how many volts to charge 4 AA nimh batts

[xboxman]

Hi

i'm going to make a solar battery charger that will charge 4 AA nimh batts and not sure how many volts i should use ?

i'm thinking of maybe around 6 volts  ? but if i use a blocking diode i may need a bit more

any ideas ??

thanks

[tanner0441]

Hi

If you look on the batteries they usualy have the current and time required to charge them, you can then work out the voltage to get the batteries to sink the required current.  I used to use a bulb in series with nicads to maintain constant voltage on them. The bulb intensity varies as the batteries charge.  There are also chips to build chargers round, which will help to preserve battery life.

Brian.

[ghurd]

With slow solar, I try to limit the voltage per cell to about 1.4V.

Takes quite a while to go from 1.32 to 1.4V.

Need about 14 cells to do a decent job.  Need about 9V open circuit.

Simpler to get the $12 / $20 Red 1.5W 12V panel from HF.

G-

[Norm]

No need to worry too much about the voltage ...

a 12 volt solar charger that I have charges about

120 Ma. into 4AA NiCad batteries (700 Ma./hr)

I have it in the south window fixed position from

11 AM to 3 PM.

voltage starts at about 5 volts total(1.25 each)

and by 3 pm. the voltage reads about 5.4 total

voltage (1.35 each)

 So going by that nimh batts at 2300 You would

need a 12 volt solar charger that charged about

350 Ma. to do the equivalent??

 If we get some sun tomorrow I'll try charging

4AA nimh batts. and let you know....

  I usually just charge my nimh batteries in one

of my 4AA charger....usually takes about an hr

or so at 800 Ma. to bring voltage from 1.2 to

1.8 v.   ......same with the ped gen. but 1 hr.

on ped gen. seems like MucH Longer !  LOL

[paborralho]

You have to consider that charged cells go up to 1.4V.

So you need more than 1.4v x 4 + 0.7v for the diode= more than 6.1v

You must know that, you should not overcharge the cells with current greater than 10% of the cell capacity.

Thats why you should put a resistance in series.

I also like the bulb as series resistor because it can dissipate more watts

(but they don't last as much as resistors).

[vtpeaknik]

I've never liked the idea of the self-contained solar chargers because it means that the batteries will be sitting someplace sunny and hot.  Rechargable batteries are expensive, and their cost reflect the energy put into manufacturing them.  Thus, a charging setup that shortens their life means that you don't actually save energy, relative to charging them from the grid, for example.

If you're setting up your own gizmo, I'd suggest separating the solar panels from the batteries.  Connect them with a cable long enough that you can put the batteries in a shaded, relatively cool place.  Also, make sure not to overcharge them, that too will shorten their life.  One solution is to use solar panels to charge a larger, lead-acid battery with a charge controller, and charge smaller batteries from the big one using a "smart" charger.  Find a smart charger that comes with a cigarette lighter adapter for use in a car - many such models are available, some for less than $15.  Rewire that to connect to your 12V lead-acid setup and you can then charge various small NiMH cells correctly.