Replacing Solar LiFePo4 with Li-ion

Can someone articulate what will happen if I replace the LiFePo4 in solar lights (2 Christmas strings and 1 deck light) with Li-ion (18650 laptop)?

I have tons of the 18650 laptop batteries and zero LiFePo4 batteries. I even have a few very low capacity, say 200-300 mAh (but state 3000-5000 mAh) 18650 that eBay sellers include with some of the flashlights; the capacity would be perfect for a something like solar lights.


Thanks,

-d

Unless your solar lights have a voltage cutoff above 2.5v, then a possible reason not to do this is safety. Consider that you'll be continuously draining Li-ion cells below 2.5v and then recharging them. While taking the cell below 2.5v is bad form, charging a Li-ion cell that has been below 2.5v, especially continuously, like every day, one might expect it to eventually. My understanding is LiFePO4 cells can better tolerate being entirely depleated and recharged, but it's still not good for them either. I thought this was the reason many solar lights use NiCd.

I'm pretty sure you can find the LiFePO4 cells you need at Home Depot or Builders Square in the US, but I expect them to be far more common in other parts of the world.

Put simply a Lithium Iron Phosphate battery has greater life cycle (capacity Vs charge cycles) over conventional Lithium ion batteries and delivers a more uniform voltage as long as capacity is not exceeded during the charge/ recharge life. They also have better chemical and thermal stability.

As to answer your question about using Li-co batteries as a substitute a bit more info would help. Like what type of charger is built in to the Solar host and at what rate does it charge?

There are a few other problems which could occur should a 3.7V cell be substituted for a LiFePO4 cell. One is overcharging. When it comes to Li-ion batteries, LiFePO4 is somehat unique in that it tolerates overcharging significantly better than 3.7V chemistries. This means that manufacturers of garden lights can get away with very minimal charging circuitry compared to what would be necessary for safe operation of a light with a 3.7V cell. In other words, substitution with a 3.7V cell could result in a rather dangerous overcharging situation.

Driving circuitry is another potential problem. LiFePO4 naturally has a lower voltage (3.2V nominal) compared to other Li-Ion chemistries. This lower voltage allows an LED to be easily direct driven from a single LiFePO4 cell. Much like the absence of sophisticated charging circuitry, this reduces costs. Using a 3.7V cell would not only overdischarge the cell (as mentioned above). It would also likely overdrive the LED, which would burn it out.

Finally, there is the issue of heat. By their very nature, solar garden lights sit in the sun and get hot. They therefore need a battery that is tolerant of heat, and will neither experience significantly decreased longevity in the heat nor be more likely to experience thermal runaway. This pretty much rules out the more delicate 3.7V chemistries.

The bottom line? Solar garden lights are pretty much designed around the lower voltage and more robust nature of LiFePO4 batteries vs 3.7V cells. If you need to replace a bad cell, it is DEFINITELY best to replace it with another LiFePO4 cell.

dlong said:

Can someone articulate what will happen if I replace the LiFePo4 in solar lights (2 Christmas strings and 1 deck light) with Li-ion (18650 laptop)?

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Most likely, they won't fit... the only solar lights that I've seen that utilize LiFePO4 cells use 18500 cells which are available online at battery space for $3 each if you need a bunch, or I've seen Duracell LiFePO4 18500 twin packs (in two different mAh ratings) at Lowes for about $10 - $14.

chillinn said:

My understanding is LiFePO4 cells can better tolerate being entirely depleated and recharged, but it's still not good for them either. I thought this was the reason many solar lights use NiCd.

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More & more solar lights are going to LiFePO4 cells, probably because of their perceived "safety record" & the extra voltage for higher lumens.

chaosdsm said:

More & more solar lights are going to LiFePO4 cells, probably because of their perceived "safety record" & the extra voltage for higher lumens.

Click to expand...

Cost is likely another reason for going LiFePO4 vs Nicad. As I said, a single LiFePO4 can easily direct drive an LED without any circuitry. Nicads require either a voltage booster or multiple cells. Also, Nicads are becoming either more expensive or outright banned due to their cadmium content.

Sorry --- I just realized that I'm not getting any emails from subscribed threads.... Anyway... besides spending money...

* These solar lights were only $5. I just hate to spend $3 (bulk) or $5-7 (lowes?) per battery for a $5 light. But then I also won't want to throw them in the trash....

* The lights do use 18500 sized batteries, but the interior design is very roomy so it will fit 18650 just fine.

* I guess, I'll just hang on to the nights and keep an eye out for clearances at the garden department and maybe they'll be $1/battery.

Thanks for all the replies. Now I have to go find the other post that I made and see if there are any replies....

-d