Alternator charging rates

[booster]

I have been looking all over for information on how much amperage batteries of various type will accept at various states of discharge. I have come up pretty much empty. All I find are recommended charge rates, and some, like Lifeline giving a maximum inrush that the batteries will survive.

What I am trying to do is get a bit of a database of what folks, including us, should expect if they put in a large, or second, alternators. Big alternators are not current limited, except at the very top end, and it varies with alternator temperature and voltage, so really the batteries are going to controlling the charging current from the alternator in many cases, especially with the very big alternators. Another topic here that was talking about charge rates of up to 1000 amps got me to thinking about it, and started me looking for the information.

I don't know about other brands, but the recent Roadtreks have wiring to the coach batteries capable of handling 80 amps (with 80 amp breakers). They came with up to 220 AH of wet cells or AGMs and usually a 145 amp alternator in the Chevies. You are talking maybe 20-30 amps to run the van, so you have more current available to the batteries than the wiring and breakers are able to handle, it the batteries will accept it. We have heard of folks hearing the breakers clicking on and off in Roadtreks. We often here of AGMs being able to accept charge faster, but how fast? Will 220AH of AGM take 100+ amps when at 20% SOC? 50% ? How much less would wet cells take? I would certainly think you would not want the breaker making and breaking continuously until the SOC got high enough to reduce the charge current to stop tripping the breaker. It wouldn't last long, I think.

In our case, with the GM alternator back in our van, we won't have the wiring and breaker problem, as we have 160 amp capable wire and breakers with the 145 amp alternator. But, when we go to the 250 amp alternator, we certainly could have issues. Plus, even if we are OK now, if we go to AGMs some time in the future we may have the problem show up. I can switch to 200 amp breakers in the wiring, and be right on max, which would help, but we still could pass that. We got the 250 amp alternator to get the high enough amperage at idle to run the microwave, so we really don't want to go much smaller if we don't have to.

Possible solutions that I can think of would include:

* Letting the van idle until the SOC came up to a point that the amperage would be below the wiring and breaker capacity

* Disconnect one of the battery banks, which might reduce the current to a level low enough

* Increase the wiring and breakers by another 50% (we have already doubled it)

* Maybe there is an adjustable external voltage regulator that would work on the 250 amp alternator, so we could turn down the voltage if needed, to reduce the current

* Maybe turning on the headlights and AC fan, and rear defogger would use enough power to get us below the 200 amp level available to the coach

Any input or opinions would be greatly appreciated, as there does seem to be a shortage of information on the topic out there, at least that I have found.

I currently have just finished running down our batteries to 30% SOC (260AH of 6 volt on one bank, 115AH of 12 volt on the other, all wet cells), and will test it to see how much they pull off the 145 amp alternator at higher than idle speeds to simulate driving. Will do them together and individually by bank, as we have disconnects at each battery. We will see what happens.

[markopolo]

It'll be very interesting to see your results.

I think the maximum charging current inflow is determined by taking the battery voltage and dividing it the battery's total internal resistance. I don't really understand it all. The internal resistance will vary depending on the state of charge........ You could take the time and figure it out for your battery banks but it wouldn't apply to my battery bank for example. There will be a variance in the internal resistance between same make and model batteries from what I've read so to be accurate it is specific to a particular battery.

Recommended charge rates vary greatly depending on the actual battery. C/8, C/3, C/20 ................ all the way to Cx4 (Concorde) if you have the wiring to handle it. source: http://www.solar-electric.com/deep-c...ery%20Charging

You can see why RV manufacturers kept it very simple for decades. 1 or 2 coach batteries for lights etc. and generator for AC needs.

[gerrym51]

Booster- i certainly do not have the knowledge or understanding of this that you do.


However the Roadtrek system-especially on the full E-trek and E-trek package does seem to work. couldn't you look at the results and work backwards.

