AGM batteries

lazza

HI

We've found a reasonably cheap source of 12V AGM 250Ah (C100) batteries.

I was wondering if the forum could explain to me exactly what the advantages and disadvantages may be of AGM over the equivalent wet flooded cell batteries??

The internet seems to give varying answers, as always

Thanks
Larry

Cariboocoot

Re: AGM batteries

As a rule:

AGM's can take/give higher current rates.
They are sealed so no water to check, no gassing into closed areas.
Not considered hazardous for transport.

They are more picky about Voltage and can be damaged more easily through improper charging.
They are sealed so you can't check specific gravity.
They cost more for the same Watt hours.

inetdog

Re: AGM batteries

Cariboocoot wrote: »

As a rule:

AGM's can take/give higher current rates.
They are sealed so no water to check, no gassing into closed areas.
Not considered hazardous for transport.

They are more picky about Voltage and can be damaged more easily through improper charging.
They are sealed so you can't check specific gravity.
They cost more for the same Watt hours.

And probably do not last for as many charge cycles as FLA of the same capacity. Some AGMs (including Telco retired) are designed for float service and use only a few times per year. Those are no good for off grid RE use.

PNjunction

Re: AGM batteries

The primary attraction is that FLA is usually limited to anywhere from C/8 to C/13 maximum charge current, where C = the 20ah rating of the battery.

AGM's on the other hand, can usually do anywhere from C/4 to C/3, which means if you have the solar power available, you can charge them faster, which in some areas of low solar-insolation may be critical. There are other AGM's which can handle MUCH more than that, and again if you have the panel power available, you can hammer them back up pretty quickly and safely. Concorde's, Odyssey (Enersys) etc, "pure leads" what have you.

They also handle larger surge-loads, and their overall voltages stay higher than fla, due to their much lower internal resistance. Peukert effect is there, but not quite as drastic as fla.

Most agm's are murdered by an insufficient ability to reach 100% charge on a regular basis. This is usually due to dropping to float too soon, or simply not having calculated enough panel to do the job. If you do it right, or are willing to compensate with a generator or charger, they are quite reasonable for RE service.

FLA's on the other hand are easier to diagnose for various issues, because you can measure specific gravity of each cell, rather than just relying on overall SOC voltage, which is always an approximation and can hide developing issues.

One tip for helping reach that fully charged state for an agm is to set your float voltage the same as absorb, in a cyclic application.

lazza

Re: AGM batteries

Are they justo as vulnerable as FLA batteries when it comes to excessive discharge?

Cariboocoot

Re: AGM batteries

lazza wrote: »

Are they justo as vulnerable as FLA batteries when it comes to excessive discharge?

Yes. It's the same battery chemistry, just a different execution of it.

inetdog

Re: AGM batteries

lazza wrote: »

Are they justo as vulnerable as FLA batteries when it comes to excessive discharge?

Yes. In one way they have less of a problem with excessive discharge during storage because the internal leakage current tends to be lower. This allows them to be stored with a longer interval between recharges than an FLA of the same capacity.
It is still not a good idea to let the SOC go too low or stay too low indefinitely.

Alaska Man

Re: AGM batteries

Mine work just fine for RE........ I got them so I didn't have to worry about Hydrogen finding my woodstove during a charge cycle. Come to find out FLA really don't put out enough to blow up my cabin, but caution is still recommended.

I've been told they last twice as long and I have been told half as long. Starting year two of hard service and so far they work better now than when they were new. Time will tell about how long they last, but I'm pretty anal about SOC and bringing them to 100% often.

gww1

Re: AGM batteries

Alaska man
You make me nervous, I have a wood stove also. I had my leggs burnt with propane, I'm aiming for the head this time.
gww

Cariboocoot

Re: AGM batteries

My flooded cells are five feet away from my wood stove.
Batteries do not produce so much hydrogen gas that it is a combustion issue if there is enough air volume around them.
It becomes a hazard in a closed battery box where a spark could ignite it. Otherwise the stuff floats up, up and away very quickly.

gww1

Re: AGM batteries

coot
5 feet is about right for my batteries and stove also.
gww