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Solar charging NiMH for uC circuits

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edeca

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I have been reading some webpages about solar charging NiMH batteries to run uC circuits from. Some of these simply feed the output of a solar cell through a diode and into the batteries, which I figure is bad.

Total current for circuits will be fairly small, a 3.3v PIC, a 3.3v radio module and a number of sensors. I figure 3 AAA batteries will probably do it (unless I can find a single specialised battery at the right voltage) with a LDO regulator.

I have come across the DS2715 which looks like it might work to regulate the charge.

Can anybody recommend any other charge controllers? Also, what voltage/amperage panel would be best? Futurlec sell 3V 45mA panels which look promising.
 
My solar garden lights came with an old-fashioned Ni-Cad AA cell that had a low mAh rating and its cheap materials rusted away within one year. I replaced some with a high quality Ni-MH AA cell which works much better and longer.

A Ni-MH AAA cell is about 850mAh. A discharged cell needs 85mA for 10 hours plus about 1 extra hour. Each cell will be 1.4V to 1.5V when fully charged.
A 3V/45mA solar cell has a voltage and current that are too low to charge 3 Ni-MH cells. The solar panel needs a diode in series to prevent the battery from discharging into it at night. So a 6V/100mA solar panel should be used to charge 3 AAA cells. It will charge them in about 3 sunny days and will not over-charge them much.
 
Neat, thanks for that! Some further reading suggests that charging at a current less than 1/60th of the total capacity will not ruin cells quickly.

How about this sort of thing:

**broken link removed**

That's enough voltage and small current (but in uC/radio sleep mode, that shouldn't matter).

Like the 1/60th current for capacity, what general rule of thumb can I apply for finding the voltage I need to trickle charge at?
 
Simply look at the datasheet of the battery on the website of the battery manufacturer. I use Energizer battery cells and their datasheets.
 

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Thanks again audioguru. I've got a datasheet for one of the cells (the Mempac is 3 x 1.2v button batteries) and it suggests that 14mA continuous overcharge is fine.

I'm still confused what voltage to charge the 3.6v battery at though.
 
I would follow the rule that was designated by the MAX712. It states that the voltage supply should be 1.5V + (1.9V x number of cells). For your application I would recommend (assuming that you are just using a diode to charge the battery) using the formula as 1V + (1.5 x # of cells).

NiMH batteries are fairly robust and can handle a good deal of punishment.
 
Thanks again audioguru. I've got a datasheet for one of the cells (the Mempac is 3 x 1.2v button batteries) and it suggests that 14mA continuous overcharge is fine.

I'm still confused what voltage to charge the 3.6v battery at though.

You don't, you charge them with current - just feeding the solar panel through a diode directly to the battery is all you need to do. Unless you have a huge solar panel, or live in the sahara desert, overcharging isn't likely to be a concern. Bear in mind the specs on solar panels tend to be the theoretical maximum, and unlikely to be approached in normal use.
 
Thanks all. I found this useful snippet which agrees with the above:

"The minimum voltage you need to get a full charge varies with temperature--at least 1.41 volts per cell at 20 degrees C.". Of course that doesn't take into account voltage drop over the diode, which is counted in your 1V.

As Nigel says, the panels aren't all that efficient (certainly not in sunny Britain) so it's not a huge issue.

I will pickup a selection of panels and see what works.
 
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