Trying to understand relay power limits (strictly for DC)
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 &amp; 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.