Meduim load tester.......

Peter KG6LSE

AS to not railroad the other thread on NiMH load testing .


xanadu and I have decided on a home brew load tester.

Here are some prelim specs ..


50A max . @ 12V ( for testing UPS AGM batts
Full A/Ds on the current and voltage .
low volt cutoff .
ran on a P1 !!!!!!!!!!!!!

( I assume a LCD screen to display data ! ?

Peter...

LoopyByteloose

50 Amps at 12 volts might be enough to make popcorn as a by-product.

I really think having 10 NiMH in series and under a high load is just going to destroy at least one cell. If the polarity reversal doesn't eat one cell, the temperature will likely go out of bounds and do damage.

You might add temperature monitoring to each cell.

xanadu

It definitely needs at least a two line LCD

Loopy it will be programmable down to 100ma. 50A would be for testing SLAs or ATX PSUs.

A safety feature easy enough to add would be that you input the number of cells and their voltage before it will work. Or it could be used only with individual cells. Monitoring the load temps and batteries temps won't be an issue so it can be relatively smart and not get anyone into trouble.

Time to finish up the current sensor project I started three years ago

LoopyByteloose

It guess Boeing and the 787 Dreamliner team thought that individual cell temperature wasn't an issue either ... until it became apparent that the cells in the middle were the ones that were bursting into flames.

Theirenew design has deployed more space and thermal insulation between cells... a passive solution. Liquid cooling might even be better if sudden high demands of power are required, or quicker charging.

NiMH in the AA package seem to have a thermal sensitivity to failure due to the thinness of the coatings on the central anode. I can't prove this, but reading between the lines in enough technical documents that explain their construction indicates that might be the highest point of failure due to use (as opposed to just leaking away while sitting unused).

You really have to think like a chemist and look at each component in terms of chemical stability to understand failure in a battery. Chemistry and ion exchange both are very involved in thermal events and the thermal events age the battery away from the optimal design. (In Lead Acid, the sulphur in the H2SO4 electrolyete begins to mix with the Lead and you have a sulphated cell that will no longer hold a full charge.)

In other words, the chemical equation that looks so beautiful on paper with charge and discharge being balanced, isn't exactly what the real world provides. That is close to how the battery begins its life, but impurities and excesses of charging or discharging create variations that don't behave. And in some cases, the geometries of construction eventually kill the battery.