# Battery recharging via solar panel

I am trying to use rechargeable batteries. If I recharge two of the below batteries via the solar panel below, how many hours will it need to recharge to 100%? Likewise, what if I recharge four batteries?

Batteries:

• Rechargable NiMH AA HR6 1.2V 700 mAh

Solar panel:

• Related Maximum Power: 3W
• Voltage at Pmax: 9V
• Current at Pmax: 0.33 A
• Open-circuit Voltage: 10.5 V
• Short-Circuit Current: 0.43 A
• Normal Operating cell Temp: 47 +/- 2 C
• Maximum Series Fuse Rating: 10 A
• Operating Temperature: -40 C to 85 C
• Application Class: Class A
• Cell Technology: Ploy-Si
• Dimensions: 306 x 148 x 15 mm

Vpanel >> Vbattery so panel acts as ~~ constant current source.

2 batteries in series or 4 batteries in series will still be << 9V so charge in the same time.

Imp < Icharge < Isc or
0.33A < Icharge < 0.43A

Say ~~= 350 mA.

T charge ~= Battery_mAh / Ichg x 100 / Battery_efficiency % NimH CURRENT acceptance efficiency probably 80%-90%

= 700 Mah / 350 mA x 100/80 ~~~= 2.5 hours

That's in full mid-day sun with panel square on to sun. Charge for longer than this and you will quickly "cook" the batteries.
Vbattery_max at that charge rate should probably not be more than about 1.5V/cell when charging.

eg Bangalore gaisma gives

    http://www.gaisma.com/en/location/bangalore.html


to get SSH = sunshine hours per day (= kWh/m^2/day)
Your panel will produce ABOUT 350 mA x SSH mAh per day

Bangalore example -

Half a day on an average day will be more than enough in any month - see below.

Chart from Gaisma entry for Bangalore as above.

• Thanks for the detailed explanation! Question : Is there any issues in charging batteries directly from solar panels? – Madhu Sep 11 '14 at 1:22
• Yes. Re-read the answer. Especially the part where he says "That's in full mid-day sun with panel square on to sun. Charge for longer than this and you will quickly "cook" the batteries." You should use a charge controller that will check the state of charge and change charging characteristics until stopping the charge when the batteries are full. Else, fire. If you stay there the whole time, then YOU can become the charge controller, you can get a decent charge, but you must monitor the batteries and not allow them to overcharge. – Enemy Of the State Machine Sep 11 '14 at 2:25
• @Madhu As EOTSM says (and I said), overcharging is a risk if left to their own devices. If you can keep the batteries out of the sun and cool then monitoring battery temperature rise and stopping when they get toasty warm (say 40C - depending on ambient temperature) is one of the easiest ways to detect charging end. Sudden rate of increase in temperature is related. Vmax = 1.5V/cell is another. The usual negative voltage dip can be hard to detect when charge energy may vary due to clouds etc – Russell McMahon Sep 11 '14 at 3:55
• @RussellMcMahon - excellent answer and very easy to follow! Thank you. I am confused regarding your Imp<Icharge<Isc. When I have played around with my home made solar panels using different resistors to measure current output, it seems like ICharge could be less than Imp since Imp is at Pmax which would be the "knee" of the power graph? or am I confused? Since the batteries have such low voltage, they would likely be at a much lower efficiency of the panel and have a lower charge rate, but unknown without knowing the graph of the panel. – Filek Sep 13 '14 at 6:30
• @Filek That's at full sun (1000 W/m^2) OR any Vpanel such that Vpanel_max_power > Vbattery + diode_paneltobattery drop. In a simple PV+diode + battery system, adding the panel when the battery voltage is < Vmp will pull voltage down and current will tend towards Isc. As Isc is always > Imp at sensible insolation levels this means that the charge current will be higher than Imp (and lss than Isc which is max possible in any given light and temperature conditions. – Russell McMahon Sep 13 '14 at 10:58