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Solar charging a small SLA

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Hello guys and gals,

I've started figuring and ordering parts for yet another portable stereo project and could use a little input. For this stereo, I'd like to use solar panels to provide charge for the battery pack. It should be mentioned that the solar panels will not be the only source of charge for the battery, and it is unlikely that the panels will ever have to charge the battery in a completely discharged state. That being said, it would be nice if the solar panels could, on occasion, completely recharge the battery if, for some reason, a wall charger were not available (due likely to poor planning and/or the coming apocalypse).

For starters, I'm thinking of using a 12v 3.2 Ah SLA for the battery, mainly because it is more simple to charge than a NiMH battery pack (I think.)

The solar panels I'm looking at are rated to provide 15.4v at 100 mA. (although, I could potentially expand the system to provide 200 mA).

The crux of my question is this: will the solar panels provide enough current to charge the battery in any state (either partially or totally discharged)? And if so, how necessary is a charge controller for such a low current source? In other words, is it even possible for the 100mA panel to overcharge the SLA to the point that it would cause it to gas? Would the appropriate approach just be to connect the panels (via diode) to the battery terminals, no controller necessary?

Thanks for your consideration,

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Yes, gassing is a problem for lead acid batteries connected to solar cells, even 100mA can cause a 3.2Ah battery to gas. In fact I've had problems with a 14Ah battery gassing when being fed with 50mA continuously.

The problem is the voltage.

Are you planning to leave the solar cells connected continiously?

You need to limit the voltage, rather than the current. If the solar cells are to be left connected to the battery continuously, the voltage should be limited to 13.8V. This could be done with an LM317 or a TL431 with a booster transistor. Unfortunately SLAs will take ages to charge if the voltage is limited to 13.8V. To charge quickly, the current should be applied until the voltage reaches 14.4V then stopped until the voltage drops to 13.8V, at which the voltage can be continuously maintained at 13.8V.

NiMH cells are harder to charge than SLAs. The voltage will rise as they're charged, then fall again just before becomming fully charges, after which the current should be reduced to a very low level. It's possible to trickle charge NiMH cells continiously, if the current is kept low. 100mA will probably be too high for 2500mAh AA cells but it probably won't be a problem for 9Ah D cells, it's best to check the data sheet.

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Awesome, thanks for the reply. The overcharging (and gassing) is exactly what I'm concerned about.

I was planning on leaving the solar panels connected essentially all the time. That being said, I plan to incorporate a LCD panel meter to monitor the battery voltage, so i suppose i could put a toggle in line with the panel and just cut it if the battery was all topped off. At some point though i'd like it to be automatic. It sounds like to accomplish this i need to incorporate a solar charge controller, which is again, part of my main question.

My reasons for not wanting to build in a charge controller are this:

1. Complexity. I've never built one, although I'm sure i could with a little research. There appear to be numerous ICs designed to do just that available from digikey.
2. Power consumption: the solar panel in this project is pretty small, and i don't want to rob any of its precious charging ability.

I've posted this same question on a couple of other forums, and many of the replies indicated that with the low power rating of the solar panel, i should be able to squeak by without using any controller, but that is clearly contrary to your experience. One poster even suggested that NiMH battery packs were fairly tolerant of overcharging, which is not something that I've heard before, but it seems like a pretty reliable source.

In response to your possible solution, i feel fairly certain that I don't want to straight up regulate the voltage down to the the 13.8v trickle level, for the same costly reason you mentioned. So perhaps i should look further into building in a solar charge controller. I actually have an unused standalone unit in my possession, but it's rated for 12v 3 A, so i think that it's probably overkill for this project.

Thanks again for the post, I'd love to hear any more ideas you might have,

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What's the voltage of the solar panels when no load is connected? It's generally much higher than 12V, mine are >17V.

An LM317 will only drop a couple of volts so won't cost you much. The only thing is the leakage current may be too high as when it's dark the batteries will discharge via R1 and R2, see the schematic, it'll make sense. You could add diodes, one in series with the battery and one in series with R2 but you'll loose an extra 0.6V or just make R1 and R2 so high the discharge current is very low.

A charge controller needn't be very complicated, a TL431 and comparator could be connected to a PNP transistor or MOSFET so the LM317 is bypassed until the battery voltage reaches 14.4V and reconnected when the battery voltage falls below 12.5V. It's also possible to disconnect the battery from the output when the voltage drops too low.

Come to think of it, trickle charging NiMH cells is fine as long as it's not prolonged, 100mA into 2500mAh cells is just 0.04C which should be fine as long as there's a break every 20 hours which there should be unless they're in the Arctic in mid summer.

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  • 4 weeks later...

Sounds good.

Another quick question:

I think I'd like to expand the solar array beyond the single 100mA panel. I also have some 15.4v 50mA panels. I'm assuming it would be okay to hook up the 100mA panel in parallel with one of the 50mA panels to build a 150mA charger. This seems like a straight forward, no problem assembly, but I've been wrong before. Do you see any problems with this approach?

Thanks again,

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