Wednesday, 11 June 2008

Battery Balancing the cheap way

I have installed a zener diode - resistor battery balancer on my 'self balancing scooter'



There are 2 x 6.8V diodes and a 5 ohm resistor across each battery.
(Thanks again for the info from Tuarn)

The Lead Acid batteries at rest are about 13.3V at new and fully charged so no power is lost when the batteries are not in use.

The Zener voltage must be greater than the battery full charge voltage

The idea is that the more voltage there is across a battery, the more current is bypassed that chargers lower batteries.

This worked surprisingly effectively.

The batteries before installing the balancer were from 12.5 to 14 volts at 'full' charge.
After balancing they were all within a few millivolts of each other, even after some discharge.

I was concerned that the bypass current may effect the automatic charger cutoff point as the batteries became fully charged however the charger still functions properly.

Given Zener voltage = 13.6
Maximum charger voltage = 15.2
Bypass resistor = 5 Ohms

The maximum current through the resistor is (15.2-13.6)/5 =.32A
The maximum resistor power dissipation is .32 * (15.2-13.6)=.5W

My ceramic 5W resistors are overkill. I will use 1W resistors in the vehicle system.

Maximum zener power dissipation will be 6.8V * .32A = 2.2W
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