Charging AGM batteries

speaking of AGM charging, I just installed some 6V AGM batteries. The brand name is 'Matrix Infinity'. I doubt they are they highest quality things but thought I'd give them a try as they fit in my battery compartment (barely)

The spec sheet doesn't mention temp compensation, my Morningstar charge controller has a default of -5mv/°C/cell, is this reasonable for the AGM's?

theoldwizard1 wrote:
Most good chargers, including solar, have a specific setting for AGM.

Betcha the VAST majority of RV owners charge with the trailer's converter - whatever it does is what the battery gets, good or bad. :W Wasting $$$ on AGMs makes little sense for RV owners who can't be bothered to also make sure the charger they intend to use meets the specific charging requirements of those expensive AGM jugs. :R

Most good chargers, including solar, have a specific setting for AGM.

Hayyyyyy Zeus!

Get wrapped up enough in trying to meet idealistic manufacturer perfection and you'll end up in a rubber room.

As far as I know Concorde Lifeline is the only AGM with a 14.4 charge limit. Others say 14.8 is I-D-E-A-L

But the 30% charge minimum is for batteries that regularly get discharged to less than 50% state of charge. Keep discharges above 50% state of charge and forget the minimum charge amperage needed.

Not one SINGLE bite at my challenge so I will spit it out plain and clear.

Raise charging voltage ONE FULL VOLT above manufacturer's specs and guess how this will affect battery life. Go ahead and make my day...

Wanna know the answer...?

Feel confident enough to bet? I do. I'll take a (lollipop) bet any day.

And I can post a link to an AGM manufacturer's printed proof. Proof by testing not by hyperbole.

Wuuwee. Forget 14.4 volts Raise that sucker to FIFTEEN POINT FOUR VOLTS.

What kind of percentage cut will it make in AGM battery life.

And you know that "---- ----" walks.

Let's hear it!

LESS THAN TWENTY THREE PERCENT REDUCTION IN CYCLE LIFE which is a hell of a lot different than a reduction in overall battery life.

And the volts per lessened life as usual with chemistry is NOT LINEAR. It's logarithmic. Think of the top edge of a giant circle angling to the right. The higher the voltage the greater the DEGREE of cycle life degradation.

It is quite similar to depth of discharge versus cycle life. Raise that voltage ONE HALF VOLT and the cycle life reduction sure as hell isn't going to be <11.5% it will be A QUARTER OF THAT. Get the picture?

The argument makes as much sense as fretting over if house paint will flake in a 9.0+ earthquake.

BATTERY TEMPERATURE is 100 times more important, than fretting over .3 volt float. When the Lifelines go to Michoacán they get placed in the direct blast of an air conditioner. 95+F versus air exit 70F temp. And THAT will increase battery lifespan 150% for standby UPS duty.

IN MY MOST HUMBLE OPINION, select a converter that most closely resembles the battery OEM recommendation then quit worrying. Charging the battery to .5 amps for every 100 amperes hours capacity IS SOMETHING AUTHENTIC to deal with. And guess what? A charger rated slightly too high of charge rate WILL MAKE DOING THAT EASIER.

Gibben a book und they eat the gotterdam pages :)

I did worry about my charger having too high amps for the one 100AH AGM battery, so I did not use my 55 amp converter on it.

I used my Vector portable which has 40,20,4 amp settings, so you can limit the amps that way. Now I have the second AGM I can use the 55 amp converter. But I will not be using my 75 or 100 amp converters that I have in my collection on those two AGMs. Maybe I could, but I see no reason to go over the specs when I don't have to.

People with those fancy inverter/chargers can limit the amps output by limiting the 120v input ISTR.

AGMS used as engine starting batteries in general never get discharged much, and therefore do not require specific absorption voltages be met for hours.

Using my ammeter, well my AGM battery eventually gets to the point where it accepts 0.0x amps at any voltage when charged as charged as it can be.

If I start the engine at chrged as charged can be, when amps taper back to 0.0x amps at 14.7v, the battery again will be fully charged.

