Why do rechargeable batteries have no short-curcuit protection?
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. 

Lithium ion safety



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.