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Posted: Wed Apr 15, 2009 2:12pm

Post Subject: What size battery charger?

I want to fit a mains battery charger and would like some advise please on the rated size that I would require. My existing set up consists of 2x110Ah wet lead/acid cabin batteries in parallel and 1x 110Ah wet lead/acid engine battery. All three are less than six months old. I expect for the next 5/6 years to have a mooring with a 240vAC shore supply and my use will be limited cruising ie long weekends/weeks holiday. After that time hopefully extended/continuous cruising. My expected power consumption would be for one person and during the first 5/6 years, morning and evening for 6 days a week plus one full day - lighting (although 240v lights available in the galley, bedroom and saloon, water pump, hot water/central heating(gas fired). The fridge needs replacing and intend to use a 240v unit run via an inverter when not connected to the shore supply at least for the first 5/6 years. NO tv, dvd. Maybe a radio/cd system. The battery chargers that I have seen (Sterling) range from 10 to 60 amp. The charger maybe connected continously whilst hooked up to the shore supply (is this OK or would being connected only when the batteries need charging be best?)and I have it in the back of my mind that batteries should not be charged at too high a rate. I would like to build in some allowance for future upgrades ie more cabin batteries. Thank you Brian


Posted: Wed Apr 15, 2009 2:38pm

Post Subject: What size battery charger?

Dear Brian, It is no good just listing the electrical items you intend to use and saying "one person". You need to do a proper power or energy audit to see how much you will take out of your batteries and even if they have enough capacity. You will find details of how to do this in the course notes on my website ( Whilst it is true the slower you recharge lead acid batteries the more effective that charge will be it is also true that if you allow the charge to take too long the batteries will sulphate and suffer reduced capacity and life. You need a multi-stage charger that drops to a float charge when it decides the batteries are fully charged so it can then remain connected to shorepower all the time. Make sure it will also operate as a power pack. That is work correctly whilst your domestic equipment is drawing current from the batteries. If it does that then you will take very little out of your batteries because most of the time the charger will be supplying the lights and DVD etc. As long as you can leave it connected 24/7 I think a 20 or 30 amp charge will do the job. Actually I think a 10 amp one would do it, but I bet you start to want more electricity as you go on. If you are not going to leave the charger on 24/7 then a battery monitoring system like a Smartguage and the 60 amp or larger charger may be better. Engine batteries tend to be pretty much fully charged within an hour or so of starting up so I would just ignore the engine battery and simply put a jump lead across the domestic and engine battery positive terminals every few weeks overnight when the engine is not in use, but you could get a dual output charger of a slightly higher output. Please note that there is no hard and fast ratio of battery charger size to bank capacity, It all depends upon how you use the equipment etc. Personally I would not be worrying about 5 or 6 years time, you will almost certainly have replaced the batteries at least once in that time. Tony Brooks

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