Jump Starting - negative terminal or body earth?

**Toxic_Avenger** · 3rd May 2016, 09:16 PM

I don't jump start cars often, nor do I plan to, but reading over the defender service manual it suggests a procedure for jump starting the tdci (puma) defenders. I thought a jump start was a fairly routine and basic procedure, but it seems that the process is very specific about the placement of the negative jumper lead:

A. Connect one end of first jumper cable to positive (+) terminal of booster battery.
B. Connect other end of first jumper cable to positive (+) terminal of discharged battery.
C. Connect one end of second jumper cable to negative terminal of
booster battery.
D. Connect other end of second jumper cable to a good earth point on the disabled vehicle (eg. engine front lifting eye, as shown in J6086), NOT TO NEGATIVE TERMINAL OF DISCHARGED BATTERY. Keep jumper lead away
from moving parts, pulleys, drive belts and fan blade assembly.

WARNING: Making final cable connection could cause an electrical arc which if made near battery could cause an explosion.

If booster battery is installed in another vehicle, start engine and allow to idle.
Start engine of vehicle with discharged battery, following starting procedure in Owners' Manual.

Remove negative (-) jumper cable from the engine and then terminal of booster battery.
Remove positive (+) jumper cable from positive terminals of booster battery and discharged battery.

Point D above is puzzling me. Is there any reason other than the warning (explosions etc) why one would attach the negative jumper lead to the engine or chassis rather than the battery negative terminal? My understaning is this risk is low... moreso if we are talking about a battery that is flat (overcharging batteries - not discharging- releases explosive H2 gas...). Surely the inrush current from a charged battery connected to a flat battery wouldn't evolve significant amounts of H2 in the time it takes to kick the car over and let the alternator take control?

Would having a + / - connection at the battery for an anderson plug Jump starting point be a suitable way to run this? This is what I've seen done on large trucks, using the 350A anderson plugs, and thought it would be a useful addition to my vehicle.

Tombie · 3rd May 2016, 09:20 PM

Mate - it's been that way since EFI came in...
It's not a new technique..

**Toxic_Avenger** · 3rd May 2016, 09:42 PM

So...
Have I been doing it wrong in the past (connecting negative directly to the battery)?
Is it a non issue, or a matter of dicing with death if it's not done correctly?
Is risk of igniting hydrogen gases at the battery the primary concern, or is there other issues with frying an ECU?

On face value I read point D, and the warning text... and all I see is a ass covering exercise on the risk of igniting a gas that probably is not an issue in the situation that requires a jump start.
Attaching the Negative clamp to an engine earth, a chassis earth, or the battery direct seems inconsequential, as they are all connected and have a huge conductor CSA between them.

**67hardtop** · 3rd May 2016, 09:50 PM

Ever seen a battery explode?
I have.
Almost seriously injured my mate.
It was a tractor battery. He hooked up a battery charger that was already turned on. The resulting spark caused the battery to explode. Big mess and lucky he was wearing safety glasses or he may have been blinded we were told. It was cold so he was wearing a thick coat at the time. Very lucky man.
We were still finding bits of battery in the workshop for months.

It only needs to happen once.

Cheers Rod

Sent from my GT-I9507 using AULRO mobile app

**Eevo** · 3rd May 2016, 11:26 PM

as there is a voltage difference between the good battery and the bad battery it will create a spark when you complete the circuit.

the idea of using the body earth point is that your moving the spark away from the battery.

a battery if broken or charged incorrectly can produce hydrogen

hydrogen plus spark in the presence of oxygen makes big bang

but im with you, i use the negative terminal.

**Eevo** · 3rd May 2016, 11:31 PM

didnt you watch the movie the martian?

remember the explosion when he was trying to make water using hydrogen.
although the movie gets the science very wrong.

[ame]https://www.youtube.com/watch?v=sFYjVsV4r0Y[/ame]

**drivesafe** · 4th May 2016, 04:37 AM

Hi Mitch, there is still a high risk even if the battery is SUPPOSEDLY FLAT.

It may look like a flat battery but it is actually a potential bomb.

Instead of being flat, it could have a shorted cell, which could then be causing the battery to gas and it could still be gassing while you are connecting to it.

Just play it safe and ALWAYS use an earth point and NOT the negative terminal.

**Tins** · 4th May 2016, 05:28 AM

Originally Posted by drivesafe

Hi Mitch, there is still a high risk even if the battery is SUPPOSEDLY FLAT.

It may look like a flat battery but it is actually a potential bomb.

Instead of being flat, it could have a shorted cell, which could then be causing the battery to gas and it could still be gassing while you are connecting to it.

Just play it safe and ALWAYS use an earth point and NOT the negative terminal.

X 2. Even to the point of making sure that EVERYTHING is switched off when reconnecting a battery. A spark is a spark.

**alien** · 4th May 2016, 12:30 PM

Not much I can add about why we don't put both leads on to the battery.

If your looking at jump start options with out pulling the seat etc.....
-The Anderson plug as you mentioned.
Fitted to trucks to protect us dumb truck drivers from exploding batteries, yep 4 of them usually to choose from.
I've seen them wired to the house battery, direct to the starter and also to the winch power feed cables.
This means with out a matching plug you can't help anyone

-Fit a USI160 from Traxide and just push the dash button, works a treat

-Fit power pins under the bonnet, I've noticed that Mack has these now.

**p38arover** · 4th May 2016, 12:37 PM

The L322 has both + and - posts under the bonnet, away from the battery. One reason is that there is an explosive device on the +ve battery terminal which is intended to disconnect the battery in an accident.

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