approach natural extends to stacked battery-strings. It appears that the longer the series strings the more forgiving of individual internal battery-voltage-spread things become. It is conceivable to charge a six-volt battery in series with a 12-volt, to get an 18-volt battery, as long as the Amp-Hr capacities are the same.

9 - Since my last Post, I have gone over the AGM E-Trek battery situation in a number of ways and come to some additional ideas.

10 - My original proposal was a direct outgrowth of the RT configuration. Although the re-wiring proposed is rather straight forward, difficulties arise almost immediately with tuning each battery-pair of the stack for current-balance, charging and discharging. It is easy enough to do optimal wiring, and check for balance, but to achieve and maintain balance is difficult and tedious.

11 - With respect to original improvement proposal I am bothered that things were getting much more complicated than they ought to be. The devil was indeed in the details. For best results, each of four battery-pairs must be individually matched. This is unlikely to be achieved with only eight batteries to choose among.

12 – One has to consider the following

a) - Accommodating the spread of characteristics present in a batch of good batteries requires a statistical approach.

b) - In a stack of multiple batteries both in series and parallel it is not advisable to make parallel pairs, triplets, etc. of batteries and then connect these in series.

13 - This addresses a question, I believe it was raised by Booster some time ago, while discussing the new battery system of his coach. If I remember correctly it went something like this. Is it better to series connect individual strings of batteries and only connect them in parallel at the ends? In the AGM E-Trek case at the chassis and the 24 volt points, while ensuring that the impedance of each string, facing the charge source and the load consists of the same jumper/cable impedance.

14 - It becomes immediately apparent that an arrangement of parallel battery strings requires fewer jumpers. What is even more dramatic is that the share of battery impedance contributed by the short jumpers is somewhere around a third as much. This is of great benefit. In addition it offers much simplified wiring. (“Less” appears to achieve “More”).

15 - From a statistical point of view even with the best efforts, batteries will have a spread in their characteristics. By picking four batteries at random to make each 24-Volt-string, it is more likely that strings will be very similar. If the strings end-up too dissimilar a clamp-on Ammeter and a Voltmeter will easily establish the differences. Exchanging as few as two strategically selected batteries between strings can bring the sharing of charging and load currents close enough. With eight good batteries a five percent or less current-differential between strings would be excellent, but 10 percent differential is probably still acceptable. It may not be necessary to relocate batteries, as good enough balance might be achieved by re-routing wires.

16 - If a permanent monitoring/warning system is to be installed only two current sensing points are needed. This considerably simplifies the installation.

17 - To exploit the approach on the E-Trek, I suggest that the long cables front to back be used to separately make the connections, such as A+ to E- and A+ to F-. With fewer jumpers needed it becomes easier to choose between batteries for inclusion in a particular battery-strings.

18 - Do the two cables connecting the A and B batteries to the E and F batteries pass through RT's under-floor termination box, via the circuit breakers and the stand-offs? Is this the only protection in place?

19 – in the new layout, failure of one string reduces capacity to half, but leaves the other string available.

20 - Having switches to select one or both strings would help in battery bank evaluation and problem isolation. For example one string draining the other.

21 - At the outset all eight batteries should be checked separately and preferably fully charged before interconnection.

22 - Please ignore my previous reconfiguration in favor of the above suggestion. Although my previous suggestion was an improvement over RT's original arrangement, the one in this Post is even better and simpler to implement.

23 - If this were my coach I would measure whether I could shoehorn four Lifeline GPL-L16T, 400Amp-Hr, six-volt AGM batteries into the rear compartment. Battery dimensions are 12 inches long, 7 inches wide, and 16 inches high. This would give me a single low impedance 24 volt battery-string. For one this would get rid of all paralleling issues, and there would be no house-batteries under the hood.

a) I would mount the batteries on their side with the narrow side down and terminals toward the rear-door. Battery manufacturers claim that AGM batteries can be mounted on the side, except not upside down. The better of the two side-mounts is narrow-side-down as the plates and mattes with electrolyte are horizontal, which avoids having gravity pull electrolyte toward the lower plate-edge.

b) There should be just enough space to access the terminals and jumpers. If necessary I would raise the cover an inch or so.

c) I would prepare the batteries in pairs. Pre-install the jumper near the bottom. Drop each pair in place. This leaves only the jumper interconnecting the pairs.. One jumper-end can be pre-installed the other end attached just before the second pair is dropped in completely. The chassis-ground and 24 volt connections, which are near the top can also be pre-installed if connection at the far end is lifted.

d) I would see if I could use the two cable runs front to back to improve alternator-charging by reducing voltage-drops.

