I only drive this car on weekends, and if I have only done short journeys there is a risk of a flat battery. This happened to me two days ago, and standing on a street in the cold waiting for roadside assistance I resolved to buy a booster pack (A new battery would be a good idea too but this one is not that old, and one of these booster things would be much easier to keep charged than either taking my battery out every few months, or running a charge cable from my house to my car, which isn't usually possible to park near enough to my house)

I was looking at the "Sealey RS131 12V 900A Emergency Power Pack" on Amazon, and at least one of the question answerers said he was able to start a 3 litre merc with it. But another one said don't trust it for anything over 2.5 litres, and mentioned the "Clarke 4000 Heavy Duty Jump Start Engine Jump Starter" ("1500 amps peak output / 700 amps boost at 12v")

I don't really understand what the numbers mean. Is there a recommended minimum volts, amps for jump starting this particular 3 litre petrol engine?

(On a side note, the roadside assistance guy first tried his booster on my car - didn't work, ended up having to hook up his van at the same time as the booster to get the car started, but to be fair it was probably very flat, I had in desperation been trying it over and over again before he arrived, which probably drained it further. Add to that the fact my car is like a human having a stroke - windows wind down, alarm goes off, etc when the battery is low - all contributing to draining it further.)

  • There is a much better solution than a battery pack. Look into a Battery Tender. There are a bunch of different versions. Just ensure you unplug it before you start your car. This doesn't answer your question at hand, but it will solve your issue. Mar 15, 2016 at 11:25

1 Answer 1


While a battery booster pack will work and just about any of them will work to an extent (more expensive, higher CCA will work better/longer), there is a much better solution to your problem.

You need to purchase a battery tender to attach to your vehicle. This will ensure two things:

  • Normal battery life without sulfation. Over time as a battery is discharged down to nothing, sulfate crystals form on the battery plates. This degrades the reserve capacity of the battery. When the battery is discharged down like that many, many times, you can expected your battery to last a much shorter period. You can expect the battery to have a lifespan of 1/2 to 1/3 of what it should have been.
  • The bigger reason to get the tender is so your car is ready at a moments notice. All you have to do is unplug it and away you go. Motorcycle owners use these all the time to keep the battery in good shape over the winter.

There are many different brands available. Look for the type which will float charge and maintain the battery, not just keep it charged. Here in the States you can get them for $20-30, which by all approximation is far cheaper than a new battery or even a jumper box.

  • Thanks. I may well get one. But I'm thinking a booster is a good thing to have in the car for peace of mind. I can't easily park near enough to my house to have a tender plugged in with wires fed into my house. I tend to only be able to park at the end of the street.
    – MrVimes
    Mar 16, 2016 at 13:46

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