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Thread: Charging a dead flat battery

  1. 16th June 2021, 09:18 AM #1
    BradC
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    Charging a dead flat battery

    This is just a thought triggered by the "charge through the 12S socket" thread.

    A month or so ago I left the iiDtool engaged on the D3 and ran the battery flat. Completely flat (<8V).

    I usually charge with a CC/CV bench power supply (0-30V 0-10A). I put the power supply on the battery and it shot up to about 9V at which time the car started to boot, drew much more than 10A and pulled the battery down below 8V where it all stopped. The car was stuck in this loop where it was trying to boot but 10A wasn't enough. I ended up disconnecting the battery to put a good surface charge on it before hooking it back up to allow the car to boot.

    Once the computers booted up, the current consumption dropped below 10A and the battery continued to charge.

    I've heard of people powering the car up through the 12S socket enough to get the doors unlocked, but even with 10A I was unable to get enough into it to allow the BCM to fully boot. It wouldn't respond to the remote.

    Has anyone managed to charge a car from dead without disconnecting the battery? A jump start would be fine, as I assume would be a battery charger with > 10A capability.
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  2. 16th June 2021, 09:34 AM #2
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    BMKal
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    I got home last week from 5 months working interstate, and mine was down to 5V.
    I used the key to open the passenger's door and bonnet. When I first put a CTek charger on the battery while it was connected, it seemed to be taking charge OK, but strange things were happening like tail lights were continually flashing etc. So I disconnected the battery and left it on charge overnight - it came back up to almost 13V.

    When I re-connected the next morning, the car alarm went off (as it should have because I had used the key to open the door). I cancelled this by pressing the button the key fob, started the engine and re-set the dash clock and "auto-up" on the windows, and haven't had a problem since. I have dual voltage meters mounted on the console (for cranking and aux batteries). Both batteries are running at normal voltage since charging.
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  3. 16th June 2021, 09:45 AM #3
    BradC
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    Quote Originally Posted by BMKal View Post
    Istrange things were happening like tail lights were continually flashing etc. So I disconnected the battery and left it on charge overnight - it came back up to almost 13V.
    Yeah, that was my thing. I could also hear what sounded like the turbo actuator and EGR valves cycling.
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  4. 18th June 2021, 03:45 PM #4
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    loanrangie
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    Usually if a battery is that dead you'd need it to connect to another good battery like you were jumping it so as to bring the voltage up so that a charger can detect it and commence charging.
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  5. 18th June 2021, 04:55 PM #5
    Hogarthde
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    Yep, that’s what I did on the farm years ago.
    Although I don’t know if my newish charger needs the same reminder, I am with Nick...connect two , one good, one flat for a while
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  6. 18th June 2021, 05:53 PM #6
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    drivesafe
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    It also depends on the type of battery charger.

    Many chargers have a Power Supply Mode, and if you have a battery that is so low that the charge will not charge it, switch to Power Supply Mode and charge the battery in this mode for a while.

    Then switch back to the normal battery charging mode, job done.

    NOTE, you may need to charge the battery in Power Supply mode for a few hours, before the charger mode will work.

    This is actually something that is require more now with lithium batteries that have been discharged low enough that their BMS has shut down the battery.

    Most chargers will not charge them, so you switch to the Power Supply Mode. Run it on the lithium battery for a few SECONDS and then you can use the charger mode.

    So something to consider when choosing that new battery charger. Future-proof your uses and make sure it has a Power Supply Mode.

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