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12 Volt system upgrade

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I am just about to upgrade the 12 Volt system on my boat, to something a bit more practical.

From new (2006), the manufacturer installed 2 x 85 AH Leisure batteries, with a master-switch (as below) to enable the selection of either battery 1, battery 2, both 1 and 2 together, or all OFF.


In principle this seems fine, but leisure batteries were not designed to provide the instantaneous high current requirements demanded by engine starting. The recommended CCA for my engine is 350A.

If battery 1, is assigned to engine starting, and battery 2 for the domestics, then in order to charge both at the same time from the engine, the master-switch needs to be switched from Batt 1 to Batt 1 + Batt 2. Unless the master-switch has the extra connections for protecting the alternator diode regulator pack (this one doesn't), then moving the switch between options, while the engine is running, could damage

the diode regulator in the alternator.

Another consideration is that if one battery is kept for engine starting, the current set-up leaves me with a single 85 AH battery for the domestics. Ignoring the freshwater pump, and lighting etc, the 12V Engel fridge draws 4 Amps, albeit in an on an/off duty cycle.

So, the plan is:

Assign the two 85AH leisure batteries to domestic duties, thus giving me 170 AH.

Buy a suitable battery for engine starting only.

Remove the existing master-switch, and replace it with two separate isolator switches (as below).


This will allow the starting and domestic batteries to be switched independently. Of course I will have to separate the ignition and associated wiring (e.g. Power Trim & Tilt etc), from its current common connection with the domestic 12V system.

The final piece of the puzzle, will be a low voltage-drop, split charge diode pack (see below), to enable both the starting and domestic batteries, to be charged from the alternator, while not allowing the starting circuit, to ‘draw’ from the domestic batteries.


I have already installed a Sterling charger, which currently charges the two existing leisure batteries, when connected to shore-power, and also acts as a 12V power pack for the domestics. I will leave this to charge only the domestic batteries, as the starting battery, will only provide starting power, and will re-charge from the alternator.

Hopefully, the revised system will allow me to moor overnight, away from shore power, with 170AH of power available to run the domestics, and not worry about having enough power to start the engine, the following morning. :Stinky:)


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Hi Dave

Sounds like a similar set-up to that I installed in our last boat, Tranquil Breeze. The difference there was that we had 4x110Ah batteries, which I split into 3 banks (1x engine, 1x inverter, 2x domestic). I used three of the isolators you picture, a three bank Sterling charger, and a 3 bank Sterling digital alternator regulator.

The digital regulator was a lot dearer than the equivalent splitter diode type, but was intelligent in that it charged the banks in priority order, and if a large drain was sensed on one of the banks, all alternator power was diverted to that particular bank.

The only question I would have, is that if you are not charging your starter battery from the Sterling charger when on shore power, you would have no way of starting thr engine should you have a flat battery. I realise this should not happen, but there is always the possibility I suppose. Would it be worth also putting in a battery link solenoid to temporarily link the started and domestic circuits for emergency starting?

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Hi Mark

I may at some time, consider the 'all singing and dancing' Sterling charger that you describe, as it is a very comprehensive unit.

My primary concern at the moment, is to remove the two leisure batteries from starting duties, as they really shouldn't be subjected to this. I have to wonder why the boat manufacturer, chose to do this :? .

Secondly, a single 85AH leisure battery isn't really up to the job of powering the domestics, especially if swmbo want's the TV on, to watch her soaps :roll: .

I take your point about being able to use the leisure battery bank, to start the engine should the starting battery ever let us down. However, a simple solution to this, would be to keep a pair of jump-leads on board (all the batteries will be contained in a single battery locker).

I have probably got room for four batteries, but I reckcon that when the time comes, I'll replace the two 85's, with 110's.

I would do the whole lot now, including the 'all singing and dancing' Sterling charger, but having just spent £6.5k on upgrades to the house, will shortly being taking our No.1 grandson on holiday to France, before he goes back to his parents in Australia, and then Sandra and I going over to Oz later this year, I'm trying not to squeeze my pension too hard :cry .


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  • 2 weeks later...

I decided after all, to spend a bit more of my pension :) , and go along similar lines to you Mark.

I didn't 'splash out' on two 110AH batteries, but I did buy the Sterling ProSplit R zero volt drop split charging system (see attached picture), and new dedicated starting battery.

Apart from the the cost of two new 110AH leisure batteries, my two 85AH leisure batteries are still very healthy, and with a custom-fit retaining system built into the battery locker for them, I decided to keep them, until such time as they need replacing.

Went down to the boat on Saturday, removed the old Battery Master Switch, installed two individual isolator switches (as shown earlier in this thread), thus keeping the domestic and starting batteries separate from each other. Then installed the ProSplit R charging system, and thankfully everything worked as it should :grin:

After starting the engine, the LED indicators on the ProSplit R, showed that only the starting battery was receiving charge from the alternator. When the energy drawn from the starting battery was replaced, the LED's then showed that the domestic battery bank had been brought 'on-line', and was also receiving the charging current from the alternator, and unlike the usual Diode Block split charge unit, the Pro Split R has a virtually zero volt drop.

So instead of having 2 x 85AH leisure batteries to serve both starting and domestic use (hardly ideal :roll: ), I now have 170AH for the domestic supply, a dedicated starting battery, and an intelligent/efficient charging system. All-in-all, a job worth doing, and one that will hopefully serve my cruising needs well :dance



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Yep, that's the unit Dave. We had no flat battery problems after installing that at all, and I'm sure you will be happy with your investment. At times of high current draw (like when we were running the inverter) you could actually hear the alternator working hard to replace the juice :grin:

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