Lawrence Rhodes via EV wrote:
I'm building a small 7 cell pack with two strings (14 total) of Nimh 4ah. I'll be using a generic 10 buck charger designed for 7 cells. 350 ma. I need to protect the pack. Is there a specific Thermistor that triggers at the right voltage to protect the pack?
You don't want to charge nimh cells in parallel. As they approach full, their voltage *falls*. The falling voltage of the fully charged cell prevents the one in parallel with it from reaching "full".
The simplest way to get around this is to add a series resistor (or something with resistance) in series with each string. The voltage drop across it allows each string to independently reach "full". However, then you have to monitor each string separately, to know when it reaches full, so you know when to stop charging.
A 10-dollar charger probably has no brains. It probably mindlessly applies some "trickle" current forever, and never shuts off. Nicads could tolerate this, but it is bad for the life of nimh cells.
The best algorithms for charging nimh cells involve looking for the voltage peak near full charge, and then shutting off. Or, some use a sensitive thermistor to sense cell temperature, and shut off when it begins to rise. Or, count amphours, and stop when you have put back 110% or so of the charge removed.
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