24 - In an recent Post (could not track it down) Booster touched on the more or less imminent appearance of 48-volt battery-systems. One can create a very high Watt-Hour battery-systems consisting of as single stacked string of batteries. For example using eight of the 400 Amp-Hr, six-volt batteries just referred to, one obtains 19,200 Watt-Hours. Which means one has 10,000 usable Watt-hours at a weight of nearly 1,000 lbs. This where lithium-ion batteries become very attractive.

25 - Had RT reached for a 48 volt system, sidestepping 24 volts entirely they could have achieved their goals by stacking the two 24 volt battery strings, and avoided much if not all the chaos, assuming that the other electronics needed could have been found.


Best Regards

GerryM

[markopolo]

Would that be something like AECD in series at 24V and BFGH in series at 24V then parallel AECD & BFGH?

As a reminder of what the current setup looks like according to photolimo, here are the voltages on solar float charge and the parallel connections in this image:

E-Trek Battery Layout how to balance.jpg

Series:
A + B + C + D
6.33v + 6.43v + 6.94v + 6.88v = 26.58V

Series:
F + E + G + H
6.34v + 6.43v + 6.94v + 6.88v = 26.59V

If balanced, you'd expect to see:
6.65V + 6.65V + 6.65V + 6.65V = 26.6V

Info Source: http://www.classbforum.com/forums/f5...html#post35767

[booster]

My question was actually very basic, for a 4 battery system primarily, but also for more batteries in theory.

My original test on the bench had two strings of two 6v batteries, then the two strings paralleled. If I read what GSM has in his last post correctly, that would be his preferred configuration. I got amp and voltage drop readings that were fairly different among the 4 batteries and banks. Lifeline said no biggy, but who knows for sure, as it may or may not be splitting hairs. The question also asked if tying the banks in the middle, as Roadtrek does, and quite few articles recommended might be worth the effort.

Packaging of our batteries into our location made it appear that having the system two 6v batteries in parallel, and then in series with another parallel pair would make for a cleaner install and better cable routing in and out of the area.

I ran another test on the bench with above parallel first setup and got considerably better matching of the voltages and currents between the batteries, which surprised me.

What surprised me even more, and was a question I have asked several places on this forum, was what would cause the return amp minimum to change to considerably lower in the parallel first (second test) It dropped nearly 50%. We are talking very low amps of something like 1.7 amps vs .9 amps for 440ah of batteries at 14.3v. At the end of a charge cycle, I immediately took off the charger and separated the batteries, let them rest a few days, and then checked their voltages. They matched nearly identically, and considerably better than the series first, then parallel test (first test).

Did the batteries actually get more full do to the better voltage match and that caused the lower return amps. I have no clue . It does seem to indicate that for our 4 battery setup, everything (that I can measure) looks better being parallel first then series, which is not what GSM has found, so there is something going on.

As to #8, it is extremely hard to charge or discharge a series 6V and 12 string. We played with this a lot in the 70s on tow trucks and jump start setups to get 18v to bump starter speed, once the starter was engaged to hold the voltage down and not fry stuff. It worked really well on the hard to turn stuff like the big GM engines. Nobody really got the systems to not be hard on batteries, but they got them to work. The best seemed to start cranking with engine booster and 12v batteries and then parallel in 3 six volt battery making 18v on single relay. The good thing was that the gas power booster would up its voltage to come close to the new reference in most cases, helping even more. The series 6 and 12v stuff never worked as well.

I do think that if you were going to do a 6 and 12v charging in series, capacity alone would not make it work, as they would need to have identical cell design so the impedance and acceptance rates on all 18 cells would match perfectly like internal cells in one battery do. If the cells didn't match, you wouldn't get the same voltage across all the cells.

Of course, the lithium folks build whatever voltage they want and the try to balance them, and it seems to work OK if the cells aren't very disparate.

[Boxster1971]

Originally Posted by booster

My question was actually very basic, for a 4 battery system primarily, but also for more batteries in theory....

