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I've got a solar panel (12V, 330mA, 2W) which I will use to charge a (12V 5Ah) lead acid battery. I'll put a voltage regulator and shottky diode in between the two.

However, could overcharging become an issue? Say I don't discharge the battery in any way, what happens when the battery is full and the solar panel just keeps delivering power?

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Over-volting will cause gassing. ST have a comprehensive application note which details a reference design for an MPPT charging system. It includes a BOM, suggested PCB layout and Experimental Results: st.com/st-web-ui/static/active/en/resource/technical/document/… – Emyr Apr 24 '14 at 17:00
up vote 2 down vote accepted

It doesn't sound like you need anything more between the solar panel and the battery than just a Schottky diode. Your panel is rather wussy compared to the battery, so it doesn't look like there is anything it can do to hurt the battery.

How much current can the solar panel deliver at 13.6 V or so? That is usually the voltage that "12 V" lead-acid batteries can be float-charged at indefinitely. The panel probably can't even put out enough power to get to 13.6 V, especially with a diode in there.

The real problem may be that the panel can't produce enough voltage to charge the battery all the way. Check the panel and battery specs carefully, and don't forget to consider the voltage drop on the diode.

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Would it be a good idea to put two of these panels in series to get 24V and then regulate voltage down to 12V just to make sure the battery gets enough? – sbrattla Apr 24 '14 at 17:09
What considerations are there when it comes to voltage drop on the diode? – sbrattla Apr 24 '14 at 17:12
@sbratt: Two of these panels in series would be running them rather inefficiently. The battery would probably clamp the voltage to a safe level, but then the panels would be operating well below the most efficient point. However, again, you have to get clear specs on what the panel can put out under maximum insolation, then compare that to the maximum the battery can tolerate indefinitely. – Olin Lathrop Apr 24 '14 at 17:38
If you use a proper switch mode charger chip, two panels in series is not a bad idea. But if you are linearly regulating them, then you are just wasting power. – Joe Apr 24 '14 at 19:28
Should one Schotty diode be across the solar terminals to help avoid hot spots with random shadows? Maybe then the controller. Then another Schottky diode in series say with the positive lead to be doubly sure of no battery discharge at night (the controller being sealed to prevent inspection). Don – user95064 Dec 21 '15 at 20:08

If you don't want complicated circuitry for the charger, you just need to clamp the voltage such that it doesn't exceed around 13.2V for a 12V cell. This is a typical trickle charger setup. You panel is very small compared to the cell, so you should be fine with this.

If you exceed that voltage, you can get to the gassing voltage for a lead acid battery, where capacity is irretrievably lost. This voltage goes down with temperature. It is around 14V at 40C.

If you want to get more efficient charge for your solar power, you can use one of the many solar fed switch mode battery charger chips, like the TI - BQ24650 This also gives you more flexibility for use of LiFePO4, which is starting to replace Lead-Acid. This would probably not make much sense, unless you were wanting a faster charge and using a larger panel. It is capable of 8A charge.

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