6v series vs 12 v coach batteries

[SteveJ]

Originally Posted by RossWilliams

There is no reliable way to accurately monitor your battery capacity while using it. The best you can get is an estimate and that is good enough if you don't need to stretch your capacity to its absolute limit. If you are pushing things to their limit you are likely better off investing in better batteries rather than a better battery monitor.

I would like to sort of disagree here. "There is no reliable way to accurately monitor your battery capacity while using it" with a volt meter. One could use a hydrometer to check an FLA battery but that is not a practical thing to do on an ongoing basis when camping

A shunt based monitor will certainly inform you of the the batteries amount of discharge. My monitors (I have two of the cheapies from Amazon) keep a running total of watt hours consumed and charged as well as the amount of current going either way and the voltage. About $32 at Amazon, all in. I have found that a real monitor system(not just a volt meter) is invaluable when boondocking, especially with solar.

https://amazon.com/gp/product/B01JOU...?ie=UTF8&psc=1

There are other solutions that are less un-elegant and a bit easier to use. Victron is one, there are many others. Searching around the forum for battery monitors should yield enough reading to kill a day in lock down.

[booster]

Originally Posted by SteveJ

I would like to sort of disagree here. "There is no reliable way to accurately monitor your battery capacity while using it" with a volt meter. One could use a hydrometer to check an FLA battery but that is not a practical thing to do on an ongoing basis when camping

A shunt based monitor will certainly inform you of the the batteries amount of discharge. My monitors (I have two of the cheapies from Amazon) keep a running total of watt hours consumed and charged as well as the amount of current going either way and the voltage. About $32 at Amazon, all in. I have found that a real monitor system(not just a volt meter) is invaluable when boondocking, especially with solar.

https://amazon.com/gp/product/B01JOU...?ie=UTF8&psc=1

There are other solutions that are less un-elegant and a bit easier to use. Victron is one, there are many others. Searching around the forum for battery monitors should yield enough reading to kill a day in lock down.

Battery monitors are an extremely useful tool for all of us that have them and have used them for longer periods of time to better understand them.


In general, a shunt based monitor will be very accurate accurate on the discharge cycle in counting amp hours. The inaccuracies of a monitor are caused mainly by other things.


* You need to have consistent 100% full charging of the batteries, or if you don't go to 100% it has to be consistent and known how full they are. It is impossible to know how much energy you have left if you don't know where you started. IMO, the best and mostly only, way to know you are full is to meet the voltage and amperage trigger points for charging voltage to the batteries that indicate a full battery of that size for lead acid batteries. Lithiums more commonly go by voltage only.



* The monitor needs to have a reset to 100% full when those charge parameters for full have been met so it is reading 100% at the start of the discharge cycle.


* You need to have the correct, and actual, battery bank capacity for your bank set in the monitor if you want to be able to read % full SOC. This is not as easy as it it appears because the actual capacity changes with the battery age, and temperature. The rated capacity is at a 20 hour discharge rate (5 amps for 20 hours and a 10.5v empty voltage point while discharging to get 100ah capacity) so it is not a true indication of actual energy stored in a battery as higher amps will cause voltage drop and give a perceived lower capacity. Having a Peukert setting in the monitor will not get rid of this issue, though. It is relatively easy to keep up with the capacity changes if you do a simple test when you put in the new batteries. Set the monitor to the 20hr rated capacity and discharge the batteries to 20%, if that is your low end cutoff choice. Shut off all loads and note the battery voltage after sitting overnight. Every year or two run the same test and see what the voltage comes out as. If it is lower you just need to adjust the AH capacity setting down a bit as the batteries have lost some actual capacity. Temp you can allow for from a temp capacity loss chart and estimate as used, it needs to vary a lot to make much real difference, but can be an issue if very cold. As in the first comment, the batteries need to get reliably full to have this stuff work.


* On the recharge cycle is where the accuracy of a monitor is inherently very difficult to get to be accurate. It takes more amp hours into a battery to recharge it than you used taking them out, based on the charge efficiency. The problem is the charge efficiency is a dynamic item that changes with SOC, charge rate, etc over the charge cycle. It can be nearly 100% when the SOC is at 20% while charging and well under 10% when the battery is near full. The integrated "under the curve" charge efficiency is what is needed at all points to tell you the actual SOC during recharge. No meters are capable of those calculations, so the recharge will not be spot on. If you have very consistent discharge depths the composite charge efficiency will be the same every recharge cycle and that can be put in the monitor so the recharge % will be correct, but RV use rarely gives consistent discharges so you are stuck with a most common type setting. It will normally take anywhere up to about 15% more ah in than came out, depending on the battery condition, type, and DOD.


