# Trying to understand relay power limits (strictly for DC)

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.

• You think you can get away without posting a schematic? Commented Jan 8, 2021 at 16:51
• Relay contacts are rated many ways, max current, max voltage, max power, and power rating can be less than simply max current times max voltage, and it would depend also what kind of load you have, resistive, inductive or capacitive. And you never mentioned the battery voltage either. Commented Jan 8, 2021 at 16:56
• two 12 V, 10 A batteries in series form a 24 V, 10 A battery Commented Jan 8, 2021 at 17:12
• how is the whole 1st paragraph of your post related to your question? ... it seems to be a waste of typing ... you are asking about switching a 10 A circuit with a relay ... you could ask that with one or two short sentences Commented Jan 8, 2021 at 17:14

In theory your relay only needs to match the battery: when the relay is closed, the voltage is zero and the current is the same as through the battery. When the relay is open the current is zero and the voltage across it is the battery voltage.

In practice you want to add a healthy margin for things like transient events, back EMF (when switching reactive loads), tolerance/variations of battery specs, aging, charging state, source impedance of the batteries, temperature drift, etc.

Most of this depends on what protection mechanism you have in place and how fast & effective they can kick in. Protection is needed to prevent battery and load from cooking, exploding, melting, catching on fire, etc if things go sideways somewhere.

If you were NOT planning to put protection in place, than picking the relays is probably the smallest of your problems.

You must choose break before make timing to prevent shorting out a cell. Then with wire and load inductance , there will be arcing on break so a metal film cap, I suggest with a small series R can be placed across the contacts to suppress glitches and reduce series Q =1 of the resonance.

Then on closing the contacts you will have wetting current that may exceed 10A but that’s OK. it’s the energy of the arc that reduces contact life in DC motors that demands debating the contact current from a pure resistive load rating.

Once you define all these variables in a “design spec”, the solution becomes obvious.

I might also consider Opto isolated FETs.