Ideally I would like to use 18650 LiFePO4 rechargeables but I have given up trying to order them. Nowadays it is prohibitively expensive getting Li cells into Canada. Thus instead of using LiFePO4, I figured I’d use the Li-ion cells I have on hand and charge them up to 3.6v — the upper limit for LiFePO4. To do this my plan is to use the LiFePO4 charge setting on my chargers — an Opus BT-C3100 and a Klarus C4.
However, is this safe to do so? (I mean safe in terms of cell longevity.) I suppose that if charging algorithms are the same between the two types of chemistries (except for the upper cutoff limit of 3.6v vs 4.2v), then under-charging Li-ion this way should have no ill effect (I suppose).
Fyi I’m hacking a pair of heated socks to take two Li cells instead of the three AA alkaline they’re supposed to take. They’re hardly warm with the alkalines, and 2x4.2v is too much for them. Ideally I’d like to get a total of 6 to 7v. I can get close to this with 2x3.6v.
You’ll have crazy-long longevity as long as you don’t run them down too far, but that means probably using them for a short while then recharging them, as 3.6V is maybe 40% charged or so (forgot the numbers).
So they won’t be harmed, but also won’t be very useful.
Spring for the 2-3bux and get a LK Lii202 or similar.
Yes it is completely safe, you will also get a longer life out of them (not that you will notice this at all)
The problem is that they will only last like 1/4 or 1/2 of what they normally would.
For longlivity you could mod the charger to do only 4-4.05V, still enough capacity compated to 4.2 but longer cell life 3.6V is not enough to really use in high power lights
Assuming original carriers are 3xAA in series, that gives you a stock voltage range of 4.5V to about 3v (there’s not much energy left in an alkaline below 1v).
Why not get a 2P 18650 holder? You’d get 4.2 to 3V but probably need some kind of low voltage cutoff.
kbear99, alkaline cells are a bad choice for medium to high drain applications. I guess your heated socks may drain the cells in less than an hour, which in practice is just a little above 3.2V average at 1A discharge for 3 cells in series. What this means is you would get a little more power output and much better runtime using 3 li-ion 14500 cells in parallel.
If you go the LiFePO4 route power output is likely to get much higher (well above 3x), runtime will suck and it may even collapse (because of heater element failure).
Can you please give instruction in laymans terms for me to do this. I want my chargers to stop at maybe 4.05 or 4.10 instead of 4.2.
I have multiple brands of chargers
Adjusting the sense resistor value alters the charging current/ratio, which may or may not be possible depending on the power supply strenght/rating.
The maximum charge output voltage is generally shifted via “adjustment” of some sort of voltage divider in the circuitry. See here, for example: LiitoKala lii – 100 CHARGER Who Likes 'em? There are related mods for that charger there.
If these socks were originally intended to be run on 3xAA cells in series, then I would think a single 18650 fully charged would be perfect. Use 2 in parallel for more runtime.
I know you are thinking 4.2V is not as high as the 3xAA in series, but after the voltage drops of the Alkalines are taken into account, you may find that the heat generated by the 18650’s is higher.
Be careful with increased voltages, power output of the heater goes up by the square of the voltage (a 20% increase in voltage will give a 44% increase in heat)
