View Full Version : What should voltage be with engine off and while running?

06-02-2014, 01:51 PM
Had a dead battery this weekend, got it charged but was nervous all day. While running it was at 14, but after sitting awhile, before starting was 11.5-12.5. Never hot an alarm.

06-02-2014, 02:18 PM
When running you are measuring the output of the alternator, which should be 13.5+, so you are good there. With engine off you are measuring battery voltage, which should be at least 12-12.5, so 11.5-12.5 is a pretty uncertain range. Was it at 12.5 when you first shutdown, then went down to 11.5 after you listened to tunes for an hour or two? If so then you are probably good, but should also seriously consider a second battery with perko switch (11.5 is dangerously close to not starting an engine). If it was 11.5 when you first shutdown then I would replace the battery.

06-02-2014, 03:01 PM
It was 12-12-5 after shut down, but I never rain radio or anything, and it still dropped with time. It would start fine when off for a few hours, but overnight it went down to around 11, and wouldn't start. Battery is new, got it a month ago, ran it, then had it on trickle until this weekend, and was completely dead, wouldn't even click. I guess the trickle charger could be bad, as it was reading fine the entire time.

06-02-2014, 03:12 PM
I had this happen to my battery last year. For a verification, I removed the battery from the boat and took a number of voltage readings. I would lose approxiatemly 1 volt a day. I took it to Autozone to have the battery checked out by their machine and it passed the test. After some time stamp pictures of voltage readings, they agreed to replace the battery.

06-02-2014, 03:49 PM
Unhook the battery and see if it will hold the charge longer. You may have a parasitic power draw some where.

06-03-2014, 05:11 PM
Check it this morning, 12.2. The last time I ran it was Sunday evening about 6.

06-03-2014, 05:45 PM
12.6 is a fully charged batt. I have a little over 14v with a new alt at idle.

06-06-2014, 01:44 PM
Im having a similar challenge with batteries- Put in new one- killed it via the new clock I put in- (not smart) and didnt run it for a week. never was able to get it to hold charge again- took it back to auto zone - they replaced it- New one 12.5 when new- ran it today several starts and 12.4 after the day of running- No radio. So 1) if I put a multimeter on the battery while running I should be able to measure the alternator output? 13-14 volts y/n? My voltmeter stays at 11.0. sounding like alternator? thoughts-

06-06-2014, 02:20 PM
First, get a good DVOM (digital Volt/Ohm meter). You do not need to spend a lot of money, but they are worth their weight in gold IF you know what they are telling you.

12V. Batteries: Knowing what the static battery voltage is important. A fully charged battery will read 12.6 v. or higher. The # after the decimal point IS important. For every .2 volt subtracted from the 12.6, you can guestimate your battey charge has lost about 25%. I.e, 12.4 = approx 75% charge.

Testing with a DVOM. Test the battery static voltage at the actual battery posts (not the post clamps). Should be 12.4 v or higher. Crank engine and observe meter. Should not drop below approx. 9.2 v. If it does, then either battery or excessive draw is happening. If meter reads 10 to 12 volts while cranking (and it is cranking slow) then you have a resistance problem.

Once started, meter should read at least 1 v. over your static measurement. I.e, static of 12.4, start and run would then be expected to be approx. 13.4 v. This indicates the alternator is putting out enough to charge the battery.

The key is to test at the actual posts of the battery. If you move the meter leads to the clamps and get different readings, well you then just discovered a problem.