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Thread: 840 Battery Drain?

  1. #1
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    840 Battery Drain?

    Fellow 8ers,

    I don't use the 840 much, but I do take it for a 5 minute spin around the block at least once a week to help keep things ticking over, although during the past couple of weeks I've had to get it jump started - is the weekly 5 minute spin enough to keep the batteries charged?

    If so, does anyone know of a common problems / trouble areas that may be may be causing a power drain.

    Cheers

    D

  2. #2
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    The 8er is notorious for being hard on her batteries, with all of the electronics in the car. I don't think that 5 minutes around the block is long enough to get a good charge into the batteries; I always thought it took much longer. You have probably been running them down even more with that short run. A battery tender is a good way to keep a good charge in the batteries when the car isn't used. Just plugs into the cigarette lighter and keeps the car charged.

  3. #3
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    No it's not enough to charge them, and anyway the alternator isn't designed to charge the batteries when badly depleted as yours are...... the load is too great. Time to get a proper charge on those and then a tender (try a C-tek), and possibly replacement of the batteries.

  4. #4
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    I was always told it took about 20 mins driving to offset the charge loss from starting the car. This would of course assume good condition batteries.
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  5. #5
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    Quote Originally Posted by wedge31 View Post
    I don't use the 840 much, but I do take it for a 5 minute spin around the block
    I think that's the source of your drain right there...

    You need a trickle charger.

  6. #6
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    A 5 min drive is one of the worst things you can do to a car, nothing gets up to temp so you end up with condensation in all your engine, trans, diff, exaust, etc. you need to go for a good drive that way everything gets up to temp and the condensation evaporates. Just my opinion.

  7. #7
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    Just want to clarify that it should be a float charger not a mere trickle charger.

    A float charger knows when to stop and start again - a trickle charger never stops charging and can boil the water out of the batteries.

    Fwiw Harbor Freight has trickle chargers very cheap.

    Regarding the 5 minute drive it's way better to keep it parked than drive for mere 5 minutes. At least 15-20 minutes with nice fast highway stretches is best.
    Last edited by Ladislav; 09-23-2009 at 03:53 PM. Reason: Automerged Doublepost
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  8. #8
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    Quote Originally Posted by cantbl8 View Post
    Just want to clarify that it should be a float charger not a mere trickle charger.

    A float charger knows when to stop and start again - a trickle charger never stops charging and can boil the water out of the batteries.

    Fwiw Harbor Freight has trickle chargers very cheap.

    Regarding the 5 minute drive it's way better to keep it parked than drive for mere 5 minutes. At least 15-20 minutes with nice fast highway stretches is best.
    I meant a "battery tender" like the one BMW sells. Definitely need to get one that knows when to stop charging...

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  10. #10
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    The battery tender sounds like the solution to this but the problem that has kept me from using one before is that my car is parked on the street. Is anyone aware of a booster type in car battery tender?

  11. #11
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    what about a solar trickle charger?

  12. #12
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    Wow who has the willpower to just drive an 8 for just 5 minutes? It takes all my willpower t come home

  13. #13
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    Quote Originally Posted by Bankswood View Post
    Wow who has the willpower to just drive an 8 for just 5 minutes? It takes all my willpower t come home
    +8!!
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  14. #14
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    Steffen, 212,600 miles? What parts have you not replaced. Also explain the Hybrid badging on the back of your 8

  15. #15
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    Transmission has been in the car since 64,000 miles when it was replaced/rebuilt under warranty. Also, interior is pretty much original, except of course the steering wheel and "new" upper glovebox where the airbag used to be.
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  16. #16
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    Hi Everyone: I too am experiencing the same/similar type of non-charging problem with the 1997 840. In my case, I replaced both trunk batteries with no-name Interstate batteries (at the same time) and made sure that they were a match in terms of CCA and brand so as to avoid issues such as I am experiencing. So, what's the next step? I think I should check the alternator charging output with a volt meter as the car is running and then when the car is off. The 840 has 108,000 miles on what I understand as the original 125amp alternator but before I start wrenching on replacing or repairing it, I want to be absolutely sure that that's the problem. Thoughts from those experienced in the matter??

  17. #17
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    You may also have the parasitic battery drain from failing capacitors in your General Module. I would check that the alternator is putting out 13.8 volts and then look at the GenMod.
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  18. #18
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    If you are worried about battery drain - measure the current with an ammeter. The easiest way to do this is to remove one of the (BROWN) battery negative cables, connect the ammeter between the lead and the negative battery post and then disconnect the second battery's negative lead - your meter will now be measuring the current demand.

    Do all of this with the key out of the ignition and then wait 16 minutes at which point the consumer cutoff relay will open switching off the trunk lights and the current demand should drop to 50mA or so. If that is the case you can stop worrying about battery drain!
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