How Long Can Water Pump Run Off Battery

Discussion in 'Freshwater Aquarium Builds' started by Rivermonster, Jul 21, 2019.

  1. Rivermonster

    RivermonsterValued MemberMember


    Im trying to calculate how long a water pump will run from a car battery.
    The battery is 50ah 12v
    The pump is 20watts 240v
    Left on 24hours a day
    How long would it last in hours or days?

    Has anyone out there tried running a water pump like this?

    Any help appreciated, thanks
  2. RSababady

    RSababadyWell Known MemberMember

    From a purely theoretical point of view, what you have is:
    1. 50ah in the battery x12V = 6000Wh
    2. 20Wh of pump usage

    So theoretically you will have 300hrs of pump time.

    However there are two things that reduce this simplified calculation change dramatically:
    1. you need to convert 12v to 240 volt which will drop the efficiency by 50% = 150hrs
    2. Car batteries are lead acid batteries designed to give 50ah bust of maximum energy to start a car engine and not to trickle feed something for a long amount of time (think of leaving your car light inside the car on overnight - it will run your battery flat within two days i.e. 48 hrs)

    From a practical point of view you will probably have 2-3 days of power to run the pump. If on the other hand you have a 50ah battery from a UPS or any other device designed to draw current at a steady rate (not like a car battery), you may well go on for 1-2 weeks.
  3. OP

    RivermonsterValued MemberMember

    Thanks for the reply,
    Im planning on using a 12v 100w inverter connected to the battery. They are on ebay, with a cigarette lighter connection on one end. Going to cut off, attach crocodile clips,the attach to battery. The inverter has a uk 3pin plug socket i can plug pump straight into. Probably uses a bit of power itself, but perhaps this gives a similar result to the 'ups' battery?
  4. RSababady

    RSababadyWell Known MemberMember

    Rivermonester - just out of interest, why are you using a set up like this? Do you not have access to 240V straight of the grid?
  5. Nobote

    NoboteValued MemberMember

    You need a deepcell can get them in 12/24/36 volt. Deepcells are what I run my trolling motor. Its 70lb thrust and 24 volt. I literally can run it 7 hours and still have about 30% battery left when I get back to the launch. And thats river fishing too...not still lake water.
    A 24 volt deep cell costs about 100.00

    A car.battery isnt going to last.long under a load.
  6. OP

    RivermonsterValued MemberMember

    The reason im asking is because i dont want to run an electric cable from house to a pond. I was going to use a solar panel/pump combo but they look unreliable.

    Ive seen a 1000w inverter used with a 900w device for several hours from a 12v lesiure battery(same as car battery just bigger- around 110ah). So a 20w device on 100w inverter with 50-80ah should last a few days. . . ?

    Its possible to charge the battery with solar but im just going to charge indoors, then place next to the pond. Then swap it with pre-charged one as nessecary. I found a device that cuts out the battery when its low and shows battery level.
  7. JayH

    JayHValued MemberMember

    Like Nobote said, you don't want to use a car battery for this. They are not designed to provide power over a long period of time. You'll kill the battery in very short order in a use like this.

    You need a deep cycle battery that's designed to be deeply discharged and then recharged. Something like a golf cart or "marine" battery should work. That may be what the "leisure" battery you mentioned is intended for. I've never heard that terminology so I don't know what category it fits in.
  8. kallililly1973

    kallililly1973Well Known MemberMember

    For what its worth i run a solar air pump on an outdoor 29 i'm sure its smaller than your pond build but this little airpump has run pretty much from the day the sun charged it about a month n a half ago.
  9. Nobote

    NoboteValued MemberMember

    Solar is a pretty good option. Most people that own boats of any size are installing a 2 ft solar panel to charge deepcells while on the water, and they actually work very well.
    30 yrs ago you couldnt have trusted a 10 ft solar panel to effectively charge a boat...the little one a saw running the other day kept up with some pretty heavy usage...running a speaker, marine radio plus charging phone and a running trolling motor,