Re: Would 2x 1/2AA batteries in Parallel Work?

by adafruit_support_mike on Sat Aug 10, 2019 7:02 pm

Diodes will keep the LiPos from damaging each other, but will make voltage mismatch problems worse.

Diodes have an exponential voltage-to-current curve: increasing the voltage across one by about 60mV sends 10 times as much current through it. A change of 18mV doubles or halves the current.

If you have a 60mV difference between the two LiPos, the one with higher voltage will send 10x as much current to the load as the one with lower voltage. A difference of more than 100mV will essentially cut the cell with lower voltage out of the circuit.

To make matters worse, all silicon devices become more conductive as they get hotter.. the "about 60mV" estimate for 10x as much current drops by about 2mV per degree C. Diodes self-heat as current flows through them, so a diode with more current flowing through it will tend to become more conductive and steal even more current away from the other one. Under worst-case conditions, it leads to a condition called 'thermal runaway' where the hot diode keeps drawing more current until it hits the limit of the power source or the load, or burns itself up.

You can make the current balance better by putting resistors in series with each diode. It's the same principle as using current-limiting resistors with LEDs, and it works, but the quality of your current match is proportional to the ratio of resistor voltage to diode voltage. If they're equal, the currents will match to within about 5%.

At that point, you're losing about 1/3 of the power you get from a LiPo to the resistors and diodes that keep the LiPos from killing each other.