Drop in battery voltage

[Guest]

Is a drop in voltage of 1.5 volts reasonable for batteries left on board for two weeks. Both mine measured just under 14 volts and fell to just under 12.5 volts.

The isolating switch was in the off position and as far as I can ascertain nothing is connected directly to either battery.

Thank in advance

Norman

 

[Guest]

14V will probably have been the voltage the battery was loaded with. How soon after switching the engine off did you do these measurements? My experience is that a battery will never be able to maintain 14V for more than an hour! I believe 12.8 volts to be the reference for a completely full battery, measured before or long after using the engine or other loading apparatus.

 

[Guest]

Martijn is correct. The 14 volt measurement was not realistic. After charging ceases you have what is often called a "float charge" which does not represent any real energy in the battery and will disappear very rapidly as soon as a load is placed on it.

This float charge is over and above the true charge level of the battery and masks getting a true reading. If it were fully charged, then the figure of 12.8 as the true charged voltage would be a reasonable assumption in which case you only dropped 0.3 volts. Depending on the battery age and condition, this may be excessive but not unusual.

However, the 12.8 starting voltage is only a guess depending on how long it had been on charge and the progress of the charging. The battery could have shown 14.0 volts after charging ceased (or while in progress) but the battery may only have been 12.5, in which case you didn't drop any at all.

Next time, make the measurement after charging has ceased, and after the voltage has ceased to drop visibly with a small load on it - say a cabin light. This reading will be a much more realistic starting point for determining self discharge.

 

[Guest]

just a couple of points that might be worth looking into,

check that the top of the battery is clean especially around the terminal posts. If the battery top is dirty it is possible to get a slight leakage between the +ve & -ve terminals.

also make 100% sure that there are no other loads attached ie a automatic bilge pump or a battery monitor. These may consume very little, but it will make a noticable difference.

As batteries age the internal resistance does drop slightly causing self discharge. it is not normally a problem apart from a slight reduction of capacity. I wouldn't suggest that replacement is required at this stage unless you are experiencing starting problems, and even then I would suggest that a discharge test would be worthwhile. (ask your local car battery centre, it takes about 20 seconds and they usually do it for free).

What might be an option is to install a small solar panel just sufficient (maybe as little as 2w) to overcome the self discharge. there are some available that are designed specifically for this job made by Exide. I know of a website with details on, but I've been asked not to promote it!

kev

 

[Guest]

Thanks for reply which makes sense.

I will take up your suggestion next time I am at the boat.

 

[Guest]

Thanks for reply.

I will try Colin,s suggestion first.

Hope you don't get any more hassle from the net police.