I currently have 2 batteries I use for trolling motor.  One is an older Cabelas that came with boat & the other I bought last season.  I do a lot of trolling for trout &salmon so I run them down quickly.  At the moment I have to disconnect the cables to switch batteries and do the same to charge both.  Is there a switch I can buy to hook up to avoid changing cables all the time?

Your marine batteries should be the same strength, age and size. The power using a weaker battery will always be equal to the weakest battery.

Tom

It sounds like you have a 12 volt TM and are switching from one battery to another. Your batteries do not necessarily have to be the same age and strength in a parallel system like they do in a series. You should rig them in parallel to draw off both batteries at the same time. One charger will charge both of them but you will have faster charging with a two bank charger. 

On 5/24/2020 at 11:37 PM, K1500 said:

Your batteries do not necessarily have to be the same age and strength in a parallel system like they do in a series.

It's really not recommended to do that. It'll work, but you could either fry the old battery, or drastically reduce the lifespan of the new one. A $40 switch is definitely cheaper than batteries. 

Run 2 batteries in paralell; ground to ground and positive to positive to double the amp hours. The switch is good to switch off the batteries when charging to protect the engine and electronics . Replace the old battery.

Tom

PS, batteries in paraelell can be charged as 1 battery if they are equal condition.