Is it ok to keep rechargeable batteries in an unplugged charger considering possibility of discharging the batteries via the charger's circuitry, etc.
Electronic – Is it ok to keep rechargeable batteries in an unplugged charger
batteries
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RB = Rechargeable battery.
NRB = non rechargeable battery.
I'll limit the following to AA and AAA cells and then comment on others at the end.
When standard cell types such as AA, AAA PP3 (9V "transistor battery) and similar are used, the use of non-rechargeable batteries rather than rechargeable ones will almost never damage equipment. Essentially never in normal situations.
Almost no equipment that employs AA/AAA cells is made specifically for RB rather than NRB and it is expected that consumers will plug in NRB on some occasions.
The range of RB of NimH or NiCd is about 0.9V - 1.3V and the range of NRB such as Alkaline cells is about 0.9V - 1.6V.
So, at the high end a NRB may make (1.6-1.3)/1.3 x 100% = ~= 23% more voltage. This could conceivably cause a problem if tightly designed but is exceedingly unlikely to in practice.
A RB may output in excess of 10A for short periods under very heavy load and a NRB probably under 1/2 that in most cases BUT RB systems do not rely on such currents as they are not consistently available and cannot be maintained for long or for a large % of battery capacity.
"Standard" AA and AAA cells are expected to produce the voltages mentioned above. There are nn standard cells available that may give MORE voltage from a NRB than usual. eg primary Lithium Thionyl Chloride, Mercury, Lithium ion all MAY be found in AAA, AA, C, D sizes but they are rare an specialised and users are usually trained in the difference. Some of these are rechargeable and some are not.
Even then - as the voltages of th standard NRBs are LOWER than the special batteries the system is liable to simply treat them as "flat".
SO while some interesting an unusual situation MAY produce damage when very special batteries are expected , this is unlikely to happen and may never happen.
There are two main types of rechargeable batteries - Lithium Ion and its children (such as LiPo) and the rest.
- LiIon cells MUST ALWAYS have protection electronics and the vast majority do have. If abused or, sometimes, just because it wants to, a LiIon cell will reduce both itself and the equipment it is in to a flaming heap.
LiIon "Vent with flame" ... - spontaneous "uncaused" laptop self dismantlement because (even) Sony could not get it right.
The no-name brand can't always either ;-) ...
- All other types can cause serious damage when shorted but are not usually prone to explosive self dismantlement. Short-circuiting may cause dangerously high thermal energy dissipation and may damage the cells permanently, but will generally not cause a run-away reaction.
LiIon (Lithium Ion) is a "slow release bomb" waiting to happen. A LiIon battery may be able to be persuaded to "vent with flame" (Gargoyle knows) by charging too fast, charging to excessive voltage, heavy discharge, spike penetration or heavy knock, charging at normal rate when voltage is low, charging at all when voltage is very low.
Protection devices for LiIon calls are the norm. These usually mount INSIDE the battery casing so their presence is not obvious. LiIon cells should ALWAYS use such devices. Most manufacturers will not sell LiIon cells without internal protection devices. Some will.
LiIon cells under fault conditions undergo heavy self discharge at a point which produces Hydrogen gas and first molten and then gaseous lithium metal. Temperatures rapidly rise to the ignition point "and away it goes". Once started the reaction usually runs to completion. Water is welcomed as an additional reactant by the Lithium metal.
Lead Acid, NimH. NiCd, NiFe, LiFePO4 ...
can all do significant thermal damage and will probably damage the cells if you short them and leave them shorted. BUT they usually do not explode and are not usually known to self incinerate.
I have never seen any of these batteries fitted with internal protection.
Note that LiFePO4 = Lithium Ferro phosphate, is in that list. A LiFePO4 cell is about as safe as they come. You can drive a silver spike through its heart, if you must. It will not thank you, but also will not self dismantle. Very briefly, the reason is that the Li metal is held in a spinel structure in the cell proper and does not migrate physically when the going gets exciting.
A lead acid "car battery" will melt about anything metallic that you place across its terminals. This would include eg large crescent wrenches. If you got badly hurt by bits of molten wrench you should not be surprised. The battery would probably never be the same again, but will probably not melt down. You CAN literally explode a lead acid battery by igniting hydrogen generated during charging, but that is a separate issue.
Add say 6 x AA Nimh 2000+ mAh cells, various coins and some keys to a trouser pocket and go about your business. On occasion you will get a short via coins and keys and misc cells so hot that skin burns could result and fire would not be a surprise. If you manage to do this you are an idiot! I've done it two or three times :-). Not again, I think!!!
NiCd about as for Nimh. More robust against shorts probably.
A fully charged 2000+ mAh AA Nimh will provide 10+ amps into a hard short.
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Best Answer
I'd expect it to be OK in many cases - BUT no guarantees as it depends on the charger designer. If the charger was powered you'd hope most designers would place the cell in a neutral mode. You'd hope. Unpowered it is not hard to draw almost zero current, should the designer care and 'know their stuff'.
An experiment:
Your question is easily answered in a specific case by an experiment.
Place a battery in the unplugged charger with a small piece of paper between battery +ve nipple and +ve charger electrode so that the battery is isolated by the paper.
Set a multimeter to lowest current range and connect probes between charger +ve and battery +ve.
A well designed charger should produce at most < 100 uA and it would be easy to get this < 10 uA and not hard at all to get < or << 1 uA.
If smoke comes out of the meter while performing this test the charger is (or was) of an inferior design.
Lacking a low enough range ammeter, use a voltmeter - play to see what range gves highest reading. Now try eg a 1 megohm and 10 megohm resistor in series with bttery and meter and see how they compare.