Alternator overcharging batteries

[Gerald55]

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?

[pianotuna]

Hi Landy,

Ok, how 'insane' is the ecm?

I have dual charging paths but can control them manually. They have 50 amp auto reset circuit breakers on them, because I got tired *very* fast of failure of the OEM 60 amp fuse (a hard to find item). I "see" 14.5 volts in cold weather and 13.3 once the starter battery has replenished itself.

Both the house charging circuits are post starter battery.

[grizzzman]

Gerald55 wrote:
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?

This may be what you are looking for Download the PDF file to see how you could use it.

Good luck!

[RoyB]

If you are using one of those smart solenoid relays like Sure Power or Blue Sea has it will only charge the external batteries when the truck start battery is being charged.

The main reason for this is to not over-charge the external wet cell flooded deep cycle batteries. This was probably incorporated because the external batteries may too far away from the alternator charge systems that the truck battery system really doesn't know it has an external battery setup connected.

The smart solenoid relay senses when the truck alternator system hits the 14.2 VDC charge state and connects to the external battery when that happens. When the truck system drops back to the 13.6VDC and 13.2VDC modes on the truck side then the smart mode solenoid will drop off the external battery connection.

This also keeps the external battery system from discharging the truck start battery system which is what you really want to happen.

I know BLUE SEA has a great writeup on the SI-ACR SOLENOID RELAY on how its works.

I am only referring to the external battery being a wet cell deep cycle battery and maybe the other style deep cycle AGM batteries need different charge voltages.

Roy Ken

[Gerald55]

Pleasureway Excel 2008, Gas 5.4L in a Ford E-350 body.

I'm not sure if the OEM alternator is internal or externally regulated, but all the suggested replacements I looked at are internal, so that points towards internal.

The isolator is a sure power solenoid based model, I think.

[landyacht318]

Gerald55 wrote:
@landyacht318 - All good points about true SoC, but they seem somewhat orthogonal to the discussion of the alternator charging. For now, I have to live with the algorithms my Samlex solar controller implements, which I believe are all time based for the absorption phase and do not the include acceptance based cutoffs.

Sorry for meandering, but AGM's are expensive, and full charge is hard to determine, so believing the solar controller is getting them full, without verification, is not wise.

Also, solar cannot meet the higher amp recharge requirements they need every so many deep cycles.

My Northstar AGM protests at 4 cycles of only low and slow solar, even if it meets the acceptance threshold each day. One high amp recharge and it returns to happy. My alternator is cheap and is easy for me to replace, so I use thick copper and do not fear maxing it out as it is cheaper than a new battery.

One can limit the amp flow by using thinner copper circuit to house batteries. More voltage drop on thinner copper, causes less amps to flow.

A digital voltmeter on the dashboard whose voltage sense lead is on the house batteries is VERY revealing and enlightening when driving.

It has shown me just how insane my ECM controlled voltage regulator is.

[wolfe10]

Tell us what coach you have and we can give more specific advice.

Gas? Diesel?

Internal regulated or external regulated alternator?

Solenoid-based battery combiner or diode-based battery combiner?

And, with 4 brand new AGM's, you are wise to "get it right".

[Gerald55]

YC 1 wrote:
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.

No generator currently, solar only.

[Gerald55]

@landyacht318 - All good points about true SoC, but they seem somewhat orthogonal to the discussion of the alternator charging. For now, I have to live with the algorithms my Samlex solar controller implements, which I believe are all time based for the absorption phase and do not the include acceptance based cutoffs.

[YC_1]

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.

[RJCorazza]

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.

[landyacht318]

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 battery

Their 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 none

With 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.

[Gerald55]

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).

[landyacht318]

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