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I have a battery which will remain connected in series, and I want to have an option to connect / disconnect the balancing leads electrically - something like this:

sample circuit

which will connect the batteries when the "active" line is driven high from an external power source (not the battery stack, only connection between the two as what's shown here, i.e. "common" ground when the connection is made, and high resistance when off, but then disconnect them when it's driven low (the resistor bridges are going to an ADC to then calculate each cell voltage)

The problem with this circuit is that when the active line is low, the gate voltages on the P-channel mosfets ends up being the same due to the resistor dividers that I want to remain there, and thus I need mosfets that can handle a large gate-source voltage difference. I am looking for a way to "chain" them or stack them in a ladder-like fashion where the voltage that the mosfets needs to handle is not related to the # of cells in the stack - what would be the best way of doing this?

P.S. - not shown here but I want to be able to actively balance the cells as well, so once I figure out their voltages I can selectively charge/drain the cells via these same balancing lines, thus I want low RDSon mosfets, which tend to have lower voltage tolerances

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What do you want is called BMS(Battery Management System).

Basically you want to protect the battery so it can withstand among many other things: over/under current, over/under voltage. Since nowadays people are putting batteries almost everywhere, IC manufacturers responded appropriately. You will find tens of IC solutions for just the thing you need. Most popular being:

  • TI - BQ76PL536(Tesla's choice)
  • Linear Technology - LTC6804(BMW's choice)

Read the datasheet carefully and you should be able to create basic circuits. Go further, connect it with MCU capable of driving FET's as well as communicate with the IC and you will have very basic BMS structure.

However, if your budget could not afford one of the above IC, take analog multiplexer along with differential amplifier and use MCU's internal ADC for measurement. Luckily, you will have 12 bit resolution ~ 1-2mV measurement error(Li-Ion) after compensating

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  • \$\begingroup\$ No, what I want is a way to electrically disconnect the BMS from the battery \$\endgroup\$ – user2813274 Jun 26 '17 at 18:19
  • \$\begingroup\$ That's a big no no making your system unsafe. Don't do that. If you want to legally ship your product by a plane, you must pass UN38.3 directive for dangerous goods. You can't pass it using a method you described. \$\endgroup\$ – Bip Jun 26 '17 at 18:33
  • \$\begingroup\$ I am not looking to ship it by plane (not sure how just a circuit would be unsafe anyways if it's not even physically connected to a battery, but that's beside the point), just looking for a schematic as to how to enable/disable the connection between the battery lines and the BMS \$\endgroup\$ – user2813274 Jun 26 '17 at 18:47
  • \$\begingroup\$ Who will disconnect the circuit? User, some other system? \$\endgroup\$ – Bip Jun 26 '17 at 18:57
  • \$\begingroup\$ I am looking for a way to connect/disconnect it electrically via the "Active" line when it changes from high/low, manually is another issue \$\endgroup\$ – user2813274 Jun 26 '17 at 20:02

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