* On top of the above charge efficiency on a single cycle issue is the one that probably bites more users and gives the most inaccurate readings in the real world. The issue is that if the batteries don't get full on a particular charge cycle, the inaccuracy in the meter of that recharge cycle carry over to the next discharge and all other cycles after that until you get a fully charged cycle that resets the monitor to 100%. In other words, every consecutive short cycle compounds the inaccuracy, and there is no way I know of to prevent it. Many short cycles of lead acid batteries will get to about 80% SOC as that is within the quicker charging rate area and up to there the charge efficiency is pretty good, so maybe 5% per cycle buildup. If you happen to have the charge efficiency setting at that 95% you can come out pretty close but if the SOC at charging varies, or the DOD before charging varies it will go off to some amount. I tend to just let the monitor do a running average charge efficiency setting, which not all monitor do, and it is OK for us a we have a big battery bank so never come close to needing to know exactly where we are on SOC. We also tend to get to a full charge reset at least every 3 days so never very far off. Those with a single battery and less capable charging systems need to allow for this issue as knowing very closely how much power is left is very important if you are on the edge of running out regularly.


With our current setup, I normally will only look at the ah we use in 24hr period once in a while, as it is a good indication if something has gone wrong like a frig issue or big inverter left on. Checking for reset occasionally is done, but it has always done what is should when charged full.



Bottom line for us is that we would not be without a monitor even though we now don't need it as much, as it is one of the main reasons we were able to get to this point. I wouldn't build a setup without one.



The idea of living with something like 20% accuracy of some idiot lights, because a monitor is only 95% accurate so not good enough doesn't make sense to me.

[RossWilliams]

Originally Posted by SteveJ

A shunt based monitor will certainly inform you of the the batteries amount of discharge. My monitors (I have two of the cheapies from Amazon) keep a running total of watt hours consumed and charged as well as the amount of current going either way and the voltage. About $32 at Amazon, all in. I have found that a real monitor system(not just a volt meter) is invaluable when boondocking, especially with solar.

There is a long discussion of the problems with shunt based monitors, but the summary is they only give you one side of the equation, how much you have consumed, not how much you have left. There are a variety of reasons why those two things won't match. For instance, a battery's actual capacity will vary from its plate capacity depending on the temperature, how it is used and how old it is.

With AGM batteries, relying on a power in - power out to evaluate the state of charge can create the illusion you have a lot more remaining capacity than is actually there. That can easily lead to chronic over-discharge of the battery, particularly as it ages. There are system's that try to account for this, but they have their own weaknesses.

My understanding is that most lithium batteries have a built in BMS that prevents battery damage and the shunt system is really the only option since battery voltage remains flat for most of the discharge cycle. But it still only gives you an estimate of your remaining capacity.

[booster]

If there is a worry about over discharging, you need a monitor more than ever so you can evaluate realistically what the problem is, as in use, charging, bad battery, unknown current drain. Once you define the problem, you can fix it correctly without just throwing stuff at it. As mentioned above, testing battery capacity is very doable at home.


If you still are worried about overdischarge, put a low battery cutout on them and the problem is gone, just like with a BMS. It baffles me how having a monitor would make you more likely to overdischarge compared to not having one. A monitor is very accurate in telling you if you are getting full.



As mentioned, I will always choose "less than perfect" over "very inaccurate". Getting paralyzed because of the quest for perfection doesn't give any results, and living with an inadequate system would be the result.



The biggest intangible and common problem, as I have mentioned many times, is that I think most chargers are not doing a good job of getting the batteries to full. IMO, it dwarfs the problems from other things because you have less energy available than you think, and it destroys the batteries making any other issues worse.

