Real capacity of cells after resting 2+ days.

[Bullzeyebill]

I have been paying attention to charging procedures, and reviewing results on CPF of various tested NiMh cells, mostly SilverFox's but others also. Has anyone done capacity tests of cells after a few days resting. I know that cells can come off of the charger as high as 1.45 volts or so, but would the results be the same or similar if the cells were tested for capacity after resting 2+ days, and the voltage has dropped to around 1.38 volts or so?

Bill

 

[Turbo Guy]

Two days means next to nothing unless the cells are really sorry ones. All NiMH cells will deliver more capacity and volt voltage better if they are warm and that is the reason the Maha C-9000 has a 2 hr. rest period after charging prior to discharge in the Break-in mode and 2 hours during the Cycle mode.

Sanyo Eneloops and Rayovac Hybrids have been tested months ofter charging and they really do retain most of their capacity. On the other hand I have had some Energizer 2500s that were flat after a week and other brands high capacity cells that were nearly as bad.

A snip from one of Silver Fox's Eneloops post

The self discharge test revealed that in 31 days there is 93% of the original capacity remaining. I do not know if this self discharge rate remains linear, but in discussing the new battery chemistry with Wayne at AmondoTech, he mentioned that he is under the impression that the self discharge rate slows down the longer the cell is stored. It does not completely stop, but it is slowed down to almost nothing.

 

[Bullzeyebill]

Thanks, I understand the longer term self discharge. I am talking about the drop in voltage that occurs with all NiMh and NiCd cells, even the new low discharge cells, within a day or so after they are charged. That is the voltage that I was asking about and what the capacity of the cells would be then. A lot of us do not go out within a few hours after a full charge and use our lights, so would a cell that measured say 10Ah shortly after a full charge still measure 10Ah after one or two days? What percentage drop would there be, roughly. For example, Silverfox's discharge graph for the AccuPpower D 11.5Ah in the NiMh Battery Shootout thread?

Bill

 

[Mr Happy]

Qualitatively, the amount of charge that produces a change in voltage from, say, 1.38 V to 1.45 V is very small. So when a rested cell drops by that much over a few days (a few weeks for eneloops) there is a decrease in charge, but it is not more than say 50-100 mAh, at a guess.

 

[VidPro]

and , , , once put on a load any high voltage from being on the charger would decline very rapidly, so any total wattage numbers wouldnt be severly effected by the temporary higher voltage, just because it wouldnt last much time,once a load is applied..

still the practical reality of testing the cells in a similar way that the cells are USED would provide a picture that is closer to the reality that we use them in. the question becomes is it practical to spend 2 more days testing a battery unless its for any major self discharge

 

[Turbo Guy]

How accurate do you want to be?

As I mentioned NiMH cells do best when warm/fresh off of charge. If you let them set on a cool location (frig) for a couple or three days ,remove,warm them then test they will be much the same as when charge complete. If you store ythem in a hot location such as a auto in 110F ambient location then they will hold voltage under load well but have reduced capacity. If they are being used in 40F location then they will perform poorly until they warm up,if they watrm up.

A AA NiMH being used under a 0.25A load in 60F will have a gradual voltage under load increase as it warms to 75-80F.

There are so many variable that effect a cell's batterie's performance that most statement are very general guidlines.

Best used freash off of charge for max. performance.
Max. storage life at reduced temp..

More general terms:

NiMH lose 10% capacity in first 24 hours after charge, 5% next 24 hours, 2.5 the third 24 hrs. etc. etc.
Some say 30% in thirty days.
I have done 60 day test on non LSD NiMH cells and my results with quality cells were much better.

Bottom line is if you truely must have maximum available capacity from say a 2000 mAh cell then use a quality 2300 mAh cell.

Intended useage should be the major consideration for cell choice. The best cells for EDC,a bug out bag, or high drain light / flash or HT are likely not the same.

