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Subject: Battery management onboard Seamaster 30

Paul Killick
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Date Posted: 11:21:25 01/12/2007 Sat

I have been receiving a few questions recently regarding charging systems on Seamaster 30`s.

The item commonly called a coil due to its appearance is a split diode. It basicaly directs the alternators charge current to both the port start battery and the domestic battery when the engine is running only and isolates the domestic battery when the engine is not running. This maintains the starting battery charge for starting the engine only.
It is important to fit the correct type of battery! A starting battery is designed for just that, delivering loads of amps very quickly to aid starting. The domestic/leisure battery needs to be suitable for deep cycling ie. able to discharge apmperage slowly and re-charge from flat. Starting batteries are not designed to go flat! If they do they instantly loose up to 50% of thier power and will never charge to maximum again.
There are batteries available that do both fairly well and are commonly known as (Semi traction).

The easiest way to add extra battery power is to double up on the domestic battery, use two batteries exactly the same.

On the Seamaster 30 a few domestic lights are powered from the starboard starting battery, you will notice that the far right hand fuse box has another thick cable in the feed side (Bottom) This is the input from the starboard battery.
Personaly i prefer all domestic load coming from the domestic batteries leaving the starting batteries to do just that, starting. I fitted a permanent jump cable between port and starboard starting batteries via a solenoid, a small button switch on the panel can be used to quickly join and isolate the two starting batteries should one of them run low. You have to connect using a 25mm2 minimum cable with preferably an in-line heavy duty fuse link on each end and from thoes to the main battery master switch terminal.The engines then require a common negative cable between them.

The modern systems are very good and split all the batteries between both alternator outputs which allows one engine running to charge all three banks of batteries if required automaticaly.

When replacing the old AC alternators with new you should only use a proper earth return marine unit and will have to re-wire the main cables as per manufactures instructions possibly through larger capacity ameters if they exceede 50 amps output! Car alternators do not re-charge domestic batteries as well as marine types unless fitted with an extra charge system control system. (IMPORTANT)If you have a petrol engined boat then you must only fit an ignition protected marine alternator! This is recommended only on Diesels but should be seriously considered. The problem is that car alternators have exposed workings with lots of sparks flying around inside the unit just waiting for some petrol fumes to ignite! Marine units are sealed and cannot ignite fumes in the engine space.

Considering this subject is complex and varies between applications i recommend you get professional advice from a marine electrical engineer and only use marine electrical suppliers, all too often i see installations that are dangerouse and not suitable fitted to boats!

(All cables and terminations must be compliant to marine standards as listed in the boat safety essencial guide as a minimum good practice standard) All main cables these days should be fitted with heavy duty fuse links. Most fires aboard boats are caused by electrical systems!

Any members requiring further advice can contact me via e-mail

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Subject Author Date
Re: Battery management onboard Seamaster 30mark williams14:00:21 01/02/2009 Sun
Re: Battery management onboard Seamaster 30Ron15:39:11 02/03/2009 Mon

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