Let's say I had three batteries in series with two SPDT relays such that the circuit goes relay1->battery1->relay2->battery2->relay3->battery3->output, such that the relays either connect to the next battery in series or bypass it and connect directly to the next relay. This would allow me to turn the relays on and off to change the voltage of the output, such that it could see any combination of batteries 1, 2, and 3 - for example, if relays 1 and 3 were active, the output would "see" batteries 1 and 3, while if no relays were active the output wouldn't "see" any of them.
If each battery were rated for, say, 10 amps, how would I go about determining what DC power the relays need to be able to handle in order to operate safely? Would it be as simple as the voltage of the next battery times the current limit (in this case, 10 amps)? Or do situations such as having relays 1 and 3 active without relay 2 mean that relay 1 would need to be rated for a power equal to the voltage of batteries 2 and 3 times the current limit?
If the latter, is there any way to achieve this functionality if I had a long string of batteries in series? It seems that the power requirements would scale fairly quickly. I'm not really sure how I'd go about speccing relays for a situation such as this, as I'm fairly new to them, so help figuring out what exactly I should be looking for would be appreciated, as I'm pretty sure I couldn't just pick any "10A DC relay" and call it a day.