# Solar cells & Batteries

I was planning on having a couple of solar cells recharging a battery!
My question for this is:

How can I recharge a battery through solar cells and at the same time let the battery do some work(Because if I setup some solar cells to the battery and then from there to let's say a lamp, then how can I be sure that all the solar cell's energy goes to the battery FIRST and THEN to the lamp)?

And how can I know if a battery is nearly full(So I know when to switch batteries)?

If I am not really clear about the question's then please say so :)

The solar cells will provide some current when illuminated. This can be applied to charge the battery. It doesn't matter if something else is connected to the battery at the same time that is drawing current, you still get to use the power from the solar cells effectively. In fact, it's more efficient to power the end load directly than to first store the energy in a battery and later have the battery power the load. The point of the battery is to be able to use the power when it's needed, not just when the solar cells can provide it.

For example, let's say the solar cells can produce 1A in full sunlight, and this 1A can be applied to charging the battery. Now let's say a load is turned on that draws 400mA. The 1A from the solar cell will be split so that 400mA drives the load, leaving 600mA left to charge the battery. This is perfectly fine. If the load were more than the solar cell current, this would still work. Let's say the load takes 1.5A. 1A of that would come straight from the solar cell, and 500mA from the battery.

In other words, there is nothing wrong with the solar cell current relieving some of the load on the battery as apposed to charging the battery and then having the battery supply the load current. As I said before, that is actually more efficient since there is some loss in storing and the retreiving energy from the battery.

Knowing when the battery is full and what to do about it is a separate topic. Some battery types are quite fussy about how they are charged. For those you will need a active circuit to monitor the battery voltage and possibly temperature to decide what current to charge it with or what voltage to hold it at.

Lead-acid batteries are pretty forgiving in this area and don't have a problem with continuous charging even when full. The simplest solution is to arrange and size the solar cells such that they can only produce the maximum float voltage at the maximum current the battery can take when full. Then you can just connect the solar cells accross the battery and be done with it. This will charge the battery much more slowly than it is capable of when it is low, but is a simple setup guaranteed not to damage it.

If the solar panel size or cost is important, you need to use the energy from it more efficiently. In that case I'd use a switching power supply driven by a microcontroller that takes into account what current the solar cells can put out, the battery voltage, and possibly the battery temperature to decide what current, if any, to charge it with. This gets complicated. You have to read the battery datasheet carefully and implement the charging regime accordingly.

• Then you can just connect the solar cells accross the battery and be done with it. What about a diode to stop the solar cell discharging the battery when it's dark? Commented Jun 23, 2011 at 14:47
• @Matt: Good point. Usually there would be a Schottky diode in series with the solar cell string. Commented Jun 23, 2011 at 16:01
• Olin, when I read your answer I noticed that you never mentioned the word "Volt", how come? is it because it doesn't matter or because you think I will figure that one out by myself? ;) Commented Jun 23, 2011 at 19:35

The solar cell's current goes to whatever load is attached, the battery is just a load like any other. So if both your load and the battery are attached to the solar cell there's no way to ensure the battery will be served first.
By the way, this is probably not a good setup. For charging the battery you'll want a constant current (depends on the technology), but your load most likely wants a constant voltage.

• If this "probably not is a good setup", can you please tell me a good setup? Commented Jun 23, 2011 at 15:45
• @Harry - not so easy. Like I said for charging the battery you'll need a constant current source, but you have no control over the charging if you simultaneously have a load attached. Charging and doing work simply don't go together. Commented Jun 25, 2011 at 12:26
• Not necessarily true: the battery fixes the voltage, biasing the solar panel which will give the current. I'm using a solution like this and it works fine if the battery has the right voltage to bias the panel at the MPP (also lower voltage works fine) Commented Apr 20, 2012 at 7:12

This task is way more complex than one might thought if you want to implement it properly. Battery voltage & solar cell voltage does not match, so you cannot connect them directly.

Solar cell have point of optimal efficiency - if you draw less or more current - total power will be lower (because A*V will be lower). You cannot maintain work on this point without micro-controller or specialized IC. Your task would be draw exactly this current from solar cell, and convert it using DCDC converter in current & voltage limited mode, and connect it directly do battery.

Then you can just attach your load to battery - and it will work automagically - if solar cell produce more power than battery can consume - it will be consumed by your device. If battery is charged (DCDC is at voltage limit) - your load will work from solar cell.

So the most complex part is this A&V limited DCDC & maintaining optimal current for solar cell.

• So let me see if I got this straight Let's say I got a 6V battery with X A, to even recharge this battery CORRECTLY I must use a converter that converts whatever the sun cell's V is at to 6V so it's a MATCH or else it won't be done correctly and will hurt the battery in some kind of way. Is this correct? Commented Jun 23, 2011 at 15:45
• You need all that to make it all efficiently. If you have 6V solar cell, and 4.2V LiIon battery, and you are ok to just throw away 30% of efficiency, you can make things simpler. Just use linear regulator to lower voltage from 6 to 4.3, then a Schottky diode which will lower it to 4.15, then a resistor (~1-5Ohm) to limit maximum current, and then you can connect that to your battery. Commented Jun 23, 2011 at 16:01
• False: why should you lower the panel voltage to 4 V? If the LiIion battery (which will be more likely 3.7 than 4.2 V) will bias the panel at that voltage plus the Schottky drop, it will be perfectly fine, and you won't be in the MPP maybe, but you'll still get an higher current and you'll be near 90% of MP Commented Apr 20, 2012 at 7:16
• LiIon battery have voltage in the range 3.5-4.2 depending on it's charge. Solar cell may have arbitrary voltage output, depending how many elements are connected in series. It is very unlikely that you would have solar cell with MPP voltage near 3.7-4.2V. Commented Apr 21, 2012 at 16:42

You need a charge controller unit to charge the batteries which will allow the battery to charge to a certain limit and turn off the charging when the battery is charged and also will start the charging of the battery once the voltage of the battery goes down below a certain threshold limit.

You cant just directly connect the Panels to Battery. The battery will blow off the panel as at night the panel will act as a load to the battery as the voltage at the panel will be considerably low as compared to the battery. (Potential difference you see mate !)

• In fact, use energy harvesting ICs that are made specifically for solar charging application Commented Apr 7, 2016 at 6:06