..... Packaging of our batteries into our location made it appear that having the system two 6v batteries in parallel, and then in series with another parallel pair would make for a cleaner install and better cable routing in and out of the area.....

..... I ran another test on the bench with above parallel first setup and got considerably better matching of the voltages and currents between the batteries, which surprised me.....

Interesting information. Are your batteries close together in your location?

I looked at this with my four 6V batteries widely separated on each side of my van. The cabling to do parallel first would give me more connectors and one long cable run to get them in series.

[booster]

Ours are setup 4 in a row, sitting on one end so the terminals face the rear of the van.



In this configuration, the parallel then series made it really nice for cabling. The plus comes in at the upper drivers corner and neg goes out at the passenger top corner. All the jumpers only go one battery over so very short and the same length.



As I mentioned, I really don't know if it is better or worse one way or the other, just that stuff I can measure APPEAR to be better. No way to check actual state of charge in AGM, so indirect is all we've got to go by. Either way, I don't think it is a big deal because I have seen information at battery sites that show both ways, and I couldn't find any information about it anywhere I looked. I hope GSM can let us know where he got his information, as it is probably an educational read.

[Boxster1971]

Nice setup booster! With the batteries that close together you can easily wire them either way. Or even change it later if you wish.

[markopolo]

Just in case there's an educational moment coming here's a sketch of four 6V batteries in parallel/series and series/parallel with the batteries labeled ABCD for easy reference.

series parallel or parallel series.JPG

[Boxster1971]

Thanks for the diagrams markoplo. I was going to add some myself as I noticed you always end up with two more connectors in the parallel then series setup as compared to the series then parallel.

[booster]

Originally Posted by Boxster1971

Thanks for the diagrams markoplo. I was going to add some myself as I noticed you always end up with two more connectors in the parallel then series setup as compared to the series then parallel.

You are correct, and the series then parallel works well in a four square setup, not so well in four in a row, for jumper length and equal length. If you add the center "balancing" jumper to equalize the voltage in the middle of the two series strings, like Roadtrek and some others do, you wind up with same amount of connections. How much difference any of this makes is probably not worrying about, but like Marko, I hope we get a "teaching moment".

[Boxster1971]

Yes the Series then Parallel worked well in my four square setup. It looks like the diagram below with a positive terminal bar and negative shunt in the center space.

[booster]

Originally Posted by Boxster1971

Yes the Series then Parallel worked well in my four square setup. It looks like the diagram below with a positive terminal bar and negative shunt in the center space.

Especially good in that style for your setup with the batteries on each side of the van. You had to get them tied together anyway, so the two positives and negatives were there anyway and you got to be able to balance the cable lengths. My original test on the bench that was series first was wired identically to what you show, with the double cables.

Very nice, and looks very well balanced.

[gsm]

Booster, Post 91; Markopolo, Post 90; Boxster1971 Post #98

1 - My Post # 89 was based on memory of discussions, books, articles, application notes I saw in the 1980s when interested in off-grid power-systems. This is all way before the internet. I am sorry that I cannot be as helpful as hoped for, but maybe this Post will get things partway.

2 - I had forgotten about what I had read back then about good and bad choices when configuring a battery-system, to take account of practical aspects needed for day to day monitoring and maintenance of life systems. Primarily, I recall some of the logic behind the choices of some system-builders. Memory is foggy and documents and notes long since gone, but what I suggested in Post # 89 is an amalgam of what I read and digested at the time.

3 - I partially recall discussion of a moderate size battery-system consisting of 36 batteries configured into 36-volt, rectangular field. The field consisted of of six separate strings of series connected six-volt batteries, that were then paralleled at the ends only. I do not know why 36 volt was chosen or what the electrical goals were. Maybe this aspect was not dealt with. I do not recall specifics of the batteries, except that they were more than likely something akin to a T-105. Perhaps it was the esoteric significance embedded via the repeated number “six” that made it memorable. There was mention that the charging system was oversized to ensure that batteries would be a frequently fully charged.

4 – It was reasoned that in the situation it was best to make strings of six batteries (36-Volt system) fuse both ends, connect the six strings in parallel to the negative and positive copper-buss. At the negative buss (the ground end) of each string had a shunt. Fuses at both ends of strings were guards against Murphy's Law.

