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Charging a flat battery off the alternator!

Discussion in 'Car Repairs / Maintenance' started by DIYspanner, 11 Jul 2018.

  1. DIYspanner

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    This is something I've always done when jump starting cars.

    When started, I check for 13.5+v across the terminals to be certain the alternator's working.

    However, I've had a look at the RACQ website and its advising the following -

    "
    After leaving the car's lights on and flattening the battery, going for a drive will recharge it
    FALSE

    You won’t fully recharge your battery by going for a drive, idling the engine, or going for a short stop-start trip. In fact, ‘surface’ charging or continuous undercharging will lower the capacity of the battery over time and shorten its life. You could also void the battery warranty by not recharging it correctly.
    The only way to reliably restore a flat battery’s charge is to use an appropriate multi-stage battery charger. The charger voltage needs to be high enough to mix the battery acid evenly in the electrolyte to prevent ‘stratification’.
    "

    I've never had bother with the alternator charging the battery!
     
  2. Keithmac

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    A comlpetely dlate car batter could take 3 days of constant charging to recover.

    Not 1 hour off the alternator!.
     
  3. Burnerman

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    As above - it depends on how flat the battery actually is as it has to provide the alternator with an 'exciter' voltage so a totally flat one will need sympathetic recharging from an external source.
    That's the good old theory anyway, but in these days of 'smart' alternators and batteries - both of which require to be coded to the car - its anyone's guess what is correct :eek:
    John :)
     
  4. DIYspanner

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    So when folk go to the local factor's shop to get a suspect battery tested, one that's a peaky due to lack of good charge, may actually be deemed as a bad by the tester when all it really needs is a thorough charge to revive it.


    On the subject of charging, I got a battery charger from Lidl some years back. It does both 6v and 12v, trickle and boost charging. It has a green LED to indicate when the charge is complete for trickle charge. The off times I use it, charging usually takes about a couple of hours.
     
  5. Burnerman

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    Apparently some 'smart' chargers are capable of reviving a totally flat battery and I can't argue with that - in my experience it all depends on how long its actually been flat for.
    I reckon if the battery has been left flat for a couple of weeks then its game over, but what do i know!
    John :)
     
  6. EFLImpudence

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    If you think about it logically, if the alternator won't recharge the battery fully, then in normal use the battery will always be at a level less than fully charged.

    Is this the case?
    If it is, why is it designed like that?
     
  7. Burnerman

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    I'm sure there have been many papers written about this one but I've always assumed that the alternator voltage was kept a little high to compensate for losses found in the battery due to heat and chemical decomposition to a certain degree - so the battery remains at a healthy voltage and current capacity to enable a good start in all weathers.
    The battery will be designed to accept a small amount of overcharge without damage to the plates or excess electrolyte gassing.
    For sure, they aren't the things we used to buy years ago - when the difference was largely the physical size!
    John :)
     
  8. Keithmac

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    Not really, the alternator is designed to run all the car systems an service charge the battery.

    A completely flat battery is a different animal to a healthy battery (12.4v/ 12.6v rest voltage).

    I prefer our 10 amp charger at work with analogue ammeter, easy to keep an eye on and when the battery takes less than 1 amp it's about fully charged.

    The new fangled ones you don't really know whats happening but the ammeter tells the full story. Plus if it won't take over two amps at any point frpm flat it's knackered imho..
     
  9. EFLImpudence

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    I'm not talking about completely dead, but -

    if the battery won't start the car and you jump start it, then ten minutes later it will start, what has happened?

    And what will happen if you drive it for another two hours?
     
  10. Burnerman

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    I'd guarantee - so long as the battery wasn't an absolute knacker - all would be well for a start the next day.
    John :)
     
  11. Keithmac

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    If it's on the brink of starting then a two hour drive would probably sort it (all depends on what the alternator can output and what the systems on the car require).

    These new SMART Ecu controlled alternators are a different story, can let the batteries drop below 12v and only charge on overrun etc. Surprised the batteries survive tbh.
     
  12. DaveHerns

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    I thought it was because the alternator read the battery voltage and only charged it back up to that plus a bit more. Probably only on new alternators, not the old Lucas 18ACR/A127.
     
  13. wgt52

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    From experience - don't do it regularly.

    Had one car with a high output Alternator and a small capacity battery; had 3 new batteries in 18months, every one had buckled plates from the regular rapid charging.
     
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  14. Peter.N.

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    Maybe if its a very modern car with one of the new fangled 'smart systems' but everything I have owned has recharged via the alternator assuming the battery is in good order.

    A modern alternator can have an output of 100 amps or more so unless you are driving with everything in the car switched on there should be plenty of spare power to charge the battery. The regulated alternator is around 14.5 volts, a fully charged battery at 2.2 volts per cell equates to 13.2 volts, measure the voltage across the battery shortly after you have started and unless you have a faulty alternator it will read in excess of 14 volts meaning its charging and will continue to do so until the battery is fully charged.

    Before cars had auto aux belt tensioners you would often hear the belt squealing after startup, this was dude to the very high output from the alternator recovering the battery form the starting load, the squeal would then stop because the battery had reached a reasonable voltage, it would then continue to charge until full charged was reached.

    If you go back the the days of dynamo's their output was only about 19 amps so it did take a long time to fully charge the battery and you could tell when it was because the panel lights got brighter but in my experience of probably nearly half a million miles of driving cars with alternators I have never had a problem with the alternator completely recharging the battery.

    Peter
     
  15. DIYspanner

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    I checked the water in an AA brand battery on a KA today.

    On with ends, next to the screw tops, there's a pull out rubber plug.

    Should these plugs be removed?

    When I bought a maintenance free one a few years back it had vent rubbers for transit which I had to remove on fitting!



    Also, when using jump leads, since using an earth away from the battery on the flat car due to the risk of sparks causing an explosion, rather than looking for a good earth point, make a meter long earth cable, 600amp crocodile on one end and lug on the other. Connect this between the earth jump lead and flat battery, crocodile end on battery first and your guaranteed a good earth.

    There is of course that a bad earth that's causing the flat battery in the first place!
     
    Last edited: 22 Jul 2018
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