Thread: Jump Starting - negative terminal or body earth?

Originally Posted by JDNSW

Don't leave us hanging - did he do it?

John

I got a talking to...

not sure why, Instructors are supposed to know more than the students and impart knowledge aren't they?

how was a student supposed to know what the strange symbology on the door of the battery box that looked something approximately and unmistakably Exactly like

CAUTION
+ve EARTH

was intended to mean?

On edit...

Acutally I guess I did learn something that day. Not sure if its what the instructor wanted or not.

Originally Posted by drivesafe

Hi Dave and if I understand what you are saying, he wants to disconnect the earth lead at the body/chassis?

That is both daft and dangerous.

If you disconnect at the body/chassis, then you have to make sure the negative lead does not rest on an earthed part of the vehicle. OR you have an earth circuit agin.

Tell me I'm wrong and what you mean.

yep, you're wrong...

Positive earth system. Spanner is on the negative terminal nut on the battery and the handle is about 3 inches away from bare metal.

For others

FFT is Fitted For Transmission, which means its old, very very old.

Negative is not always earth in DC stuff.

Earth is your 0V to chassis/frame/body back to supply. very broadly you can have

+ve earth (which is what the FFT trailers were and had the return for the lights independant all the way back to the trailer plug)

a center tap earth which on a 24V 2 battery system gives you 3 possible voltages -12v 0V +12v and 24V and is very uncommon except for some fixed plant installations

and the now almost ever present -ve earth

the correct disconnect procedure is to ALWAYS disconnect whichever Terminal/cable is the earth cable first, protect that (which most people dont because they just tuck it out of the way of the battery) and then disconnect the rest.

IF you're only attempting to isolate the batteries and its got a single cable from terminal to the Chassis/Earth Bus you can disconnect it from the the Chassis/Earth Bus if you protect that end as well. This wont work for you on a modern vehicle that has multiple earth paths.

Here's another tip: When working with Batteries, it is a good idea to insulate that part of your tools which are not actually needed for turning nuts etc.

I once saw a Hydrogen explosion caused by arching. They were very large batteries with caps removed for charging, in a space which just happened to be ideal for creating the ideal Gas Air mixture for ignition. The man who caused the ark was not an Electrical Tradesman, so was not equiped with insulated tools. He was propelled a distance of about Ten Meters but luckily, apart from a few abrasions and bruises, was not harmed.
Note it was only the gas which exploded, not the batteries, and car batteries are not likely to create any where near as much gas, but you could still be injured.

I am reminded of an incident when I was in PNG. We had a coastal transport (MV Possum, if my memory is correct) about 60ft brought in a load of supplies. They anchored for several nights, using an auxiliary to charge the batteries each day. Unbeknown to the skipper, one of the main battery connectors had become loose, and sparked when the main engine was started.

The resulting explosion lifted the battery access hatch out of the wheelhouse floor, and removed the tops of three out of the four very large six volt batteries, exposing the plates and electrolyte. After tightening the connection, the batteries remained serviceable, but the skipper was hoping for calm weather for his return to Port Moresby.

John

Don't know if I've missed it, but you should check the flat battery before connecting another battery to jump start. If you know the reason why the battery is flat i.e lights left on,than the static charge would be low. The best way to jump this would be to connect the battery and run the "donor" vehicle for about ten minutes,switch off,disconnect and if the flat battery is any good it should start.If you've just pulled up and the truck won't start i.e ign lights on, turn key, lights go out,it could be dirty terminals but if you have a voltmeter and the battery voltage,static is good but drops to zero when you turn the key,this indicates an open circuit in the battery. Driving with the battery in this condition replicates driving with the battery disconnected and would cause a voltage spike which could/probably would damage the alternator and ECU

Originally Posted by cuppabillytea

So could have been my Austin 1800 Mk1 or my Rover 2000 SC or both. Thanks.

The LandCrab Mk1 almost certainly, the Rover I'm not sure, it would depend n the year.

Originally Posted by JDNSW

t the skipper was hoping for calm weather for his return to Port Moresby.

John

I'll bet he was! One of rhe main dangers faced by submariners in older times was sea water leaking into the battery compartments. The chemical reaction gives off chlorine gas. That stuff will spoil your afternoon.