Please Remind Me--AGMs to Full by Ammeter Action?

[BFL13]

Probably LY, maybe Mex, but there was something about the way an AGM will initially be seen to get to Full by getting to lowest amps at a voltage--say 14.8v (charging spec for my AGMs) then --this is where I can't remember the post(s) on this---amps can rise again?

I finally got an ammeter (Trimetric) on my two 100ah AGMs in parallel during a recharge and they worked their way up to 14.8 then after a while, amps tapered down to about 0.9 for the pair.

I turned the voltage down to 13.7 for the night and this morning (at 0.01 amp at 13.7 for the pair) turned it back to 14.8 to see if amps would drop to zero like Phil gets as his idea of Full. (Only he does that with 13.8) So amps were back up to 0.9 again at the higher voltage, but later I noticed 1.0 and then 1.1 so amps were rising slowly. Same voltage.

I remember something was posted about that and what it all means, or at least I think I remember something about that 🙂

Anybody know about this? Thanks.

[MEXICOWANDERER]

Where did you read AGM batteries must be charged at >14 volts?

Recharging is a game with all batteries. Depth of discharge and this can be expressed as percentage, versus total time needed to recharge. It is anything but a linear formula. Let's keep this limited to AGM.

Discharge 200 ampere hours from AGM. At 1n initial 10 ampere recharging rate it will take something in the area of 25-29 hours to obtain a charge rejection ratio of .50% amperes at full charge where the voltage of course has risen.

With unlimited time plus a guarantee the battery does not end up undercharged the limiting factor is time to sulfation - usually several days. The fuller the battery is the less likely it is to sulfate. A logarithmic scale in interpretation is neither needed nor desired. Problems arise when a recharge cycle takes, say, one week. The damage aggregates. It is cumulative over time.

But again, repetitive failures to fully recharge the battery is the thing to avoid. Being chemical, it too is a cumulative process, tempered by time spent at less than full recharge.

I must have missed something here. GENERATOR RUN TIME is the bugaboo for AGM. I trade max charging potential for time spent to fully recharge. It would be ideal to charge hard for two hours with a generator, then allow the rest of the day to attain point five percent full charge level via solar voltaic. Just make sure the point five percent happens - every day.

With long-term outage I can run the Frightening Harbor 2-cycle for eight hours on a tank of gas/oil at a very light load. This is why I seek those solar panels so I can play hide and go zap with the afternoon sun. As small as that buzzbox generator is, it still consumes fuel and makes noise for a living.

Here's a could be joke - two thousand amp hours of AGM. One half of one percent is 10-amps. Yeah yeah yeah, save it. The idea was presented. Any mention of lithium hexes your converter. Let's keep things organized - the world needs more orgas... ops I mean organs.

[BFL13]

Phil, if they won't take any more amps at a higher voltage then AFAIK (not much!) that means they are truly full.

I don't know what the story is on charging at less than the rated charging spec with AGMs. Seems it is ok in your case--it just takes longer.

With Wets it is different, where you must get them to "gas" and 13.8 won't cut it--needs 14.x for gassing. My "learner set" of Wet batts never got up to proper SG with the 13.8 converter no matter how many days I left it on them. Only saw proper SG when I got a charger that could do 14.x.

My particular AGMs want around 14.8 for charging and around 13.7 to Float, so that is what I do. They also specify max of 27 amps per 100AH, so I also stick to that limit. I have no reason to go outside their specs. If I did have a reason, I would have to deal with that (get different batteries or take my chances with these) but I don't, so no problem. 🙂

[pnichols]

BFL13 wrote:
I turned the voltage down to 13.7 for the night and this morning (at 0.01 amp at 13.7 for the pair) turned it back to 14.8 to see if amps would drop to zero like Phil gets as his idea of Full. (Only he does that with 13.8) So amps were back up to 0.9 again at the higher voltage, but later I noticed 1.0 and then 1.1 so amps were rising slowly. Same voltage.

After "charging" (fixed voltage converter) at 13.7-13.8 volts until the current flowing into my two paralleled AGMs is near zero ... and then starting up the MH engine to hit them with 14.2-14.4 volts for a minute or two ... the current into them does not change - it still reads near zero.

So this means I've not done the "full check" test as BF did, because I only left the higher 14.2-14.4 voltage on them for a minute or two after charging them for hours at only 13.7-13.8 volts while observing the ammeter.

Do all the comments in this discussion thread make it clear to me that I've never been charging my AGM batteries up to full by only using my stock converter (most of the time) and the engine's alternator (part of the time) even though the ammeter indicates that current flow into them drops to near zero after hours on the converter - NO.

I've still never read a definitive technical explanation stating unequivocally that to fully charge a dry (AGM) lead acid battery one MUST charge using voltages in excess of 14 volts. All I've ever read implies only that one can charge FASTER using voltages in excess of 14 volts for AGM batteries.

So far per our camping style and being sure to use AGM batteries with a 13.5-13.8 volt float spec, our charging method has kept me from not having to replace our stock 7345 Parallax converter.

