acceptance charging rate AGM batteries

BFL13 wrote:
We had measurements and reports before on here that you could draw far more in amps from AGMs than with Wets (especially 6v Wets) for the voltage drop that would trigger the inverter alarm at 11v.

So the idea was (I thought!) :( that if you had a small rig that could only carry say two batts, but wanted to run a 2000w inverter to run the microwave, toaster, etc, you should make those two batts AGMs.

Now we are hearing you should not pull high amp draws from AGMs?
Which is it?

It depends, again, on the construction of the AGM battery. I run a surplus Telecom Battery. It is unique in it's construction. It's designed to run a mild steady state withdrawal, when a power outage occurs at a cell phone site. It is in no way designed to handle high surge loadings. Very few plates. Thick heavy plates that take up a lot of space inside the battery, hence a lot less surface area for chemical reaction with sulfuric acid. Slow and steady as she goes is what at Telecom battery is designed for. 10 years life, because it's expensive to run crew up to remote sites to R&R all the battery packs, so the Telecom co's want batteries that will be warrantied for 8 years, and hope they get 10 years out of them.

My situation is unique to MY battery. Ask the manufacturer of your battery. Make the phone call, find out what it will or won't do for surge loading. I'm in a small 21 foot travel trailer, solo. My needs are unique to me, and me only. Everyone's needs are different, I don't have a wife's electrical needs to satisfy daily when dry camping. To me, it's still camping, I like it a little rough.

@Niner

This is my second set of telco agms
Just over five years and approx 2000 cycles on the first set
A 20 percent discharge is 135 amps
Nobody here is taking about thin plate starter batteries, except Mexi and his automotive comparisons PT has 4 telco agm, I have 5
The specs for my batteries say what they can produce ...not , what not to do,
I have no intention of doing a 1000 amp discharge, 200 amps per battery
But the specs say it's possible
I prefer generator for breakfast
But sometime late at night I want a cup of coffee or MW some bacon
The Bacon take less time than The coffee

Hi BFL13,

Ummm sorta both. It does depend on the cell count per jar and the number of units.

Cells are designed to provide 25 amps, so twin six volt jars in series can do 25 amps @ 12 volts comfortably. That can probably be doubled to 50 for short periods of time.

Why 25 amps? Because the battery companies all agreed to measure at that amperage.

Twin 12 volt jars have double the number of cells and are wired in parallel so they can do 50 amps (each cell is still only doing 25 amps). That, too, can be doubled to 100 (short time periods).

My (limited) understanding is that, because of lower internal resistance, starved acid AGMs exhibit lower voltage drop under load. So a large inverter could easily push a pair of six volts over the low voltage "cliff". That will happen less with 12 volt units, and even less with AGM. This is assuming all banks have the same number of amp-hours.

Starved acid run out of "bricks and mortar" before the plates can be damaged. i.e. the acid becomes "weak as water". It may shorten life span. Surplus acid (mine) you better not do that or you may end up with door stops pretty darn quick.

There are two reasons I went to surplus acid AGM. I got tired of laying in the snow to check electrolyte levels. And they were cheap because they had seen service in a cell phone tower.

In short, all AGM's are not created equal, and what is good for Lifeline (a premium battery) may destroy my surplus acid AGM jars.

We had measurements and reports before on here that you could draw far more in amps from AGMs than with Wets (especially 6v Wets) for the voltage drop that would trigger the inverter alarm at 11v.

So the idea was (I thought!) :( that if you had a small rig that could only carry say two batts, but wanted to run a 2000w inverter to run the microwave, toaster, etc, you should make those two batts AGMs.

Now we are hearing you should not pull high amp draws from AGMs?
Which is it?

MrWizard wrote:
ok i'll confuse the issue a little
What do you mean by 'c'
batteries have an AmpHrs capacity
often called C20
which is total capacity from full charge to depleted, normally a cell voltage below 1.7 or approx 10.42 for the whole battery
a battery with a c20 rating of 100ampHrs
a c/5 charge or discharge is 20amps

so to me 2C means a discharge rate of 200amps
a 45ampHr gel discharged at 2c would be putting out 90amps

is this what you mean Naio ?

Yeah, that is what I mean. In the particular case, it was 66 aH batteries, with momentary loads that blew 100amp fuses.

