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Thread: Battery Charger Recommendation- Min. Capacity

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    Battery Charger Recommendation- Min. Capacity

    My CTEK MULTI US 3300 chargert cannot keep battery voltage high enough with the scanner enabled, receive msg. like"no communication with dashbord". Measured battery volts =11.9 when connection drops out. Battery is in good condition and otherwise works fine. cranking voltage is above 10.5. My conclusion is that the charger output is too low. Another charger (cheap) which claims to have 2/8/17A range also fails. Car is 2015 Audi S3.
    Can someone recommend a charger/power supply that works. Thanks.

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    Benevolent Dictator Uwe's Avatar
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    Hi and welcome.

    This query does not belong in Product Support.
    Moved to General VW/Audi Repair.

    I've never had good luck with cheap chargers.
    Xantrex Truecharge are good, but they are not cheap.

    As for capacity, 20 amps should be more than enough for your car -- assuming you turn off big current consumers like seat heaters, lights, and the HVAC system. However, lots of cheap chargers lie about the amount of current they will actually produce. In addition, I've seen some that will happily produce plenty of current until they think your battery is charged and then stop and not re-start when the voltage drops again until you power-cycle the charger.

    Something I've had surprisingly good luck with is a 30A fixed voltage (nominally 13.4V) power supply. It wasn't cheap either though.

    -Uwe-
    Last edited by Uwe; 03-05-2020 at 07:31 PM.
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    An update which might be useful. VCDS worked OK for 30 min on new 70 Ah battery w/o battery charger.
    Repeated cranking test using new battery:
    Start/stop cycles 4
    Bat. volts before test 12.6
    Bat. volts after test 12.6
    Min/Max volts on 4th crank 10.7/14.7
    Ambient temperature 7 C

    Previous crank test w/old battery gave 10.5/14.7V for 2 cycles. Youtube The Trainer suggests 2 cycles and 9.5V is cut off.
    Conclusions:
    Old battery was weak, on the way out
    Crank test needs to be min 4 cycles

    A current draw with only ignition on, main lights and AC off measured 1.8 A. done by battery shop on low range clamp meter, about 1 min long.

    Found that NOCO make a charger rated 10 a/13.7 V as a supply voltage (open circuit). Much cheaper than Xantrex. Have not bought it.

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    A typical battery charger wont work.It cant supply enough ampage to keep the control units alive. You need a battery support unit. I have a Ring rated at 50A and easily see 28A+ regularly being drawn when ignition is on and vcds running.
    Telwin, Ring,Gysflash, Sealey etc all make decent ones so get a decent one.
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    Quote Originally Posted by morski View Post
    A typical battery charger wont work.It cant supply enough ampage to keep the control units alive. You need a battery support unit. I have a Ring rated at 50A and easily see 28A+ regularly being drawn when ignition is on and vcds running.
    Telwin, Ring,Gysflash, Sealey etc all make decent ones so get a decent one.
    28A draw is amazing. Can you speculate where the current is going? I ask b/c I am considering buying the 10 NOCO charger based on the low measured dash draw.

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