Adding a 2nd / services battery

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MacD68

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Hi,
This is probably a pretty basic question, but any input appreciated. My current boat came with just a starter battery fitted. Electrics are currently limited to lights (nav and internal), but I plan to add instruments and a heater (Mikuni) shortly, and possibly an Autohelm later in the year. To avoid concerns about draining the starter battery, it seems like a reasonable precaution to install a second battery to handle all other services, and leave the original battery to deal with starting. The questions is how (electrically)?
I see reference to everything from basic manual switches to switches based on diodes, to decide which battery gets charged when the alternator is running? Any thoughts on the best way to go?
Should I consider an option to connect the house battery to the starting circuit, or starting battery to the house circuit, in emergencies?
Any hints on a useful website to improve my knowledge also appreciated.
Thanx.
MacD
 
There are all sorts of systems of varying complexity and price, but for a simple, inexpensive set-up, a simple split charge relay works fine. It's what I use and has saved me hassle a few times when I've left something switched on.

You can use a cheap relay suitable for car headlamps, or spend a bit more for a Voltage sensitive relay. The difference is that the simple relay switches the domestic battery into the charging circuit as soon as the alternator starts to work, while the VSR waits until the starter battery has recharged to a certain level. As a confirmed tightwad, I use an ordinary relay.

PM me with an email address if you need a circuit diagram.
 
personally, if designing a low tech system, i wouldn’t bother with the 1,2, both switch to link the batteries incase of a flat engine bat, i would rather carry a pair of jump leads and save the extra wireing. im sure other people have different views re speed of change over in emergency ect, but it depends how simple you want to go. Matt
 
Probably easiest done using a battery switch cluster with voltage sensitive relay

like this http://www.power-store.com/?id=198

the start battery will connect to the start battery switch and the existing starter circuit connects to the other side of that switch.

your new house/service battery connects to the house battery switch and all you need to do is trace where the feed is taken off for the nav lights etc. (hopefully a feed to a fuse box of some sort) disconnect this and reconnect into the other side of the house battery switch.

the voltage sensitive relay looks after charging when the engine is running but the start and house circuits are separate.

these switch clusters also have an emergency parallel switch in case the starter battery does have a problem.
 
In simple terms you set up the service battery entirely separate from the engine battery exceept the negatives are connected together with heavy cable (for starter current). You have a separate isolation switch for both batteries.

Now you fit a heavy current switch between the positives of the 2 batteries.
This is connected to charge the service battery when the engine is running and opened when you stop the engine and want isolation. It can also be connected to boost the engine battery for a start if the engine battery is dead.
From there it is simple to connect a Voltage sensing relay across the isolation switch so this disconnect /connect function is done automatically.
Some VSR contain a high current connect function for manually connecting for emergency engine start to avoid buying that manual switch. ollewill
 
Get a copy of :

Alastair Garrods - Electrics Afloat. ISBN : 0-7136-6149-6

It's a PBO associated publication, it's simple, it's accurate, it's worth it's weight in gold as THE book to help you through all this.

You will have advise to fit split diodes, VSR's, 1-Both-2-off switches ... the variations will be such that by time you get to end of thread of 20 or so replies - you are still not sure.

Me ? I bought above book and a number of others ... 12V Bible being one. Garrods book made most sense and was illustrated very well.

I still have the old traditional 1-Both-2-Off switch and 2 battery system. It has worked well for years powering as much or more than you mention. I'm not about to change.
Garrods book will explain that as well as VSR's ... Diodes etc.


It can be as complicated or as simple as you want - honest.
 
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