A Matter of Battery Balance
Dear WoodenBoat,
I’ve just read the excellent article on repowering classic boats with electric motors by Ben Parson (WB No. 312). The breadth of coverage, the level of abstraction, and the depth of information are all outstanding. But Ben writes that gaining a 48-volt capability by connecting 12V batteries in series “can have balancing problems.” No. The balancing problem arises in parallel battery configurations, where one underperforming battery can hold its neighbors below full charge.
The word “battery” itself provides a clue to how we arrange several low-voltage cells to increase voltage. The individual cell voltage is dictated by its chemistry. Increasing the size of the cell increases its charge capacity (energy in amp-hours) but not its voltage. Physically larger batteries of the same rated voltage have larger cells.
A battery is any interconnected set of cells, but particularly one connected in series, in order to obtain a higher voltage. We now talk about battery banks, but in the olden days those were just called batteries, because cells only existed individually. Now the cells are commonly packaged together in a common enclosure, and we call the resulting package a battery no matter how many cells it holds.
In other words, when we build a 48V battery bank out of four 12V batteries connected in series—or equally, 24 nominally 2V cells—we’re doing exactly what the manufacturer of a 48V battery is doing. Only the packaging is different, unless the battery has the type of internal Battery Management System (BMS) that monitors individual cell voltages, and that same type of BMS is available for external management of a battery bank.