|Re: OT-N cells hide & seek|
Message #11 Posted by Luiz C. Vieira (Brazil) on 26 May 2012, 9:36 p.m.,
in response to message #10 by Les Wright
I have a standard charger I want to modify and use it for charging specific batteries. I just want to re purpose the case, line transformer and batteries compartment. This standard charger has just rectifying diodes, some current-limiting resistors and one small current meter, and I want to build something more reliable and precise inside of it. It's been hard to find some time to get my classes ready to carry on, what to say about time to a single project like this one...
What I use to recharge the NiCads and NiMH is single pieces of double-sided, fiberglass PCB's with 1/4 resistor soldered on them (picture). These little PCB's are placed between the batteries and the charger contacts, like this, so the current is limited to approximately 1/10 nominal battery current. I use the 47-ohm resistor for the ones below 200mAh (I have 12 of 190mAh and 4 of 150mAh) and the 27-ohm resistor for the 400mAh and 500mAh NiMH batteries. I let them charging for no more then 10 hours, and from time to time I measure the voltage across the resistor to make sure it has the average 1/10 charging current. As you can see in the picture, I use two N-size batteries at once for each AA-size battery holder. I do this mostly because I measured the voltage in the charger contacts while unload and I found about 2.35Vcc. When I inserted one single 1800mAh rechargeable NiCad AA-size battery I measured about 300mA, and that freaked me out! So I decided to use two N-size batteries (about 2.3Vcc) with no series resistor and I measured almost 60mA. After a few tests and some computing (Ohms rule, nothing fancy) I got to the resistor values.
I do not care for charging batteries this way. It is a once-in-three-months operation, give it or take, so the full extent of the batteries lifetime expectancy will be probably more than my own (I'm 50 Y.O., now). I have some almost 5 Y.O. batteries still running smooth after as many charges as I have submitted them to. In fact, I believe they will live long because I am not exceeding their specs.
Hope this helps.
Edited: 26 May 2012, 9:41 p.m.