Don Terrill’s Speed-Talk

Apologies if this is the wrong place to put this...

I’m having an issue with my 1999 C2500 suburban. With a fully charged battery, if it sits for 48 hours there isn’t enough juice left in the battery to start the truck. It will just turn over very slowly for a few revolutions and then just click. With a hot battery it starts very easily.

With the ground cable disconnected and my digital volt meter set on 10 amps, and connected between the battery post and the disconnected ground cable, I see an initial draw of 110-200 milliamps for about a minute, then it drops to 20 mAmps and stays there even after monitoring for 20 minutes. From what I’ve read anything under around 50 mA is good.

Battery is 18 months old, fairly new starter, new positive and ground cables. Nothing is on that I can see with the key off. It does have an aftermarket radio that only works with the key on, no amps or anything additional there.

I’m stumped at this point. The battery had been tested multiple times at a couple different places and it always comes back good. Any ideas???

If you disconnect the battery for 2 days, does the voltage on it drop? I've seen quite a few batteries test good only to go dead after a few days.

20mA is good. No problem there, unless one of your kids is slipping his phone charger in the accessory port without you knowing it.

How many times has the battery been allowed to run flat enough that it won't start?

Get the battery tested with a device that can tell you the actual capacity, in AH or RC. Any chain parts store should have a Midtronics or equivalent tester that can do this. A dumb tester that only shows 'good' is useless, as the battery will test 'good' since it's able to start the engine when it is decently charged.

Might sound like a dumb question, but does the battery hold charge if it sits for 48 hours with the cables disconnected?


Either the draw is greater than what you've measured, or the battery capacity is way low.

Look up TSB #02-06-03-010A: Info - Battery Parasitic Drain

The recommendation for maximum parasitic drain is around 30 mA (0.030 amp). A typical drain today actually falls into the 7-12 mA range, even though some vehicles do approach the maximum. Multiply the drain (in amps) by the time (in hours) the battery sits without being recharged. The result is the amount of AH consumed by the parasitic drain. The actual drain may be small, but over time the battery grows steadily weaker.

When disconnected, the battery voltage only drops a very tiny amount. I don’t remember the exact numbers but it was very small.

Unfortunately the battery has ran down quite a number of times, as I don’t drive this thing hardly at all. I’m probably going to put it up for sale for this reason...sitting seems to be a lot harder on them than actually driving them.

The battery capacity issue is what seems to be happening. I guess I need to find some place with a proper battery tester.

Thanks!

My 2006 Envoy will sit for 3 weeks without being touched and still start - though it's clearly right at the limit. The battery is a no-name of unknown age that was in it when I bought the truck at auction 3 years ago. You have a really serious problem if it won't last 3 days.

I have a Trailblazer that would go about a month before failing to crank the engine--and that was with driving nearly every day. Charge the battery overnight, it'd go about another month. New Optima battery. I tested parasitic draw, alternator output volts/amps, starter draw volts/amps, cleaned the battery case so it couldn't self-discharge through any conductive slime (there really wasn't any slime build-up) and polished the battery cable ends. Even looked for voltage drop between alternator and battery--tested fine. I put an alternator and starter in "just because" they had over 150K miles, maybe closer to 200K on the originals. No change.

In the end...I changed batteries and all the problems vanished. And they've stayed vanished for four years or more. I will have a difficult time finding the courage to buy another Optima battery.

When I was 16 I bought a new battery for my car. It worked fine all day but I would have to jump start it every morning. The people I bought it from kept telling me it wasn't the battery, they were wrong.

An internal self-discharge (wow, that sounds dirty) is tricky to spot with a tester. When fully/freshly charged the battery will have good capacity. But, the way to pin that down is to check if it still goes flat over X amount of time with the cables disconnected.

I had an Optima that decided one day to silently turn itself into a 5-cell 10.5 volt battery. Bad things happen when you feed a 5-cell battery 14 volts at ~100 amps from a properly functioning charging system.

your specification on parasitic draw is correct, 20 mAmps is very normal. When you watch the amperage drop after a few minutes, you are seeing the modules on the vehicle go to sleep. Lots of modules have keep alive memory, so that is why the draw never goes completely away, Keep alive memory is very important, for leaned fuel trims, transmission shift points and line pressure, idle control, fuel trims, if you have the fancy options, things like seat and mirror positions, etc. Any normal modern vehicle will discharge a good battery in 15 to 20 days. I would be testing your battery. The old load tester is getting obsolete, most technicians now use a digital conductance tester, which is more accurate.

The simplest way to proceed with this is to take a known good battery of similar capacity and substitute it for a few days

Swap yours onto the donor vehicle as a double blind test, if possible

Then you'll have a direction

Travis,
Take a look at the mirror lights in your sunvisors. Make sure they are not staying on. My 99 Tahoe was doing almost exactly the same thing and I finally figured it out after walking out into the garage with the lights out and caught a glimpse of it

It doesn't matter if there is a light staying on if it's drawing only 20mA. 20mA will not kill a good battery in 48 hours.

It is one of 3 things.

One, the draw is actually higher than what was measured.

Two, some module is waking up after more than 20 minutes and drawing far more than 20mA.

Three, the battery is so damaged/worn out and the capacity is so low that it can't put out 20mA for 48 hours.

I decided to try testing the battery again, as I couldn’t remember any exact numbers from testing it last time.

Charged the battery for a while, got it up to 12.43V. Left it in the garage, unhooked from the charger...24 hours later it was at 12.00V. 48 hours later it was at 11.56V. Took it back to the O’Reillys where I bought it, and guess what? It tested OK But since the voltage was low they offered to charge it again for free. Argued with the manager about it...they would not replace it. Went to Autozone...it failed immediately on their tester. Lost my ass on the core charge but don’t even care at this point. So...we’ll see if this fixes it.

And apparently batteries are made of gold now (it does say “Gold” on it). $178 for a standard 78 series battery???

Travis, keep this place in mind Mr Battery in Garland Tx. There at Garland Rd & Jupiter. I had the same issue if I drove the car every day it would start but real lazy. Let it sit 2 days and would barley turn over then just click.
went to Mr Battery got a brand new no name any group size $45. we'll see how long it lasts????

jed wrote: ↑Tue Oct 20, 2020 6:00 pm Travis, keep this place in mind Mr Battery in Garland Tx. There at Garland Rd & Jupiter. I had the same issue if I drove the car every day it would start but real lazy. Let it sit 2 days and would barley turn over then just click.
went to Mr Battery got a brand new no name any group size $45. we'll see how long it lasts????

That’s good to know...I’ve never heard of the place.