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      10-09-2021, 08:17 AM   #1
Ascanio
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Normal battery voltages with engine ON and OFF

I just put in a DieHard H7 a few days ago.
Am reading 12.3V with engine OFF and 13.9V with engine ON from OBD.
Is that normal?

Cranking speed was super fast the first few days then started slowing down.
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      10-09-2021, 10:31 AM   #2
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That sounds about right.
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      10-09-2021, 10:46 AM   #3
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I guess it depends where you measure it?
I am noticing the cranking is slower after a couple days since battery was new, could that be also due to driving style?
Driving downtown I basically stay between 1500-2500rpm, I guess this is not what the S54 engine is designed for hahahaha

Might want to find a spot with good visibility to flor it without being arrested so I can let the engine loose?
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      10-09-2021, 11:29 AM   #4
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Quote:
Originally Posted by Ascanio View Post
I guess it depends where you measure it?
I am noticing the cranking is slower after a couple days since battery was new, could that be also due to driving style?
Driving downtown I basically stay between 1500-2500rpm, I guess this is not what the S54 engine is designed for hahahaha

Might want to find a spot with good visibility to flor it without being arrested so I can let the engine loose?
If after sitting for a few days the battery seems to have less ability to crank you could have a number of issues. There may be a drain, or bad ground somewhere that is pulling power from the battery while it's sitting. If when you drive it your trips aren't very long the alternator may not be having enough time to fully recharge the battery.
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      10-09-2021, 12:21 PM   #5
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It may very well be the second.
Using a big big big engine for running errands around town. My god the S54 engine looks huge. I can imagine it takes a lot of energy to crank it up.

I might use a battery tender overnight every day.
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      10-09-2021, 04:02 PM   #6
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If you are consistently getting 12.3V after sitting just overnight. then there's definitely a drain somewhere. Just for comparison, I get 12.8V after driving and measured from engine terminal posts. It would still show 12.4V after sitting a week without driven. My commute is 30 minutes of surface streets to work and same on the return trip. I'm also running a H7.
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      10-15-2021, 10:39 AM   #7
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I made a point of taking scenic routes on the way to work and back, and that definitely helped.
Also NOT using Sport driving easy around town (my guess is that amplifying throttle blips is just going to wet/dirty the engine if not actually dialing in full heat cycles).
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      10-16-2021, 08:55 AM   #8
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When it comes to battery health it's not so much *how* you drive; it's much more about *how* *long* you drive. It takes a buttload of amps to turn an engine over, and it takes time for the charging system to replenish the battery. A lot of short trips will kill a battery. If that is what you do, keep it on a tender.
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      10-16-2021, 09:10 AM   #9
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Quote:
Originally Posted by wdb View Post
When it comes to battery health it's not so much *how* you drive; it's much more about *how* *long* you drive. It takes a buttload of amps to turn an engine over, and it takes time for the charging system to replenish the battery. A lot of short trips will kill a battery. If that is what you do, keep it on a tender.
Agreed, my consideration is about separate things that might come into play:

1) Short drive time -> not enough to properly charge battery
2) Low RPM drive -> not enough current to do everything properly, including charging battery
3) Low RPM + rapid RPM gradients -> might wet and soil engine, especially plugs, making it tougher for the engine to start and run correctly

I also imagine that low RPMs might very well fail to insure proper lubrication of he head. So I am making an effort to do longer trips, and to keep engine above 2000rpm even when coasting in lower gears.
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      10-16-2021, 09:34 AM   #10
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Modern charging systems work well at anything above idle engine speeds. You shouldn't have to rev the car to make amps. Do your RPM calcs based on the engine, not the battery. S54 needs the oil to be warm before beating on it so drive easy until the oil temp gauge comes up.
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      10-16-2021, 09:35 AM   #11
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Of course! I am talking about 2-3k rpm, not 8 haha
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      10-16-2021, 10:45 AM   #12
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Quote:
Originally Posted by wdb View Post
Modern charging systems work well at anything above idle engine speeds. You shouldn't have to rev the car to make amps. Do your RPM calcs based on the engine, not the battery. S54 needs the oil to be warm before beating on it so drive easy until the oil temp gauge comes up.
Agreed. If you monitor the voltage with the engine running, you'll see that it doesn't vary much between RPMs. Mine is mostly sitting between 14.0V-14.1V from idle all the way to redline.
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      10-20-2021, 10:27 AM   #13
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Voltage with the engine running should be between 14 & 14.5 volts. If it's lower than that I would have the alternator checked.
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