I have a solar-panel-and-battery kit with a built-in inverter and what not built into the battery box. (Goal Zero Yeti 400) The battery box has Anderson Powerpole connectors for chaining it to external batteries. As far as I know, these are connected directly to the battery inside.
The battery built-in is a 33 Ah 12V sealed lead acid battery.
Suppose I'm out camping, and someone joins me and brings another 12V, perhaps of different capacity, and at an unknown charge level. How can I safely connect these batteries together in parallel?
I assume that if the voltage difference between them is too great, the current flow between them will be a lot and potentially damaging to one or both of the batteries. Therefore, I'm thinking that some sort of resistor is called for, even if just until they're balanced.
Is this a good idea? And if so, how do I choose what resistor to use in this case? I suppose I could use something of very high resistance and let the battery trickle up to the right voltage over a long time, but it would be nice if it didn't take days. What sort of current can these batteries take before they become damaged?