I have 4 AGM batteries in my rig with a total capacity of 440AH (actually 880AH @ 6V, but of course they are wired to get 12V).The data sheet for my Haze batteries species a bulk -> absorption cutover point of 14.4V. Both my solar charge controller and my progressive dynamics converter are smart and can get close enough to the required profile, so I don't have many concerns there.The alternator charging, however, is "dumb". As far as I understand it, an alternator is a voltage source, and seems to put out about 14.3V or so. So when the batteries are charging on this source, during a long drive, for example, will the alternator overcharge the batteries? Also, will the 4 batteries accept 4x more current than the 1 battery that came originally in my coach, and will this put undue strain on the alternator?

If your voltage regulator in your vehicle only allows 14.3 maximum, then there is no risk of overcharging as long as the batteries are not in the engine compartment exposed to engine heat for hours and hours on end.However that much AGM capacity will max out your alternator and its circuit shortening its lifespan.My Single group 27 AGM maxes out my alternator when depleted. It states 14.46 is the absorption/max voltage, but it regularly gets 14.9v when driving and is over 2 years old with over 200 deep cycles on it.Make a forced cold air intake for the alternator with a bilge blower if your charging circuit is thick enough to allow the batteries to overwhelm the alternator

Is the voltage regulator an integrated part of the alternator?I'm not sure 14.3 is the max. I measured that at the battery, while driving, but the batteries were partially depleted, so it was probably not at max voltage. I guess I should measure it when driving and the batteries have been topped off?What amperage do alternators typically top out at? I know that the charging current is usually less than 30A, since for a while I (accidentally) had a 30A breaker in-circuit with the charge line and it only tripped when the batter was quite low (below 12V). That was when the charge line had a smaller gauge wire for much of its run, however, so the currents now would be higher.I'm interested in ways of protecting my alternator by limiting the current. Are there cheap current limiting devices? Alternately, maybe I just install a manually operated SPST switch which disconnects the alternator from the battery, and leave it off except when I really need it (usually the solar is enough, so the alternator charging is often redundant).

Alternators can have internal voltage regulators, or external. It is vehicle/ platform dependent. What vehicle do you have? alternator? Mexwanderer knows his alternators.My '120 amp alternator' maxes 62 amps at 800rpm cold, and 32 amps max hot at 525 rpm and I have seen it touch briefly on 110 amps into my depleted batteryTheir output varies greatly, the alternator rating established in a lab, when cold, at high rpm with large loads and a circuit capable of passing huge current.Voltage when driving will be changing with rpm and battery state of charge, and the decisions of the voltage regulator, which can be as loony as a meth addict.AGM batteries enjoy high recharge rates, even those which say to limit charging amps to 30 per 100AH of capacity.Low and slow Solar alone and deeply cycle agms do not make for happy long lived AGM's. They need the occasional high amp recharge from their most depleted state. For best AGM battery life on a deeply cycled battery, this is not optional.Just because your solar charge controller drops to float voltage, does not mean the batteries are fully charged. It only means they attained absorption voltage and held it as long as programed to do so.proper absorption voltage and duration are moving targets. A one size fits all charge algorithm is more a one size fits noneWith AGM's, full charge can be determined by the amount of amps they can accept at absorption voltage.Lifeline, topdog AGM maker, says when a 100AH battery can only accept 0.5 amps at 14.4v, only then it can be considered fully charged.Do not believe blinking green lights that supposedly indicate a fully charged battery. They are insane and mock any human who believes them when they first alight on a deeply cycled battery.Just because lots of people have faith in blinking green lights, does not make them right, no matter how vocal they might be/become, when their faith is questioned.Trust if you want, but I prefer to verify.

Your concerns make sense about taxing your alternator, but I never gave it any thought on my previous rig (E350 Born Free MH). That was with 230ah of batteries and the alternator would have about 80 amps draw initially, rapidly decreasing to 30-40 amps in a few minutes. Maybe test the draw on your alternator at a depleted battery level and compare to the alternator capacity? You do have double the battery capacity I had though...As far as alternator voltage is concerned, the voltage is temperature compensated to a small degree. Hot summer days and/or a warmed engine compartment I only see about 13.6-13.8v and perhaps 14.5v on the coldest days here in Maryland.

If you have an onboard generator then run it for a couple of hours if you have been boondocking. This will keep the heavy load off the alternator.

Alternator overcharging batteries | Good Sam Community

27 Replies

pianotuna
Nomad III
Jan 09, 2016
Maybe this summer I'll finally use something like this:

https://alternatorparts.com/quicktifier-external-bridge-rectifier.html
pianotuna
Nomad III
Jan 09, 2016
Hi Robert,

I quite often see my solar sending power to the engine. I do have control of the charging circuit relays. I figure I get a bit of savings and it eases the load on the alternator.
mchero
Explorer
Jan 09, 2016
Don
Cant give you a solid answer on that one. I was originally looking at a Balmar 3 stage alternator but never got as far as pricing. I do see that their regulators can run anywhere from $200 to $300.
I recently installed 1K of high voltage panels with a Morningstar 200 light charge controller. I'm still thinking about adding a relay circuit to the Morningstar that will disconnect the aux start relay while driving down the road "IF" I have enough solar.

