Remote Living > Lighting
organic lithium ion battery trouble
dnix71:
I have a portable led light bought at Home Depot that charges directly from 12vdc to help light my bathroom off the grid. It was nice and bright when new, and still is when fully charged, but it doesn't stay charged long so I opened it up. The base unscrews to reveal two 3.7v 0.55ah nominal organic lithium cells in series.
One cell has 2.93v and the other 3.5v. Am I correct in assuming that the 2.93v cell is bad? There is room in the base for double the number of cells, so I would like to replace the pack with 4 cells (2x2) for double the run time.
Simen:
Don't know what chemistry your 'organic' Lithium are, but based on your stated 3.7V, i'm guessing Lithium Cobalt or Lithium Manganese.
These cells should be charged with max 4.20V +/-0.05V, and should never get below 3.00V. In series, they should be kept within 0.10V of each other at all times.
If a cell are kept below 3.00V, it would deteriorate very fast. Cells below 3.00V should be trickle charged to above 3.00V before full charge.
So, yes; the cell @ 2.93V are weak. ;) I'm guessing your voltages are when the cells are discharged?
Out of curiosity; what size are those cells? :)
OperaHouse:
There is a lesson here. Simple series charging has life issues. I have access to lots of computer packs. The chargers on these are complex monitoring each cell to get maximum life. I don't worry about it because cells are free. Doubling up series parallel may onlu serve to kill more batteries faster if that extra time is not needed. How many years did these last?
dnix71:
The cells are 4/5 of a AA size, and the light is only a few months old. I was dissapointed when I got this home and opened the package and read the fine print. It says the battery pack is only good for 300 cycles :'(
300 cycles is poor even by lead acid standards. Since organic lithium is expensive, these may have been 'seconds' or rejects from RC pack manufacture.
Is there something better for this light, like maybe 3 AA NiMH in series? The nominal voltage would be very close and the cost per pack should be less.
Simen:
Please define 'organic'... Are you talking about cells in a non-rigid container? In that case, those are suspectible to swelling, and in doing so; loosing capacity. Also, these cells are often charged really fast ( >2C-30C), which reduce life considerably.
You'd be better of replacing them with some 'laptop' cells (18650 cells - 18mm diameter, 65mm length), if you can make them fit. Those are more durable, and comes in up to 3Ah (maybe 4Ah nowadays...)
The cells used in my e-cigarettes are 'soft' lithium cells, rolled into a cylinder. These last only 3-6 months... I've swapped those cells with some used laptop cells, and they last several days between charge, and doesn't loose capacity... (so far... ;) )
And charging;
As soon as you connect rechargeable Lithium cells in series, one really should make sure there are a BMS connected (a Battery Management System); one can get a simple BMS for 2 cells for a few $'s... (check the RC market...)
Edit;
2x 3.7V in series would be 7.4V... = 6 AA NiMh cells... ;) Could you fit that? ::)
I would check the circuit board where the led's are; there's probably a small current regulator there. find the number, and check on the internet what voltages that regulator can handle. Maybe you can run it directly from a 12V battery... ;)
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