Problem Charging AAA Batteries With MH-C9000?

[Zephrus]

I didn't know if it would be good forum etiquette to resurrect such an old thread, so I opted to reference the old thread and ask this question in a new post. REF: http://www.candlepowerforums.com/vb/showthread.php?154756-Maha-MH-C9000-Wizard-One-Charger/page2

Hello Hoggy,

Yes there is a 2 hour top off after the charger shows done. The top off charge rate is 100 mA.

With AAA cells, it is not a problem if you charge at higher charge rates. For example, if you have 800 mAh cells, you should charge in the 500 - 800 mA range. If you do this, the charger will terminate on high voltage (1.47 volts) and utilize the top off charge to complete the charging process.

If you charge AAA cells at 200 mA, you could run into some problems with overcharge from the top off charge. Of course this would depend on the capacity of the cells you are charging.

Tom

If this is the case, then wouldn't it also be detrimental to AAA cells to do a "Break-In" cycle on them?

For example, If you're charging an 800 mAH AAA cell, the charge current during the Break-In cycle will be 80mA. At this current, the cell will be "fully charged" in approximately 10-11 hours (90% duty cycle of the C9000). Since the C9000's Break-In charge cycle runs for 16 hours, that would mean an addition 5-6 hours of charging at 80mA.

If, as stated above, an extra two hours of "top off" charging at 100mA would be detrimental to the cell, wouldn't an extra 80mA for six hours also be?

I'm asking because I'm about to do a Break-In cycle for several sets of AAAs, and wanted to know if there is anything to be mindful of during the process that might potentially damage the cells.

 

[Wrend]

They're full from empty after about 12 to 14 hours of a 0.1C/h charge rate (80mA for AAA Eneloops on 800mAh break-in). Charging efficiency isn't 100%.

16 hours is the industry standard for this type of a full charge and capacity test, however; and it also works for slightly older cells that might take a little longer to charge due to higher internal resistance.

But to answer your question: No, it isn't a problem, and neither is the top-off charge. I have tested this, and the full top off charge with an additional 1 hour trickle charge afterwords does not overcharge AAA Eneloop cells. This is partly because the C9000 terminates the main charging phase so conservatively. The top off charge just helps to balance the cells more at a more fully charged state.

Just discharge your cells on the C9000 first before doing the break-in (the lower the discharge rate, the better), and you're all set.

For general charging, I recommend charging AA Eneloops at 700mA to 1000mA on the C9000, and 300mA or 400mA for the AAAs, and letting them go through the top off charge after the main charging portion is "done."

If anything, undercharging might be an issue for the AA Eneloops. This is partly why I've more recently been charging them at 700mA (letting more capacity "soak in" before 1.48V is reached) and making sure to let them go through the full top off charge.

Notice how your cells never get more than just a little warm to the touch on the C9000. There's a reason for that. :thumbsup:

 

[Zephrus]

They're full from empty after about 12 to 14 hours of a 0.1C/h charge rate (80mA for AAA Eneloops on 800mAh break-in). Charging efficiency isn't 100%.

16 hours is the industry standard for this type of a full charge and capacity test, however; and it also works for slightly older cells that might take a little longer to charge due to higher internal resistance.

But to answer your question: No, it isn't a problem, and neither is the top-off charge. I have tested this, and the full top off charge with an additional 1 hour trickle charge afterwords does not overcharge AAA Eneloop cells. This is partly because the C9000 terminates the main charging phase so conservatively. The top off charge just helps to balance the cells more at a more fully charged state.

Just discharge your cells on the C9000 first before doing the break-in (the lower the discharge rate, the better), and you're all set.

For general charging, I recommend charging AA Eneloops at 700mA to 1000mA on the C9000, and 300mA or 400mA for the AAAs, and letting them go through the top off charge after the main charging portion is "done."

If anything, undercharging might be an issue for the AA Eneloops. This is partly why I've more recently been charging them at 700mA (letting more capacity "soak in" before 1.48V is reached) and making sure to let them go through the full top off charge.

Notice how your cells never get more than just a little warm to the touch on the C9000. There's a reason for that.

Hey Wrend, Very informative reply.

