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Battery Advice

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Hi folks

So I've bought this boat and whilst the batteries charge and shore power works, this that take a decent amount of current on 12v seem pretty sad. The fridge has an on light but doesn't get cold, if I plug in a 12v TV it comes on for a few seconds but then the power dips below 12v and goes off.

I stuck a multimeter on the batteries and they say they've got 12.6ish volts coming out of them. I don't know anything about the age of the batteries but they don't look too old.
Should I be looking more closely at then or are my woes more likely caused by something else I should inspect?



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On 10/08/2019 at 17:03, dnks34 said:

If your engine is cranking ok it sounds like your domestic battery may be weak, is it the type you can top up? 


So we moored up last night, ran the lights and radio but fridge was off and TV etc. This morning the boat jumping.


I'm not sure if the batteries are split it doesn't seem like it's got a dedicated starter.


Either way should I just cut my losses and replace them and see how I get on?


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If the Alternator is putting a good charge to them while you are running then Id say they have had it. 

Its worth sussing out how your charging system works, some might have a battery changeover switch others could have a hidden relay that connects the 2 batteries when the ignition is on or they may be both connected to the starter as you suspect.  Thick cabling on the positive between the 2 batteries would suggest they are connected together as one bank providing no split charge diode or smart splitter is in the line.

I always go for the cheapest leisure batteries available from a reputable retailer (usually online).  Then if they only last 2 or 3 years you wont be so mad when they inevitably begin to fail.  

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When cruising the battery charge flips to full I guess because it's measuring the charge output, they also were around 80% capacity when I went to bed last night, but it certainly seems anything under 60-80%% on the battery charge and it won't fire up. Also I wouldn't expect just lights and radio to zap 2 90ah batteries down to 50%.


I'll buy some new ones and see how it gets on, worst comes to the worst I have 2 new batteries and something else to diagnose.

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1 minute ago, Smoggy said:

I'd change it so you have a dedicated starter battery, at least then you can always get it running.

Yeah absolutely. It does also have a battery selector switch when turning the boat on, 1, 2 or both. I guess technically when moored, I could flip it to a single battery which should leave the other one charged and then flip to the other when kick starting the boat? (Guessing at this point.... :) )

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On our boat we had both banks connected via a 100amp relay and suitable cabling which was switched on with the ignition so both banks charged via the alternator with the engine running and were separate from eachother with ignition off, simple cheap and effective.

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If your lights are "older" style incandescent or fluorescent, then each may be using between 8 to 24 (or 32) Watts.  The newer LED bulbs use less energy.  If you have several lights on, the fridge and the TV, then you might be using 10 Amps (120 Watts).  Over 4 or 5 hours that's 40 or 50 Amp-hours, which will hammer a battery and use almost half of its capacity.

|f one battery has been discharged more than the other, then switching to "both" is likely to cause current to flow from one to the other, and that will detract from their ability to start the engine. 

It may be true that the batteries need replacing, but unless you understand how the batteries are wired and how much energy you are taking from them, you may find that new ones are not much better.  Would it be possible to get advice from someone local to your mooring who understands boat "electrics"?


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