Amp hours to fully recharge?

Re-reading the Trimetric blurb on charge efficiency, I see it uses 94% not 96 as I have been saying lately.

They stress this is until the SOC is at the top of the range, where there is gassing and charging efficiency is lower. They measured charging efficiency with some Trojan wet cells at less than top range and got 95-98% charge efficiencies.

So their 94% is a conservative amount in their view. It is hard to say if that comes out even when you go to full SOC.

The 94% is adjustable if you find you are always over or under on AH when the charging system you have indicates the batts are full and that is when you want the AH to be at Zero.

They also mention that choosing your amps point for when they are tapered to some low amount that you can call that "full" could be C/50, but C/90 would be closer to True Full. then they throw in what I have been saying elsewhere too, having seen this happen:

"....(C/90) ...however this may require more solar time (for example) than is usually available."

With a 460AH bank, C/50 is 9.2a and c/90 is 5.1a No way would I call my 458AH bank full if it is still accepting 5 amps. Then you have whether that 5 amps is at 14.8v or 13.8v. (my controller would have dropped to 13.8 by then, so 5a would indicate a lower SOC than at 14.8)

I have learned that at 14.6v, 5a per battery is at about 90% SOC, and 3a per battery (12a) is near 97%, so C/50 at 9a would be past 97% but not by much.

HiTech wrote:
Ditto. Really seems to be the manual charger and/or solar that will get my AGM bank up to 100% for me. My DC-DC power supply will as well (actually quite well with 10-15a available), but I still consider it just another manual charger.

Jim

IMO, I think solar with that everyday "absorb voltage" is what does the trick. That and the shallow discharges. I have found that if I keep the discharge above 85%, the converter will bring the batts back to 100%.

Ditto. Really seems to be the manual charger and/or solar that will get my AGM bank up to 100% for me. My DC-DC power supply will as well (actually quite well with 10-15a available), but I still consider it just another manual charger.

Jim

HiTech wrote:
The easiest way to tell though has been the behavior of a battery at rest voltage on a manual 2 amp charger for me. Mex told my that the delta V goes up very quickly when fully charged and that is the easiest one for me to nail down on an AGM.

My Schumacher smart charger kicks into float significantly before the battery is full, even when the 2 amp rate is selected.

That delta V going up quickly happens on my batts as well. I noticed it a few times but didn't really know what that was until MEX explained that to you. I wasn't really concerned because SG was right where it should've been. About the baseline, I had a lot help with that and knew I needed that needed to be done before I bought these batteries. That was the first thing I did.

I have used two converters and my portable B&D1093 and none have gotten my batteries to 100% full. Probably more like 95-97% (IOTA ~95...Paramode ~97...B&D1093 ~97). Even let my batts sit on float for a month. No dice. 15V (8-9A) for 2-3 hrs after a "full" charge gets them right up there.

LOL not knowing that much to start with, 90% of the solutions i found while learning DO point to " and verify with the specific gravity" which makes it a little harder to come up to speed. From where I sit now I would say (as many other AGM users have) that the good ones are pretty tough. They seem to tolerate moderate to severe discharge pretty well and are surprisingly recoverable. They tolerate repeated overcharging in moderation better than I expected, with progressive capacity loss but it is slow. They absolutely dispise continuous overcharge. Solar actually seems a great application for them since even if set for some overcharge, it falls back pretty quickly when they are charged, so the amp hour (watts really) per day excess is controlled.

I have not read this anywhere, but to put a new set of AGMs into service my gut feel would be to get a good overcharge on them in their production configuration one time to clear the grids from sitting around (even if brand new or stored at a battery shop doing monthly top charges), set up a float charger for maybe a tenth or two under spec, and watch how it responds. Then I would go about chacterizing the bank (as a single unit), checking it under load and charge by looking at amp hours in and out, temps, voltage...etc. I would work my way up to optimal float charge value from the low side over time, but top charging enough to get back to full until the float charge is actually tuned in enough to keep the string fully charged.

Jim

It would drive me crazy to have sealed batteries and not be able to know what is "really going on" by using an hydrometer. Hi-Tech can build a gizmo for his AGMs but that would be impossible for most of us. Shows what can be done though.

If your SG is really back to base line absolutely you are there -IFF you are quite sure the battery was full with no sulphation when you established the baseline.

It has been a little more challenging with AGM. I've been using baseline 24 hour temp adjusted voltage and amp hours out vs returned, as well as battery temp.

The easiest way to tell though has been the behavior of a battery at rest voltage on a manual 2 amp charger for me. Mex told my that the delta V goes up very quickly when fully charged and that is the easiest one for me to nail down on an AGM. It is better than measuring voltage to just a tenth since at 12.85v I still have amp hours to go. But at 12.93v it's done.

My Schumacher smart charger kicks into float significantly before the battery is full, even when the 2 amp rate is selected.

Jim

Hi mena,

But how do you measure SG on an agm battery?

How can you have a battery that's full per SG (your established baseline when new) not be full? If you're back at baseline, temp compensated, then it's full.

At low current it is supposed to be considerably more efficient charging (I.e. fewer amp hours to get back to a given state of charge) than at high amps. Efficiency is not always the objective (Pt for speed for example per your stats), but it is important to be aware that the ratio of amp hours in to amp hours out goes up using high current charging.

New AGMs typically have some of the best charge efficiencies of lead acid batteries, and even they are only 90% charge efficient new. They may look charged when they get the same Ah back in, but As far as I can tell that is just not possible. It's probably not even all that close to full. I suspect a battery that looks full (e.g. back to same resting voltage and specific gravity) with 104% of Ah returned to it is not starting out from a full charge. I cannot really think of another way a charge of less than 110%-115% can get you back to starting conditions except starting lower down the SOC curve than full where charge efficiency to get back to that point is better.

Jim