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Battery conundrum

  • 10-02-2009 7:17pm
    #1
    Seanieke
    Registered Users, Registered Users 2 Posts: 853 ✭✭✭


    Just a quick one to get the gray matter workin...

    How does one calculate the Ampre Hour of a battery??

    I went into Halfrauds today to look at a battery, but I couldn't see the usual 80AH sticker on it, Instead it read "640 Amps starting current"

    Now I know with generators etc its Watts divided by Voltage equals Amps..

    Am I to presume its 640 / 12 in this case giving us 52Ah or what.....????

    If anyone wants to try fish an answer from Google have a look at this..

    http://www.powerstream.com/battery-capacity-calculations.htm

    then turn it into English for us simple folk!!

    Thanks


Comments

  • #2
    crosstownk
    Registered Users, Registered Users 2 Posts: 12,863 ✭✭✭✭crosstownk


    that website wrote:
    Amp-hours means amps times hours. Divide by amps and you get hours, divide by hours and you get amps. So it isn’t amps, and it isn’t amps per hour, it is amp-hours.

    You calculation of 640A/12V does not result in ampere-hours. It is not possible to have a time unit result by dividing amps by volts.

    Ampere-hour is a rating of battery capacity - you don't really calculate it. 80AH simply means that the battery can deliver a current of 1 amp for 80 hours or 2 amps for 40 hours - you get the picture. This has little to do with the 640A starting current which is the current it can deliver at a specific temperature - zero celcius iirc. So if you use a lot of accessories with the engine off or if you leave the car on the driveway for long periods then a high AH rating would be preferable.

    Physics aside, if the battery is more or less the same size and the posts are correctly orientated and sized then the chances are it will do the job.
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  • #3
    Seanieke
    Registered Users, Registered Users 2 Posts: 853 ✭✭✭Seanieke


    Need a bigger battery... VW Passat Diesel with 60AH battery..needs more poke for the accesories


  • #4
    crosstownk
    Registered Users, Registered Users 2 Posts: 12,863 ✭✭✭✭crosstownk


    I'd the same problem with my 1.4 Bora. It came with a small battery (can't remember the rating). I fitted a 1kW stereo and the poor battery would die in 15 mins if the volume was turned up any way loud. So I got a battery that was intended for a 2.4 Transporter which gave me a fair few extra minutes.

    What sort of accessories are you running? Are they high current consumers?

    Maybe try another supplier - you local friendly motor factor may be able to help you out.
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