NiCd 2.5Ah discharge curve. Terminal voltage increase during 1A discharge? - Page 1
Forum section: electronics / general

author: AlfBaz
author_rank: Super Contributor
timestamp: May 25, 2016, 02:23:49 am
content: I'm "reconditioning" a Nickel Cadmium 2.5Ah battery pack out of a piece of test equipment I'm repairing for someone. The pack is made up of 10 cells and was found at 0V when I first measured it.Initially I put 100mA through them and checked for major shorts. They seemed OK so I gradually increased the current to half an amp and checked the temperature (finger thermometer) of the cells as the voltage increased. Only a slightly perceptible increase in temp above ambient on a few of them. Once the voltage reached 13V I disconnected the current supply and the voltage held.I then connected the pack up to my Maynuo M9812 dc load, fired up its software set the safe voltage to 6V (0.6V per cell) and drew 1A out of them. The pack quickly ( a few minutes) dropped down to about 8.8V where it sat for the majority of the discharge. Whilst sitting around this voltage and drawing 1A I started seeing the terminal voltageincrease. Not much mind you, around 300 to 500 mV so from 8.7 to 9.3V all whilst drawing 1AI haven't played much with NiCd's before and certainly wasn't expecting to see a battery's voltage increase whilst discharging it. Anybody know what's going on here? I don't know whether this something to do with the chemistry or a mix of damaged/unbalanced cells coming to some form of equilibrium

author: johansen
author_rank: Super Contributor
timestamp: May 25, 2016, 05:53:53 am
content: if you can figure out how to add water to those cells you could get another life out of them.hmm.. ~0.050" carbide drill bit.. inject water with a needle.. solder over the hole...

author: Kaptein QK
author_rank: Regular Contributor
timestamp: May 25, 2016, 08:30:54 pm
content: Quote from: AlfBaz on May 25, 2016, 02:23:49 amInternal resistance lowers as the cells heat up during discharge.

author: AlfBaz
author_rank: Super Contributor
timestamp: May 26, 2016, 01:51:10 am
content: Quote from: Kaptein QK on May 25, 2016, 08:30:54 pmAhh ok, thanks

author: AlfBaz
author_rank: Super Contributor
timestamp: May 26, 2016, 01:57:47 am
content: Quote from: johansen on May 25, 2016, 05:53:53 amPulled the pack apart, one cell was dead. In the process of deep cycling it. It's getting better but I don't think it'll get anywhere near its rated capacity.The pack must be really old. It's an RS Components branded pack. I found reference to the RS catalogue number in a pdf published by a vintage radio mob, showing obsolete RS catalogue numbers along with possible RS alternatives. The replacement number shows up on RS as obsolete

author: NiHaoMike
author_rank: Super Contributor
timestamp: May 28, 2016, 12:34:26 am
content: Replace with NiMH?

author: Ian.M
author_rank: Super Contributor
timestamp: May 28, 2016, 12:57:02 am
content: The charging circuit may well not be NiMH compatible. E.g. the typical 0.1C tricle charging current for NiCd will kill fully charged NiMH cells fairly quickly.Its still possible to source most types of tagged NiCd cells, so, if the economics justify it, I'd rebuild with new (or at least cycle tested NOS) cells.

author: AlfBaz
author_rank: Super Contributor
timestamp: May 28, 2016, 07:13:33 pm
content: Thanks guys.Ian, It appears the tabbed sub-C cells a readily available the only problem I might have is wether or not I can get away without having to spot weld them, like the originals. I'll contact the owner of the unit to see what he wants to do.I could look for a similarly rated NiCd pack though the form and size of the original fits into a nicely engineered holder which I'd like to re-use

author: Ian.M
author_rank: Super Contributor
timestamp: May 28, 2016, 07:26:26 pm
content: Unless the cells are a very tight fit in the pack, you should be able to simply solder to the tags. Maybe use copper braid to get enough CSA without too much thickness.