How Electric Cars Work

­An important feature of the charging process is "equalization." An EV has a string of batteries (somewhere between 10 and 25 modules, each containing three to six cells). The batteries are closely matched, but they are not identical. Therefore they have slight differences in capacity and internal resistance. All batteries in a string necessarily put out the same current (laws of electricity), but the weaker batteries have to "work harder" to produce the current, so they're at a slightly lower state of charge at the end of the drive. Therefore, the weaker batteries need more recharge to get back to full charge. Since the batteries are in series, they also get exactly the same amount of recharge, leaving the weak battery even weaker (relatively) than it was before. Over time, this results in one battery going bad long before the rest of the pack. The weakest-link effect means that this battery determines the range of the vehicle, and the usability of the car drops off. The common solution to the problem is "equalization charge." You gently overcharge the batteries to make sure that the weakest cells are brought up to full charge. The trick is to keep the batteries equalized without damaging the strongest batteries with overcharging. There are more complex solutions that scan the batteries, measure individual voltages, and send extra charging current through the weakest module.