Typically I'll charge my Eneloop AAs at 900-1000mA (C/2), and the AAAs at 500mA in order to facilitate an early and proper charge termination ... the idea being that the subsequent top-off charge will do just that, top them off (AAAs) rather than overcharging them. You believe a lower 400mA rate will work just as well? I'll give it a shot.

For the C9000 I figured (correct me if I'm wrong) about a 90% efficiency based on its charging method, and then another certain negative efficiency component due to heat loss, which I can't figure for any one cell since different brands of cells have different internal resistances. So for the Eneloop AAs, using the C9000 16 hour Break-In, that would seem to put the actual charge at closer to 2880mAH (1.6 x 2000 x 0.9) plus a small degree of loss due to heat.

I've been using Maha/Powerex since 1999, i.e C777, C777Plus. Always loved their chargers and they've always done a good job for me. The C9000 had a bit of a rocky start but after its few revisions, I consider it top rate. So I know what you mean by "that reason" the C9000 is so good :thumbsup:

Thanks for the extra info, and nice to know that I haven't been inadvertently/unknowing damaging my AAA cells.

 

[Wrend]

You believe a lower 400mA rate will work just as well?

Yes. It shouldn't be a problem; even 300mA should be fine for the AAAs. I've done a hand full of tests with the AAAs (because I was also initially worried about overcharging them with the top-off charge on the C9000 and had heard others speculate that it was bad for them), comparing their break-in capacity results to their normal charge discharge results, and they were not as fully charged with the normal charge, including the 2 hour top-off charge and an additional hour of the trickle charge (about 9mA, if I remember correctly). I believe the top-off charge is about 90mA, which is very close to the 0.1C/h rate, which is generally considered a safe enough rate to continue charging the cells at after they're full (though of course, I would recommend avoiding that when you can).

The AAs are proportionally even less fully charged after the top-off and trickle charge, which is one of the main reasons I now charge them at a 700mA rate, and make sure they go through the full top-off charge.

I only use and charge my cells together in sets, and having them more balanced at a fuller state of charge is important to help avoid the risk of reverse charging and to help keep them more evenly used (at least in theory).

For the C9000 I figured (correct me if I'm wrong) about a 90% efficiency based on its charging method, and then another certain negative efficiency component due to heat loss, which I can't figure for any one cell since different brands of cells have different internal resistances.

I've measured the charging efficiency to about 50% from after the power supply before the charger to how much the cells discharge when used (taking into account the efficiency of the charger and the efficiency of charging and using the cells) to calculate their continued cost of use after being purchased. (Of course they very easily beat out non-rechargeables in regards to total cost effectiveness and often performance too.)

Though I haven't tested this specifically, from the charger to the cells on a normal charge, I would estimate the efficiency to be closer to maybe 80%. The nature of NiMH/NiCd cells is such that to "fully" charge them you'll actually have to, in a sense, "overcharge" them a bit. This is normal, and when kept within proper time limits and charging rates, won't add any significant damage to the cells. It's just part of their normal wear and use.

I guess ideally they could be charged to maybe 80% to 90% full, but then there is the problem of determining how to actually do that and insure that they're balanced. The C9000 comes close to this, in a way, because of the 1.48V termination, but then there are slight temperature and cell variations that the C9000 can't account for.

If I remember correctly, Eneloops have a slightly higher voltage average compared to traditional NiMH cells, so the main charge terminating at 1.48V on the C9000 might be a little more conservative for them than for other NiMH cells.

Thanks for the extra info, and nice to know that I haven't been inadvertently/unknowing damaging my AAA cells.

No problem. You should be fine. My oldest Eneloops are the fist generation ones and I've been using them for over 3 years now, and they're still performing within the range of brand new cells after a discharge and break-in on the C9000. (I estimate maybe a 2% reduction in their capacity potential at most so far. When I tested them a year ago it was close to around 1.1%. This was just an estimate comparing them to newer cells though, because I didn't have my C9000s yet when I first got the Eneloops.) So that bodes pretty well for Eneloops and using the C9000 to charge and maintain them. At least well enough for me. I expect them to last at least 5 years, and maybe 10 or more.