NiMh 1C ok?

[Dvnts]

Hi all, a quick question on charching my NiMh cells in my Radio.
They are eight 2500 mah AAs. I tried charging them at 1C once or twice, but they get very warm. My charger has the option of changing the batt type, C rating, and there is another number which can be changed from 1-5 with no description. I have no idea what this is for. I just leave it on 1.
Any advice appreciated.

[Hughes500Bob]

http://www.powerstream.com/NiMH.htm

Here is a good source for understanding how to properly charge a NiMh battery.

They should just start to get warm when charged .... but never ever get hot.

Read the article!!!! Charge rate varies from .1C for overnight trickle charge to 1C for really fast charge. Having the right charger for the job required is paramount to success. Battery temp monitoring by thermisters (or other) is essential. When you buy that little AA NiMh charger with the clear cover ..... it is essential that the cover be CLOSED when charging so the thermister works properly. The new Sanyo Eneloop charger (my favorite NiMh rechargeables for portables!!!) has to even be plugged in vertically to a wall outlet to position the thermister correctly!!

Unless you have the proper charger for a fast charge, trickle charging overnight with a back-up timer to make sure you do not overcharge is the best way to recharge and assure long battery life.

Heat kills batteries!!!!!!
[edit 5-25-2007]

[M Carr]

I would recommend charging AA Nimh cells at a current of 1 amp or less.
1.2 amps MAX. reason is you dont really want to heat them up when charging.

I am guessing you are saying 1C means you tried charging them at 2.5 amps? If so that is WAAYYYY to much current for those little AA cells.

Like hughes said, " they should just start to get warm when fully charged"

[SilverFoxCPF]

Hello M Carr,

It seems that the battery manufacturers do not agree with you...

Sanyo suggests charging their 2500 mAh cells at 2.5 amps. Here (http://sanyo.wslogic.com/pdf/pdfs/HR-3U-2500.pdf) is their specification sheet.

This seems to be pretty much a universal charge recommendation. GP, Panasonic, Energizer, and Duracell all give similar recommendations.

It seems that the end of charge detection signal is not strong at lower charge rates, so the 1C charge rate seems to give a strong signal. You are allowed to go as low as 0.5C, but 1C gives a stronger end of charge signal.

When charging at low charge rates you can do a lot of damage to your cells by overcharging, should the end of charge signal be missed. This damage can occur even if the cell does not heat up.

The problem with 1C charging comes from the battery holder and wiring in the Radio. I have had battery holders melt down at just over 1 amp. If you are charging at rates over 1 amp, it is best to remove the batteries from the holder and charge them separately.

The 1 amp limitation is caused by the battery holder and the radio wiring and connector. It is not a limitation of the cells.

Let's see now, 0.5C on a 2500 mAh cell is 1250 mA. You can most likely get away with charging at 1000 mA, but you should keep an eye on things in the event the end of charge signal is missed.

If your cells are overheating at a 1C charge, it is a signal that something is wrong with them. It's time for a 16 hour 0.1C charge to get them back into shape. If they are still heating up after this, I would no longer consider them "healthy."

Tom

[M Carr]

Tom,

Are you sure they arent talking about a sub-c sized cell. If they are than "1c" might be fine. I still would'nt and wont charge AA size cells much higher than 1 amp.

Is it possible that maybe 1c charge rate for AA cells is fine for them as long as it is closely monitored for temps? I dont know that for sure.

Charge them at whatever rate you prefer but I will continue charging at the rates that I have been doing for years.

I cant understand why everyone seems to insist on pushing the charge rates on their batteries just to get it charged only a few minutes faster.

I dont claim to be a battery expert but I have never had any issues or failures of any of my batteries including lipo's and I would like to keep it that way.. :)

But thanks for the info... :)

[SilverFoxCPF]

Hello M Carr,

You should take some time and browse the various battery manufacturers web sites. You will find that they all suggest charging in the 0.5C to 1C range. The Sub C cells charging recommendations are the same as those for AAA consumer cells. The Sub C cells seem to be able to handle 2C charging, but it is not recommended by the manufacturer.

The reason is not for time saving, it is to insure you get a strong end of charge signal.

Cell temperatures should always stay below 140 F, with 104 F being ideal. Heat damages NiMh cells, so you have to watch the cell temperatures.

There is nothing wrong with charging at lower charge rates, as long as your charger doesn't use negative delta V for a charge termination signal. There are some termination signals that will somewhat work at lower rates, but usually you end up with some degree of overcharge every time you charge at lower rates.

It is also less efficient to charge at lower charge rates. At 0.1C, you need to put about 160% in to get 100% out. At 1.0C the input is reduced to around 105 - 110%.

If you completely empty your cells before charging, you can use a timer to charge them, however, this does not work well for partially emptied cells.

I am glad that you are getting good life from your cells. I am merely suggesting that you may be able to get improved performance and life from your cells by following the manufacturers guidelines.

Tom

[M Carr]

I definatly will do some research on it.

appreciate the info. and I may have learned something new.. :)