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Thread: Normal volts drop during start?

  1. #1

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    Normal volts drop during start?

    My battery cranks good, but drops from 12.6 to 10.5 volts as I crank the engine. What is normal drop on a good battery?

  2. #2
    wireweinie's Avatar
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    9 to 9.5 volts maximum is the range most items call out. The starter motor may still crank but most electronics, including electronic ignitions will stop functioning below around 9 volts.

    If you are measuring this voltage directly from the battery posts, the voltage drop is due to the internal resistance of the battery itself. As batteries age, this will increase until the battery is no longer usable.

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    Quote Originally Posted by supercub1999 View Post
    My battery cranks good, but drops from 12.6 to 10.5 volts as I crank the engine. What is normal drop on a good battery?
    Battery voltage load test using the starter. No less than 9.6 volts. Alternator/generator charging rate of 13.6 up to 14.6 or 14.8....don’t recall the exact upper number. If those two items are healthy the third part of the charging system is the starter. Measure the amp draw on the starter. Shouldn’t be greater than....thinking 250 amps... don’t exactly recall.. when testing you’re looking for the max starting amperage and cranking amperage draw....should drop to less than 200 amps...


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    Thanks, I think I am ok than. Yes the volt gauge is connected to the posts of the battery and it dropped to about 10. or 10.1 volts immediately upon hitting start, but than almost as quick as it dropped it came back up to 10.5 and stayed at 10.5 while cranking for 3 seconds.
    Last edited by supercub1999; 08-01-2021 at 09:01 AM.

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    wireweinie's Avatar
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    Your battery should be ok with those readings. If you are running any kind of electronics, it's always a good idea to shut them down during engine cranking. Mostly to protect them from voltage spikes but also to keep them from shutting down each time the bus voltage momentarily drops below minimums.

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    Quote Originally Posted by wireweinie View Post
    If you are running any kind of electronics, it's always a good idea to shut them down during engine cranking. Mostly to protect them from voltage spikes but also to keep them from shutting down each time the bus voltage momentarily drops below minimums.
    For those with full glass panels, or even just an electronic engine monitor, the TCW Technologies Integrated Battery Backup System (IBBS) does a great job of protecting against sags during engine start. It's standard fit on Carbon Cubs with G3X system. For engine start - IBBS ON, Master On, Avionics Off. No need to wait for the system to boot after engine start to check engine parameters and all system parameters are being logged before and during start.

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