If I'd stayed with liquid acid batteries there are solid electro-chemical reasons - beyond shorter recharge times - for at least some of the time hitting them with higher voltages than what a stock converter outputs.

[MEXICOWANDERER]

BFL, but...but...but...you realize of course squeezing every last nickel's worth of performance and life out of your batteries means ...cheating... a battery distributor out of his just due income?

The wackos do not realize that maximization of service life and performance is the greenest thing individuals can do....wait...Oooommmmmm...Kumbayah...don't want to be ostracized.

[BFL13]

I tried Mex's trick above to see if they were truly full and it worked. Took more than a few seconds to taper back down so maybe they needed a bit more, no idea. Anyway they are happy now at 13.7 and 00.0 amps holding at that with the charging light mostly on, so they are getting some tiny amount of positive amps in when the charging lamp is lit, but not enough to show on the Tri's amps number. Looks good to me! 🙂

Thanks again, everyone, for the good info on AGM care.

[landyacht318]

My Northstar when newer would eventually taper to 0.0 amps at any voltage, Now it is pretty rare that i see it take less than 0.2 amps at 14.7v but it is up in the 700 deep cycle range, many of those well below 50%.

My meanwell to battery wiring is a bit stretchy and stretchier depending on load. Dialing in 13.6v after 0.5% of capacity it reached, well sometimes i dial it a bit too low as I am unwilling to wait for it to turn back on as voltage naturally drops. Hours at 13.5v and then dialing upto 13.6v the amps will raise from 0.0 to 1.5 or so, and upto 14.7 goes back up to 2+ amps.

My Northstar defnintely wants 13.6v float, adjusted for battery temperature if more than 20 degrees away from 77f.

Lifeline's manual states 0.5% of capacity at absorption voltage. I seek this goal every recharge.

This tapering 0.5% or less is taking nearly twice as long as when the battery was new. I imgaine it would have gone cliff diving long ago if I did not adjust accordingly.

The tapering to 0.5% duration can be lessened after a deep discharge and a solid 40+ amps is fed to it until it hits absorption voltage and then is held until 0.5%, but this high amp blast restoring some performance is becoming less effective as cycles accumulate.

[MEXICOWANDERER]

Pound-A-Peg grade verification of float voltage and temperature compensation validity can be had by simply re-initiating absorbsion voltage value. The near zero amperage reading should resume within seconds. If it does not, tweak float voltage upward a tad.

Inverse testing works too, but be cautious about dialing down float voltage too far. A few hundredths at a time is optimum. Note temperature has a significant effect on pound a peg tweaking hard numbers.

[BFL13]

Seems to be working out. I do leave the charger at 14.8 until the batts are full (as noted by amps being down to near zip), then switch to Float at 13.7 per these batts' specs.

That is better than the automatic chargers that hold Vabs until batts are maybe 97% and drop to Float to let the batts come up the rest of the way--if they ever do! I can get them to 99.9% and then drop to Float.

There is no load on these batteries at all while on Float except to run the Trimetric's 12v power. They have been at 13.7 for a couple hours now and amps are holding at 000 amps on the Trimetric with the charging light mostly on, goes off sometimes and comes back on.

You can't measure anything without changing it (as learned in science class) and here is an example where the Trimetic takes power from the battery so it can't quite get to total resting. I don't have a switch for the Tri, but could if it really mattered in the big picture.

[MEXICOWANDERER]

Charge to absorbsion voltage until amperage slumps to < 1% total ampere hour capacity. The key word is absorbsion.

[BFL13]

No, these are the ones I got a year ago for the 1981 truck camper. The battery box is inside (bad design--not a sealed, vented box) and the fumes from Wet cells would set off the LP alarm I installed.

I have not yet decided if I want/need those other AGMs.

EDIT--this morning I found the Tri saying 13.5v and 000 amps with the charging light flickering. (When you adjust the voltage with the batteries still connected you don't see on the charger's voltmeter exactly what you dialled in until later when the batteries adjust to the new voltage, so I didn't quite get 13.7 as intended). So I dialled it up to 13.7 and amps went to 1.4 on the pair to get them up to 13.7, then started to taper. I will see where that all settles later. I'll get their Float right eventually, allowing also for temperature. Love having an adjustable voltage charger working with the ammeter! 🙂

Anyway, having the ammeter on the job means I can now look after the AGMs properly. (Thanks to folks here with AGMs describing what to look for on that.)

[brulaz]

Are these the used ex-telecom AGMs you were looking at awhile ago?
Or?

[BFL13]

Thanks Mex!!! Ok so back to 13.7 Float and hope amps stay the same and don't rise. I guess if they do, that means the Float voltage is too high and I should lower it?

Thanks goodness I now have an ammeter on the job with my AGMs. (They were in the truck camper for a year with no Trimetric, but are now in the MH with a Trimetric so I can actually tell what is going on with them---I hope!)

[MEXICOWANDERER]

The rise you see is normal and to be avoided as a nuisance. Overcharge. Stop when the ammeter bottoms out. When it starts rising again all it's doing is causing needless chemical reaction and heat.