[Winston]

Booster, we think you've identified one of the subtle advantages of lithium. Most of the 'contributors' to shunt-based SoC monitoring inaccuracy you discussed seem less of a concern for lithium. Reaching 'full charge' (for resetting the SoC to 100%) is comparatively easy. Our charge/discharge efficiency is very close to 100% and can largely be ignored. Our overall capacity has remained essentially unchanged over the first three years of use (although to know this, we've had run full capacity tests each year). Lithium capacity doesn't temperature-sag as much as lead-acid (our inside-mounted batteries rarely dip below 45 degrees in any event). The only uncertainty is whether our capacity would change under greater loads - - our capacity tests are made at 10 amps. But, except for those short-term blasts of 150 amps (induction stove or hot pot), a typical load is 10 amps (0.02C). We wouldn't expect much capacity variation when most operations are conducted at such low 'fractional C' levels.

[booster]

Originally Posted by Winston

Booster, we think you've identified one of the subtle advantages of lithium. Most of the 'contributors' to shunt-based SoC monitoring inaccuracy you discussed seem less of a concern for lithium. Reaching 'full charge' (for resetting the SoC to 100%) is comparatively easy. Our charge/discharge efficiency is very close to 100% and can largely be ignored. Our overall capacity has remained essentially unchanged over the first three years of use (although to know this, we've had run full capacity tests each year). Lithium capacity doesn't temperature-sag as much as lead-acid (our inside-mounted batteries rarely dip below 45 degrees in any event). The only uncertainty is whether our capacity would change under greater loads - - our capacity tests are made at 10 amps. But, except for those short-term blasts of 150 amps (induction stove or hot pot), a typical load is 10 amps (0.02C). We wouldn't expect much capacity variation when most operations are conducted at such low 'fractional C' levels.

I totally agree, and in general, even getting to reset isn't that big a deal with lithium as the charge efficiency is so close to 100% in most cases that you could do many, many, short charge cycles without doing much to the accuracy. You also get very good charge accuracy on recharge with very high charge efficiency.



Since for the most part I think Peukert use in RV systems is not useful for many past stated reasons, I totally agree with your choice of amperage for the capacity testing. The best thing to do, with lead or lithium, IMO is to use the amperage that would be most likely to be being used when you get to cutoff. That said, I haven't done that as we have enough capacity that the difference to just using the 20 hour rate is to the conservative side as we are actually closer to the 5 hour rate. It is a lot easier if you are going for an 80% down point to just pick an amp hour out based on the 20 hour rating and just repeat that at future tests. Lazy way out Since the lithium is voltage controlled at cutoff, you have the luxury of just going to a voltage and then seeing how many ah it was as you don't have to let them rest as long as lead and that is what you control by anyway while in use.

[SteveJ]

Originally Posted by RossWilliams

There is a long discussion of the problems with shunt based monitors, but the summary is they only give you one side of the equation, how much you have consumed, not how much you have left. There are a variety of reasons why those two things won't match. For instance, a battery's actual capacity will vary from its plate capacity depending on the temperature, how it is used and how old it is.

With AGM batteries, relying on a power in - power out to evaluate the state of charge can create the illusion you have a lot more remaining capacity than is actually there. That can easily lead to chronic over-discharge of the battery, particularly as it ages. There are system's that try to account for this, but they have their own weaknesses.

My understanding is that most lithium batteries have a built in BMS that prevents battery damage and the shunt system is really the only option since battery voltage remains flat for most of the discharge cycle. But it still only gives you an estimate of your remaining capacity.

I don't get all that anal about it. I just use the monitors as a guide in conjunction with the read outs from the solar controller. When I get past 500 watts hours of discharge(cloudy days, too lazy to deploy the solar when only staying a night or two) I'll start to pay closer attention as I want to charge from some source by the time I get around 700 or so. The battery was good for 1200WH when new and is two years old. It's mostly a best guess scenario but with real data the guess can be closer to reality.

Charging systems include 150 watts of portable solar, genny(not an Onan), and chassis alternator. Battery is an Everstart Maxx group 29 "maintenance free" LA. This set up has worked quite well for a few years. If/when I have to replace the absorption fridge it will most likely be something with a Danfoss compressor. At that point I will likely build a bit more capacity into the system. Probably go to two batteries and add another 100 watts of solar and maybe switch to an MPPT controller depending if I go series on the panels.

Bottom line for me. It is a lot easier to observe what is going on with a shunt monitor. I used the solar for about 2 months of boondocking without it when first installed. I'm not going back.

