Hi,
I'm trying to protect a battery from overloading when too much current is
drawn from it. Is it possible to do so with a thermistor? Or are there
better ways of doing this?
PTC thermistor devices can work in some applications but not all. It
doesn't take much of a battery to be able to output enough current to
destroy the PTC device if the load side shorts. The ceramic ones have
more resistance and thus hold the current at a lower value while they warm
up.
Making sure that there is enough wiring resistance to protect the PTC is
the easiest way to protect the PTC from over current. Using wiring
resistance spreads the heat over a large amount of material.
The trip point of a plastic PTC moves around depending on its history.
For the Bourns ones at least, the trip point is higher on one that is
fresh out of the box. When it is soldered into the PCB its trip point
tends to decrease slightly. If you take a fresh one and use clip
leads and a power supply to trip it, you will see a reduction in the trip
point after the first trip or perhaps two. After that it settles out to a
final value. You have to be careful to let the thing cool for a long
time. A small difference in temperature makes difference to the trip
point.