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Ariadne
Ariadne

Posted: Tue May 06, 2014 12:03pm

Post Subject: battery capacity - cruising fridge

Hi, I have 3 x 110ah domestic batteries and am constantly running out of A/c overnight running an ordinary domestic 230v frig on the inverter. Is it worth considering one of these expensive low power fridges - Isotherm? or do I just need more domestic batteries?

Tony-B
Tony-B

Posted: Tue May 06, 2014 6:16pm

Post Subject: battery capacity - cruising fridge

What you need is to first find out why this is happening. Then we can decide on the best course of action. I assume "running out of A/C means the inverter shuts down on low volts and although that usually means the batteries are well discharged it sometimes means that the DC inverter wiring is too thin. If we ignore that for now the other possibilities are that the battery capacity is not large enough and if the batteries are more than a few weeks old they have almost certainly lost capacity. That you are not charging for long enough each day so the batteries are never fully charged, but that quickly leads to damaged batteries with a very much reduced capacity. Next there is the possibility of faulty battery cells self discharging the batteries. Finally, as you say, your bank MIGHT not be large enough. Before we can go any further you need to carry out a power or energy audit for YOUR BOAT and the way you use it. You will find worked examples on (www.tb-training.co.uk/16elect.htm#bmn68). That will tell you how much electricity you are using each day but remember all inverter driven loads have their wattage divided by 10, not 12. From that do the calculations that will give you a fair ideaabout the amount of electcity you need to store and thus the bank size required. If you can not be bothered to do that take the energy audit figure and multiply by 4 but then you will not learn why the calculations are as they are. Now you will know if you need another battery or not. Lastly do the calculations to find out how long you need to run your engine for to recharge the batteries to a tolerable degree. Only after all that can we decide what the next step is. without any information about running times, age of batteries, alternator output and battery voltage to at least one decimal place in the morning and about half an hour after engine shut-down with no large electrical loads on (a lamp or two or a radio will be ok.) I have nothing to base any calculations or diagnosis on. Purely based on your question and my past experience of similar questions in the past my guess is that you are never getting anywhere near close to fully charging your batteries each day and you have sulphated to causing a massive loss of capacity OR the resultant deep discharges has damaged a cell or cells. If they are wet open cell batteries and they are kept topped up then a hydrometer at least two charges after topping up will give a fair idea about the bank condition. Send me the 18 readings split up into three groups of six. You need between 12.7 and 12.8 volts half an hour after stopping charging having applied a small load. If the voltage is lower than 12.25 V before you start to charge the next day you are over-discharging the batteries. Please send me your calculations and the voltage readings/hydrometer readings to Tony@tb-training.co.uk and we can take it from there. Also include the battery type and age. FWIW I run a 12V fridge and Alde central heating overnight without excess discharge and no other problems on a 330Ah bank but I do make sure the bank is properly charged each day.

Tony-B
Tony-B

Posted: Thu May 08, 2014 8:59am

Post Subject: battery capacity - cruising fridge

An update for our readers. I have received a detailed email about this problem and as far I can tell the questioner has, as so many do, fallen foul of an amp hour counter type state of charge instrument. The fridge consumption figures that he has given me certainly seem very high but than could be caused by a lack of ventilation around the fridge or lack of refrigerant gas. Once the actual cause is established then a new A++ rated mains fridge or a 12V one should go a long way to reducing consumption. Of more concern is the state of the batteries after a year or what looks like persistent undercharging and over discharging. I have a horrible feeling a new set of batteries may be required. The thing a lot of boaters should learn from thsi is NEVER to blindly trust an amp hour counter type gauge. Check its veracity against voltage readings (see above) and when fully charged by the charging current. This would ideally be at 1% of the bank size but a cruising boat would probably find it more economic, fuel wise, to stop at 2%. That's between 3.3 and 6.6 amps for a 330 Ah bank. Tony Brooks

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