So how long does it take for amps to taper back to 0.0x at 14.7v after starting my 5.2 liter gas V8 engine? Under 45 seconds.

Manufacturer prescriptions/recommendations as to recharging are what they decided will yield the least amount of warranty returns. They will tweak them as needed to maximize their potential maximum profit. The CEO's mistress' jewelry collection depends on it.

Obsessing over precise charging voltages is not healthy, especially when one cannot control their vehicles voltage regulator anyway, or if their converter does not exactly meet the specs listed by the battery manufacturer. BEsides, without battery temperature compensation, which very few chargers have, well how many batteries are precisely at 77F every time they are charged?

An AGM that exceeds the absorption voltage limit at its given temperature is not an instantly dead AGM, and perhaps even cumulatively seeing a higher voltage, could last longer than an AGM chronically undercharged from too timid a absorption/max voltage.

While some le$$er AGMS say to limit charging amps to 30% of capacity, well, these are the higher resistance AGMS's. How hot will they get if that 30% is exceeded? How often can they exceed this before they vent the electrolyte? How many times can they do this before getting too dry?

I don't know. I have a Northstar AGM which regularly saw 14.9v and much warmer than 77F temps early in its life before I could control my alternator's target voltage. I have over 500 deep cycles on it now, and overnight voltage under load is still impressive and it is by far the most impressive lead acid battery i have owned.

I'd not stress exceeding ideal absorption voltage by a few tenths, unless perhaps ambients were 110F and I had 100 amps available of charging source into a well depleted 110f AGM.

I'd stress undercharging more, and in the case of my Northstar AGM-27 at 90AH capacity, I'd stress never being able to exceed a 30 amp charging rate as low and solar only recharges have performance( judged by voltage held under load, and morning voltage) begine to tank after 5 low and slow solar only recharges.

My battery seems to crave high amperage rechrging from its most depleted state every so often, and of course the true 100% recharged, determined when amps at 14.7v taper to 0.45 or less.

The time it takes amps to taper to 0.5% of capacity at 14.7v has increased a lot in the last 100 cyles.
Prematurly initiated float voltage equals premature battery death, in my opinion.

Use a hydrometer or ammeter with battery at absorption voltage to determine full. Far too many people believe that their charging source dropping to float voltage indicates full charge. Without verification this is extremely unwise, if one cares about achieving good to excellent battery longevity.

If one does not care, then why bother reading on an Rv tech forum in the first place?

Ammeters are now cheap. Seeing how many amps are flowing at that voltage is extremely enlightening during both charging and discharging.

BFL13 wrote:
I think that is correct--they spec a limit of 30% charging rate on some, so the voltage won't spike too high. AGMs do heat up and you don't want them to blow out their seals. The unsealed flooded type will vent ok and you just add replacement water--can't do that with AGMs.

And unless you have a very smart charger how do you user adjust the current limit to only 30% .... by setting the voltage profile.

Realistically most people don't have a large enough charger and/or have too much resistance in the battery leads to do any damage to an AGM, which is just as well as I found I am over current limited (75 AMPs) to 14 volts even with only one group 24 battery for the first portion of the charge cycle. Most of the time I have more battery than that, so the charge rate is really set by the current limit on the charger if you work get the resistance down on the battery cables and I think that is the typical situation.
With my stock setup, I have now moved the battery to be only about 2' from the charger, even at 14.6 volts output from the charger I only got 45 AMPs into the battery, I was losing .85 volts in the long wires to the front of the trailer at 45 AMPs. I suspect that is a bit more than most, but I think a lot of people lose more voltage than they think they do on the way to the battery.

I think that is correct--they spec a limit of 30% charging rate on some, so the voltage won't spike too high. AGMs do heat up and you don't want them to blow out their seals. The unsealed flooded type will vent ok and you just add replacement water--can't do that with AGMs.

cubbear wrote:
U can buy a AGM battery and put in your car U don' worry about all that stuff

Is there a car that charges at 14.8 volts ..... ?

U can buy a AGM battery and put in your car U don' worry about all that stuff