Engine battery voltage fluctuation

[moe58]

Hi  I am wondering if it is normal for the engine battery voltage of a 2 year old AGM battery to be in the 12.5V-13.0V range when the engine has not been running for several days or more. I also noticed that when I turn the engine on to idling speed the engine battery voltage fluctuates between 14.1V and 14.4V, but when I increase the rpm the fluctuation decreases in range and frequency. Is that also normal? Thanks

[sitara]

Where and how are you measuring your battery voltage? Best measurement is at the battery posts with a multimeter, and jiggle the probes to make sure you have a good contact. With everything off and a battery that is fully charged and has been sitting idle for several days I would expect a steady reading of 12.8 volts, maybe a little lower depending on battery condition. The fluctuation you are seeing could be due to poor connections to the battery.

[moe58]

Thanks Sitara. Actually I wasn't taking any voltage reading, just reading the screen display on the Scheiber DC panel.

[sitara]

Hi Moe, the Schieber panel readings can be suspect, there are plenty of threads on them, although the panel on my boat has been pretty good.

[rene460]

Hi Moe, the 14 V + when the engine is running just indicates that your alternator on the engine is working.  It has to raise the voltage somewhat to drive the charge into the battery.  The reading will drop back to something in the 13 V range when the battery is charged, then back to the 12 V range when the engine is stopped.  Sometimes called a surface charge effect.

To get a voltage reading that is useful in indicating battery condition, you then need to leave the battery isolated so no charging and no load for a while.  I suggest 15 minutes is absolute minimum but an hour or two is better.  And you need to use a voltmeter on the actual battery terminals for a repeatable consistent reading.  Always worth having one on board for trouble shooting any electrical problems.

But from what what you have said so far it all looks reasonable.  

It is always worth keeping an eye on the voltage to pick up on battery or charging problems, and so long as you are mindful of the accuracy, the panel is a convenient way to do a frequent quick check.  The reading may vary a bit with battery load so take this into account with your observations.  Get to know what your voltage looks like after a period of isolation, like before you switch on the battery charger or engine when arriving at the boat.  And what you see with various loads, especially big loads like the fridge.  Then a deviation will help you get to it quicker, before any real problems.

rene460

[moe58]

Hello rene460  Thanks for the detailed reply. What I noticed is that the engine battery voltage is usually 12.8V when I check it after several days of not running the engine. Based on your response, this is normal. FYI; I never switch off the battery charger. At the dock my boat is connected to shore power for 6 months during late fall and winter as I run a space heater and dehumidifier on AC power when the boat is not in use. In spring and summer I don't connect shore power at all and rely on 300W solar panels only. The fridge is always turned off when the boat is not in use.

[panoramix]

To give you some idea of battery voltage

Battery condition after minimal 3 hours rest
10% 11,6V
20% 11,9V
30% 12,0V
40% 12,1V
50% 12,2V
60% 12,3V
70% 12,4V
80% 12,5V
90% 12,6V
100% 12,7V