5 - What was interesting were covers, to expose only one battery at a time.
There was a plank, a movable trestle, and a length of two by two with hooks as lever to lift and replace any battery in the field.

6 – Individual strings could be disconnected for service. It also made it convenient to detect, isolate and correct battery failures such as cell-shorts and open conditions. Fusing of each string could be for one sixth of the total current making it possible for a cell-short to drop a string. The onset of a fault-condition and the final interruption could be easily detected by monitoring shunt currents, followed by voltage measurements and load-tests along the string, and replacement operation.

7 - The argument was that paralleling six batteries at each voltage level in the stack made problem-isolation/correction a nightmare, and would likely necessitate service interruption.

8 - Furthermore paralleling batteries up the stack wastes copper. If each battery does its job, there is no appreciable lateral current flow. In any case the lateral current would not go through the load, where it is needed. The lateral current would try to maintain one battery at the expense of another. Substantial lateral current is a sign of a problem, which gets masked.

9 - If one string drops out capacity for the system suffers less than a 20 percent loss. Not a big deal. If this matters than the system was too small to start with. At worst it should mean a temporary deeper discharge until the string was restored.

10 - There was some discussion of the statistical differences among batteries, suppliers, battery ages, differences in service conditions, etc.. Taking care of the gross differences by avoiding installation of disparate batteries, all else tended to be averaged out (buffered) within and between strings, if not immediately, it would over time.

11 - It was not the track of individual battery-terminal-voltage versus SOC that was thought important, but that capacities of the batteries in the string were similar enough to come to full charge together, and when discharged reached equal discharge. Ultimately the only thing that matters for each string is how it behaves at the end terminals. Furthermore since all strings used the same batteries, the paralleled strings shared charging and discharging current as equally as possible. Any departures here could be found via the shunts.

12- The argument went, since the six by six setup started with a batch 42 batteries, all from the same production-run, random selection would likely give very similar terminal behaviors among the strings. Individual batteries would be tested and installed fully charged. There were pros and cons on various aspects, but the bottom line was not to worry about things until the system could be tested. At that point it might be decided based on measurement to attempt to match the strings somewhat better by exchanging batteries between strings. I did not try to follow-up on operating results obtained over the years.

13 – Six of the 42 batteries were to be setup in a 12-volt system. Over-kill for the 12-volt needs, but regarded as a ready source of a six healthy spare batteries for the main system. The intent was to use the best four batteries of what was left of the failed string to reconstitute a 12-volt system. The string replacement was of aged but healthy batteries, that would have been generally kept at full charge.


NOTES: -

14 - With respect to Booster's Posts # 38 and # 91 dealing with 1.7 Amps versus 1.0 Amps. My suspicion is that two of the batteries have a slightly higher capacity than the other two. One battery is located at the top of one string and the other at the bottom of the other string. When the strings are tied together at the midpoints, a diagonal bridge is inserted allowing these two batteries to more fully charge, approaching the combined leakage resistance of the batteries. Without the bridge each string has the leakage resistance of a fully charged battery in series with the lower resistance of a still slightly thirsty battery, giving a higher current. A current that is so little above the leakage-current that the higher capacity batteries would likely never be able to charge fully. This is made worse because only a fraction of this already small current is turned into battery-charge. (These two batteries probably also exhibit a few tens of milli-volts lower terminal-voltage). In any event this combination has a lower resistance, and “fakes” a higher finishing current. Over time, without the bridge-jumper these two batteries will give up this slightly higher capacity, and all batteries will come into line.

15 - If both of the slightly higher capacity batteries had turned up at the bottom or the top of their respective string, there would have been no noticeable change in finishing current when connecting the bridging-jumper, and no awareness that two batteries started with a slightly higher capacity, and were gradually giving up a slight amount of capacity. How much is that slightly higher capacity? I speculate it may be as little as two percent. It is one aspect of the spread in a production run of batteries. It is never specified, but is nevertheless there, and ultimately does no matter.

16 - On the other hand, if the two batteries, that I suggest have slightly higher capacity, had instead a lower leakage resistance (more leakage current), adding the bridging jumper could have significantly increased the finishing current, depending on the relative sizes of the leakage resistances of the four batteries.

