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I'm designing a fast-charge NiCd battery charger. To determine when to terminate the charge, I'll be using the standard technique of monitoring the charge profile of the battery and terminating charge when the battery voltage peaks and begins to decrease, as shown below.

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I'll be using a 10-bit ADC to monitor the battery voltage, which gives me approximately 5 mV resolution with a 5 V supply. I'd like to terminate charge when I see a 10 mV drop in voltage. However, I'm a little worried that 5 mV resolution won't be enough to reliably detect the end-of-charge condition, especially with some noise thrown into the mix. Would it be a good idea to use an instrumentation amplifier with one input set to a steady 1.55 V and the other input connected to the battery? Then, that approximately 30 mV rise and fall (above 1.55V) would look like, say, a 3 V rise and fall. Then, charge termination would occur when I detect a 1 V drop instead of a 10 mV drop, which would be a lot easier on the ADC.

Any problems with this solution that I'm overlooking?

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  • \$\begingroup\$ Looks like 5mV would indeed be very close to the limit. Battery university says the change is about 5mV, so just barely detectable with your ADC. \$\endgroup\$ – JRE Dec 16 '17 at 20:40
  • \$\begingroup\$ I think maybe set your reference to something lower like 1.3V. You can still amplify to get a larger variation, and still have a large (enough) usable range. \$\endgroup\$ – JRE Dec 16 '17 at 20:49
  • \$\begingroup\$ Seems like an OK plan. I think it will make it so you can only sense battery voltage near 1.55V, though. Some other charger functions may require you to sense the full range of battery voltages. \$\endgroup\$ – mkeith Dec 16 '17 at 21:50
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You could use a comparators so you get a 1 when you've achieved the target voltage and the add some hysteris so it goes off when the voltage drops by 10mV (or whatever value you want to set). A windowing function comparator is maybe what you need.

Or get a 12 bit DAC ?

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  • 1
    \$\begingroup\$ No target voltage. This charge method is called negative delta V. You charge at a constant current and monitor the voltage it takes to force the constant current through the cell. When the cell is charged, the voltage required will drop - but you can't set a fixed voltage limit because there isn't one. You are looking for a change, not a fixed level. A comparator won't do. \$\endgroup\$ – JRE Dec 16 '17 at 19:58

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