Smart Charger for NimH Circuit

[tonyb]

I thinking two thermistors one for ambient temp. sense, one for batts. temp., going to a comparator when it detects a tempature rise ( battery charged condition)it triggers the charger to go off. Anyone have a good circuit? Some years back I remember working on a batt. charger for a mil. contractor it used a chip I cannot remember who made it now it was pretty sophisticated though.

 

[JSWrightOC]

Sounds like a start!

Another method that would eliminate one of your temperature sensors would be to sample the temp every minute or so, and when the change (delta) over that time period (T) is higher than a reference, the charging stops. This will eliminate ambient temperature dependencies. I think this is the basis behind all dt/dT chargers.

 

[Doug Owen]

Temperature only may not be \"Smart Charging\"

There are lots of ways to do this, from relatively simple bridge circuits using thermistors, to straightforward ones using any number of neat low cost temperature sensor ICs to systems using changes in forward voltage drops on silicon diodes.

However, none of the makers would seem to support a hot rock charger depending on such measurements alone to end charge. They all recommend this as 'one of several' such triggers. Consider that if the detector somehow fails (for any number of easy to forsee reasons), a *fire* is the next likely result.

Caution is advised.

Doug Owen

 

[tonyb]

Re: Temperature only may not be

I built one that measured voltage but that only worked for one voltage which happened to be 12 volts, it went to trickle charge when it reached 14.4 volts. I want this one to be programmable current for any batt. or batts. I connect to it.

 

[nitelite]

Re: Temperature only may not be

I think the real reson most products use voltage slope is the hassle with measuring cell temp. The thermistor must be built into multicell packs or clipped on or something to single cells.

The best route might be to use both. The maxim 712 fast charge chip can use both voltage and temp to fast charge cells.

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/1666/ln/en