# Does the voltage of a solar panel have to be greater than that of a battery pack to charge it?

I have a 6 volt solar panel and a 4.8 v battery pack(4 AA nimh). Will I be able to charge the pack? The solar panel is 100 milliamps and batteries 2300 mah.The solar panel is like the one here: http://www.ebay.com/itm/301018398863?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT

• Might work. Can you give full information on the solar panel and batteries? The things we would like to know about the solar panel are make and model (if known) and Vmpp, Voc, Isc, Impp. Edit your original question to add the requested items. May 7, 2016 at 20:39
• The required information is not available. For sure, if you just directly hook up the panels to the batteries, they will charge to some extent. How long will it take? Will it ruin the batteries? Will they ever charge fully, or get stuck at some midpoint of charging? Can't tell. If you add a diode in series, they will be less likely to charge fully, but at least no current will flow from battery to panel at night. May 7, 2016 at 21:04
• Can you put the panel in full sun with a resistor as a load and then measure the Voltage across the resistor to find the actual loaded Voltage? At full summer sun 60 to 100 ohms will work. Use a bigger resistor for winter. Panel Voltage will drop as temperature rises in summer heat, so give it a few minutes in the sun to warm up. If 6V is the 25C Voc rating then it's probably a bit too low when loaded, as 4x NiMH will need 5.2 to 5.6 Volts to charge. Sep 10, 2020 at 14:15

Although this is an old question the answer may be useful to others.

Summary: The PV panel suggested is of too low a voltage and power rating to be more than very marginally useful in this application.

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To charge a battery the applied voltage must be at least equal to the highest voltage the battery reaches. In this case either the PV panel voltage must be as high as desired or you need to add a boost converter. I'll deal only with the direct PV panel connection.

The maximum possible charge rate is 100 mA into a 2300 mAh battery so the maximum rate = 100/2300 = C/23.
A NiMH cell charged at such a low rate will have a fully charged voltage of about 1.4V, so 4 cells will require about 5.6V.

The PV panel has 12 cells. As Voc (V open circuit)is about 0.5 - 0.55
then V/cell Voc of the panels is ABOUT 6 to 6.6V.
Vmp (V max power) will be 80 to 85% of Voc = 4.8 to 5.6V - with the 5.6V being an optimistic high value.

The combination of needed Vmax_battery of about 5.6V and the panel Vmp of probably less than 5.6V tells you that while you'll get some degree of charging the battery would not fully charge in sensible timespans.

Even at 100 mA the 2300 mAh battery will take 23 hours of full sun to charge. Typical equivalent full sun days vary with geography but in eg the continental US not too far north you may get 1 to 2 hours a day in Winter and 4 to 6 hours/day in summer.
So with the panel clean and well aligned and IF the battery had say 3 cells you'd need 4 to 12 days for a full charge.

In reality the charge current will be less to much less. It could take several times longer than the above figures. As a keep-alive for very low powered or occasional use equipment the PV panel MAY be of some use but it is really grossly underpowered and of too low a voltage for the task.

Adding a series diode would make matters worse again.

If that's all you've got, no, you can't just connect them together. To charge batteries, you need a battery charger.

Does the voltage of a solar panel have to be greater than that of a battery pack to charge it?

To answer this question: no. That's what boost converters are for. Also, keep in mind that the 6V/100mA rating of the solar panel doesn't happen simultaneously. I.e. the 6V is probably open-circuit voltage, and the 100mA is probably short-circuit current. The ideal way to deal with a solar panel is a maximum power point tracking converter, which presents an optimal load to the solar panel at all times.

• In all the sources where I have searched, batteries are charged from a solar panel with a diode in series to prevent current from going to the solar panel from the batteries. May 7, 2016 at 20:51
• It sort of looks like it is a 12 cell panel, which implies Voc of around 7V. But without at least knowing Voc, Vmpp, Impp and Isc, it is hard to say much that is useful. May 7, 2016 at 21:08

It's possible, but as others have suggested, more information would be necessary. I have charged batteries using this method, though additional circuitry was required. Current will only flow with a difference in potential, so the charging voltage needs to be consistently higher than that of the discharged battery, and higher or equal to the potential of the desired charging level. I have used a switching power supply (boost-converter) in order to raise the potential over that of the batteries. Current management can be a bit more complicated depending on the charging profile of the battery.