 

[Bullzeyebill]

Part of reason for my asking is my experience with three Tenergy 10 Ah D NiMh's for use with my Malkoff Triple which draws about 3.2A's. Was using my Vanson charger and not getting complete terminatiion; cells were not reaching above 4.1 volts. I switched to my Triton charging the three in series, using charge discharge setting. Voltage came off charger in 4.4 volt range. Cells rested for 20 minutes, and I noticed while discharing at 3A's that the voltage dropped fast to the 3.6+ range under load within 40 minutes or so. Voltage held better down to cut off, 2.4 volts. Capacity noted was 8800mAh. Total discharge time was 168 minutes, pretty short I would think.

So the question, would these Tenergy D cells hold up within two days or so, and I could probably answer most of that question myself by doing a discharge after 2 days. Probably would get better performance with the AccuPower 10 Ah LSD cells, initially and after a few days using my Malkoff Triple. The Tenergy cells are probably not robust and good for holding voltage/current at 3A+ drain.

Bill

Bill

 

[VidPro]

on the vanson charger check the manuel, depending on which one it is it will not fully charge a fully discharged 10Amp D cell, without putting it back on the charger, because it times out.
or like find out what the rate of charge is, and when the timer kicks in, and figure it out from there. then of course after stuffing it back on, you need to know if its going to go over specs for overcharge, or stay within the capability of the cell.

at 3A's that the voltage dropped fast to the 3.6+ range under load within 40 minutes or so.
as it usually would, check the discharge graphs for the battery, basically what you said is sorta normal, along with the batts having about 9000ma not 10. any of the Dcells of about any type, will handle a nice 3amps just fine, its just right for it. except for the faux ones with a AA cell hiding out in a D case.

 

[Mr Happy]

I switched to my Triton charging the three in series, using charge discharge setting. Voltage came off charger in 4.4 volt range. Cells rested for 20 minutes, and I noticed while discharing at 3A's that the voltage dropped fast to the 3.6+ range under load within 40 minutes or so.

This is completely normal, IMHO. Under load the discharge voltage of NiMH is about 1.2 V per cell, so 3.6 V is exactly where it should be.

Voltage held better down to cut off, 2.4 volts. Capacity noted was 8800mAh.

I would say this cut off voltage is much too low, corresponding to 0.8 V per cell. You are in danger of causing cell damage from cell reversal by doing this. The recommended cut off voltage for series packs is about 1.0 V per cell.

Total discharge time was 168 minutes, pretty short I would think.

Well you discharged a pack at about 0.33C and got a discharge time of about 3 hours. Isn't this exactly in line with expectation?

So the question, would these Tenergy D cells hold up within two days or so, and I could probably answer most of that question myself by doing a discharge after 2 days. Probably would get better performance with the AccuPower 10 Ah LSD cells, initially and after a few days using my Malkoff Triple. The Tenergy cells are probably not robust and good for holding voltage/current at 3A+ drain.

I'm not seeing anything badly wrong from your test results. A D cell should comfortably manage a 3 A drain current; many AA cells can manage this.

A capacity of 8000 to 9000 mAh is not unreasonable for a labeled "10,000 mAh" cell, especially a cheaper brand. You nearly always get less real-life capacity than the label. The label represents a theoretical maximum capacity under perfect and undemanding laboratory test conditions.

 

[Bullzeyebill]

Thanks Happy. I am familiar with the Vanson's timer. I did the Triton setup charge/discharge cycle which cuts off at 0.8 (X3). I will do the manual discharge function which I can set for 0.9 volts next time. Yes, it is apparent that the Tenergy cells are not performing like the better cells, and no where near the AccuPower 11.5Ah cells which attain the 1.2 volts per cell underload at about 3A's, between 2 and 3 hours, not 40 minutes, into the discharge. These are the kind of cells that I want, and I would perfer the AccuPower 10Ah LSD's.

Bill

 

[Mr Happy]

Tenergy 10.0 Ah D cell: ~$7.50 ea.
AccuPower 11.5 Ah D cell: ~ $12.00 ea.

You gets what you pays for, I'm thinking?