Darxus

How charged does a car battery need to be to start?

How charged does a car battery need to be to start?

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2009-09-29
Dead battery, 38 amp hours, 0.8 amp charger = 47.5 hours to fully charge. Will I be able to drive to work tomorrow? Or will trying just prolong the process? 2007 Accord. I left the interior light on over the weekend.
  • In all likelyhood
  • Honestly, you can probably get enough of a charge after 4-6 hours... After that, just let the car run for 30 minutes and you should be good...
  • The math is actually a bit worse, as the charger is not 100% efficient. That said, you will likely have enough charge in the battery in a few hours (3-6) to run the starter, unless you really killed the battery to the point of needing replacement. Overnight should be more than enough.
  • Current available out of the battery is a function of battery temperature (this is chemistry talking here) If it's in the dead of winter with a cold battery and cold engine you need a lot. When everything is warm not as much.

    Once the car is started almost all of the current is coming from the alternator and not the battery. Keep all your peripherals off and limit your idle time (alternator is very inefficient at idle) I usually just put the charger on and wait a while and then try and start it, if it goes then put a brick on the throttle and keep the engine over 1000 RPM to get more efficiency out of the alternator. At that point it is charging the battery a lot faster than your trickle charger. You can do all of this before you need to finally try and start it that last time.

    Now comes the tricky part. Every time you discharge a normal car battery (not a deep cycle marine one) you remove a lot of lead from the plates. You may have blown through one or more of them and now your battery is toast. AAA will come out and do a battery check for you for free if you have the service and if the battery isn't any good anymore they can replace it in-situ for about the same as NTB/Sears.
  • Would you like to borrow a 10 amp charger? I forget whether you're in greater camberville or even other points local.
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