How do 2nd batteries connect?

When people run a second battery, what ways can they be connected? For example, directly connected to the main battery in parallel.

What other ways are there?

 

Depending if you're wanting to add more voltage or amps to a system, you can connect batteries together in series or in parallel. Parallel will add more "capacity" (amps) and keep voltage the same. Series will add more voltage to the system.

But I'm assuming you mean how are they actually being connected? Usually with a selector switch. So then you can draw power from Battery "1", Battery "2" or Both (or kill power to circuit completely "Off").
Usually one battery would be your starting battery, usually one is your accessory battery (deep cycle).

 

If one battery is very low like maybe 10v and the other 12.5, is there any worry of over charging the dead battery? Assuming the total resistance is 50mO, 2.5v is 50amps.

 

I'm using a Blue Sea Automatic Charging Relay with Manual Control PN7622

 

Simple battery isolators will disconnect the two batteries when voltage is below charging voltage and reconnect when above so the alternator charges both.

Once the vehicle is running, the alternator supplies juice unless you’re exceeding its rated current (unlikely under normal circumstances)

Some fancier options like the RedArc can charge an AGM 2nd battery at higher voltage than the stock battery when it detects the alternator is running. The RedArc also accepts solar panel input directly.