# Lead-acid vehicle battery discharge time

Discussion in 'Power Electronics' started by flippineck, Feb 11, 2018.

1. ### flippineck

269
9
Sep 8, 2013
Assuming a car battery starts out at time zero, three-quarters full and 10% into it's useful lifespan, how long would you expect it to last, switched off with the key out, before it's no longer able to supply enough current to turn the starter motor?

Drain with ignition off - 8 mA,

Battery rated capacity - 68 Ah

Ambient average temperature - 6 degrees Centigrade

Engine size - 3.5L Petrol / Carburettors

Last edited: Feb 11, 2018
2. ### duke37

5,329
750
Jan 9, 2011
I think this is impossible to calculate. A petrol engine needs a certain voltage for the ignition, the voltage is lowest at TDC when the spark is required, this is alleviated if a capacitor discharge ignition is fitted since the capacitor is charged before the maximum current drain.

I think that your time is way out, I have heard of a combine harvester (diesel) being started after many months.
If you discharge 8Ah, then 8mA would take 1000h or 6 weeks.
A short charge will redistribute the contents of the electrolyte and will reduce the internal resistance.

Do not leave a lead/acid battery discharged, the plates will sulphate and will not recharge.

3. ### flippineck

269
9
Sep 8, 2013
You must have caught my post before I'd edited out the time calculation, which I'd erroneously had at 4 days I think.. yep, I rechecked my maths & came up with 68 Ah / 0.008 A = 8500h or approx a year.

My battery went flat after one day of non use & having measured what's being taken with the key out of the ignition, it's only 8mA.

It's a very new battery so I'm now wondering if the alternator's not charging and my guesstimated 'three quarters full to begin with' was wildly optimistic. Prior to fitting the new battery I'd been propping everything up with a solar panel that you sit in the windshield & plug into the cigarette lighter to keep the battery topped up. I kept using this panel after fitting the new battery until 3 lads broke in and stole the panel.. it was not long after that the battery went flat in a day.

Going to charge it up halfway, start the truck & then measure if the alternator is sending any current toward the battery.

4. ### kellys_eye

4,277
1,146
Jun 25, 2010
The manufacturers will have details of self-discharge, discharge capability/temperature etc.

5. ### duke37

5,329
750
Jan 9, 2011
The voltage to charge the battery should lie between 13.8 and 14.2V. 14V is a good value to aim for.
My neice had a low charging voltage and changed the alternator with no improvement. The problem was a poor, rusty earth connection.
If you get the voltage up at the battery terminals you will not need to measure the current.

6. ### flippineck

269
9
Sep 8, 2013
Just got me thinking there.. I recently changed the starter motor. When doing so I noticed an extremely heavy black-sleeved cable terminated in a big fat lug, under one of the starter mounting bolts.

Without really giving it much consideration I just thought "big powerful motor, big powerful earth" and bolted it down tight. I gave the cable a little wiggle after tightening just to check it was tight and something didn't feel right, like the conductor was loose inside the lug crimp. Wiggled again and decided "ahh, it's just the sleeve loose" and concluded the job.

Now I'm thinking hang on, is that big fat black cable, the main / only connection between battery negative and vehicle?

Maybe I need to get back under there and re-terminate that lug.

7. ### Hopup

253
35
Jul 5, 2015
To improve the charging connect big cable from alternator to battery and ground the alternator, both using big cable.

8. ### flippineck

269
9
Sep 8, 2013
Not sure by any means but I'm guessing at the issue being 'sulphation'.. ?

After coming to the end of the line cleaning and tightening terminals, insulating loose ends of previously chopped off loom etc, I took the battery off the vehicle and gave it a good long charge at 14.7V followed by a heavy discharge to virtually flat using an old truck starter motor. Repeated the process a few times & each time the charging time to reach the charger's idea of 'full voltage' (IIRC 13.6V) got much much longer. First charge took about 20 mins, last charge took a day and a half.

Everything now operates absolutely fine.

I was suprised because I haven't had this battery long from brand new - but on the downside I don't know how long it sat on the parts shop shelf for before I got it, and also it's been used very sporadically on an old 4x4 used as a farm tractor. So it's had plenty of long sits without charging, and I guess what charging it has had has been only brief (After engine start the jobs it usually does are over in no more than half an hour.