Design, installation and troubleshooting of marine electrical systems            PETER KENNEDY YACHT SERVICES
                   Marine Electrical Systems




How do I charge two battery banks and still keep them separate? 

There are a number of ways of doing this:



Should I hook up batteries in Parallel or keep them in separate banks?

There may be some disadvantages to hooking up batteries in parallel but there are also considerable advantages. The main disadvantage as I see it is that as the batteries get old one will fail first, quite often by getting a shorted cell.  It will very quickly damage the other batteries by causing them to be overcharged. When not charging the good batteries get worn out trying to charge the shorted battery. The end result is the entire bank quickly fails. One way to guard against this is to use an Amp Hour meter and measure the calculated efficiency of the batteries from time to time.  Any drop in the level could indicate a bad battery. I have found from experience that at an efficiency reading of  85% that the batteries are still working but on their way out.  Replacing batteries at an 85% efficiency rate is a good way of avoid unpleasant breakdowns out on the water
The advantage of having batteries in parallel of course is that it makes a larger battery bank which will be less discharged in percentage terms and so will have a longer life.  It will also make a simpler electrical system, less switches, combiners, isolators, expense and potential operator errors.  So in reality everybody has batteries in parallel and just has to deal with the consequences.

We only sell a small selection of batteries at PKYS but please feel free to take a look at what we have to offer including the new Victron Energy Lithium Ion 24 volt battery



How big a battery bank do I need?

For most people to answer to this is "as big as possible".  Charlie Wing's book the "Boatowners Illustrated handbook of Wiring"  has a very good explanation of how to calculate the capacity you need. The calculations start by working out what your daily load will be both at anchor and while underway.

You need to remember that you cannot use all of the capacity your batteries have without shortening their life considerably.  You should aim to not discharge your batteries past 50% of their capacity.  In addition if you are charging batteries from your alternator while underway you will find that the last 20% of the charging cycle is painfully slow.  All of this means that the amount you have to work with for continued operation while underway is 30% of the rated capacity of your batteries. 

So for example to provide for a consumption of 100 Amp Hours per day and charge once per day you would need to have a battery bank rated at  333 Amp Hours.  

I enclose a sample table below for you to complete in order to calculate what your actual power consumption is..  I have filled in a couple of lines with a hypothetical example.  Every boat will be different of course and you will have to make a best guess at the answers.  You should do a separate table for when underway.  Often at anchor the loads will be higher as there are more lights left on for longer times and they are not offset enough by lower navigational equipment loads.

Power consumption At Anchor



Watts/12 = amps


Amp Hours

   Galley 30 2.5 3 7.5
   Salon 60 5 5 25
Navigation lights        
   Tricolor  or




   Running lights        
Fans and Blowers        
   Cabin fans        
   Galley or head        
   Propane valve        
   Electric winch        
Total Amp Hours per day at anchor.




Does anyone out there know where I can find a 'complete' listing of battery dimensions according to group or whatever?  I have a French built boat and cannot find anything to tell me what US equal size mine are.  It really matters when your battery boxes are as tight a fit in the scheme of things as ours are.

The Battery Replacement Data Book is a free publication published by Battery Council International of 401 N. Michigan Ave, Chicago IL 60611  and you should be able to get a copy from your local battery shop, I did. It gives dimensions for the common group sizes, but it is aimed at automotive uses and there are many batteries out there that are not standard. For those you will have to check with the various manufacturers

Nominal dimensions for common sizes are as follows: 

Group size Length Width Height
G24 10.5" 7" 9"
G27 12" 7" 9"
4D 21" 9" 10"
8D 21" 11" 10"
Golf Cart (6v) 10.5" 7" 11"






Peter Kennedy Yacht Services
    Marine Electrical Systems

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