Yesterday Mike Bennett and I worked on a load tester for my nicad pack in my shop. It loads each cell one at a time with around 40 amps. Which is 1C. The goal is to find bad or failing cells when the pack is discharged down to the first drop in pack voltage. This occurs around the 5 mile mark. So I know many cells are croaking at that point.

We bought a 100 amp, 12v lead acid load tester from Harbor Freight for $25 on sale. We ended up using bailing wire as an additional load element. I’ve used bailing wire before for discharging cells. It works fine. It’s a soft wire that when it turns red, it still soft. So it stays consistent. We did some empirical testing to see how much bailing wire we would need to add to get 40 amps at around .6 volts. We came up with two 11″ pieces in parallel. We crimped on 1/4″ yellow lugs and bolted them to the large copper crimps already in the load tester. That worked great!

The meter ranges from 0-16v. As it turns out it was fairly accurate around the .5-1.5 volt range. But that was too small of a needle swing for me. So at a later date I will recalibrate the meter by 10x. It will then read 0-1.6v. Perfect for nicad testing. It has a 51 ohm 1/8 watt resistor across the meter movement. So that needs to be changed to a 470 ohm pot so it can be calibrated nicely.

We load tested a couple of cells continuously to see if the bailing wire would fail. It got hot, but never glowed. So I think the only weak link is the rocker switch moving 40 amps through it. Since having a rocker switch would require 2 hands to operate the load tester, I may very well bypass the switch. Then the load tester can just be rocked only the 2 cell terminals and that will activate the load tester. There are 250 cells to test. So speed is everything!!

This load tester could be tweaked for use on lithium too.

Yin wrote me today to let me know that I saved him $1230 in repair bills on his THS720 scope. He had the same problem that mine had back in 2009. He found this blog with the repair entry through Google. It felt great to hear that my work helped someone else! Then he went and bought my other THS720 scope! Thanks Yin!

I am selling a great Tektronix THS730 scope. It’s here on Ebay.

My old faithful Fluke 83 from the mid 1990’s had a display problem that is sometimes described as ghosting. It was much lighter in color than normal. Some segments were nearly faded completely away. I found this thread. All I did was take a polymer eraser, the white ones from many mfr’s, and cut it to a more pointed shape. Then carefully cleaned the places on the circuit board where the lcd makes contact. These contacts were made visibly brighter by the eraser. I then cleaned up the contacts for the push buttons as well. They were never a problem, but worth the cleaning since I was in there anyway. I cleaned the carbon tips of the buttons gently with the eraser as well. I never took the old original pink elastomer off of the lcd nor did I clean the lcd. Just gently cleaning with the polymer type eraser of the pcb contacts was all that was needed. The display has never looked so bright and crisp!! We will see how long this repair lasts!

There is a replacement kit on Ebay from a vendor named A-fluke. The kit these days is about $24USD. It supplies instructions and the newer gray elastomeric parts for the 80 series of Fluke meters.