Car Battery? Watch it!

[Canberra Man]

Hi.
I've seen many mentions of a car battery to power up home built radio's. Be very, very careful. It is sometimes not realised that short circuit current of a car battery irrespective of capacity can be in the region of 1,000 amps.

Ken

[ben]

Due to the virtually unlimited current capacity , I wouldn't consider using such a battery without fitting an in-line fuse rated correctly for whatever is connected to it.

[Mike Phelan]

If you really want a 13v supply and must do this, as well as what Ben rightly says, do not have any sort of metal jewellery on you - watch bracelets, et al.

I wouldn't bother - a bench power supply is safer.

[Boom]

I was using a lorry battery as a supply in a shed once and all of a sudden there was one almighty BANG. When I recovered I saw that the battery was split in half! Hydrogen from the battery must have been set alight by a spark.

I had to use a hosepipe to wash myself and my lad who was then aged 4 down with and couldn't hear anything for two days.

I've been very wary of car batteries since then.

Dave

[Peter.N.]

Many years ago I had a glass two volt accumulator explode in my face, not a pleasant experience. A loose connection on a car battery can cause a spark a spark which can ignite the hydrogen when charging.

Having said that, I have a bank of batteries under the bench as a back up for my ham radio, but they are sealed type. Car batteries will not last very long if they are cycled, they are designed with a lot of thin plates to give a high current for starting and then to be immediatly recharged, if used as storage batteries they will quickly disintegrate, you need deep cycle batteries which have less but much thicker plates.

Peter

[AlanBeckett]

Some years ago my son was changing the starter solenoid on his Landrover. I went out for a few minutes, and when I got back there was a Fire Engine outside - very impressive. He'd 'forgotten' to disconnect the battery, and shorted the live solenoid terminal to the chassis with the spanner. This had apparently glowed red followed by the cables catching fire. His mother panicked and dialled 999. After a new battery and cables the Landrover was/is OK, but he hasn't done it again. Be careful.
Alan

[kalee20]

All the above is true - very high current capability, plus the explosion hazard when charging. Although, the risks can be managed. After all, nearly every car has a lead-acid battery!

The big advantage of a battery is: No output ripple or noise; and complete isolation to earth (capacitance to earth is a few tens of pF only). And for some experimental purposes, this can be invaluable!

[dave walsh]

Good advice but I suppose it's about managed risk. The sort of old car battery that I've used occasionally [by definition] has not had much amperage left in it!. The gas explosion aspect is the most worrying but the battery doesn't have to be in a confined space or in close proximity. There are a couple of portable "jump start" units on sale in the uk currently that contain a medium size battery. One unit has live leads with crocodile clips that have nowhere safe to clamp on to. Not sure how this passed inspection. The more expensive one has the vital isolator switch. No reports of melt downs so far! Dave W

[Michael Maurice]

Originally Posted by Mike Phelan

I wouldn't bother - a bench power supply is safer.

This thread has gone off topic more or less from the start. Car batteries should be put to the purpose they were intended for ie to power cars.

To prevent a short circuit and a big explosion we are closing this thread