IIRC the specs on mine said 496 watts per cell
thats over 200amps for something like 45 minutes to depletion
the 8hr capacity is 135amphrs thats 16+amps for 480 minutes
31 amps for 2 minutes is almost nothing

Oh, great, thanks for posting the numbers for our batteries! Very useful. Is that per cell or per battery?

Two minutes is nominal time and the 12.1 volts proves all is OK at your batteries. You purchased enough capacity to put your batteries on Easy Street as far as loading is concerned. PS Ihave seen cables melt and batteries explode from a dead-shorted starter and a negligent driver. Most were Fords and the contacts in the solenoid welded. Blew the lid off the battery and spewed acid.

Here's something to chew on: 5,000 starter cycles. One engine fires in 2.2 seconds of cranking the other requires 8-seconds. These are realistic numbers. Extrapolate the ramifications on the battery and starter motor. Now enters the Problem Child that demands 15-seconds.

Hi Mex,

My real life is 160 amps of draw for 2 minutes and 22 seconds. Battery voltage remains at 12.1. The bank was not at 100% for this test.

MEXICOWANDERER wrote:


An exercise

BCI load test of a 800 CCA starting battery

50% of rated BCI CCA amps is the formula = 400 amps.

A 400 ampere load is imposed.

For 15-seconds

Voltage at the end of 15 seconds of load must not have drooped to less than 9.6 volts (temp comp).

While this example is not a real-life example of RV battery discharging it should show the relationship of how a battery reacts to a load. 9.6 volts is far too severe a load for usage (it's only for testing the battery).

My personal choice for discharge maximum is 20% of total amp hour rating, and that is for limited time loads like heating coffee in a microwave.

MrWizard wrote:
actually the chemistry of batteries is
tilted in favor of discharge
it always has been
the amount of power needed can vary, and can be extreme
its reversing the chemical process that is harder to do and uses more power

yes you can dead short a battery
but in all automotive vehicles
the intense starter drain, is much higher than the recharge rate
the battery is not hurt by this
unless the car won't start and you totally drain the battery

That's fine with thin plate batteries, where there is a surplus of very thin plates designed to handle that surge capicity for starting.

My AGM is very special purpose, being telecom surplus, has few plates, but there are very few of them, they are thick and very heavy and are pretty much 10 year life design batteries. It's the "few Plates" that both limits the charge and discharge rate, not enough surface area for the battery acid chemistry involved to allow more amps in a completed circuit, without voltage drop, which excacerbates the need for more amps drawn. Those more amps drawn to compensate for voltage loss makes expensive parts inside a ham radio overheat, burn up, blue smoke, and fail.


I have a workaround already... run the Honda Generator with the Megawatt 30 when I want to run barefoot with 100 Watts on the ham radio. Radio pulls 23 amps tx, Switching power supply unit makes 30. Plenty of surplus, and the battery acts as a buffer when keying the mic.

Maintaining a safe discharge voltage level is easier. The longer the duration however the fewer the amps permitted. When that magic voltage limit is reached discharging must cease.

An exercise

BCI load test of a 800 CCA starting battery

50% of rated BCI CCA amps is the formula = 400 amps.

A 400 ampere load is imposed.

For 15-seconds

Voltage at the end of 15 seconds of load must not have drooped to less than 9.6 volts (temp comp).

While this example is not a real-life example of RV battery discharging it should show the relationship of how a battery reacts to a load. 9.6 volts is far too severe a load for usage (it's only for testing the battery).

My personal choice for discharge maximum is 20% of total amp hour rating, and that is for limited time loads like heating coffee in a microwave.

What many folks do not have experience with is the fact that excessive discharge loads cause a flooded battery to gas. The same gas as developed when overcharging. A foolish person who tries to start an engine using a shorted starter motor can make the battery electrolyte actually boil. This damages the battery.

actually the chemistry of batteries is
tilted in favor of discharge
it always has been
the amount of power needed can vary, and can be extreme
its reversing the chemical process that is harder to do and uses more power

yes you can dead short a battery
but in all automotive vehicles
the intense starter drain, is much higher than the recharge rate
the battery is not hurt by this
unless the car won't start and you totally drain the battery