Recently [DiodeGoneWild] got his paws on a purportedly IP44-rated outdoor solar light with PIR motion sensor that also contains an 18650 Li-ion cell. Since it’s being sold in Czechia in a supermarket by a Czech company you’d expect it to meet at least some quality standards, but once he got inside the device a true horror show unfolded.
The best guess here is that someone took an originally cheap NiMH cell-based solar light circuit and simply replaced said NiMH cell with an – unprotected – Li-ion 18650 cell, as there is absolutely no charging management or cell protection. This is important as while NiMH will throttle charging current by rejecting it as heat the more it is in a charged state, Li-ion is not as tolerant.
Even ignoring that the ‘IP44’ enclosure has more gaps and holes in it than the average slice of Swiss cheese, the 5.5 V-rated PV solar panel happily tried charging the Li-ion cell at around 6 V. This raises the potential that on a particularly sunny day with an already charged Li-ion cell sitting at 4.2+ V, it’ll effectively grenade this cell.
During an outdoors test the 18650 cell was charged to 4.8 V before the sun vanished behind the building, which probably isn’t an amazing sign if you are expecting for this light to keep working for more than a few weeks. Or not violently explode/catch on fire on the side of your house for that matter.
Of course, treating a NiMH cell better than in this particular circuit will extend its lifespan as well, but the failure mode of that battery chemistry tends to be a bit less aggressive. Hence why people keep getting away with dodgy circuit designs like this.
I’m surprised. Not by the degree to which it is cheap and nasty; but by li-ion getting the nod for a product being cost optimized straight into dangerous dishonesty.
Maybe I’m just old and out of touch; but I still think of this as an application where a vendor who doesn’t really care about the fact that they are probably also lying about how long the light will run from a day’s charge will just shove a nickel cadmium cell in there and call it good: tepid energy density and the cadmium issue; but cheap and takes abuse pretty well. NiMH if they care slightly more, since the self discharge won’t be a huge deal on a 12-24 hour timescale and the energy density is better.
Has Li-ion just come down that much across the board; or is it a case where proper Li-ion would still be more expensive; but the volume manufactured for more demanding applications with actual QA means that irregular lots turn up for a pittance in quantities large enough to bodge in but not necessarily large enough to re-engineer around?