Brett would have more experience with the Xantrex regulator.
pianotuna
Nomad III
Jan 09, 2016
Is the Xantrex Alternator Regulator a replacement for the OEM or can it be used at the same time as the OEM regulator?

mchero wrote:
How about something like this?
wolfe10
Explorer
Jan 09, 2016
mchero wrote:
How about something like this?

An excellent unit. Have one on the sailboat. May be overkill on a motorhome, but on a boat where an engine can be running for 24 hours at a time or the opposite (engine just run for charging batteries) a smart regulator is a big asset.
mchero
Explorer
Jan 09, 2016
How about something like this?
RCL
Explorer
Jan 09, 2016
If your two banks of batteries(house and chassis) batteries are charged through a diode isolator there is one scenario that I haves experienced. In my case, the alternator output was determined by the voltage seened at one bank. This method adjusted the alternator output to compensate for any resistive drop in the high current charging line to that battery. It turned out that the chassis battery line had a bad crimp connector in it.The alternator upped the output voltage to compensate, but the house batteries saw this higer voltage and were cooked. This was on my '83 Southwind.
Dick L
Gerald55
Explorer
Jan 07, 2016
I'll be boon-docking pretty much everywhere between Mexico through Colombia, so I expect high temperatures to be the norm. I'm thinking my manual disconnect for the charge line is a bare minimum for that kind of weather.
MEXICOWANDERER
Explorer
Jan 07, 2016
Wuuwee first time I've noticed this thread :)

Yep, MoPar does indeed have ECU Voltage Irregulators.

To cure the long long week's long vacation hotfoot, remember a silicon rectifier drops voltage automatically by roughly point seven .7 volt. A standard 70 amp three bolt finned isolator tolerates 140 total amps when BOTH OUTPUT STUDS are paralleled. This goody would be installed near the batteries. The regular incoming voltage cable would go to the center terminal and BOTH outboard studs would be bridged and pass into the AGM positive stud.

Dropping voltage by .4 volt is a little trickier. It requires passing the alternator - AGM charge lead line through an adequately sized SCHOTTKY rectifier. Ambulances use isolators with Schottky rectifiers but if you should price one you may end up inside the ambulance.

This gets even funner....

14.2 volts is far too high if battery temperature is say 90F. A standard silicon rectifier isolator would do the trick just fine. If you want to get boondocker fancy wire in a switch with appropriate gauge cabling that bypasses the isolator with the flick of the wrist.

GYRO GEARLOOSE TYPES
Re-read this post AGAIN. CAREFULLY before you unleash with ISOLATOR DIATRIBE. This is NOT a standard silicon rectifier isolator hookup. Questions I will answer politely. Criticism gives me license to bite with needle fangs filled with sarcasm venom.

I am doing this precise thing with a psychotic WFCO. Only with a 500 ampere rated Schottky rectifier. But the Schottky rectifiers sold on eBay need to be insulated and heat sinked and fitted with bolt terminals and case and forget it - it's not a project suitable for novices.

An isolator with switch makes perfect sense for full day after full day on the road. If the days include high heat I would rate the modification as "essential".

FURTHER NOTE. THINK BEFORE CRITICIZING
Converter, solar, squirrels in a cage charging gets connected BETWEEN the isolator and battery.

Gripe, Snivel & Whine...

14.3 volts is too high of voltage for an antimony flooded battery. 14.3 volts combined with hot summer temps and marathon driving BOILS flooded batteries. Yeah! Gurgle Gurgle. This is why I shook my head at all the neurotic grade micro-management of converters, solar yadda and this subject was not touched upon.

Not everyone fits into the marathon hot weather driving scenario. But those who do can be battery killers ESPECIALLY WITH RV FLOODED BATTERIES.

And for the short-term memory types this is regarding vehicle engine alternator charging ONLY.
Gerald55
Explorer
Jan 07, 2016
full_mosey wrote:
Gerald55 wrote:

...
OK, the need for high amp recharges was news to me. Or is this basically another way of saying that AGMs do need equalization? ...

No. Equalization is a flooded(FLA) thing; meaning to use excess VOLTS to force electrolysis so that the stratified acid is re-stirred EQUALLY throughout the liquid.

Well, not anymore. When AGMs were just penetrating the market, the conventional wisdom was "don't equalize", but that's been changing. Today, several (most?) of the top-line AGM manufactures recommend some type of equalization. Often it seems to be a "vent-less" type of equalization - i.e., not gassing enough to pop the vents on the AGM, so no electrolyte is lost.

In any case, it's clear that some of the benefits of equalizing carries over from flooded LA to VRSLA AGM as well.

As to 'the need' for high Amp charging, there is partial truth to that. The lowest actual mfgrs spec I have seen is to limit Amps to 30% of the 20hr rating. The highest I have seen is unlimited Amps. Some of this is lawyer talk and some is marketing talk.

OK, but that only seems to support the idea that charging too fast can be a problem, not that charging too slow can be?

AGMs do benefit from temp comp chargers. These chargers limit Volts by reading the battery temperature via a probe. IMHO, the purpose is to maximize Amps while at the same time keeping Volts below electrolysis. Allowing electrolysis is simply a waste of energy in an AGM and results in slower charging.

Yes, I use a temperature probe to enable temperature aware charging in my setup.