bad battery?
Thread Starter
Advanced
Joined: Dec 2017
Posts: 52
Likes: 21
From: Modesto, Ca
Finalist 2022 C1 of the Year - Unmodified
bad battery?
My battery seems to be weak. (not holding a charge). I measured the voltage at various times and watched it lower.
Day one... full charge 12.73v
Day 2.....12.37
Day 8.... 12.29
Day 12 ... 12.25
Day 18..... 12.21
the car was not started/run the whole time while testing. battery drain with key off was 0.014v
Battery is less than 2 yrs old.
Cables are tight and clean
engine is 350, with electric fan radiator. (big draw)
Day one... full charge 12.73v
Day 2.....12.37
Day 8.... 12.29
Day 12 ... 12.25
Day 18..... 12.21
the car was not started/run the whole time while testing. battery drain with key off was 0.014v
Battery is less than 2 yrs old.
Cables are tight and clean
engine is 350, with electric fan radiator. (big draw)
Le Mans Master
Joined: Nov 2015
Posts: 7,479
Likes: 2,650
From: Really Central IL Illinois
I have found that a new acid core battery is about 12.6vdc, fully charged 'rest voltage'. When you put a charger on the battery, it may raise the voltage to 13vdc or more. It will discharge over the next 24 or 48 hours at a fairly fast rate until it returns to the 'rest voltage' of 12.6vdc +/-, but then it will loose voltage at a slower rate. Some say a good acid core battery may loose 3 - 5%/month which would be about .6vdc or less a day. Li-Ion batteries seem to discharge at a faster rate, up to 10% /month. You might want to GOOGLE your battery manufacturer's web site for the best answer.
I would assume your rest voltage is your day 2 reading if the car has not been driven. However, 12.37vdc would be low for a new battery.
If you have modern accessories, like a radio or dash cam, this will of course discharge the battery faster depending on the device. Some devices can sap a battery in just a couple of days if not charged.
I have had batteries discharge to less than 10vdc and then they would not hold a charge.
I would assume your rest voltage is your day 2 reading if the car has not been driven. However, 12.37vdc would be low for a new battery.
If you have modern accessories, like a radio or dash cam, this will of course discharge the battery faster depending on the device. Some devices can sap a battery in just a couple of days if not charged.
I have had batteries discharge to less than 10vdc and then they would not hold a charge.
Team Owner
Joined: Jul 2006
Posts: 22,200
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From: Kannapolis NC
Or a clock or aftermarket radio. A battery will lose small bits of voltage sitting on a shelf. I don't think you have a problem. On my 60 it'll drop down to 11 1/2 V if I don't drive for a while, but it still starts and runs fine.
Thread Starter
Advanced
Joined: Dec 2017
Posts: 52
Likes: 21
From: Modesto, Ca
Finalist 2022 C1 of the Year - Unmodified
yes,usually starts when cold. more difficult when hot.
can a battery have decent voltage but not much power (amps?) to start?
I am a novice at this and I thank all for your help.
can a battery have decent voltage but not much power (amps?) to start?
I am a novice at this and I thank all for your help.
Burning Brakes
Joined: Jul 2009
Posts: 1,240
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From: ESCONDIDO CA
2025 C2 of the Year ('63 and '67) Finalist - Unmodified
C2 of Year Finalist (appearance mods) 2019
It's been my experience that both head and winter cold affects battery output. And, age plays a roll too. You should get more than two years out of even the cheapest battery. Old motors have usually been rebuilt with different carburetors, starters, and ignition systems that can change starting characteristics. A hot motor can be **** to vapor lock which will make it difficult to start. A couple of easy fixes; let it cool as long as you can, depress the accelerator fully when trying to start, and at last, retard the timing a little. Not gonna talk about old wiring or alternators! Good luck.
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good strong batteries should handle a small milliamp discharge from clocks, radio station presets, etc... for a couple of months.
when I park my car for the winter the battery is pretty low after a couple months.. this is normal..
do your test again without the battery connected to the car and check your voltage.
dirty batteries will discharge between the posts externally.
bad batteries will discharge between the plates internally.
batteries lose 10% of capacity due to age per year.. so a 5 year old battery will go dead quicker. and may not have the capacity to even crank the car.
when I park my car for the winter the battery is pretty low after a couple months.. this is normal..
do your test again without the battery connected to the car and check your voltage.
dirty batteries will discharge between the posts externally.
bad batteries will discharge between the plates internally.
batteries lose 10% of capacity due to age per year.. so a 5 year old battery will go dead quicker. and may not have the capacity to even crank the car.
Team Owner
Joined: Jul 2006
Posts: 22,200
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From: Kannapolis NC
I fear you're going to spend a lot of money and worry over an issue that doesn't exist. The voltages you listed are better than most car batteries. After 19 days you still have over 12v. You can get a lower voltage by how clean the ground or probe on you meter are connecting. Worry when it sits for a couple of weeks and engine rotates slower than normal when starting. I assume you have a clock, don't know about aftermarket radio with station and time memory, but those will drain a battery slowly by necessity.