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LED drivers seem like an easy and cheap way to provide a constant current source for NiMH charging, but most need a sense resistor between the load and ground, which won't work when charging a cell in circuit. Here's an example.

Is there an alternative way to use these chips that doesn't require the sense resistor to be between the load and ground?

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  • \$\begingroup\$ Battery charging is actually complex and difficult, but has also been the subject of billions of dollars of design evolution. Don't ever try to wing-ding something else as a battery charger. Use a battery charger ASIC or other kit tuned for that specific pack chemistry and number of cells. If you must mess around, use NiCd as it's the most forgiving. \$\endgroup\$ Jun 20 at 18:20

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Is a Shunt connected like this problem for you?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ I believe that would be okay- I'm not sure I completely understand how to wire it, though. Would the GND for the LED driver then have to be isolated from the load GND? \$\endgroup\$ Jun 20 at 17:00
  • \$\begingroup\$ @AndrewMowry It should. Otherwise the shunt would be shorted. \$\endgroup\$ Jun 20 at 17:07
  • \$\begingroup\$ In my case the voltage source for the charger is also used to power the load when charging the battery, so I believe it has to share a ground. Do you know if there's any other way to make the feedback work? \$\endgroup\$ Jun 20 at 17:15
  • \$\begingroup\$ @AndrewMowry Circuit I posted: The charger supplies the Load during charging. The 36mA flows to load and 64mA charges the battery. \$\endgroup\$ Jun 20 at 18:17
  • \$\begingroup\$ Thanks-- I understand now. I'll have to think about this a little, because I'm not sure in my case the implications of having two different GND levels on my PCB, for example the charging voltage powers an MCU when charger but the battery powers it when the device is unplugged. \$\endgroup\$ Jun 20 at 18:45
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LED drivers usually constant current output with varying voltage output. It can be used as charger with some additional circuit, for voltage control, to protect battery from overcharging. Resistor in series setting the output current. Best way to discontinue charging at some voltage.

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  • \$\begingroup\$ Thank you-- I plan to have charge termination implemented with an MCU, but I'm wondering more specifally about the sense resistor on the LED drivers, and how to use that when charging a battery in circuit. \$\endgroup\$ Jun 20 at 17:35
  • \$\begingroup\$ @andrewmowry read the datasheet. The IC just keep some voltage at that resistor. Since it has constant value, you can add that to measuring. \$\endgroup\$
    – user263983
    Jun 20 at 17:43
  • \$\begingroup\$ Sorry, I'm not sure I understand. Are you saying that I should leave the sense resistor in series with the battery? That may be fine, but I too much of a novice to know for sure :). Are there downsides to having the resistor there? \$\endgroup\$ Jun 20 at 17:58
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    \$\begingroup\$ @andrewmowry resistor is necessary, because it works as current sensor. There is a lot of integrated circuits designed to control battery charge, just choose one. \$\endgroup\$
    – user263983
    Jun 20 at 18:23

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