Charge level indicator for small 4x AA Solar Battery Setup?

Is there a simple way to build a charge indicator for 4 AA NIMH 1.2V 2000 mAh batteries? I thought a small 1-10 volt indicator would do it, but was told it would depend on battery type, etc...

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AndyGadget5 years ago
 
Chargers don't come much simpler than that!  For a simple 'OK' indicator, I'd use a 1K resistor, a 2.7V zener diode and a green LED in series.  The LED will start glowing when the battery voltage gets to around 4.7V, getting brighter up to full charge.  The resistor limits the current so as not to drain the battery too much.
A 1.2V NiMh discharge curve looks something like THIS so at 4.7V (4 x the graph voltage) it's about the end of useful charge.
rocktot (author) 5 years ago
Ok, and for 4x 2000mAH batteries, use a lower Zeno-Diode, like a 2.0 ?

Thanks!!
 
No, use the same 2.7V zener diode for 2000mAH.  The charged voltage will be the same, but the 2000mAH will hold 20% less charge than the 2500mAH ones in the graph.  The 'band' of the zener should be towards the +ve of the batteries.

2000mAH will (roughly) drive a circuit at 200mA for 10 hours, or 100mA for 20  hours, or 20mA for 100 hours etc.
rocktot (author) 5 years ago
Ok, thanks, this is the battery charger I would be using: It seems it would just take too long to charge with the zeiner diode example, also shown on your link. Also, not sure if this little thing could power so many light bulbs. But something for overcharge is definitely needed

http://www.reuk.co.uk/Basic-4-AA-Solar-Battery-Charger-Plans.htm
AndyGadget5 years ago
 
THIS is close to what you're looking for, based on the LM3914 bargraph IC
The problem with battery voltage monitoring is the supply voltage to the monitor changes as the batteries discharge, so any ratiometric measurement is useless.  The LM3914 compares to a fixed reference so can be used.  You'd need to change a couple of values to suit your 4 x 1.2V batteries.

I'd suggest setting the limits as 'fully charged' at 5.0V and 'discharged' at 4.0V.  That would give you a good indication of exactly where the battery was in its discharge curve.
 
As I said, you'd need to modify the resistor values for the range I suggested.  You can work them out from the linked datasheet.
rocktot (author) 5 years ago
Ok thanks, but that is for 12volt.

Lets start over. How do you tell if an AA battery is fully charged?
Why wouldn't a 1-10V indicator do the trick?

If you where trying to charge Li-Pol batteries then yes you would need more then just a voltage indicator.