Charging 72 volt battery
Have you wondered if batteries in series get charged properly? Recently a main supply surge resulted in smoke, soot and an awful burnt plastic smell coming from my car’s built-in 72 volt charger. Time to investigate.
Why 72 volts?
My car uses six 12volt batteries in series to power the motor with 72 volts. Why do we use 72 volts anyway? well the reason is that if we have a 10 kW motor powered by a 12 volt battery the current consumption from the 12 volt battery is very high. 10,000 divided by 12 is a current of 833 Amps at full load. No battery can stand this sort of punishment so by increasing the battery voltage to 72 we get a more manageable current at full load of 138 Amps. Full load is not used constantly. This is only taken from the battery when using maximum power, such as pulling away or the steepest hills.
Back to charging those batteries. For a long time I have wondered if a 72 volt charger for a pack of 6 batteries was the best way to go… or not?
The affordable mini electric cars often use the 72 volt option as a matter of course and provide a built-in 72 volt charger to do the job of refilling the batteries. A few months ago, I decided to check each battery after a full charge and was not surprised to find that the state of each of the batteries was far from equal.
72 volt Charger readings
Two of the six showed full charge, One showed 90%, another 86%, a fifth stood at 81% and the sixth at 77%. When the voltage from the charger equaled the design voltage for the 6 batteries, it simply switched off, telling me I had a full charge. The result was that the charge didn’t do too well running around the town for my shopping.
A replacement 10 Amp 72 volt AGM charger was listed at several hundred dollars, which I really didn’t want to pay, but were there any options?
What seemed to me as a viable option was to charge each 12 volt battery to full then retry the car. I was delighted by the result, but to get there I had to charge the batteries individually over a couple of days. This was OK for a test but impractical for a car which is in use virtually every day. The obvious answer was to provide individual chargers for each battery.
72 volt batteries individual chargers
I went ahead and bought five chargers of the same output as my old faithful AGM charger and built a bank of chargers. The chargers were all fitted onto a board which neatly mounts directly onto the underside of my rear seat. By tipping the rear seat into its second position ( which I rarely do anyway) I am able to read the state of each of the individual batteries As each battery is refilled to its best state, its charger switches off automatically, but I know from the state meter on each charger, how that battery is performing.
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