[Davydd]

gerrym51,

Where online is there any postings in any detail as to the performance of the E-trek? I mean something up to the caliber Booster has been posting here. I've seen the journalist's happy talk.

[booster]

Originally Posted by markopolo

It'll be very interesting to see your results.

I think the maximum charging current inflow is determined by taking the battery voltage and dividing it the battery's total internal resistance. I don't really understand it all. The internal resistance will vary depending on the state of charge........ You could take the time and figure it out for your battery banks but it wouldn't apply to my battery bank for example. There will be a variance in the internal resistance between same make and model batteries from what I've read so to be accurate it is specific to a particular battery.

Recommended charge rates vary greatly depending on the actual battery. C/8, C/3, C/20 ................ all the way to Cx4 (Concorde) if you have the wiring to handle it. source: http://www.solar-electric.com/deep-c...ery%20Charging

You can see why RV manufacturers kept it very simple for decades. 1 or 2 coach batteries for lights etc. and generator for AC needs.

Yep, that is similar to what I have been finding, and like you say, it doesn't apply across different systems. I do think there probably could be some "general" rules that might be good enough for wire and alternator sizing, though. Maybe a graph that would show approximate amps a battery of given AH capacity will take at something like 13.5 volts when starting at 20% SOC (highest amps). Hopefully, wet cells would be relatively consistent between brands, same with AGM. Don't know though.

What I found was that our batteries took about 98 amps max when they started charge at 30% SOC. Voltage dropped to 13.2 volts at the batteries from 13.9v at the alternator. Just the 260AH of six volts took about 75 amps at 13.7 volts, the single 12 volt took 45 amps at 13.8 volts.

Judging from the above, and the fact that AGMs will except charge faster, would say that a Roadtrek with 220AH of AGMs almost certainly will go over the 80 amps that the breakers are at. Same would be true with two 12 volt 105 amp AGMs, I would think. Anyone who runs their batteries down low would be tripping the breakers. This may explain why there have been few folks with later model Roadtreks complaining that their shore charger doesn't charge the starting battery, even though they have a separator. They may well have a worn out breaker and aren't charging the coach batteries off the engine either.

The obvious solution is to have the alternator capacity and wiring match so that the batteries can't pull too much, but if you do like us, and size for idle output, you would need huge wiring, breakers, and separator. I can't imagine wiring and other things capable of handling a 350 amp alternator, it would be huge and heavy to do.

[booster]

Originally Posted by gerrym51

Booster- i certainly do not have the knowledge or understanding of this that you do.


However the Roadtrek system-especially on the full E-trek and E-trek package does seem to work. couldn't you look at the results and work backwards.

There is a very big hole in information on the etrek and how it works, although we do know that they have changed components a few times and some folks have done modifications and improvements to the systems. We have also heard of possible battery failures for whatever reason. All unconfirmed do to lack of information, however.

It does appear that the very large output of the etrek from the engine generator is at 24 volts, which would cut current in half, and wire size with it.

At this point, I don't think any of us can determine how well the etrek system works, or doesn't work, as there really hasn't been anyone do any real testing on one to see how it all perfoms. For any kind of "normal" user, having 800AH of battery with 245 watts of solar would make it highly unlikely to run out of power before you would be driving again (without using the air conditioning off batteries though), so deficiencies would not likely be noticed. The problems would show up as over or under charging of batteries, and shortening of the useful life. From what I have learned from our system, getting good charging (not over or under charge) when you have multiple battery banks, multiple voltages, multiple charging sources, differing wire lengths, etc can be very difficult to achieve.

As more manufacturers get into the big alternators (engine generators for sales types), I think we will be seeing some interesting setup, perhaps with 12v to 12v chargers or regulators to control the big outputs when not needed or when they would be too large for some of the other systems.

[markopolo]

Do you think the 98 amps was the max that the 145 amp alternator could output at the test RPM's?

75A + 45A = 120A combined at 30% SOC. It looks like you sized the wiring and breakers just about perfectly in your van for when you reinstall the 250 amp alternator (unless I'm missing something).

