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Posted: Mon May 23, 2011 9:13pm

Post Subject: Battery or what?

My tub is 20 yrs old - Mitsubishi K3 - no charger - has a battery manager - (1,2,both) Three batts (Starter & 2 leisure) in series - we always charge with both on and stop with just the leisure. Last year I changed my gas fridge for a shoreline 12v. Apart from a CD player and usual pumps thats all the powere we use (no TV etc. 6 months after the fridge was fitted the 2 leisure batts were done for I bought 2 more and within a week I took them back - tested at the yard and they were done for also - sold me 2 more and put a larger cable on the fridge whgich they said could be the problem. They bench tested my alternator (It was a strong suspevt)but found it fine. My alternator must be good (?) because the starter battery is fine (5yrs old - kicks 1st time even after last winter) and I have plenty power when running. Yesterday we ran for 6 hours and within 2 hour of a stop the fridge has stopped. Today - charging up looks OK - hovers around 13V - on my voltmeter but when I stop running and turn the switch on - it shows only @ 10 volts - I think I have yet another leisure battery that has gone dud on me but I don't know why. (Wife thinks it is the fridge she had on too high or thernmostat is done) but surely that would not destroy the battery. If the alternator was working intermittently it wouldn't stiff the leisure batteries permanently would it? Jim


Posted: Tue May 24, 2011 7:59am

Post Subject: Battery or what?

Despite what many people thing starter batteries have a very easy life when compared with typical domestic batteries. They are normally fully charged within half an hour of starting so would stand up far better to charging problems. If your 13 volts after 2 hours of running is correct I think there is a major problem with the charging system PROVIDING the alternator belt is tight (please check - no more then 10mm flex under moderate finger pressure in the centre of the longest run) AND the engine was running at 1500rpm plus. Your alternator should reach a charging voltage of at least 14 volts and 14.2 to 14.4 volts is even better after several hours running. Once you rule out a slipping belt you need to check the voltage at the alternator's main output terminals and again across the battery. There should be a maximum difference of no more then 0.3 volts (less is better) and if you have more you need to look at loose/dirty terminals, undersized wires, worn out charge switch. As a very rough guide the fridge itself will require perhaps 160Ah of battery capacity (using accepted discharges etc.) so if your two batteries are less than this you have the problem. A faulty battery could pull the charging voltage down but as far as 13 volts after 2 hours is very unlikely (as long a those two hours were run at full charging speed). Regrettably you give no details about who tested the alternator, its make and model, how the batteries are wired etc. so I can not give much more help. I strongly suspect the alternator, especially if it was the marina that bench tested it. I think you may need more and detailed guidance for further tests and if so please email me on giving FULL details about your system. Tony Brooks

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