Battery: AGM vs. Flooded Cell

I’m happy with my wet T125s and the additional AHs they have.

Not with those, but note the AGM requirement to get them full every so often (often as possible), which means charging at 14.4ish volts constant ( with no dropping to 13.6v before you get there like some converters will) to 0.5 amps per 100AH of battery. Now you can go to 13.6v.

So how are you going to observe that? I can see the amps taper down to that with my Trimetric monitor. Point being that going AGM also means getting an ammeter you can monitor.

You also need a charger that will hold the voltage at the right level until amps are down to spec.

I like AGM batteries because you don't have to check the acid level. Living in a dry climate the acid in my batteries always evaporated.

Price wise you can look into used AGMs. I am in a good place where we have a retailer that sells AGMs that are rotated out from industrial use on a regular cycle.

$75 for 110Ahs.

I recently got 4 x 155Ahs to build a 620AH bank at $428 OTD. Hardly more than the price of a single new retail AGM. They are dated Mar 2013. I charge with a 2200W/75A inverter/charger, plus 400W solar. In another system I have 2 of them dated Oct 2009. I expect the last 4 to last 10+ years as well.

Flooded batteries are not as much a worry over extensive maintenance; whereas the penalty of letting the plates run dry is catastrophic.

An ammeter is the key to knowing the SOC of AGM during charging.

HTH;
John

AGMs hold a charge longer when sitting. And no watering or corrosion is really nice. Forget the money.. just cry once and get them. You'll never look back.

2oldman wrote:
AGMs hold a charge longer when sitting. And no watering or corrosion is really nice. Forget the money.. just cry once and get them. You'll never look back.

That's been my experience too with AGMs. I charge them at the end of a trip and months later when I prep for the next trip, I go to charge them and they are still good to go.

Make sure your battery charging system(s) can handle AGMs. Slightly different charging voltages compared to flooded lead acid.

AGMs also have a lower Peukerts constant so if you discharge them at higher rates, they actually have more capacity for people who do that. Since they have a lower impedance than FLA, they can also be charged faster.

2oldman wrote:
AGMs hold a charge longer when sitting. And no watering or corrosion is really nice. Forget the money.. just cry once and get them. You'll never look back.

That is about it. They are widely used in the marine world where vented gas from wet cells has nowhere to go but the bilge. They are used in aviation too. Planes are not always level & acid does not go well with aluminium.

I got rid of the wet cells after a couple of years & went straight to a single AGM 4D by Lifeline. A 4D gives very similar amperage to 2 6V in series.

That 4D gave 9 years service life of over 150 nights use each year. It also sat, disconnected & with no maintainer, for 7 months straight each of those years.

Subject to Covid 19 our 2nd AGM will begin it's 3rd year in a month or so time, after more than 8 months no maintenance this time.

If money is going to be spent on an AGM it might also be a good time to upgrade what is probably a cheap & nasty converter/charger to a multi stage smart one with a wet,AGM,gel ability.

You can't charge any faster than your charger's max amps.

There is a slight time saving once you get to the Absorption Stage where the AGM has a little more acceptance rate per SOC, but Bulk Stage timing is about the same.

Some figures from past examples on this topic, noting the %SOC when Bulk ends using various charging rates. (The lower the charging rate, the higher the %SOC when Bulk ends after a time at constant max charger amps)

Wets- 32% and 65%SOC
AGM- 40% and 63%SOC

Wets-19% and 72.5%SOC
AGM- 15% and 75% SOC

So if you are off-grid doing 50-80s you are going to shut off the gen not long after Absorption starts anyway, and you will not realize much of any theoretical time saving by having AGMs.

Solar usually has a very low charging rate, so a high SOC when amps start to taper. This is true with Wets and AGMs. Hard to say if you are any more likely to get to 100% before dark with either type.

Li batts do shine in this regard. At 100% charging rate, they reach 65% SOC when Bulk ends, and at a less stressful (and recommended rate with Battle Borns) 50% charging rate, Bulk ends nearer 80% SOC. Solar with Li means you won't see much of that faster charge with them either due to the low charging rate, unless you have a very large solar array.

We were looking at all this a while back when discussing the wild claim seen in advertising, that AGMs charged "five times faster" :)