VARTA Battery :
OK it's been resting for 24H now, so I just measured the voltage again : it settled for 9.4V . Was 10.3 just after charging, and down to 9.75V just 5 minutes later. So it was quickly going for it's final state.
OK so now it's had its first full slow charge since 30 years of storage... discharge it slowly now ? Using my adjustable current source or just sticking a resistor across it ?
I guess it does not matter, as long as current is reasonable. So I will go for C/10 again, and check back on it in 10+ hours.
C/10 is fine just don't let it get below 7V (1V per cell).
It looks like it has survived. it long sleep. A couple of slow discharge and chare cycles should brng the capacity up.
This is routine maintenence on ni-cads used on aircraft. On most aircraft batteries you have access to individual cell connections (even small packs with D cells). For a capacity recovery and cell balance You discharge until the first cell reaches 1V then clip a 1 ohm resistor across that cell. This continues to discharge the cell but stops it being reverse charged by the main battery discharge current. Next cell to 1V clip a resistor o and so on. If the voltage of a cell goes below 0.5V whill still discharging the whole battery the resistor is replaced with a shorting clip. When about 50% of cells are at 1V a 1 ohm goes cross each cell and total battery discharge turned off. Typically leave overnight at this point. Then all cells have shorting clip and left for a day. Clips remved, left for nother 12H before bing put on a C10 charge. capacity chcek and re-charge.
Seems a lot of bother but the batteries are very expensive. Individual failed cells can be replaced. When I started this was all done manually including motitoring and adjusting the current with a rheostat. Now they make fully automatic systems. And batteries are slowly moving to Lithium.
Interesting, thanks for that !
Just plugged the battery to the current source to discharge it.
Took two attempts... my first idea of how I should connect the battery to Sir Wilson to discharge it, was not successful, zero current registered.
Then tried something else and it appears to work... see pics / schematics below, and have a good laugh if so you must... I can take it.
Bench DMM is monitoring battery current, and the analog meter monitors battery voltage.
So loaded with C/10 ie 11 mA, as planned. That made the battery voltage go down to 8.8V at first, then after 30 minutes or so... it's down to 8.6V, so is sinking fast eh ? Hmmm...... not looking too promising, but we shall see how it goes !
Will do a second charge/discharge cycle regardless. It sat for 30 years so it deserves some kindness on my part !
EDIT : Oops my bad ! Wiring is messy, the schematic originally showing what I THOUGHT I had done to get it to work... was incorrect. Apologies to those who looked at it. Schematic corrected / re-uploaded.
In the end there is no messing around required... battery is simply connected up as if it were any regular load, between the output of the mirror, and ground, just with the positive terminal connected to ground.
So I was just overthinking it, it's all very simple in fact !
Only thing is that I messes up the adjustment range. With no load , i.e the the mirror short -circuited, range was as I said earlier, 3.7mA up to 13.5mA IIRC. Now with an active load such as this battery, range is about 4.7mA up to 22mA. So a wider excursion.
EDIT #2 : Oh, and the joys of making experiments in a controlled environment..... let's do something cool shall we ? What is the output impedance of a 30 year old NiCd 9V battery when it spits a few mA ?
Since I can adjust the current at will, and watch battery voltage at the same time, it's easy to do ! At least a decent approximation of it. So :
At 20mA battery voltage is 8 Volts.
At 7mA it's 8.4V.
So a 0.4V drop for a 13mA increase in current. If I have my PhD maths right, then that's about 30 ohms. How good is that, any idea ? Sounds a bit high to me ? Maybe the battery did degrade over the 30 years after all... We shall measure again when we do the second discharge/discharge cycle tomorrow.