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Would there be any potential problems with the below implementation of a rudimentary li-ion balancing/(overcharge protection) circuit? Would a resistor need to be in series with the zeners?

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ most cheap/avalible zeners are 1 watt \$\endgroup\$ – Daniel Caoili Feb 1 '16 at 17:43
  • \$\begingroup\$ From what I've observed, voltages differences across the cells is usually no more than 0.1v \$\endgroup\$ – Daniel Caoili Feb 1 '16 at 18:16
  • \$\begingroup\$ A little background: I was trying to figure out how to charge a laptop battery (hp) externally. I found that I could draw power from the + and - terminals by just wiring them to a load. However when I applied a regulated current source (the battery wasn' \$\endgroup\$ – Daniel Caoili Feb 1 '16 at 18:48
  • \$\begingroup\$ t fully charged) I no current would flow into the battery. I thought I might ground one of the 5 pins on the battery to open a switch (JFET?). when that didn't (I tried all pins except the positive power suppply) seem to work I tried conecting the + terminal to one of the middle pins (not supposed to be the ground) and fried the internal battery circuit. \$\endgroup\$ – Daniel Caoili Feb 1 '16 at 18:53
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A big problem is that zener diodes have a fairly soft reverse knee. This is usually compensated for by using them in tandem with a current limiter. Since there is no reasonable way to limit the current in this case you are better off going for a part with more precision such as a TL431.

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    \$\begingroup\$ Plus you'd be lucky to get better than +/- 5% accuracy between diodes and the zener voltage will alter with temperature. \$\endgroup\$ – Andy aka Feb 1 '16 at 17:44
  • \$\begingroup\$ "soft reverse knee" what exactly does that entail? \$\endgroup\$ – Daniel Caoili Feb 1 '16 at 17:45
  • \$\begingroup\$ @DanielCaoili: The bends in a diode characteristic curve are colloquially called "knees". The reverse voltage of a zener diode can vary by several hundred mV over a difference of less than 1A. This could easily lead to the destruction of the cells. \$\endgroup\$ – Ignacio Vazquez-Abrams Feb 1 '16 at 17:51
  • \$\begingroup\$ The TI datasheet has a circuit intended for high-current uses. \$\endgroup\$ – Ignacio Vazquez-Abrams Feb 1 '16 at 18:23
  • \$\begingroup\$ The TL431 does have snags .You can still use it but be careful .It is featured in www.badbeetles.com \$\endgroup\$ – Autistic Feb 1 '16 at 19:38
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You could test various zeners and pick out the ones that trigger at 4.1 volts or so. A small pack is only gonna be charged at 1 to 2 amps. The resistor could just be a small piece of nichrome or stainless steel wire. Stainless at 30 AWG would be about 0.25 ohms per inch. At 0.1 volts, that would be 40 mW of heat dissipation. Plus, at 4.1 volts, the pack will last much longer in cycles.

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