Car battery charger woes

Discussion in 'Electricians' Talk' started by spaceman76, Dec 29, 2020.

  1. spaceman76

    spaceman76 Active Member

    Hello I hope someone can help as I not sure about this. I have a 8 Amp car battery charger; so in theory the time it should take to charge a 36Ah battery is (36/8) = 4.5 hours. The thing that's throwing me is the needle on the charger; when I first hooked up the charger (to an almost dead battery) the needle was reading 5 Amps, now about 7 hours later the needle is reading 2.5 amps. So in a nutshell is my 8A charger really not charging at 8A, as the needle shows 2.5A, and so it will take longer to charge that 4andahalf hours wont it. Thinking about; as the needle progressively drops down towards 0 it will slow down in charging. Am I right about this it really should not be this confusing should it.
  2. FlyByNight

    FlyByNight Screwfix Select

    Basically, yes it is right.

    It will start at a high current and then drop off as the cell voltage increases.
  3. spaceman76

    spaceman76 Active Member

    Ah I see, its just that when reading the internet about various car chargers and sizes ect it says to work out how long it will take to charge a battery, you divide the Ah rating of the battery by the rating of the charger . That is obviously not the case then !
  4. FlyByNight

    FlyByNight Screwfix Select

    Never really the case with car chargers, most are fairly basic and you are "fighting" the battery voltage and as it charges the voltage rises effectively increases impedance. Then there will be losses through heating. With my 12A charger, on one car it starts at around 10A, another about 6A and on a previous one, it would max out.

    Even with semi intelligent chargers for small NiCd and similar batteries, you charge at say C/10 - capacity divided by 10, however it will take around 14 hours becasue of increased impedance and heating.

    With yours, taking a guess at a linear decrease, I would say an average so far of around 3.5 - 3.8 AH and you are probably up to around 25 AH so far.
    Banallsheds likes this.
  5. The Happy Builder

    The Happy Builder Screwfix Select

    Once it’s at 85% it will slow right down, which is a good idea as it avoids cooking the battery.
  6. spaceman76

    spaceman76 Active Member

    Yes thats great
  7. Banallsheds

    Banallsheds Well-Known Member

    Many things on the internet are wrong. They are often written by people who don’t know what they are talking about.
  8. Arthur Dent

    Arthur Dent Member

    Pot and kettle
    Bazza-spark likes this.
  9. Banallsheds

    Banallsheds Well-Known Member

  10. Bob Rathbone

    Bob Rathbone Screwfix Select

    This would be correct if you used a constant current charger as is found with electric vehicles and fork trucks. These chargers increase the charge voltage as the battery voltage rises giving a full charge in the fastest time. It is possible to overcharge the battery if a timer is not included or some kind of charge sensor. Ordinary car battery chargers are constant voltage types, that is the voltage stays constant throughout the charge period, usually around 14 volts for a 12 volt battery. This charger will charge quickly initially, but as you have observed, the charging current trails off as the charge progresses.
    jimbobby likes this.

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