What does it mean to “balance” a battery pack?
“Balancing” is a term used when charging LiPo batteries. When a pack is “balanced,” it means the cells have the same voltage. For example, a 2-cell, 7.4 volt pack is balanced if both cells have 3.7 volts. If you don’t balance the cells, their voltages may drift after a few charge/ discharge cycles. Why does this matter? Because LiPo cells don’t tolerate being over-discharged well. For this reason, modern speed controls have low-voltage detection systems to prevent the pack’s total voltage from dropping past a certain point (let’s say it’s 3.3 volts per cell, or 6.6 volts for a 2-cell pack). However, the speed control only “sees” the total voltage of the battery, not the voltage of each individual cell. So, if the pack is unbalanced, that 6.6 volts might not represent 3.3 volts per cell; you might have one cell at 3.6 volts, and another at 3 volts — that cell will be over-discharged, and now your pack is compromised or even ruined. Balancing ensures the cells always have equal voltage. Happily, balancing is easy to do; in most cases, all you have to do is make certain the pack’s balance plug is plugged into the charger.
The small white connector you’ll find on most LiPo packs is the “balance plug.” It’s wired so the charger can read each cell’s voltage individually and charge accordingly.
Depending on your charger, there may be a built-in balance plug port, or an external “balance board.”