Pre-Charge resistor

TreeBass

So, from the battery thread I discovered, thanks to JohhnieB, that I may not have one of these.

Here's a pic of my solenoid



The wires look like they either were snipped off or it corroded?

Not knowing anything about this thing, is there a chance I can get shocked if I start taking the cables off to clean them??

And of course, do I need this resistor?

BGW

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sunoco

Disconnect main battery cables first. Then take of solenoid to clean.

I dont think you NEED it but it reduces arcing of the solenoid contacts

JohnnieB

1. Not the classic electrical shock you get when you stick your finger into an empty lightbulb socket, but a galvanic response (muscle contraction) is possible is you touch both a positive and negative at the same time. Fortunately, most golf carts are "Ungrounded" electrical systems, so you have to touch both sides on the battery instead of just standing on the "Ground" or touching the body/frame.

The perceived pain would be akin to what is felt when you touch your tongue the the terminals of a 9V disposable battery, but maybe not as intense (or none at all) since the epidermal electrical resistance of the hands is far greater than that of the tongue.

The greatest danger is arcing and damaging the terminals.

I carefully disconnect one of the interconnecting cables within the battery pack, touching only the one battery post, and leave it disconnected while working on the other connections.

FWIW: People were connected to the machines I used to work on, so I had to understand the interaction between the two.

2. The resister is not needed for operation. Its sole purpose is to prolong the life of the high current contacts.

The controller contains a dozen or so MOSFETs that are switched on and off about 18,000 times a second and generate a lot of electrical noise (RFI/EMI), so there are a bunch of filter capacitors in the controller to prevent them from disrupting the operation of all electronic gadgets in the vicinity. (They are still noisy, some more than others, which makes installing electronic gadgets, such as radios and audio systems, in some electric golf carts difficult and next to impossible in others.)

When the controller is powered down, the filter capacitors discharge and the next time the solenoid contacts close, the instantaneous peak current flow approaches infinity. This wouldn't be much of an issue if the contacts closed and stayed close, but they bounce several times before they settle and while bouncing, they arc. The arcing pits and burns the contact surfaces and wears them out sooner.

The resistor keeps the filter capacitors charged while the solenoid contacts are open and more or less eliminates the arcing problem.

If the pedal switch and throttle dead-band are adjusted correctly, there is zero current flow through the contacts until after they have stopped bouncing.

scottyb

Excellent explanation Johnnie

TreeBass

Thanks guys, I'll take it apart and clean them up since my power cables are currently disconnected.

I just don't like grabbing things that might bite me back

JohnnieB

It varies greatly from person to person, but typically, DC voltage under about 60V won't bite, up to about 150V will make you jump and serious burns don't occur until about 200V. (Unless you let the current flow for several seconds.)

Fairly low AC voltage can cause cardiac fibrillation, which is deadly because the heart doesn't pump blood when the muscles are contracting out of sequence. Ironically, DC voltage is used to stop the heart when it is beating erratically (fibrillating), hence the name Defibrillator.

The original DC Defibrillators used a dampened half sinusoid pulse that was about 2,000V and lasted about 2ms and they often left burns and broke ribs from the radical muscle contraction, but it was better than the alternative.

Later units use waveshapes, voltages and durations that cause less secondary trauma.

yurtle

My 48 volt buggy "tingles" if I clean and/or check torque on a single post while barefooted. No problems at all with shoes on, but I won't wear shoes unless required by law, like in a restaurant or store. I just learned to deal with the tingle. However, this isn't recommended, as a sudden tingle could cause a wrench to inadvertently be jerked into a neighboring post, which could be BAD.

Fortunately, I've got nearly half a century of jolts, and don't spaz when they occur.

yurtle

I can't tell you how many arguments I got into with EEs on their design of 24 and 120 volt lead/acid station batteries for switchgear controls. The station packs were probably as much as ten times the size of our packs (maybe more). Even ignoring shock hazards, just the potential of a wrench or wire falling into the wrong place made me nervous. I had them cover the terminals, even knowing it would make maintenance a little harder, but recognized the need for visibility and ventilation.

simple man

Inadvertently place your sweaty arm across B+ and B- and I'll guarantee you'll feel it! 36V can shock you under the right, or might I say, the wrong conditions! I imagine 48V would be worse!

JohnnieB

I wouldn't make that guarantee, but it is a pretty safe betting odds.

Statistically, the average threshold of perception for 67% of the population is 1ma for AC and 5ma for DC and Ohm's Law applies. However the resistance between any two points on the human body varies by the distance between the two points, as well as gender, age, time of day, state of mind and several other factors, such as sweating.

Also, the threshold of pain comes into play, which is subjective as well as variable.

I have a relatively high tolerance for pain and low threshold of perception for AC current, so AC current flows that many people find painful don't bother me, but I can detect an AC current flow entering my fingertips in the 300ua range while most people cannot.

If a sweaty arm is placed across the main terminals of a 36V or 48V battery pack, many people, perhaps most people, will feel it. Whether they describe that feeling as a bite or not, is unpredictable.