# Need help with a simple solar battery charger

I need some help with a circuit that I'm trying to build for an engineering class. We are making these solar cars to practice soldering, and most people just used solar but I want to make it better. My plan is to add rechargeable batteries to it, so that the solar panel will charge the batteries. I don't know too much about electronics components and circuits, so I need some help.

I currently have

• 1W, 6V, 167mA solar panel (I also have 2 cheap 1v panels)
• 1N4001 diodes
• 5v motor
• 4 AA 1600mAh NiMh batteries
• switches

However, I am not limited to these. I have a Radio Shack available to me.

I drew a picture of how I suppose it should look. I got a lot of the info from this article. One concern I have is, will the 6v from the solar panel destroy the batteries? Would capacitors work in place of that? Also, where would the ideal place to put the switch be?

Any help is appreciated, thank you!

• Charging NiMh batteries:batteryuniversity.com/learn/article/…. Jan 18, 2017 at 0:43
• And the 1N914 is simply not up to the job. Get a diode with a higher current rating, such as the 1N400X series. Jan 18, 2017 at 3:07
• Please confirm that Vpanel = 6V FULLY LOADED in full sun. Jan 18, 2017 at 10:56
• What is the open circuit voltage of the panel? What voltage does it produce at max sun and no load on the motor? Jan 18, 2017 at 11:11
• Remove the diode. Put the switch between the battery and motor. Jan 18, 2017 at 13:23

4 x NimH cells i series will work very well here. 2 cells are too low in voltage.
Full charge of a NimH cell needs about 1.45V so 4 x 1.45 = 5.8V.
IF the PV panel is 6V at full power (ie > 6V in full sun unloaded) then it is a good match to 4 x NimH cells.

A switch should be inserted between motor and battery so that the cells can still charge when the motor is not operating.

If you wanted to optionally operate the motor from solar alone you can add a second switch in the vertical wire from battery +ve to disconnect the battery and allow PV to motor direct powering.

If possible use a 1N518x Schottky diode.
Any of eg 1N5187 5188 5189

If not available a 1N400X is OK and VERY available and cheap
(x = any of 1 2 3 4 5 6 7)

The 1N581X will drop about 0.4 V at full current (1A) but drop to 0.2 - 0.3V as battery approaches full charge - and in your application at about 170 mA max the drop will be very low. .

The 1N400x will drop about 0.7V at full current (1A) but maybe 0.4 at full charge so in yout 170 mA max system it will work OK if Vpv = 6V LOADED in full sun.

Smaller capacity AA NimH are more robust and more able to withstand trickle charging. 1600 mAh AA are robust. 1600 mAh AAA are not.

If Eneloop are available they will work well - they keep nearly full charge for long periods.

• A diode is not necessary. The panel itself blocks reverse current (there may be some leakage but with a small panel it will be less than battery self-discharge). Jan 18, 2017 at 13:20
• @BruceAbbott "Necessary" is such a strong word :-). I agree - you "can get away with it", but, a diode is a good idea (and "proper design"), reduces the chances of undesired consequences, costs about nothing, and is easy to implement. | It's interesting to note that in bottom end Chinese manufactured equipment where every achievable corner is cut, and a few more besides, a diode is always* included, and it's always* a Schottky. (* for values of always assymptoting to 100% :- ). | Worst case is probably when illumination falls to the point where Vpanel is < Vbat but a significant portion ... Jan 18, 2017 at 13:54
• ... thereof. Say high 10's to low 100's of lux. I've explored this region with large and small panels. I sat i a Chinese factory trying to match voltage at low lux with production spread panels so dock lights would all turn on at about the same time as the light faded. Not totally successful. | IF he used Eneloops or other good quality low self discharge NimHs then PV leakage could exceed battery loss depending on environment. But, yes, it could be done. Jan 18, 2017 at 13:58
• The batteries that I have are Radio Shack brand. Jan 18, 2017 at 22:58
• @RyanKnutson The important points are: 4 batteries should work well. 2 batteries may be overcharged and will not run the motor well. Use of a diode is " a good idea." The 1N914 diode is not really suitable. The 1N400x or the 1N581x is. Jan 20, 2017 at 14:32