Brain twister? Polarize an amp-meter?

This had me thinking I was getting senile. Heck . . maybe I am.

I have a Harbor Freight 500 amp battery load-tester. Had it a few years and always worked fine. I have an AC Delco I bought new for $200, 40 years ago. This HF unit looks to be a clone and only costs $50.

Just an adjustable carbon-pile with a voltmeter and ampmeter.

So, here is the story. Always worked fine. Yesterday - I was teaching a class on solar electric to some teenagers. Brought the HF load-tester along so we could drain batteries when needed for certain tests.

All of a sudden - the amp scale started reading backwards. If I put a 50 amp load on a battery, it would read minus 50 amps but the analog voltmeter still worked fine. I then reversed the connections to the battery. Neg tester lead to POS on battery, etc. Now- the amp-reader read correctly but the analog voltmeter did not work at all.

This is a pretty simple device and I do not comprehend how this is possible.

So, I really was thinking I was going a little nutty and maybe it never worked since new and I never noticed?

I pulled it apart and as I expected, the amp meter is shunt wired. So, I reversed the leads. Tested it again and it STILL read backwards.

To add insult to injury, I put it back to the way it was before and NOW it is working fine.

What am I not seeing here?

Any chance you were testing a battery that was charged backwards, then later tried it on a different (correctly-charged) battery?

Could it have been a bad connection? Have seen some crazy stuff where they "sample" off of a shunt or some such. Electricity can be weird.

Nope. Same battery at the time of the tests. If someone else had described this situation, I would of figured it was not possible and maybe the person telling the story was a moron. Hmmm. Hope I did not just insult myself.

Even if there WAS a backwards charged battery - the voltmeter could not work since it is analog.

had the same thing happen..on that exact model..... I wiped off the lens with a rag and put a super static charge on the plastic lens... And it was this model. Finally found a lens cleaner for big screen tv's that stop/cleared/cleaned the charge on the lens and the meter then read normally. seems to only happen when cold.

It happened because you had an audience.

sotxbill, I think you are on the right track.

Place I used to work had a motor amp load meter on one of the machines.

I could wipe a rag across the plastic cover and the needle would go nuts!

It might go somewhere in the middle, or draw down below zero.

JDEM, FWIW, I've had the same HF tester as you since last summer, and I'm satisfied with it.

Seems to work as well as an older (collectable) SUN unit that I've semi-retired, don't drag it around outside or on service calls anymore.

That's why I bought to, to "save" the 'ol SUN unit.

The HF unit works fine. Even when reading backwards. Just an odd thing. I have two older carbon piles. Both are 30-40 years old. AC Delco
and an Auto-Meter. When beating around, I take the $50 HF. Never had any of the older one read backwards. Might be some different
circuitry in the modern HF. I admit I cannot see the electrons inside that amp panel meter.

In both of your pictures showing negative current readings the voltmeter is indicating about 12.8 volts so there is probably not a heavy load on the battery. In the picture showing +50 amps the voltmeter shows much lower voltage indicating there is a heavy load on the battery. When the ammeter was reading backwards did you try increasing the load far enough to make the voltmeter drop? Perhaps the static charge on the plastic meter face caused the negative ammeter reading & increasing the actual current would have overcome that to make the meter read more normally. This theory + 50 cents might be worth a cup of coffee!

The ammeter is really a sensitive voltmeter measuring the small shunt voltage. You already knew that. It will not take much erroneous voltage at the shunt for it to read incorrectly. The voltmeter side of things gets its power from a separate set of wires running from the battery clamps next to the main cables so it is going to get 12 volts no matter what. The source of the problem has to be on the 12 volt side, not the ammeter or shunt side. The electronic circuit board is the timer since it has that buzzer. The timer would get 12 volts from the voltmeter side and has to be triggered by something which would have to be on the ammeter side of things. Has to be triggered by the shunt voltage. I assume that is what the red wire with a white cover is doing. It connects the shunt to the timer and triggers the timer as the current start to flow through the carbon pile. I expect the fault is with the timer circuit feeding some of its 12 volts into the shunt. If it continues to read backwards the just cut that timer wire and count the 15 seconds. I think that will fix the problem.