understand boat's battery system


Old 06-17-09, 10:40 AM
Thread Starter
Join Date: Jun 2009
Location: mi, usa
Posts: 6
understand boat's battery system

Need info & advice. Bought a 1990 Fisher SV-18 BT. It has 3 12v batteries. One is the cranking/starting battery. Looks like bilge & aerator pumps, fish finder & all accessories run off from it. Could/should I regularly top it off (charge it) before a day's fishing? The other two are solely dedicated to the elec trolling motor. They appear to be set up as a 24v system. Maybe not, I do not know for sure. A single wire connects the pos terminal on one battery to the neg terminal on the other battery. The elec trolling motor leads connect to the free pos terminal on the one battery & the free neg terminal on the other battery. Until I get an onboard charger installed, how do I safely charge these two? Cells appear low in one of the two. Is this distilled water? I thought about disconnecting them & charging them seperately. How to avoid sparking/shorting while doing so? I have an old deep cycle charger which runs off house current. Thanks for any feedback, pos or neg. Dan G.
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Old 06-17-09, 05:34 PM
Join Date: Jul 2008
Location: Montgomery MN
Posts: 541
Well, I don't charge my starting batt before I go out, the motor will charge it as it runs, but you can with a low amperage charger it won't hurt it.

It sounds like your trolling motor is a 24 volt, most trolling motors above 55 lbs of thrust are 24v. As far as charging the deep cycle batts. you can charge them like any other batt pos. and neg. on the same batt without unhooking them.

You can fill the cells with distilled water.
Old 07-22-09, 04:35 AM
Thread Starter
Join Date: Jun 2009
Location: mi, usa
Posts: 6
Samuari, thank you. It seems to work for now. Still amazes me how it can be done without disconnecting them. Dan g.
Old 07-23-09, 01:08 AM
Join Date: Apr 2007
Location: Near Buffalo, NY
Posts: 4,239
Originally Posted by dagross View Post
Still amazes me how it can be done without disconnecting them.
Technically you are disconnecting them ... electrically. By connecting to the positive and negative of a single battery you're cutting the other one out of the mix. The charger sees only the one it's connected to.

If you want to charge both simultaneously with a 12v charger you have to physically disconnect the cable that jumps from the neg on one to the pos on the other.

Many people use a cut-off switch to quickly disconnect the 24volt jumper. Combined with a three-stage charger you can charge all three batteries from house power.

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