I'm currently designing a small application with a bipolar power supply. As the circuit needs to be portable, I've included two Li-ion batteries in series with the midpoint grounded to generate the positive and negative supplies.

I need the batteries to be able to be recharged without removing them from the circuit, but I'm having trouble designing the circuit to charge them. Initially, I thought about adding a DC input followed by an OPAMP-based virtual ground driver to split the DC voltage into a positive and negative rail, and use each rail to charge each battery using some IC like the MAX1555, but this would only work for the positive battery.

Is this the best approach, or is there some other better way to accomplish this?

Edit: A 12V wall adapter will be used for the DC input. Also, the device will be connected to an Arduino board using a different power supply. The device will not draw the same current from both batteries.

  • \$\begingroup\$ What power source do you intend to use to charge the batteries? Will the device ever be connected to something else (eg. USB port) while charging? While in operation, does the device draw identical currents from both batteries? \$\endgroup\$ Jul 12 '16 at 5:58
  • \$\begingroup\$ @BruceAbbott: I was thinking about using a 12V wall adapter. Also, the device will be connected to an Arduino board using a different power supply. The device will not draw the same current from both batteries. \$\endgroup\$
    – puq_87
    Jul 12 '16 at 6:36
  • \$\begingroup\$ You could charge them in series from a supply which is not otherwise referenced to the output AND use an of the usual balancing techniques. eg if you can detect when a cell has reached full charge you can shunt the current from that cell and prevent further charging. \$\endgroup\$
    – Russell McMahon
    Jul 12 '16 at 11:42

Theoretically you could use a two cell charger to charge both batteries in series. However If your device is not drawing identical currents from both batteries then they may become seriously out of balance during use, so the only safe option is to use two 1 cell chargers that charge each battery separately.

If both chargers shared a common ground then that would have to be connected to the battery center tap, and one charger would have to put out negative voltage and current.

Alternatively you could use two identical chargers, but with each one having its own isolated supply so their 'grounds' don't short out the lower cell.

Dual isolated DC supplies can easily be created using a mains transformer which has two output windings. You simply attach a rectifier and filter capacitor to each winding. Here's one that I designed and built many years ago:-

enter image description here

  • \$\begingroup\$ I was hoping to use an IC for the battery charger to keep the size of the circuit small. Is there any IC capable of outputting negative voltage and current? \$\endgroup\$
    – puq_87
    Jul 12 '16 at 16:47
  • \$\begingroup\$ I don't know of any Li-ion charger IC that works on negative voltage. You could make a charger using a negative voltage regulator with current limiting but you still need a dual voltage power adapter. To charge from a single voltage power adapter you need to generate a negative voltage on board. Some switching regulators can do this, but need an inductor and other parts which will make the circuit larger. For smallest possible size make the charging circuit external and connect it to the device with a 3 pin 'balance' plug like I did. \$\endgroup\$ Jul 12 '16 at 19:16
  • \$\begingroup\$ If none of these ideas are acceptable then you might have to consider redesigning your circuit to work with a single cell, either using a small on board voltage generator or configuring the circuit to work on positive voltage only. \$\endgroup\$ Jul 12 '16 at 19:20

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