i have addressed the following question to the retailer concerned, they have a normally very good technical department,but after a month and several attempts i have recieved no reply.

' i have purchased a set of your------- 2000MA HRS AA RECHARGABLE batteries, a charge rate of 480 ma is stated on the battery.
i have made my own charger and can select any charge rate, but 480 ma seems very high,the batteries get very hot at LESS THAN half that.'

can i please ask the forum for their opinion?

regards
dougal

 

Hi, you don't say what type of rechargeable they are, I assume NiMh?

The main question is how long do they take to get hot, if charged from a fully discharged state?

480mA does not sound unreasonable for fast charge cells, but the charger should be cutting off after about four hours, starting from dead flat.

With that type of charge rate, any attempt to charge past full capacity will cause rapid heating and permanently damage the cells as they will start to vent electrolyte.

Traditionally the threshold for continuous charge is 1/10 C so they should take 200mA for 24 Hours without harm, as long as they have not already lost capacity due to overcharge. A 14 hour charge is generally taken as sufficient to give a full capacity.

 

Your battery charger is supposed to monitor the charge of the battery then detect that it is fully charged then it should shut off. If you try to charge battery cells that are already charged then your battery charger should sense it and reduce the length of charge or refuse to charge a fully charged battery.
Battery charger ICs do this.

I have a stupid charger from a battery manufacturer. It is simply a timer. It severely over-charges (and makes very hot cells) if the cells are already charged.

 

i have addressed the following question to the retailer concerned, they have a normally very good technical department,but after a month and several attempts i have recieved no reply.

' i have purchased a set of your------- 2000MA HRS AA RECHARGABLE batteries, a charge rate of 480 ma is stated on the battery.
i have made my own charger and can select any charge rate, but 480 ma seems very high,the batteries get very hot at LESS THAN half that.'

can i please ask the forum for their opinion?

regards
dougal

For continuous charge, I never go higher than C/10. That would be 200mA in your case. In the case of NI-MH, some don't like more than C/20 continuous charge so you could use 100mA. It will take about 22 hours to recharge a fully discharged cell at that rate but the batteries will live longer because they won't get as hot. In general, when the cells are getting very hot you are overcharging them which will damage them. Slight warming is OK.

 

Hi tibbles,
As far as battery charging goes, for the most part, charging at a high rate to the point that the battery gets hot will significantly reduce the life of your battery. In fact, many sophisticated chargers have an optional thermocouple to monitor the battery temperature during charging to ensure that it isn't getting too hot, as well as to adjust the max charging voltage as the battery temperature changes. Now, since I don't know what kind of charger you have, the problem could be overcharging (continuing to charge after the battery voltage has reached the proper value) instead of a too high charge rate that's getting your battery hot. Also the word "hot" is relative and charging at a rate that gets the battery warm to let's say about 35 to 40 C is probably OK (check the battery manufacturer's datasheet for details), but if it's more like 60 C, that's almost certainly too hot. Again, go by the datasheet for the battery and if you can't get one from the mfg, then get one from a manufacturer that makes a similar battery of the same type and size.

Good luck,
Kamran Kazem,
Senior Member of IEEE with 30+ years of power electronics design. BSEE & MBA from UC, Irvine
kkazem

 

thank you very much for your replies on this. will get back tomorrow
regards
dougal

 

In addition to sensing temp., as someone already mentioned, I believe some high end supplies will pulse current to the batteries, which helps prevent them from overheating.

 

I run an energizer 15min charger off my home made solar panel with the energizer batteries and it works like a charm charges batteries free of charge in no time flat could not ask for a better setup.

One negative is the charger does not support 9v batteries...

Maybe someone has a schematic or something for a design such as this I understand your issue is you want faster charge times and be sure you do not blow anything up in the process or ruin all the cells very promptly.

I would also like to note that the energizer charger uses something similar to a laptop processor fan to cool the batteries as it charges.

I have tried several batteries with this charger but the BTY batteries from overseas don't cut the mustard the main batteries that seem to work are rayovac, energizer and duracell.

 

In addition to sensing temp., as someone already mentioned, I believe some high end supplies will pulse current to the batteries, which helps prevent them from overheating.

That's a scam. DC current is fine as long as you don't overcharge.

 

I run an energizer 15min charger off my home made solar panel with the energizer batteries and it works like a charm charges batteries free of charge in no time flat could not ask for a better setup.

One negative is the charger does not support 9v batteries...

If you mean it charges from dead to full in 15 minutes, that is going to be very hard on a NI-MH battery. NI-MH has an exothermic charge reaction and they don't like ultra fast charge. It will definitely shorten the life.