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Posted: Sun Sep 04, 2011 3:16pm

Post Subject: Battery capacity

Tony, I have 3x110 a/hr deep cycle lead acid batteries as a domestic bank. Although 2 years old they have been constantly on float charge (sterling 4 stage charger) when our boat is not in use. They are checked and topped up regularly, never needing much additional water. We cruise on average every 6 weeks for a week to 10 days. When cruising the batteries are charged daily when running the engine (Sterling advanced alternator regulator) They come back from fast charge and achieve float charge within 1 - 1.5 hours. The alternator fitted to my Beta 38 is rated at 100amp. The main power consumption is a 12v fridge. We view the 240 volt TV for a maximum of 2 hours in any given day, it is rated at 45w. When cruising recently we broke this habit and remianed at a mooring for 26 hours without charging the battery bank. The TV was on for a total of 7 hours, other than that the only power consumed was by the fridge, and a little lighting in the evening. I was suprised when the inverter low/high battery voltage alarm sounded (according to the Sterling manual this is set at 10.3v). The TV was immmediately switched off and th engine ran to recharge the batteries, again float voltage was reached in just around 1 hour. I have since tested both the domestic and starter batteries voltages under no load approx. 1.5 hours after charging, and obtained a voltage reading of 13.1 volts, this would appear to indicate that they are fully charged, however how can I determine the actual capacity in these batteries?

Tony-B...
Tony-B...

Posted: Sun Sep 04, 2011 5:04pm

Post Subject: Battery capacity

I would first recharge them and let them stand overnight. If you got 13.1 volts off charge after 1.5 hours they were still showing signs of surface charge. If they then show 12.7 volts or above you can consider them as fully charged as it is reasonable to expect. However that tells you nothing about their capacity so get hold of a float type hydrometer and measure the relative density of each cell (after standing over night the battery will be cold). Expect between 1.28 and 1.3 on each cell. The lower they are from 1.28 the more the battery is sulphated and I suspect that or one of the old batteries has got an internal short as the problem. You may well have less than 50% of the original capacity left. If any cell differs from any other by more than 0.03 consider the battery faulty. If those tests show the batteries to be OK then inspect the ends of each battery, especially the old ones. The more swollen they are the more sulphated they are. The only way a boater can get some idea about battery capacity is to charge them for a long time on float - say 24 hours - to ensure they are as fully charged as possible. Then put a known load across them (say a 55 watt halogen bulb giving a discharge current of about 4.5 amps) and keep measuring the battery voltage. When the voltage has fallen to between 10.5 and 11 volts multiply the time taken for it to fall to that level by the current and that will give you a rough guide to the available capacity. Tony Brooks

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Posted: Sun Sep 04, 2011 6:49pm

Post Subject: Battery capacity

Tony, Thanks, I will invest in a hydrometer and test as you suggest. By the way all 3 batteries are the same age, fitted new 2 years ago. Duncan

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Posted: Wed Sep 07, 2011 12:48pm

Post Subject: Battery capacity

Tony, Can you confirm or otherwise that leaving these type of batteries on "float" whilst away from the boat for up to 8 weeks is good practise. Thanks Duncan

Tony-B...
Tony-B...

Posted: Wed Sep 07, 2011 1:33pm

Post Subject: Battery capacity

I am afraid there is not a straight forward answer. If the charger float at less than about 13.7 volts then it is probably OK but if above them there is a possibility of plate erosion but this depends on things like battery construction and materials. I would expect a Sterling charger to be fine. Even if it was floating a bit higher you have to weigh up the balance between shorter battery life caused by plate erosion and the longer life gained by fully charging the battery so minimising sulphation. One thing to bear in mind is that if a cell goes faulty and starts gassing on float you have a potential problem. It will be fine as long as you keep that cell topped up but if it goes dry there is a chance that eventually a separator will fail or the build up of plate materials in the base of the battery will be such that a spark occurs inside the cell causing an explosion. This is rare, especially if you keep an eye on the electrolyte level. ON balance I think it would be better to leave them on float than to leave them partially discharged. Tony Brooks

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Posted: Thu Sep 08, 2011 1:14pm

Post Subject: Battery capacity

Tony, Thanks, it is all about risk assessment in the final analysis. At least I have the facts and possibilities to base a decision on. Duncan

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