Charging NiMH with Solar Panel

[FlashGordonDownUnder]

Hi,

I want to be able to charge my 1.2V NiMH batteries while I am bushwalking.
Or more correctly I should say that I just want to be able to top up their
capacity while I'm walking as there will not be an oppotunity to be in the
one place for 10 (20?) hours with correct orientation to get the best
output.

My digital cammera and poratble HDD use 4 x 1.2V NiNH so I'd like to top up
the batteries as a group using a 12V 5W solar panel (Uni-solar flexible
type).

Could someone tell me if I can connect the panel directly to my batteries
(they will be in series = 5V) with a diode to stop reverse current back to
panel or should I be limiting the voltage with a zener or very basic
regulator to get the voltage capped at say 6 - 8 V. I'm not too worried
about the current as I'll never get more than about 230mA out of it and even
at this rate would be for a few hours maximum hence never reach capcaity of
batteries (which are 2300mAh).

As I'll be having a number of sets of batteries (4 per set) I could put two
sets in series and then charge that way but would my top up charging be
halved this way and become rather useless?


Thanks Gordom (from Downunder in sunny Sydney, Australia)

 

[Gymmy Bob]

NiCads and NiMh batterries are designed to take a current charge forever.
Usually this the 10 hour rate. 550mAh / 10 = 55mA for 14 hours for full
charge (40% loss in charging)

Their voltage does not increase much and therefore is a lousy way to control
charge. Maximum current is what you want to control. They also get warm when
fully charged. Feel them and see if they are done.

 

[FlashGordonDownUnder]

Bob,

Thanks for the info.

But could you tell me if I can connect a 12V solar cell directly to 4 1.2V
batteries connected in series as I know the solar cell can put out anything
from 0V to about 24V.

Thanks Gordon

 

[Gymmy Bob]

Again. The voltage of the open circuited solar cell does not matter. The
circuit voltage will be the voltage of the battery bank. You will have to
connect them experimentally and measure the current and adjust with a series
resistance.

 

[FlashGordonDownUnder]

OK, I shall try that.

I thought the voltage was important also, but I did measure with 8 batteries
and noted that it was around 10.5 - 11V I think.

Thanks Gordon

 

[boB]

After the batteries are charged, a big enough un-regulated PV module
can bring the battery voltage too high and destroy them.

I'd use some kind of charger to control the end of charge. Otherwise
you have to watch them closely.

boB

 

[John Rowell]

Might use a voltage regulator. You can get an LM317 chip at Radioshack for a
couple bucks. Add two resistors to adjust the voltage to 6 volts. This will
make sure the voltage stays within a safe level. The current from the solar
panel is low enough you shouldn't have to concern yourself with
overcharging.

--John

 

[Gymmy Bob]

These are NiMh. If he doesn't have to wory about overcharging, he doesn't
have to insert a regulator chip either. NiCads and NiMh are never charged
with constant voltage.Voltage does not indicate state of charge well.

 

[FlashGordonDownUnder]

I'll be trying it this weekend if it does not rain on me!

Will be using some oldish NiCd's so if they get distroyed it will not matter
too much, they have had a hard life, been shorted out a few times with the
holder catching alight, luckily was around when this happened, otherwise may
have burn the house down!!