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SteveO

Battery switches

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The last 2 boats I have owned ( A Freeman 23 and a Seamaster 813) both came with 2 batteries, wired in parallel , and which both presumably charge at the same rate, as there appears to be nothing to prevent this. In both cases the system seemed to work very well. However, I have always had niggling worries about the possibility that over-use of the batteries in the evening could leave me unable to start the engine the following morning. Books on the subject recommend the use of a battery switch so that the batteries can be isolated and/or charged selectively. Has anyone got any experience of fitting and using these gizmos and are they worth the extra complexity, given that I have never had a problem with my simple, unswitched systems?

Regards

Steve

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Do you not have seperate batteries for the starter motor and domestic? Starter motor batteries tend to be designed specially for starter motors (high current), whilst domestic batteries use considerably less current.

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Hi Steve,

Good question, well put.

I have 4 batteries in my boat, wired in parallel but in two pairs. (I assume because I have two engines, one pair for each.) Each pair has a switch, on which I can select battery 1, battery 2 or both together. (And off)

I have never tried using only one battery, and have no idea what would happen if I did. But I guess it must be all right, as there is a switch allowing me to do so.

But like you, I do sometimes worry about overusing the batteries while the engines are off, but am never sure how to prevent this and maybe using one battery instead of the pair together may be an answer.....?

I look forward to seeing any informed replies to this one.......

Nigel

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Re: Battery switches

Postby Pete on September 27th, 2008, 10:52 pm

Do you not have seperate batteries for the starter motor and domestic? Starter motor batteries tend to be designed specially for starter motors (high current), whilst domestic batteries use considerably less current.

Both batteries are 110 ah. One of them claims to be dual purpose.

Regards

Steve

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Nigel! all you need do is find out what position on the switch runs what battery/s if you have two 110amp batteries in parallel that effectivly gives you 220amps for each circuit, on mine No1 is the starter and No2 is the domestic, when i'm running I switch it to both so that both batteries get charged, then when I'm stopped I switch it to No2, that way No1 is always fully charged for starting, I have a normal 85amp car battery for starting and a 110amp for domestic and that 110amp will last me all weekend, but it will depend on what your actually running off your domestic ones, one tip never ever switch it to off when your engine is running you'll blow you alternator, my Perko switch has Alternator Field Disconnect, to help prevent damage to the alternator if accidentally switched to the off position while the engine is running,

Steve! they are easy to fit, I changed mine from the relay system to the change over switch in less than an hour, I measured the cables I needed and had then made up at out local auto electricians, because they were longer than the standard cables, but that was only because the BSS won't allow the cables and clamps that are held in position by a big screw in the clamp, they have to have an eye in the ends and be bolted to the clamp, in total it cost around £70 for the switch, cables and the clamps, but worth every penny because even if I run the domestic battery flat I still have a full battery to start the boat, :-D try and get a switch with the field disconnect built in,

Frank,,,

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Thanks Mowjo,

So if I fit a switch, I will need to invest in an 85AH starting battery and relegate my 2x 110 AH batteries for domestic use. This, at least, will give me a bit more peace of mind when Mrs O leaves the fridge on for too long.

Regards

Steve

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Steve, I would certainly separate them.

A simple and cheap solution is to use one for engine and one for domestic and charge via a split charge relay or diodes . There are many articles on how to do this on the web.

The relay idea is the same as used in caravans, the engine battery is directly connected to the existing charge circuit but the domestic battery is isolated from the charge circut until the engine is running. The isolating relay is operated by a feed from the alternator, again details on the web. There are also diodes available to do this, some purists argue that this does not give a full charge to the domestic battery, my own boat uses diodes and I can see no problem.

I guess most of the older hire boats use one or other of the above systems, as well as using dual purpose batterys for both jobs.

You will be able to acheive what you want and the security of knowing you will always be able to start your boat. And all for about a fiver for a relay!!

Remember if they are separate if your starter battery fails chances are your domestic battery is ready willing and able to swap over albeit if you have to do it manually with a spanner.

If sod's law struck and you had flattened the dom battery the night before the engine one failed... well you would have had to been a very bad boy in an earlier life!!!

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Steve

I went down a slightly different route, but it did cost a bit more.

We have 4x110ah batteries, which are divided into 3 banks. 1x for the engine, 2x for the domestic supply and 1x for the inverter. We separated out the inverter battery incase it was overused (240V TV and microwave) so we would only kill the one battery, but our lights and fridge would be okay. In order to charge these, I bought a charging unit from Brian Ward Electrical of Brindall in preference to a split diode. This is basically relay switches and a timing circuit which channels the full power from the alternator to each of the battery banks in turn rather than dividing it between al four batteries at once.

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So if I fit a switch, I will need to invest in an 85AH starting battery and relegate my 2x 110 AH batteries for domestic use. This, at least, will give me a bit more peace of mind when Mrs O leaves the fridge on for too long.

Steve! if both your batteries are in good condition you can use one for each circuit, because if your boat is diesel you new a few more amps and a 85amp one might not be enough, if you have the room you can fit as many batteries as you want for domestic use and just have one 110amp as you starting battery, as you can see from the other postings there are several ways to split them, I went the simple route as there's less bits to go wrong,,, have a look at this site and decide what way you want to go, it covers just about everything on a boat, but has a good electrics section, http://www.tb-training.co.uk/index.htm

Frank,,,

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idealy a 110ah battery should not be used to start engines they are designed to be charged and discharged constantly and not provide the ammount of amps required to start engines where 200amps and more on diesel engine in the cold pull to start them this can cause warping of the plates in a leisure battery and so lose its use prematurely your better having a 644 model battery for starting or a 645 (poles reversed)

http://www.querycat.com/faq/64d0cd44de3 ... 496e025182

above gives lots more info

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