Acceptable Battery Voltage Drop?

[DennisG01]

Not sure where to post this - go ahead and move it if need be.

I had an issue with (2) 9-month old batteries (1,000cca). The Sub wasn't driven for about a month and the batteries went completely dead. I would've thought they would last longer. So, I charged them up individually - took crossover cable off. Ran the truck for about a week and everything was fine. But, I was still curious if there was an extra draw somewhere so I did both a voltage leak-down test and an amp-draw test.

Here's what I got (I disconnected the crossover cable during the time that I let the truck sit):

1 hour after engine shutdown:
Driver's Batt = 12.83, Passenger's Batt = 12.86

6 hours after shutdown:
Driver's Batt = 12.40, Passenger's Bat = 12.76

26 hours after shutdown:
Driver's Batt = 12.39, Passenger's Batt = 12.68

About 4 1/2 days after shutdown:
Driver's Batt = 12.08, Passenger's Batt = 12.51

I then disconnected the negative cable and put my meter between the cable and negative post. The driver's side initially read .234A, after about 3-4 seconds it dropped to .088A, then after another 3-4 seconds, it settled in at .067A. The passenger's side read 0.000.

Is all this normal? I assume the drop from .234 to .067 is explained by the the ECM powering up, the chime/buzzer coming on then off, etc. so I'm not concerned about that. Anyone else with an OBDII ever done a amp-draw test (especially on the passenger's side with the ECM, radio memory, etc)?

Is the amount of voltage drop over time consistent with the amp-draw (0.0 for passenger's side and .067 driver's)?

[DieselDavy]

Hi Dennis,
As an electrical engineer, I should be able to give a good answer for how much voltage drop can be expected, but that's not something I've looked into before. Hopefully, one of the GM techs can chime in here. I will say that I've just had a similar incident occur this winter. I let my 2d Tahoe sit all winter, after about 3 months I checked and had 0.037V on each battery!
The batteries are quite old so I'm not upset, but when it gets warmer, I'll have to look into it! Either a cell shorted and took both batteries down or I've got a load I didn't know about!
Good luck!
Dave
DieselDavy not DieselDavey

[Kenneth]

I recently took an automotive elctrical class at college and my books say "the total current draw of key-off loads is usually less than 30mA (.030A). It isnormal for some vehicles to have higher than 30mA draw forup to an hour agter the ignition switch is turned off. This allows the computers to perform their administrative tasks." My instructors said (who all have field experience) 20-45 mA is acceptable.

I would say you definitely have a parasidic drain. Visually check your glovebox and underhood lights are not staying on. If they are not the problem, set up your ammeter again and start pulling fuses. Watch your ammeter. When the drain decreases, the circuit protected by the fuse you removed last is the source of the problem.

I hope you find the problem.

[DennisG01]

Haven't pulled fuses, yet, but I will do that.

If anyone has a few minutes, can you check what type of draw you have (OBDII)?

Here's another thing I was wondering about. I understand things like AV=W, but how does .067A convert into "how much it draws down a battery over time"? I mean, is .067 the amount of amps per hour? That would mean over the course of a month, I would lose about 48 amps. Can this .067 be converted to loss of volts/hour?

[rhsub]

This is a quote from GM service manual
To calcuate acceptable parasitic load
Note the reserve capacity of the battery, if the vehicle has a diesel engine with two batteries, add the reserve capacities together. If equipped with an aux bat use only main bat. (aux bat is defined as a bat that is only in the system when the engine is running, not used on diesels)
Divide this number by 4
Compare to ammeter reading
Current drain should not exceed this number
Example: reserve cap 100 minutes, current draw not to exceed 25 milliamps (0.025 amps)
I remember checking mine about a year ago and I think it was close to what you have
The reason the drivers side bat is dropping quicker than the pass side when the crossover cable is disconnected is that the only load connected to pass side is the starter whereas all the computers/radio etc are connected to the drivers side battery
Let us know how you make out and if you need more info
If i get a chance I will recheck my parasitic load
good luck
Ron

[DennisG01]

My reserve cap is 115 per battery, so that puts it at .058 as acceptable. Close.

I pulled fuses one at a time, replacing after pulling:

On the side of the dash:
--- #19 (Radio Batt) dropped it by .049

Under the hood:
--- IGN B dropped it by .014
--- BATT dropped it by .053 (which also took #19, above, out of the loop. meaning with BATT pulled, pulling #19 did nothing - it was dead).

With both BATT & IGN B out, the reading was .001.

Other various fuses dropped the readout by .001 - I didn't bother making a note of those.

I do have an aftermarket radio which may be pulling a bit more than stock. I'll be curious to see what some of you are pulling.

Thanks so far, everyone!

[DennisG01]

Bump.

Any other thoughts? I know it's not a common problem for most, but it could be useful in the future (for others - as well as me! :D - to use as diagnostics) if we had a "general consensus" of normal amperage draw. Probably'd need about a half dozen people to post their findings.

For anyone wanting to try this that hasn't done it before (and assuming you have a basically stock electrical setup):

1) Be sure all doors closed, interior/exterior lights off/any accessories should be off - be sure the underhood light is disconnected. You may have to wait a bit for the interior lights to go off if you just opened the door.

2) Disconnect the battery crossover cable from one of the batteries (wrap it in a towel so it doesn't contact anything metal)

3) Set your DVM to "Amps" - 1 Amp should be sufficient. Most $10 DVM's nowadays come with a 10 Amp setting - that'll be fine, too.

4) Remove the negative cable from Driver's side battery, connect your DVM between negative post and cable. Give it about 20 seconds for the ECM to cycle, record the final reading.

5) Reconnect negative cable and repeat #4 for Passenger's battery. This reading should be accurate almost immediately as nothing on this side will be "cycling".

This should take all of about 5 minutes - the hardest part is removing the battery cables.

KEEP THE KEY OUT OF THE IGNITION DURING THIS PROCEDURE

[Robyn]

Parasitic load can vary a bunch depending on the rig and the equipment thats on it.
The ECM probably has power to keep some stuff alive constantly.
The stereo (Factory) with LED clock and such is powered up to keep the memory alive.

Depending there can be other items that draw some current but I am going to say that what you are seeing is about right.

You should be able to leave the rig standing for a month and be able to start right up without issue. (good batteries)

Robyn

[DennisG01]

You should be able to leave the rig standing for a month and be able to start right up without issue. (good batteries)

Robyn

That's sort of what I thought - especially since I've got two batteries. I mean, I can't believe that in a month's time 70 milliamps will drop 2 batteries to the point where the starter won't even click. Just wanted to be sure before I went back to the parts store where I bought 'em to have them checked. I don't have a load tester, myself.