17 - When stacking several jumpered-battery-pairs as on the AGM E-Trek there is a capacity spread among the pairs. Pairs of higher or lower capacity that will over the long term either be reduced in capacity or will reduce others in capacity. If batteries are close to start with, loss of a couple percent in capacity until things stabilize will not be noticed except on the laboratory bench.

18 - One would have thought that battery manufactures would have adopted some form of electrolyte PH sensor, to replace the specific gravity measurement that is impossible on sealed batteries. I would not be surprised that patents exist. One could imagine this as a couple of electrodes in the electrolyte (perhaps between layers of separator) through which a small AC voltage can be passed to detect a change in electrolyte conductivity as the cell charges/discharges. This is probably against their interests as it would increase cost, and expose more of the vagaries of current battery technology, such as differences between individual cells. In the end it probably makes no real-world difference.

19 - The intent of # 8 in my previous Post # 89. Starting with two identical 12-volt batteries. If one charged all six cells of one battery in series with only three cells of the second battery it is quite doable. All nine cells are well matched electrically, physically and chemically. If an actual six-volt battery is used, it essentially has to be constructed as half of the corresponding 12-volt battery. In effect one ends up with what amounts to three 6-volt batteries in series. I fully agree with what Booster states. I did not state it as clearly as I should have.

20 - What I was trying to emphasize is that we operate strings of cells in our sealed batteries, and some times can operate them in what appears on the surface as a contrary way, without puzzling too much about what happens internally to individual cells. We are not worried when paralleling say two 12-volt batteries. So why are we getting so concerned about lateral jumpers when connecting strings of groups of three cells (that is six-volt batteries)?


Markpolo in response to your Post # 90

21 - Establishing two series strings on the AGM E-Trek. Your Post captures the matters well, except that convenience and wire-length might dictate whether battery A or B becomes part of a particular string. I chose to describe this once more addressing some of the details and unresolved issues.

22 - In the rear compartment batteries F, G & H are adjacent to each other and easily form the upper three batteries of a string. The upper part of the other string consists E, C & D. Output is taken from D+ and H+ via fuses. Under the hood A- and B- go separately to chassis/ground via fuses and shunts. The fuses could be mounted on the battery terminals rather than on the chassis, also saving a jumper. The owner may prefer to do current monitoring in the rear via magnetic sensors, that is two hall effect loops through which string-current passes. The two long cables to the rear compartment connect either A+ or B+ to E- or F- depending on convenience and available wire length. In any case the result is two strings of four batteries, without jumpers between intermediate batteries.
23 - I do not know how the front to rear cables run. I suspect that they go through RT's terminal and breaker box, mounted underfloor behind street-side rear wheels. I could be totally wrong about this. I asked about it in previous Posts, but got no response to date.


Boxster1971 Post # 98
24 -I did not follow the evolution of your battery system. Are you using the chassis as the negative connection to your batteries or a cable-run? Perhaps both the negative and positive runs are on the same side of the coach?

25 - I am sure you chose your layout for sound reasons to suit your particular coach and needs. I am addressing others eying your setup with the intent of creating something similar for their coach.

26 - I am glad you chose the parallel string setup. You do not show a fuse in the negative end of each string, as a short to chassis at the positive end of the bottom battery or above would be dangerous. It would also involve both strings. An internal failure of one string should not affect the other string, which should survive and continue to serve.

27 - If the chassis is used for the negative run to the battery-bank and depending whether the distribution panel for 12 volts is on the street or curbside, the layout could be configured a bit differently to take advantage of the battery terminals as 12 volt take-off points. Furthermore the bottom battery of each string would be on one side of the aisle and the upper battery for each string on the other side of the aisle.

28 - Fuses for one half the total current demand on the battery bank, mounted directly on the negative terminal of the lower battery of each string, are in turn connected to chassis/ground.

29 - In place of the negative and positive buss-bar/stand-off, the two cables across the aisle could be one piece, joining the positive of the battery forming the lower part of each string on one side of the aisle to the negative on the battery making the upper part of each string on the other side of the aisle. Power in and out is taken from the tied together positive posts of the upper batteries of the two strings.

30 - If additional battery capacity becomes necessary, and there is space for one additional battery on each side of the aisle, the system can be expanded to three parallel strings each carrying one-third the total current. Weight continues to be evenly distributed left to right.


Best Regards to All
GerryM