[RossWilliams]

There are lots of people who basically do the same thing using idiot lights or voltage as triggers. As long as you aren't trying to push the system to its limits, it doesn't really matter. But if you are trying to squeak out another half hour of air conditioning by taking your batteries down to 80% depth of discharge instead of 65-70%, no monitor is going to give you the ability to do that reliably.

[booster]

Originally Posted by RossWilliams

There are lots of people who basically do the same thing using idiot lights or voltage as triggers. As long as you aren't trying to push the system to its limits, it doesn't really matter. But if you are trying to squeak out another half hour of air conditioning by taking your batteries down to 80% depth of discharge instead of 65-70%, no monitor is going to give you the ability to do that reliably.

Not true because if you know you got a full charge (which the monitor will tell you accurately) and have tested the capacity of your batteries (which monitor allows you to do easily), the discharge cycle will be very accurate. Voltage and lights can't duplicate that as first off you don't know how much capacity you have and secondly you don't know if you started full, thirdly you don't know how much energy you used. You are much likely to go under on SOC with the lights than with a monitor as some of them will show good until they are very dead, which I know for a fact because we had one. They also will show full when on charger, as will a voltmeter even if they are not anywhere near full because of the charger voltage.



I don't think Ross has a monitor or ever used one personally for a length of time like the rest of us have. As far as I can remember, we have never had someone who went from no monitor to having one ever say it wasn't a good idea and most say it is now a necessity for them. I think that kind of speaks for itself.

[SteveJ]

Originally Posted by booster

Not true because if you know you got a full charge (which the monitor will tell you accurately) and have tested the capacity of your batteries (which monitor allows you to do easily), the discharge cycle will be very accurate. Voltage and lights can't duplicate that as first off you don't know how much capacity you have and secondly you don't know if you started full, thirdly you don't know how much energy you used. You are much likely to go under on SOC with the lights than with a monitor as some of them will show good until they are very dead, which I know for a fact because we had one. They also will show full when on charger, as will a voltmeter even if they are not anywhere near full because of the charger voltage.



I don't think Ross has a monitor or ever used one personally for a length of time like the rest of us have. As far as I can remember, we have never had someone who went from no monitor to having one ever say it wasn't a good idea and most say it is now a necessity for them. I think that kind of speaks for itself.

Agreed with all of it.

[RossWilliams]

Steve -

Here is a link to an explanation of the limits of battery monitors. The author is NOT against them to wit:

"This article is long and in-depth. Please do not misconstrue the points here and think we are trying to talk you out of an Ah counting battery monitor, nothing is further from the truth. Coulomb counting battery monitors give you great information but we also strongly believe owners should better understand; ..."

Despite that statement, my conclusion was that a shunt based battery monitor was unnecessary for someone like me who was only interested in knowing I would not run out of power or kill my batteries when off the grid. And like idiot lights and voltage meters the information you get can be easily misinterpreted by people seeking certainty where there is none. This author, based on actual experience, gives a very detailed description of where people go off the rails interpreting information from a shunt based monitor.

Edit: The article is very long and thorough. It convinced me that unless you are VERY anal eventually your shunt based monitor will be misinforming you more than it is informing you. You need to be prepared to do a lot of tinkering.

[booster]

That link furnished on the downsides of monitors is brought up every time this comes up by this poster. The article and it's weaknesses in technical issues has been discussed to very long lengths so we don't want to go there again, I can assure you.


Bottom line for me from that article, a lot of the normal how they work stuff is fine, but his conclusions and particularly the assumptions of horrible practices being used to get those bad conclusions make, IMO, the condemning of monitors not valid for a huge majority of users.



The other discussions state all if well, but long and technical to be sure. Look them up if you dare. Many of us have real first hand on the road use results that we have gotten personally, and as mentioned before we don't hear of people having all these huge issues claimed, but we hear it all the time from those with idiot lights and voltmeters who have issues. I don't put much stock in claims of unusability by someone who has never had one, I guess, but just my opinion.

[Bud]

"Many of us have real first hand on the road use results that we have gotten personally, and as mentioned before we don't hear of people having all these huge issues claimed, but we hear it all the time from those with idiot lights and voltmeters who have issues."