[booster]

Originally Posted by markopolo

Do you think the 98 amps was the max that the 145 amp alternator could output at the test RPM's?

75A + 45A = 120A combined at 30% SOC. It looks like you sized the wiring and breakers just about perfectly in your van for when you reinstall the 250 amp alternator (unless I'm missing something).

I think I may have confused you a bit on the 75+45 amps. Those were the amps with the other battery disconnected. I think the higher total at 120 amps is higher because the alternator held higher voltage by about 1/2 volt at the batteries.

The amperage to the batteries stayed at the 98 amps when the engine was revved, so I don't think the alternator was at max amps, just limiting on the batteries. Looking back, I should have done one other test by turning on a 12 volt load to see if the amperage would go up. I will do that next time.

Sizing wise, I think we are fine with the batteries we have, unless the big alternator holds voltage better and increases the current a whole lot (60 amps more). That doesn't seem like it could happen. We might have an issue with AGMs if the took charge a lot easier, which is what they are claimed to do.

[gerrym51]

Booster- again my knowledge of this stuff is not as good as yours. I do know that some early E-trek getters had a problem because ironically-12 volt devices.

as it has been described on some peoples posts i've read-the original e-trek worked at 2 24volt batteries(4 6 volt ) .there are 2 sets of these to be charged.

the problem was that even the 2 sets of 24 volt were charged equally. this worked fine if just inverter. however the van also has some 12 volt devices-refrigerator/fantastic fan/heater etc. so besides the batteries be segmentd as 2 6's- 2 of these 6's to make 12 volt also seperatly wired.so all battieries not discharged together.

this caused charging problems. 2 of the 6 volt batteries were discharged more than the other six. so the other six would get to full charge but the other 2 never got there.

i do not know how this problem was solved

[booster]

Originally Posted by gerrym51

Booster- again my knowledge of this stuff is not as good as yours. I do know that some early E-trek getters had a problem because ironically-12 volt devices.

as it has been described on some peoples posts i've read-the original e-trek worked at 2 24volt batteries(4 6 volt ) .there are 2 sets of these to be charged.

the problem was that even the 2 sets of 24 volt were charged equally. this worked fine if just inverter. however the van also has some 12 volt devices-refrigerator/fantastic fan/heater etc. so besides the batteries be segmentd as 2 6's- 2 of these 6's to make 12 volt also seperatly wired.so all battieries not discharged together.

this caused charging problems. 2 of the 6 volt batteries were discharged more than the other six. so the other six would get to full charge but the other 2 never got there.

i do not know how this problem was solved

There was information, not confirmed, that the original etrek setup charged a 12 volt battery bank off of the Sprinter alternator, and that bank ran the 12 volt things in the etrek. A second bank was at 24 volts,was charged off of the engine generator, and ran the inverter circuits. There was also some sort of "balancer" to charge any bank that gone run down too far, from the other batteries. I have no idea if this was accurate, but I if it was, you would be limited to one bank of battery for the air conditioner, not the full capacity unless the balancer had very high capacity. Have to doubt the description a bit do that fact.

[gerrym51]

You know the strangest thing about all this- I have no interest in owning an e-trek or the full e-trek package.


I do intend to order a Roadtrek Ranger-with some e-trek elements. solar panels/total of 4 6 volt agm batteries/3000 watt inverter/engine generator(actually high amp alternator).

i have no illusions about this system 2 hours of AC on battery max. NOT having to listen to an ONAN-this makes it worhwhile to me. i don't think it's nirvana.

[Boxster1971]

The E-Trek 12v to 24v "balancer" discussed above is probably a DC to DC converter allowing the 24v side to charge the 12v side when needed. I doubt it goes the other way. Hybrid and Electric cars use this technology to allow the high voltage (300+) battery pack to provide 12v for standard automotive systems. The 12v loads on an RV are not very high if you don't include an inverter.