Computing Peukert, Charge Efficiency and Discharge Curves

Question

So Deka's official exponent for their AGMs is 1.2. That is high/conservative for their published discharge data and my observed results. Is it a worst case end of life number or an average number? I know it gets worse as batteries age. T they keep it on the low side because it can be referenced for warranty claims?

My discharge curves line up with their published discharge data, or maybe just a hair better. If I compute the Peukert exponent from their data which is slightly worse than my partial curves but very close, I come up with 1.12.

I'd like to make a set of discharge curves vs. state of charge (really handy for using just a volt meter to estimate SOC under some conditions), but it seems to be a chicken and egg problem without the luxury of running the experiment to the anchor point of total discharge.

If I can get an accurate Peukert number and charge efficiency of the batteries, I can calibrate the discharge curve end points to state of charge by using amp hours removed/put back as a cross check, without a having to do a full discharge (Peukert would let me adjust measured amp hours out to guess state of charge, charge efficiency would let me adjust amp hours back in as a second check on true state of charge that was reached).

My working number for charge efficiency is 87.2%, but I need some more runs to full charge with accurate total amp hours back in to confirm it.

I feel like I am trying to nail jello to a wall LOL.

Jim

pianotuna · Answer

Hi Jim,Part of the jello is the ambient temperature. 77 F is nominal--but how often are you at 80 (or at -20, like me)?

hitech · Answer

Yes. I am correcting for that as well with my handy dandy IR gun. Batteries are at 82 right now. They vary from 72 to 86 for me this time of year. I briefly quit even float charging them above 85 and certainly quit charging if they are at a high state of charge, because at that temp they gas easily when approaching full. I spend a lot of time flipping into iBooks for a quick check of the temperature compensation tables. I think it was easier to charge in the cool temps.

My handy little buck and boost converters let me adjust voltage and current pretty precisely. Currently I'm back to the point where I cannot charge any more in the charge voltage range (13.8-14.6v) so I am at temperature compensated float, soaking in around. 0.06 amps. When i get the batteries there, 0.03 amps at float will be greater than 97% charged. It seems like charging at the low spec of temp compensated 13.8v until all cells are giving off at least a few micro bubbles (valves under distilled water) here and there, then falling back to float to finish the charge is the most thorough and least violent way to get charged, at least in these warmer temps. I think I still have some stronger cells from being new and not having been on sustained float that many hours. But full charge is important as a starting and ending point for these discharge curves to help me much.

I let the Schumacher on AGM cycle run to recharge them and it did quite well. When running a full cycle from somewhere in the 35%-45% (depending on Peukert value) discharged state and letting it run it's whole charge cycle (including many hours past when it falsely indicated 100% charged), it ran the batteries between 28 amps and 0 amps with a little of it's normal dancing around. It never bubbled any cells, and got the batteries back to about 90% charge, but only including many hours at 2.5amps, when it indicated full and no charging indicators were on.

From there the 8 amp charger took over for a few more hours, then the 2 amp setting regulated through the buck converter for fine tuned control, finish charge and float.

Jim

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Computing Peukert, Charge Efficiency and Discharge Curves

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