Recharging Li-Ion/Poly cells safely
Hi all.
Having had at least one cell nearly explode due to overcharging, I would like to point out that a simple voltage regulator WILL NOT work.
Li-Ion cells (and their polymer cousins) NEED a proper charger, the precise voltage varies depending on type (CoO, spinel, etc) but is normally around 4.05V to 4.1V and no higher.
In addition if the cell is below 3.1V is needs a 1/20C (i.e. for 500mAh cell 25mA constant current) to 3.2V then current limited constant voltage Vmax until the drawn current drops below 1/10 capacity (i.e. 50mA for the above cell)
the cell then needs to be disconnected from the charger as even ten minutes of charging can damage it by plating out lithium metal onto the interior of the cell causing potentially dangerous instability even after several discharge/recharge cycles.
if you happen to see a "blown" (i.e. bulging) lithium cell then in all likelihood it has been either overcharged or overdischarged causing the electrolyte to destabilise and making the cell unsafe to use.
additionally if a cell is discharged below 2.75V then copper shunts form and cause internal leakage resulting in a dangerous temperature rise during any attempt at charging. You absolutely cannot "zap" them back to life so don't even try.
this is a different mechanism to NiCd whiskers and zapping does not work because the copper just ends up as debris in the electrolyte and contaminates it resulting in capacity loss and in worst cases a catastrophic internal short at or near full charge (read- BOOM).
LiFePO4 cells (easy to detect as their open circuit voltage is typically 3.25 to 3.35V) need a different charging algorithm, the "maximum" charge voltage here is 3.65 to 3.7V.
Under overcharge or overdischarge they just lose capacity and the internal resistance goes up so they still need a dedicated charger.
Interestingly with some of the newer generation this is less of a problem, as long as you don't charge them for days on end they somewhat self regulate and the current falls off to less than a mA once charged.
-A
memo to self:- $2 a unit for phosphate cells = counterfeits.