1983 Porsche 911 SC Targa

Tuesday, October 9, 2012

Balancing the Cells

The first charge of the new batteries is the most important. Getting all of the batteries to the same voltage is the goal, because once they are connected in series, out of balance cells end up overcharged or undercharged. The cells are connected together in parallel – all of the positive terminals are connected together on one conductive path, and all of the negative terminals are connected together on another. Current will flow from batteries that are higher in voltage and into cells that are lower in voltage, arriving at a uniform average voltage on each cell. Then the pack is connected to a power supply that is used to charge up the pack. The voltage of the pack is monitored with a programmable volt meter. When the voltage reaches a set point of 3.42 volts, the voltmeter opens a relay that interrupts the charger. If the voltage drops below 3.4 volts, the volt meter closes the relay and the charger will resume. Eventually the pack will settle at 3.4 volts on each cell. I estimate this process will take 15-20 days. The charger I purchase for regular use will be more powerful and will only take 8 hours to charge.

In an ideal world, it would be just that simple. However, the 12 AWG wire I’m using and the 60 crimped ring terminals have small incremental resistance along the pack, and each cell will not rise in voltage uniformly. The cells that are wired closer to the power supply will rise in voltage faster. I have made several extra connections, evenly spaced along the pack, back to the power supply to try and minimize the voltage variation in the pack. Once the pack is nearly fully charged, I will need to allow the cells to stabilize with the power supply off, and over the course of several days the voltage of each cell should balance. 

As the cells are charged, the voltage of each cell increases, but resistance in the wire connecting each cell is causing the voltage to rise at different rates for each cell.  I added more wires in the middle of the pack to reduce the variation, but cells that are closer to the charger are rising faster.  The pack will need to stablize after charging, and the voltages will equalize.


  1. All very interesting Joey. I'm eager to see what you do, how the batteries behave and how it all turns out. Please keep us posted!


    1. I will post details as they materialize. I expect it will take a few days to equalize, once the power supply is switched off. I'm going to try to get the cells to match to within a milivolt, at resting voltage.