Charging NiZn AA Cells With a Hobby Charger

This is an experiment, not a recommendation!

A while back, in post #35 of Mr Happy's Ultralast NiZn thread, I brought up the idea of using a hobby charger to charge NiZn cells in series. Well, since having recently obtained 8 PowerGenix AA cells, I thought I'd give it a try, just to see what happened.

For my initial test, I used two AA NiZn cells that had been basically discharged, their rested OC voltages measuring 1.56 and 1.57 Volts. I set up my Dynam Supermate DC6 (link from Dynam-RC) hobby charger to "LIPo Storage". This is the setting used to either charge, or discharge a single (or multiple cells, with balancing leads, depending on the setting) Li-Ion cell, to about 3.80 Volts, when preparing LiPo, or modern LiCo cells for long term storage. When charging, a CC/CV algorithm is used. With the charge rate set at 2 Amps, this provides a 2A/3.8V CC/CV charge, with a 200mA cutoff current (0.1C). This is of course, meant for charging a Li-Ion cell, but I substituted two NiZn cells in series, in place of a Li-Ion cell, for this experiment.

The charge process appeared to work exactly the same as if there were a single Li-Ion cell being charged. The current started at 2A, and the circuit voltage around 3.3 Volts. When the voltage reached 3.80 Volts, the CV stage commenced and the current began to drop, as expected. When the current eventually dropped to 200mA (0.1C), the charge terminated.

During the charging process, I periodically checked the individual cell voltages with an accurate DMM. While the cell voltages were rarely exactly the same, they were always within 0.02-0.03 Volts, or so. The OC cell voltages (removed from the circuit) at charge termination, were within 0.01 Volt, reading 1.83 and 1.84 Volt. This may seem a little low, but I attribute this to the fact that the charge was terminated at 200mA, as opposed to the recommended 90mA, due to the 0.1C cutoff of the Supermate. At no time during the charging process did either of the cells become detectably (with my hand) warmer than ambient temperature, nor did either cell's voltage read over 1.92 Volts within the circuit, during the charge.

I repeated this test with cells that were initially 1.54 volt and 1.63 Volt, and observed similar results. Again, while cell voltages were never exactly the same during charge, the measured OC voltages off the charger after termination of charge, were within 0.01 Volt.

My next "test" will be with two cells that are significantly (but not extremely) different in SOC, as the previous two pairs of cells were in a basically discharged state. If there is going to be a problem charging NiZn cells in series, this is where it will show up, and I'm almost certain it will. Possibly a slow charge rate such as is used with NiCd/NiMH will work in this situation.

If the results look promising, I will try charging 4 cells in series, without balancing leads (as NiCd and NiMH cells are series charged), and if that is successful, I'll try 6 and 8 cells. Keep in mind that two NiZn cells in series = one Li-Ion, in this experiment.

It would appear, admittedly in the early stages, that this is possibly a viable solution for charging NiZn cells, at least from what I've seen so far. Results suggest that it may at the very least, work for cells that are initially close in SOC.

I haven't been able to spend a lot of time with these NiZn cells, as I have other responsibilities to attend to, as well as other light projects. I'd also like to point out that I'm aware that this particular endeavor lacks any resemblance to having been performed in a "laboratory environment", with data logging of voltage, current, temperature etc. Expectations should be based more along the lines of what one might expect from someone who is the product of a Hillbilly and a Hoosier, with a bit of Cherokee Indian thrown in, more of a "hands on approach". At any rate, any comments, or suggestions are welcome.

Dave

Great tests Dave. I look forward to following your progress.

Having done some testing on the PowerGenix NiZn cells, I think that it is possible to charge these cells in series without damage if one considers the following three issues:

1.) The cells should first be charged individually using an an identical charge method, either with a PowerGenix charger or (perhaps preferably) with a CC/CV power supply. This insures that the cells all start off at 100% SOC. As long as the cells are discharged in series, the exact same number of amp-hours will be consumed in each cell, so that upon subsequent charge all of the cells should (theoretically) all come back to 100% SOC.

