Battery Temperature when Charging

[max52]

While cycling Sanyo 2700 Ni-minhs in my Maha-9000 charger at 1000 Mah charge, and 500 Mah discharge, I noticed that some of the 4 cells felt hot. Using a high end Craftsman Professional multimeter I measured 2 of the cells at 45C. and 47C. This cell temperature exceeded the 40C temperature charging recommendations with which I am familiar.

I seem to remember a recommendation to not exceed 139 degrees Farenheit during charging, but I can not remember the source. These batteries are well below the 139F reccommendation, but over the 40C advice.

These are older cells, and may be developing higher than normal internal resistance.Can someone clarify the temperature issue. I searched the site and coud not find the answer.

 

[LeifUK]

Last night I charged 2 half empty Panasonic 2700mAh AA batteries in a Panasonic PQ392 charger, and after 1.5 hours they were full, and too hot to hold. I have no idea if that is normal.

 

[T0RN4D0]

I don't think 45C is too hot for 1amp charge / half amp discharge... :thinking:

 

[Mr Happy]

NiMH cells must get hot by the end of charging when being charged at a higher rate and using -dV charge termination. At the time of charge termination the cell has mostly stopped accepting charge and all the current is being converted to heat inside the cell. A temperature of 45°C is not exceptional, although some chargers with more sensitive end of charge tests will keep the cells cooler than others. Cells with a higher internal resistance will also get hotter due to the I²R relationship for heat generation. If you start seeing temperatures over 50°C it is time to start charging at a lower rate.

 

[Billy Ram]

I don't believe it's good for batterys to get as hot as your batterys are getting. My ni-mh chargers have temp probes that I use and will terminate the charge before the batterys get hot. At the end of the charge with the voltage high and the battery fully charged the temp can rise quickly.
Billy

 

[mfm]

1) The 40C "limit" for charging at 1C is for the ambient temperature, obviously the cells can get warmer. 45C is not too much during the end of charging. Even my slow eneloop 550mA charger made one eneloop cell reach 42C.

2) The temp limit at which a charger will terminate the charge is usually 60C (139F).

 

[max52]

Thanks mfm. I knew I had read that 139F number somewhere. It appears my cells are within normal ranges.

 

[davidefromitaly]

I don't believe it's good for batterys to get as hot as your batterys are getting. My ni-mh chargers have temp probes that I use and will terminate the charge before the batterys get hot. At the end of the charge with the voltage high and the battery fully charged the temp can rise quickly.
Billy

this is the right way to charge a ni-mh battery. i charge at 1.5A some AA batts and they stay cool under the charge, only at the end they become a little warm and in the same time they perform a little -dV (i stop usually at 4-6mV of deltaV)

if you charge at 1A and the battery become hot there are two answers:

1) you are overcharging
2) the battery is crap or have increased the internal resistance

just for comparison, when i charge at 1.5A the peak voltage under load is around 1.6V, with the pass of cycles the battery degrade and you can see the peak become even higher, when arrive at 2V i think you can recycle the battery :laughing: