Marine Electronics Forum

3 batteries

Old 03-23-2007, 09:26 PM
Junior Member
Thread Starter
Join Date: Mar 2007
Posts: 2
Likes: 0
Received 0 Likes on 0 Posts
Default 3 batteries

can anyone tell me how to add a 3rd battery to my twin outborads
Old 03-24-2007, 06:23 AM
Senior MemberCaptains Club MemberPLEDGER
Join Date: Feb 2005
Location: Kent Narrows, MD
Posts: 7,144
Received 774 Likes on 366 Posts
Default Re: 3 batteries

Welcome to THT.

Some info would be helpful (type of boat, type of motors, existing batt setup) Adding your location to your profile is often helpful in answering questions

Start here

Old 03-25-2007, 11:21 AM
THT SponsorCaptains Club Member
THT sponsor
Join Date: Jan 2007
Location: Beaufort, SC
Posts: 692
Likes: 0
Received 68 Likes on 47 Posts
Default Re: 3 batteries

Why do you want a third battery? If you need a battery to run instruments or other loads and want to avoid running down a starting battery you should use a Combiner. A Combiner will divert excess alternator charging current to the third battery as it becomes available after first charging the starting battery.

There are a number of ways to do it. These methods make sure the load on the third battery will never discharge a starting battery. Although you could use a manual switch, the extra uncontrolled load the switch would put on the alternator(s) may overload them.

1. A combiner between just one of the starting batteries and the third battery. Whenever that engine is running the third battery will get a charge. Charge is only delivered in increasing amounts as capacity becomes available from the alternator after satisfying the starting battery first. But if the running time is too short to get enough charge to it you could consider #2.

2. Uses two combiners - one between each starting battery and the third battery. Now either engine or both if both are running will provide alternator capacity to the third battery as power becomes available.

3. But my preference is to dedicate one of your existing batteries for starting and the other for the auxiliary functions. Just put both starter cables from the OB to one battery, and then just one combiner between it and the second battery which is now dedicated to house loads. There is little justification for having separate starting batteries when one will be adequate so this saves the cost and weight of the third battery, plus it allows operation with just one combiner. If you are concerned that both engines are now dependent on one battery for starting, install a $15 emergency parallel switch that would allow the second battery to be put in parallel with the first one so both batteries in parallel are now available to provide starting power.

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are Off

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright © 2021 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.