charging a 16 aa nimh pack

[jmpaul320]

just a question about charging a high drain pack of 16aas in a fm battery holder... i am using a Universal Smart Charger (1.5A) for 19.2-24V NiMH... i am a little concerned because the pack gets very hot... almost too hot to touch... i unplugged the charger after checking how warm the cells were...

is this normal? i have the temp sensor electrical taped to the side of the battery pack (i believe it cuts off power at 70C... that seems high... i thought 45c was the cutoff for nimh cells)

just wanted to get some feedback before i continue charging...

the charger is charging at 1.5A and i am charging 16 aa 2000mah nimh high drain cells.

i tested the voltage when it was getting hot and i got more than 24v from 16 cells

anyone?

[arek98]

If it is charging with 1.5A then it certainly is not a smart charger! NiMh may be charged while connected in series, but only with low current. Below 0.2C is recommended. Otherwise you will cook any cell in pack that will get full charge before others. Don't ask how I know :-(. It is because NiMh can tolerate being charged after they are full only if current is low.

[alpg88]

i,m sure it is smart charger, they are cc chargers, a have several from 0.7a to 4A.

what is the brand of cells?

some AA cells can be charged with up to 10A, but it doesn't mean they can supply 10A
other AA cells can supply 10A but can't be charged with that much current.
all nimh cells regardless of size can be charged with 0.2c, but it doesn't mean it is the only current you can charge them with.
that is why it is best to look at the cells datasheet to be sure the max current for charging or discharging.

but that is for single cell, as arek98 said once you charge a series that is when thing get tricky, i agree, 0.2c is the safest current for nimh in series. 1,5A for high drain cells, is very close to 1c.

[arek98]

Sorry, I should make it clearer. I meant that charger that charges 16 cells in series with 1.5A is not a smart charger (IMO). There is nothing wrong with charging individual cells with 1.5A or even higher but then charger will terminate when cell is full (assuming it works as it should).

While charging cells in series charger is checking voltage across all of them. It has no way to detect fact that one or few cells already reached full charge if other cells are still not full and have lower voltage. It will “think” that pack is still not charged and will supply given current. Now, current will flow thru all cells including already charged ones and will cook them.

I have such “smart” charger for 12AA and while it works ok many times eventually I destroyed few cells in pack. I think these types of charges may be used for fast charging when needed but pack needs to be watched and temperature raise above certain level is an indication of trouble.

I believe it can be assumed that NiMH can be left on charger without any problems if current is below 0.1C. So, for NiMH battery pack I would use slow (0.1C) chargers. Charger with higher current may be used but only under supervision and maybe not to full capacity, just to get enough juice into cells to get going.

Disassembling pack once in the time and charging cells individually also helps keep them balanced and in good shape. Best on real smart charger like MH-C9000 or similar to check if they are still in comparable state.

[Illum]

Universal Smart Chargers are not always as "smart" as it should be. As a long term Tenergy USCs myself, I can't tell you how many Elite1700 NIMH packs I have built and destroyed by the chargers, yet the same cell size, same quantity, and using eneloops I have never had a problem. Ask around on the forum to see if there are any cell incompatibilities before putting that much in an investment

[kramer5150]

Very informative thanks!!! I was charging my 5S eneloop pack at .5A. I'll back that down a bit closer to .15 - .2C

[jmpaul320]

they are elite cells... i have a bc700 so maybe ill just use that, pity, i was hoping i didnt have to take apart the carrier everytime i wanted to charge, especially considering they last 10-12 minutes in my mag 458 lol

i do know in fact that the temp sensor is working, because it turned off twice yesterday due to temp. i hope i didnt cook the cells too much, they were nearly too hot to touch... i guess lesson learned... it wouldnt be a hobby if you didnt learn something by breaking it right? lol.

after resting the cells dropped to around 22.5v today, so i dont think i overcharged them, maybe they just got a bit warm and hopefully all is good

i will keep the smart charger maybe just for a quick charge here and there and i will carefully monitor it when i do so, but i will switch to using my bc700 at a lower current... the cells should last longer that way, that is if i didnt cook them

thanks everyone for your input, much appreciated

[Wrend]

For charging loose cells in series, I wouldn't recommend charging at a faster rate than 1/10 C/h (200mA for 2000mAh cells) in general.

If I really wanted to fast charge them in series, I'd watch them closely and take them off the charge when they get warm, then top them off at the much slower 1/10 C/h rate to help balance them at a more fully charged state.

Charging the cells on a good smart charger that has independent port charging at rates up to 1/2 C/h should be fine. In fact, you probably wouldn't want to charge them too slowly (less than 1/3 C/h) because of the risk of missed termination.

[jmpaul320]

Does anyone know of a charger for a 19.2 v pack that will charge at low currents? (200ma) and that is adjustable?

[Wrend]

Many of the hobby chargers should be able to. My Passport Ultra will charge up to 14 NiCd/NiMH cells in series. The FMA PowerLab 8 will do up to 21 NiCd/NiMH cells in series, and is more or less the top hobby charger you can get.

[jmpaul320]

the fma is a very good looking charger, on the expensive side though

going back to my battery pack... i wonder if eneloops would have been a better choice... i have read that they are good for roughly up to 10 amps... going buy lux's spread sheet for the 64458 bulb it looks like the most it would pull would be 10.8 amps...

i wonder if the 3rd gen eneloops could handle this?

[arek98]

Ok, I had the feeling that 0.2C is incorrect; it actually is 0.1C (or C/10). I updated post above.

I believe charging NiMH batteries was covered on CPF multiple times already (as well as other types). NiMH are actually pretty hard to charge correctly and maintain well. With good charger and few loose cells there is no problem but packs with many cells are cumbersome.

Here are some additional interesting information:
http://www.powerstream.com/NiMH.htm
http://www.camlight.com/techinfo/techtips.html

[jmpaul320]

Ok, I had the feeling that 0.2C is incorrect; it actually is 0.1C (or C/10). I updated post above.

I believe charging NiMH batteries was covered on CPF multiple times already (as well as other types). NiMH are actually pretty hard to charge correctly and maintain well. With good charger and few loose cells there is no problem but packs with many cells are cumbersome.

Here are some additional interesting information:
http://www.powerstream.com/NiMH.htm
http://www.camlight.com/techinfo/techtips.html

some great info there... large packs seem to be hard to maintain