help on charging batteries from solar panel

[buju357]

hi all

I have a 4v 30ma solar panel that Im connecting to 2 x 1.2v 600 ma ni-cd batteries with a small diode , the batteries are in series. what is happening is the "first" battery (one connected to the positive of solar cell) is being charged but the 2nd battery in the series is not.

Ive measured the voltage accross the first battery and its 1.5v , then measured the voltage acros the 2nd battery and its 0.6v.

from what I understand the 1st battery acts like a resistor and sucks up all the voltage only letting a little voltage go through to the second battery.

I cannot charge them in parallel cause when my circuit switches on it needs the 2 batteries in series to give me +- 2.4v.

what should i do , any suggestions ???

thanks

 

[Nigel Goodwin]

If the batteries are in series, then they both get EXACTLY the same charging current, if the voltages across the batteries are different, then you either have a faulty battery, or simply that one is already charged to a much higher level than the other. But at 0.6V, it looks more like a duff battery.

If it'll make you happier, swap the two batteries round, the 'first' one will then read 0.6V, and the 'second' 1.5V.

I presume you have the solar panel outside in direct bright sunlight?, and even at that it will take a long time to charge the batteries.

 

[buju357]

thanks for the reply

I will definitely check the status of the batteries , even swop them around. strange thing is that I fully charged both batteries before putting them in.
BTW , this is a garden light that I have managed to put together using a "joule thief" circuit to power the leds.

this is my calculations:

2 x 600ma batteries = 1200 ma
charging amperage is 60 ma ( not 30 ma)
2 x leds using 20 ma / hour

so presume 8 hours sunlight at 60ma = 480 ma charge
10 hours darkness leds consume 400ma
the difference is 80ma that it should increase by every day.

I will want to do a chematic and put it on my blog.

 

[MikeMl]

The low battery has developed a "dendrite" (partial short) and should be discarded. Actually, if they are of equal age, chuck them both.

 

[cachehiker]

Continuing to charge the two cells past 3 volts is guaranteed to shorten their life. I had a tough time getting ni-cads past 100 charge cycles doing that. Leaving the cells connected to a load after they've essentially dropped zero volts is also hard on them. Simply running LED's will already prevent that to a limited degree though.

I'd suggest a low dropout regulator adjusted to ≈2.95 Volts. A circuit that stops charging at that voltage and doesn't restart until the cell voltages have dropped to a volt or so would be even better.

 

[Mr RB]

It's most likely one or both batteries are faulty.

But you might be able to equalise them by putting a 10 ohm resistor across each battery, and subjecting them to numerous charge/discharge cycles leaving the resistors in place at all times. Ie; discharge through the resistors, then leaving the resistors there when charging.

 

[buju357]

one of the batteries was "dead" , replaced them both and now Im getting 1.5 volts through both batteries

thanks for all the help guys

 

[Mr RB]

NiCds with shorted cell (ie low voltage fault) will often come good after some charge discharge cycles.

Anyway good to hear you got it sorted.