These 2 batteries are on my recently acquired boat.

The boat maintenance guy, informs me that I can only charge one at a time, the switch up, charges the 200AMH bat, the switch down the 150 AMH Battery.

The engine starts off either.

Has told me I cannot link both together positive to positive negative to negative to charge both whilst the engine is running and remove the wire linking both together after the engine is stopped.  

I would have thought and in my experience he is incorrect.

Please advise me.

Thank you

Tom 

7 minutes ago, tomtaylor1 said:

Has told me I cannot link both together positive to positive negative to negative to charge both whilst the engine is running and remove the wire linking both together after the engine is stopped.  

Best to only connect the same make and size in parallel.

How about a solar cell to charge the smaller battery?

 

Photo of boat?

We like looking at boats.

You need a charge splitter/isolator. One of many on aliExpress https://th.aliexpress.com/item/4000124712547.html 

 

Lazada may have too.

 

 

 

As above: two batteries in a boat or camper is almost universal so the solution is very, very common.

 

 

Hi All, thank you for the information.

Tom

I had a camper van with two batteries 12v same size wired together as above without any Battery isolator, and the alternator charged both. 

3 hours ago, fredob43 said:

I had a camper van with two batteries 12v same size wired together as above without any Battery isolator, and the alternator charged both. 

That's standard for camper and most dual-battery vehicles: if the battery/batteries dies, you get some jumper leads or a push start, go to the garage and fix the problem.

 

However, if your battery/batteries or alternator dies on a boat...

Typically on a boat or RV with at least two batteries the starting battery is separate from the house battery. The starting battery is a normal battery and the house battery is a deep-cycle battery. When operating correctly, the battery isolator will charge the starting battery first, and only charge the house battery after the starting battery is fully charged. 

 

Further, the battery isolator will allow the the house battery to discharge fully, but never use the starting battery for anything but starting the engine. This allows you to run your lights, radar and bait tank all night without worrying about the boat not starting in the morning