tomtaylor1 Posted October 25, 2020 Share Posted October 25, 2020 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 Link to comment Share on other sites More sharing options...
BritManToo Posted October 25, 2020 Share Posted October 25, 2020 (edited) 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. Edited October 25, 2020 by BritManToo 1 Link to comment Share on other sites More sharing options...
Crossy Posted October 25, 2020 Share Posted October 25, 2020 You need a charge splitter/isolator. One of many on aliExpress https://th.aliexpress.com/item/4000124712547.html Lazada may have too. 2 Link to comment Share on other sites More sharing options...
VocalNeal Posted October 25, 2020 Share Posted October 25, 2020 As above: two batteries in a boat or camper is almost universal so the solution is very, very common. 2 Link to comment Share on other sites More sharing options...
tomtaylor1 Posted October 30, 2020 Author Share Posted October 30, 2020 Hi All, thank you for the information. Tom Link to comment Share on other sites More sharing options...
fredob43 Posted October 30, 2020 Share Posted October 30, 2020 I had a camper van with two batteries 12v same size wired together as above without any Battery isolator, and the alternator charged both. Link to comment Share on other sites More sharing options...
NanLaew Posted October 30, 2020 Share Posted October 30, 2020 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... Link to comment Share on other sites More sharing options...
Yellowtail Posted October 30, 2020 Share Posted October 30, 2020 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 1 Link to comment Share on other sites More sharing options...
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