What's an acceptable daily voltage drop for a battery if the truck is just sitting?

[MarcusOReallyus]

A volt a day keeps the diode away?

I'm chasing a drain on my batteries, which were recently installed. If I don't run the truck for a few days, they get pretty weak.

I unplugged the alternators on Saturday afternoon, so see how well the batteries would hold a charge. I left everything else connected normally.

When I unplugged the alts, I had 13.2 on the front, and 12.9 on the rear.

Today, I have 12.7 on both. That's not much of a drop.


Here's the weird part: I checked for current draw with the alts unplugged, and there is zero drain. I then plugged in the alts, and there is... zero drain.

Okay, maybe one of the kids had turned on the blackout lights, which would certainly kill a set of batteries over a few days.

So, at this point my question is about the discharge of a few tenths in a few days. Problem? Or not?

 

[Scar59]

Something had to be left on. These truck are designed to sit for long period with zero drain. I have several that sit for weeks and the voltage drain is not measurable. They fire right up on the first rotation of the crank. Wire it a 200 amp keyed master switch for the battery and isolate the batts when not in use. Kids will be kids, again and again.

 

[Matt Kahle]

Yeah I had the same problem some one left the black out lights on, and it drained the batteries.

 

[glassk]

A problem in the vehicle's wiring or electrical system is usually the cause of a drain on the battery that discharges it overnight. The clue to self discharge, which takes longer, is the presence of an oily substance on the top of the battery. That is acid that has boiled out of the battery and condensed on the case. That acid is conductive and will let current flow from one battery post to the other. You can prove this with a voltmeter.

http://randysrepairshop.net/basics-of-batteries.html

 

[FloridaAKM]

If you lose a volt a day, there is something seriously wrong in the battery/wiring system. The fact that you even have such a short term problem might warrent a main battery cutoff switch till you can find the real culprit. Make sure that the new main cutoff is keyed you you are also adding a security feature & not just wasting money chasing a small current drain.

 

[MarkM]

You need to use your multimeter on the amp scale to see what’s going on. Voltage drop only give you the coarsest idea there is a problem and it only will show over time.

As others have said chances are something was left on or you have corrosion built up on a terminal somewhere and it is flowing power to ground. To check for a parasitic draw using a multimeter set the meter for DC amps at the 10 amp range and disconnect the pos power cable for the cable terminal for the truck and attach the pos of the meter to the pos battery terminal and the neg lead to the pos cable clamp. This will show you if the problem you have in the the trucks wiring. If you have a big draw it will show up on the meter on the 10 amp setting. If you do have say a 2 amp draw then start pulling fuses or opening breakers one at a time until you find out which circuit is the problem. If you don't get any movement on the meter then I would focus on the batteries. If you have a small defection then switch the meter to a lower scale. Cars will always have a little parasitic draw but military trucks should not. Clocks, alarms and what not will cause a small draw on cars.

If you believe the problem may be in the batteries I would disconnect the batteries from each other and the truck and see how they hold a charge over a few days. If you have on battery on it's way out it could pull the other one down with it when connected together. When using multiple batteries in series they must always be replaced in pairs with the exact same battery and around the same date code so they both have the same potential. This keeps one battery from over charging trying to bring the lesser potential If you have a old battery and corrosion has built at the bottom of the cells high enough to touch the plates then it will now have a current flow to ground.

Hope this helps

Mark

 

[MarcusOReallyus]

You need to use your multimeter on the amp scale to see whats going on. Voltage drop only give you the coarsest idea there is a problem and it only will show over time.

Done:

Here's the weird part: I checked for current draw with the alts unplugged, and there is zero drain. I then plugged in the alts, and there is... zero drain.

As others have said chances are something was left on or you have corrosion built up on a terminal somewhere

Nope. The batteries are clean and new.

If you believe the problem may be in the batteries I would disconnect the batteries from each other and the truck and see how they hold a charge over a few days.

That's what I'm doing now.

If you lose a volt a day,

It's not a volt a day. That was a joke. Here are the numbers:

I unplugged the alternators on Saturday afternoon ... I had 13.2 on the front, and 12.9 on the rear.

Today, I have 12.7 on both. That's not much of a drop.

In fact, it's 0.5 and 0.2, respectively, from Saturday to Tuesday.


Which brings me back to this:

So, at this point my question is about the discharge of a few tenths in a few days. Problem? Or not?

 

[phil2968]

Remember that a fully charged battery that has rested unused will read 12.5 volts. If you check battery voltage after shutdown it will be higher. Alternators typically put out 13.8 volts. Check for amp draw at the battery terminals with an amp meter. The only way to find it.

 

[MarcusOReallyus]

Check for amp draw at the battery terminals with an amp meter. The only way to find it.

*sigh*

You need to use your multimeter on the amp scale to see what’s going on. Voltage drop only give you the coarsest idea there is a problem and it only will show over time.

As others have said chances are something was left on or you have corrosion built up on a terminal somewhere and it is flowing power to ground. To check for a parasitic draw using a multimeter set the meter for DC amps at the 10 amp range and disconnect the pos power cable for the cable terminal for the truck and attach the pos of the meter to the pos battery terminal and the neg lead to the pos cable clamp.

Done:

Here's the weird part: I checked for current draw with the alts unplugged, and there is zero drain. I then plugged in the alts, and there is... zero drain.

I have checked for amp draw at the battery terminals with an amp meter. There is zero draw.


Which, again, brings me back to this:

So, at this point my question is about the discharge of a few tenths in a few days. Problem? Or not?

 

[phil2968]

What is your resting voltage? Let the batteries set for a day and monitor the voltage every day. I think you have a weak or dragging stater. Check amp draw when cranking.