diy solar

diy solar

Cheap ebay solar LED lights with li-ion 18650 batteries.

Big Dummy

New Member
Joined
Aug 29, 2021
Messages
6
I have a bunch of cheap ebay solar LED lights with li-ion 18650 batteries.

If I replaced the 3.7v li-ion batteries with 3.2v LiFePO4 18650 batteries, would it work just the same? Would the solar panels and circuitry that are designed to charge 3.7v batteries overcharge the 3.2v li-ion batteries? Will the lights be the same brightness with the .5v lower voltage?

Also the li-ion batteries are 2000mah, the LiFePO4 batteries are 1500mah. I think that means the solar lights won't stay lit as long of time through the night?

I know you guys are in to higher tech stuff than what I'm asking, but I'll appreciate any time you give in commenting.

Thanks
Dave
 
These are the solar lights.
 
BAD IDEA. The reason is that when charging a 3.7v nominal li-ion battery, the charging circuitry will take them up to 4.2v.

With LiFeP04, which has a nominal 3.2v, the upper limit for charging this chemistry is 3.6 to 3.65v.

Ie, the charger inside the light will overcharge the LFP replacements you want to use.
 
No worries - but it pays to actually check the light! Most garden solar lights that used lithium are using LiFeP04, not other chemistry types for obvious flammable reasons. :)

18650 is just a size, not an indicator of chemistry. So it *may* be possible if you tear the light open, or check the manufacturer spec if they have any just what they use. A "nominal voltage" rating might help - with 3.2v being for LFP, and 3.6v nominal being all other lithium types.

You may have to deep-dive for the actual specs though.
 
I got the LiFePO4 batteries and have been doing some testing.

Charging the two batteries (Lithium Ion and LiFePO4) in identical lithium Ion solar lights, in full sun all day with me rotating them for most sun, the lights getting around 8hrs of charging sun (something they'll never see just mounted on the wall, getting a max of 3hrs sun a day). The result is even though the lithium Ion solar lights/chargers put out 5v without a battery (tested at battery compartment), with battery in, the battery seems to buffer it, and will only allowed the LiFePO4 batteries to charge to 3.25v (they have a max charge of 3.65v).

After charging I function tested; when I set them to medium light/constant (the only constant on mode, not triggered by motion), the lights with LiFePO4 batteries are dimmer than the Lithium Ion batteries for approx the first hour, then the same (not measured brightness, just what I can tell by shining around shaded room) brightness after. They both ran the same 4.5hrs, both turning off within 2 minutes of each other from discharge (the Lithium Ion won by 2 minutes).
The LiFePO4 batteries are brand new and are 3.2v/1.8ah, the Lithium Ion batteries are used and are 3.7v/2.0ah. I'll test again with new Lithium Ion batteries next time, but I only have new Lithium Ion batteries that are 2.2ah, so am expecting significant more running time.

I also tested the lithium Ion vs LiFePO4 batteries on full charge in cheap identical Lithium Ion flash lights and got the same result -LiFePO4 dimmer for first hour, then the same run time.

One other test I did was to put the LiFePO4 batteries in a charger that doesn't support them (an unmonitored/dumb 4.2v Lithium Ion charger), and they charged to 4.18v, but within approx 30mins the voltage dropped to 3.35v without using them. Pretty sure a lot of that abuse would ruin the battery, but they barely got warm during charging.
 
Well, now you are just being unsafe and unscientific mixing and matching and crossing fingers things will work.

What you have learned is that LiFeP04 gets it reputation of being "more resilient to higher voltages" by charging them to 4.18v, but what you don't know is that you ARE damaging them. More specifically, you are "plating" them with the higher voltage, and causing secondary reactions to occur that release non-organic materials clogging the SEI layer.

Simply put, it is akin to wiping up a small gravy spill with a sponge. Eventually, while most of the sponge is ok internally, the pores on the outside get so clogged that they won't accept any more gravy. :) Basically reducing capacity every cycle.

The flip side is that if you are just as cavalier about using the non-lfp batteries, they won't be so kind if you create a "vent with flame" incident overcharging them by 600mv like you did with the LFP.

Please do a bit more research and actually think about why we say the things we do. Getting away with it once, doesn't mean it isn't set up to bite you in the ass next time.
 
Well, now you are just being unsafe and unscientific mixing and matching and crossing fingers things will work.

What you have learned is that LiFeP04 gets it reputation of being "more resilient to higher voltages" by charging them to 4.18v, but what you don't know is that you ARE damaging them. More specifically, you are "plating" them with the higher voltage, and causing secondary reactions to occur that release non-organic materials clogging the SEI layer.

Simply put, it is akin to wiping up a small gravy spill with a sponge. Eventually, while most of the sponge is ok internally, the pores on the outside get so clogged that they won't accept any more gravy. :) Basically reducing capacity every cycle.

The flip side is that if you are just as cavalier about using the non-lfp batteries, they won't be so kind if you create a "vent with flame" incident overcharging them by 600mv like you did with the LFP.

Please do a bit more research and actually think about why we say the things we do. Getting away with it once, doesn't mean it isn't set up to bite you in the ass next time.
Hi
Thanks for the post, but it was only two LiFePO4 batteries that I experimented with. Done, because the solar lights designed for lithium ion batteries, in a full day of direct sun only charged the LiFePO4 to just above their nominal voltage (3.25v -not overcharged) -I repeated this result for the last 3 days in more sun than they'll ever see mounted to house. Those same solar lights will charge Lithium Ion batteries to 4.12v to 4.19v in a full days light. I wondered why the 3.7v lithium ion designed solar lights weren't overcharging the 3.2v LiFePO4 batteries.
So I sacrificed those two LiFePO4 batteries by putting them in a regular Lithium Ion charger to see the results. It overcharged them. and I don't plan on doing it again.

I'm not being so cavalier about using the Lithium Ion batteries. I would not have attempted to overcharge them. I've seen the results in online videos of overcharging Lithium Ion, and have also seen the results of overcharging LiFePO4 batteries. That's the reason I felt safe enough to overcharge the LiFePO4 batteries, outside on a concrete driveway.
The reason for this topic is to get the fire hazard Lithium Ion battery powered solar lights off of my house, and I've determined a very safe solution -replace all of the Lithium Ion 18650 batteries with LiFePO4 18650 batteries. :cool:
 
Back
Top