How Does Connecting a Battery in Series Affect Current Flow

Question

Background:

I am creating a circuit to run electronics and motors. The some of the electronics are 5V some are 12V and the motors are 24V. I have a small universal 12V battery for the electronics I run it through a 5V regulator for the 5V electronics. In addition I have a huge Optima Yellow Top 12V battery. I want to create a 24V circuit for the motors by connecting the small 12V battery to the large optima 12V battery in series.

My Question:

How would current flow through this circuit? In other words, do the motor take an equal amount of current from each battery? Would the current in the wire from the positive terminal of the smaller battery to the negative terminal of the Optima battery be the same as from the optima positive terminal to the smaller battery negative terminal (the motors will be between the optima positive and the smaller battery negative). Will the optima battery be able to supply the majority of the current? If I hooked another 12V battery in parallel with each of the 12V batteries, how would current be drawn from that battery?

Thanks for any help, Joel

Answer 1

In the simple series connection, the current will be limited to the current from the battery with the lowest current rating. Note: that may not be the smallest of the batteries, either in physical size or in total energy storage. Just for example, Nickel-Cadmium batteries generally have very low internal impedance, so even a fairly small NiCd battery usually has quite high current capability (but will discharge very quickly at maximum current capability).

When batteries are connected in parallel, you add together the current capabilities of the batteries.

For your series/parallel connection, you'd want to connect at least enough of the smaller batteries in parallel in match the current of the larger battery (or at least to match the current requirements of your circuit). If you connect more in parallel than that, you'll get the same total current flowing, but less through each individual battery, so they'll discharge more slowly. That'll give longer battery life unless current draw is so low that the batteries are already limited by their shelf life (which is unlikely where they're powering motors and such).

In most cases, however, you want to ensure that any batteries you connect in series are as close to identical as possible--regardless of overall current capability, if some of the batteries discharge at different rates from others, things can get fairly ugly.