Help replacing battery for solar light

[ejt]

I could use some help with a battery replacement project. Three years ago I bought a solar spot light setup from Amazon, which I use for the address marker at the mouth of my driveway. The LED lights are really nice, about 3000K and there is a small solar panel with electronics and a battery inside. A while back I noticed that the lights were only staying on for twenty minutes after dusk, after a full day of bright sunlight with no obstruction to the panel which was angled correctly. This made it seem like it was probably the storage battery getting weak and needing replacement.

I took the panel apart and removed the battery, which had no identifying marks. Trying to get the info for replacement has been impossible. I've emailed the seller in China and they don't respond. I've posted questions twice on Amazon and have gotten the usual type of answers: I don't know what the battery is, but I answered anyway. Thanks. One guy said they sell them at Home Depot but nothing like my battery is shown on their web site.

I'm posting a pic of the battery. It had a light purple heat shrink which I took off. The silver covering has a couple small QR codes but my phone won't read them. Its size is 65mm by 18mm or 2-1/2" by 11/16". There isn't anything in writing near the circuit board inside the panel. I'm thinking that it might be a 3.7 lithium ion but that's a guess.

Any help is appreciated. Thank you very much.

 

[Dave_H]

It certainly looks like an 18650 Li-ion cell and there are many available, can't give you the best source for replacement. Unknown is capacity but you might be able to estimate.

Possibly it could be Lithium Iron Phosphate (LiFePO4) type which is different from regular Li-ion; has a lower voltage (and capacity). These are sometimes used in solar garden lighting. (On that note, HD and others here seem to have switched to shorter 18500 cells).

If the cell is still good and can be fully charged, you can measure its voltage (need a DMM, handy for work like this). Regular Li-ion would top around 4.1v to 4.2v, LiFePO4 lower at around 3.6v or so.

Dave

 

[Lynx_Arc]

As Dave said that is by dimensions an 18650 cell and you can get them all sorts of places likely it is a lower capacity cell I know Walmart sold a 2 pack of them for yard lights at one time for about $10 and you can find an old computer laptop pack or 20v lithium tool battery pack that went bad and scavenge a good cell or two from them. If the battery is over a year old then likely it is just worn out from continual trickle charging all day.

 

[ejt]

Thanks for the replies. I'm thinking of getting a higher capacity 18650, maybe two cells with a Molex connector. Newark sells this one. It might hold a bit more juice for a longer runtime and I don't think it will hurt. Thanks again.

 

[Lynx_Arc]

Thanks for the replies. I'm thinking of getting a higher capacity 18650, maybe two cells with a Molex connector. Newark sells this one. It might hold a bit more juice for a longer runtime and I don't think it will hurt. Thanks again.

You need to figure out how much capacity the solar cell can supply each day putting a battery larger then needed to capture the power in a day the extra capacity is a waste and IMO smaller capacity cells tend to be more durable than higher capacity cells often yielding more cycles. Likely 2 batteries would be a waste unless they are low capacity.

 

[Dave_H]

A rough estimate of charging rate and usable cell capacity is possible to determine overkill. It all depends on amount of light per day, subject to changing conditions e.g clouds, shadowing etc.

Solar panel looks to be about 200 sq. cm (0.02 sq. m). polycrystaline (about 14% efficient). Sun at 1000W/m^2 peak yields 2.8W available power; average lower. With illumination 8 hours per day with good visibility I estimate about 15 watt-hours, likely optimistic as the panel may not be operating at its maximum power point (MPP).

Li-ion cell having 3.7v average voltage, 3000mAh can store 11.1 watt-hours (new). Given inefficiencies in charging, you might get one charge on a good day, or better part of. Capacity will decrease over lifetime.Two 2600mAh in parallel (5.2Ah, 19.2Wh) might not be too much excess capacity.

LED lamps appear to be 1-2W each, running two (say) for 8h per night takes 8-16Wh, not counting inefficiency of drive circuits; probably around 75-80% for a direct-drive (linear current regulation or simple resistor).

Anyway it's a bunch of numbers, assumptions and approximations but hopefully sheds some light on the subject.

Dave

 

[ejt]

I ended up getting the dual 18650 with molex connector from Newark. The molex was not the same as the ones in the solar panel but it was an easy fix to splice the one from the old battery onto the new battery's wires. Plugged it all in and it worked just fine. I'll have to wait for longer days to see what the run time is, but I'm happy that it's going again.

Much thanks to the help I received here, and a shout out to Dave_H for the above and beyond calculations!