What kind of internal protection (if any) do cell phone batteries have?

[Daravon]

I'm using a 1000mAh lithium battery that I took out of an old LG cell phone.

What kind of safety/cell-saving protection do these batteries have built-into them? Will it auto-cutoff when a cell-damaging low voltage is reached, or ever?

 

[rmteo]

Typically, none. Charge and/or discharge circuitry is contained in the device - or in the charger if the battery is removeable (eg. in a digital SLR camera). In the case of single cell phone batteries, they usually have 3 terminals - pos, neg and a temp probe. In the case of devices that use multiple cells (such as laptops and camcorders), there are usually additional taps to the individual cells for proper cell balancing during charging.

 

[carling]

I've "played" with quite a few cell phone batteries from different brands/model and have not yet encountered one which didn't have over-charge/discharge circuitry built-in.

 

[moldyoldy]

any protection devices in the cellphone battery represent a last-ditch effort to save the owner. The charging smarts are in the phone circuitry. Interestingly enough, the different phone manufacturers have markedly different philosophies about charging the batteries.

 

[Fallingwater]

Typically, none. Charge and/or discharge circuitry is contained in the device - or in the charger if the battery is removeable (eg. in a digital SLR camera).

No.
Charging circuitry might well be included in the device, or even charger, but there are always protection circuits in LiIon batteries for phones, cameras and most other things. Just about the only cases when you don't have them is in some (not all) devices whose batteries aren't expected to be replaced by the user. In that case, everything is on the device's board, and you just get a bare LiIon cell connected to the circuit.
When you think about it, this is obvious. Removable batteries almost always have exposed terminals that would be trivially easy to short. Do you really want to trust Joe Average Clueless User not to put his camcorder battery inside the same pocket where he keeps his keychain?

In the case of single cell phone batteries, they usually have 3 terminals - pos, neg and a temp probe. In the case of devices that use multiple cells (such as laptops and camcorders), there are usually additional taps to the individual cells for proper cell balancing during charging.

Again - no.
The terminal configuration you describe is common in NiMH cell phone batteries. In LiIon ones, the third (and often fourth) terminal is used by the phone to talk with the circuitry inside the battery.
And as for multiple-cell devices, in most (all?) cases the taps go to the circuit inside the battery, which worries about balancing each cell.
The many pins on laptop batteries do not act as balancing taps; they are there to let the computer communicate with the battery's circuitry. Example: pinout of an IBM ThinkPad battery.

Daravon: keep in mind that batteries meant for use in cell phones may have circuitry that doesn't let the cells deliver as much current as they could. Your 1000mAh battery has a cell inside that's good for a 2A maximum discharge rating, but a cell phone can't possibly suck two amps unless something's seriously wrong (like a short). If the circuitry knows this it's likely to open the circuit before you get to that. You can get around this by cracking open the battery casing, ripping the cell out and using it bare, but in that case you do give up any sort of protection.