Bingo. Long sentence above with multiple truths I think!

Those with monitors tend to learn them, understand them, what the monitor is saying to them. Those with idiot lights and even battery voltage meters do not tend to learn them, understand them, know what the lights/meter is saying to them As Often.

I would personally like to be tested with my batteries and voltages meters vs a booster managed monitor, like what does 12.05 volts mean with a furnace, tv, sat rec, and cell phone charging simultaneously mean? I state the approximate battery soc then booster. Heck, I would pay money, what good fun. I would bet I come within 8%+/- soc when below a 70% soc.

Bud

[booster]

Originally Posted by Bud

"Many of us have real first hand on the road use results that we have gotten personally, and as mentioned before we don't hear of people having all these huge issues claimed, but we hear it all the time from those with idiot lights and voltmeters who have issues."


Bingo. Long sentence above with multiple truths I think!

Those with monitors tend to learn them, understand them, what the monitor is saying to them. Those with idiot lights and even battery voltage meters do not tend to learn them, understand them, know what the lights/meter is saying to them As Often.

I would personally like to be tested with my batteries and voltages meters vs a booster managed monitor, like what does 12.05 volts mean with a furnace, tv, sat rec, and cell phone charging simultaneously mean? I state the approximate battery soc then booster. Heck, I would pay money, what good fun. I would bet I come within 8%+/- soc when below a 70% soc.

Bud

If you have a history with your batteries and your use is very constant, you probably don't even need the voltmeter by now, and if you have had experience to what happens to the voltage when the stuff is turned off to get a rough idea of SOC based on not or short time rested batteries, you would know even better.



Before we got the monitor, we did the "fixed known load" vs voltage, vs SOC because we had wet cells and could compare voltage to actual state of charge. At the time that was one of the commonly tried fudges to get more accurate SOC readings from voltage without having to rest the batteries all the time. I think we used a couple of the ceiling lights that were still fluorescent and the safety monitors running only so it would be easily repeatable. It worked pretty well, but it took many days to accumulate the data and make a chart to look at. It takes a lot of specific gravity testing. It would take much longer with to develop the same for AGM as you would have to rest the batteries and go be rested voltage to get SOC at each data point and it would still be less accurate than the specific gravity reading for the wet cells. Oh the bad old days, I think we will stick with a monitor. I would guess with the wet cell load and voltage test, we were pretty easily within Bud's 8% most of the time, but I am not sure we could be that close with the AGMs unless we did huge amounts of testing points.



Back in the day, everybody needed to get familiar with how long the could go with typical use (that was a lot, lot less than most use now) with learning usually done be killing the battery multiple times. With all the electric stuff in use now, typical use can vary quite a bit for many people, so makes it tougher to do the learning curve without at least doing the Bud test of load with voltage either intentionally or by accident

[Jim86301]

The Elite DCM-120100C measures both charge and discharge current in a single unit and cost around $30 from eBay (https://tinyurl.com/t6qc3d5).
Jim

[booster]

Originally Posted by Jim86301

The Elite DCM-120100C measures both charge and discharge current in a single unit and cost around $30 from eBay (https://tinyurl.com/t6qc3d5).
Jim

Do those units do a reset of the state of charge after a full recharge cycle? Charge efficiency settings?

[GeorgeRa]

I have a few years of user experience with batteries in RVs and boats. Long ago voltage and acid concentration were the only options. I even used nixie tube display voltmeter, remember those? Then I got HP multimeter with LED display, still have this antique meter but Lead Acid battery is dead. Then a gamut of idiot LEDs which were practically useless except True Charge remote with higher resolution.

My first shunt battery monitor was the venerable Link 10, wow, it worked. For my current system I paid attention to accessibility of information, so I ended up with 3 components for my 230 Ah house and engine batteries:

- Primary monitor for house batteries is Magnum ME-ARC, ME-BMK. Works well with a reasonably good viewing access, see the picture. I usually sit down on the sofa to use it.

- On the galley, visible from outside high-quality voltmeter showing house batteries voltage

- On the galley, visible from outside shunt-based ammeter showing current flowing alternator <> house with sign indicating direction.

These 3 meters give me good visibility to the state of the batteries system. If I would start over, I would likely change battery monitor to a smaller one located in a place visible from outside and inside, perhaps on the galley.