[kring]

I have a battery I need to charge, it's outside the car. the charger I have gives instructions that I must connect a 2 foot cable between the negative charger post and the negative chargers clip, no direct attachment. the positive can connect directly to the post.

Does anyone know why, this seems silly from a common sense perspective as what could 2 feet of cable possible change. So I am guessing it has something important to do at more of an electrical engineering level. and is likely simulating how you would connect the negative to a ground point in the car as far from the battery as possible.

Here's link to PDF of instructions, and I also found a few more articles and videos like this that also repeat these instructions... seems pretty universal and I'll begin now following it because I haven't done it this way for decades and never had an issue, but I'm reasonable enough to change my ways

Would really like to understand why this 2-foot cable is so critical.

Instructions

YouTube Video

[HighlandPete]

Originally Posted by kring

Would really like to understand why this 2-foot cable is so critical.

I'm reading the instructions as a safety measure. Reducing the risk of sparking near the battery. Keeping the final connection approximately 2' away from the battery.

Instruction is under the following heading.

WARNING: A SPARK NEAR THE BATTERY MAY CAUSE A BATTERY EXPLOSION. TO REDUCE THE RISK OF A SPARK NEAR THE BATTERY:

[n-hampt4]

That's probably a generic warning that comes with the charger, more to do with sparking around flooded batteries. AGM batteries give off almost no fumes under normal conditions.

The battery management in the car does require you to connect the negative to a chassis earth though. It has to measure the current going in and out of the battery, and if you connect directly to the negative post, it will not see the battery being charged.