2.) The charge rate should be relatively low above ~1.85V per cell. I know that it is widely assumed that the NiZn cells do not have an internal chemical mechanism to compensate for overcharge like NiMH and NiCd. However, the exact same overcharge reaction (i.e. oxygen recombination) does occur in a NiZn cell, but at a much lower rate. Therefore, it seems prudent to charge these cells slow, at least near top of charge.

3.) Overdischarge and polarity reversal should be avoided as this will almost certainly put the pack out of balance (and probably damage the cell).

Good luck with your tests. I can't wait to see your results.

Cheers,
BG

Well, I have a gut feeling that if this is really going to work consistantly, that the cells will have to be in a similar SOC, and probably will have to have a similar usage history, as well. Mr Happy said it likely wouldn't work and he's usually right, I just thought I'd try it and see what happens. While I'm not very far along, as far as "testing" goes, so far, so good.

As I said, I'm not real "gung ho" on this project lately, due to other obligations and projects, so it's going to be going rather slowly, for now anyway.

Dave

FMA/Revolectrix recently released new firmware for their multi chemsitry chargers.

Their PwerLab 8 can balance charge, discharge and cycle upto 8 NiZn batteries.
Multi 4 can balance charge upto 8 Ni-Zn batteries.

BTW, for discharging a single cell SLA battery must be used as a power source. With DC power supply discharge is only possible for cells with 3V or above.

Although, it is somewhat late to come up to post such news, anyone who wants to perform test with NiZn will greatly benefit from these chargers.

I should have come back to this thread with an update, long ago. I had mentioned elsewhere however, that using the "LiFe storage charge" method for charging NiZn cells, turned out to not work all that well. Cells during charge were never at the exact same voltage during charging and at the end of charge were never at the same voltage. I also noted that the more cells in series (I attempted charging 4, and 6 cells), the farther off the voltages would become.

Interestingly, cells charged with the black "5 hour" PowerGenix charger (model F100027-00) that charges cells in pairs, seems to work much the same as when using the hobby charger, charging only two cells. Cells during charging, and when they come off the charger, are at slightly different voltages using it also.

My guess is, that if you limited charging to only two cells at a time, you may be able to use a hobby charger. However, since I obtained two PowerGenix chargers that will charge two, or four cells at a time each, it seemed rather pointless to continue testing. As a result, I have no long term effect report regarding the use of a hobby charger to charge NiZn cells. I think the best charging solution for NiZn cells is probably the white PowerGenix charger that will charge 1-4 cells, as it can tend to the charging needs of each cell separately.

Kudos to FMA for including NiZn cells in their charger programing. I doubt we will see others follow suite though, as the present generation of NiZn cells seem to have been discontinued. But, in the future......

Dave

I really appreciate for your test results. I've been using and testing NiZn batteries for a while(while many people already tried and stopped using it).

Although, it is such a safe battery, much safer than NiMh, capacity discrepancies between the cells and capacity reduction after each use seems to be quite heavy.

BTW, when I was testing with FMA chargers, I could only charge them at 500mAh in order to do proper balancing between each cells. If I was trying to charge them at 750mAh, weaker cell may rise too fast during CC phase and prematurely reaches 1.9V.

Also, rest period between charge and discharges were affecting capacity reduction.
When I was charging at 500mAh, 15 min rest than discharge at 1500mAh(1C), I could see 15mAh ~20mAh reduction for eacy cycle.
While, if I try to charge at 500mAh, 30 min rest period than discharge at 1500mAh, I will get 10mAh ~15mAh reduction for each cycle.

It was strange to see first PowerGenix's black chargers charging two cells in series without any balancing features. Also, those black chargers were charging 4 x AA at 300mAh while their datasheet says to charge them at 1/2C(750mAh).