equipment malfunction with old Nicads - Page 1
Forum section: electronics / general

author: loop123
author_rank: Frequent Contributor
timestamp: January 20, 2024, 05:23:52 am
content: I have an equipment that just cant run or work if the old Nicads are present. Can old Nicads somehow cause a short or something?Also is it not when the charging is on while circuit is being used. The circuit is using the voltage from the charger? So how can old bad Nicads make the circuit not work as if the circuit is depending on the batteries even when charging? Is this possible?Anyone has similar experiences?

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 20, 2024, 05:56:58 am
content: Old NiCads do tend to fail shorted.Many of those old circuits used a constant current charger (charged at 10% of battery capacity) that supplied enough current to charge the batteries but not enough to run the product. So with something like that, you'd need a good set of batteries for it to work.

author: loop123
author_rank: Frequent Contributor
timestamp: January 20, 2024, 06:30:39 am
content: Quote from: Kim Christensen on January 20, 2024, 05:56:58 amI used a 24V Meanwell GST60A24-P1J AC/DC switching adaptor. The batteries are 2 sets of 7.2v Nicad batteries. So there must be a constant current charger within the circuit. But even when the adaptor was used, the equipment couldn't run. It was only after I removed the Nicads they it could run again. Are their circuits where if there are batteries inside, the 24V adaptor DC was suppressed, and only when the batteries were removed, that the AC/DC adaptor got reactivated?

author: tggzzz
author_rank: Super Contributor
timestamp: January 20, 2024, 10:59:30 am
content: Quote from: loop123 on January 20, 2024, 05:23:52 amYes.Here's a writeup of one piece of equipment showing how the designers protected against the consequences.https://www.eevblog.com/forum/testgear/fluke-8125a-military-dmm-teardown/msg2540730/#msg2540730There's a worse example of NiCds being a pain... Tek 1502 TDRs have a (fortunately) unique PSU where the NiCd charging is controlled by a PUJT and SCR. The equipment is protected against failure by preventing the internal SMPS operating if the NiCd battery voltage is too low (i.e. discharged) or too high (i.e. missing). That means it won't work on the mains without an operational battery.To compound that, even when switched off, the batteries are discharged at 1mA. That leads to the requirement that the battery pack must either be removed or charged once a month, but not for more than 16 hours since the charger's crest factor damages the battery.Consequently, if you obtain a Tek 1502 with a battery pack, the cells have usually disintegrated to produce a hazmat toxic waste site.Shame, because the 1502 is still adelightfulinstrument to use. Oh, make sure the sealing is still intact before you operate it in driving rain or store it under 30cm of water

author: AVGresponding
author_rank: Super Contributor
timestamp: January 20, 2024, 11:25:08 am
content: Quote from: loop123 on January 20, 2024, 06:30:39 amHow are you connecting the external PSU to the DUT?

author: RoGeorge
author_rank: Super Contributor
timestamp: January 20, 2024, 11:48:29 am
content: Quote from: loop123 on January 20, 2024, 05:23:52 amYes, quite often. Metallic dendrites can grow inside, and short circuit the battery.Sometimes, the metallic whiskers can be melted by feeding for a couple of seconds a very high current through the shorted cell (the dendrites are thin, they will eventually melt, similar with a fusible), and the cell can regain its functionality for a short period. This is a survival trick only. May be dangerous to try it, and most of all, it won't last. Other dendrites will grow back and short-circuit the cell in a couple of days, or weeks at most. Tried it a few times myself, it never last, and it can't be repeated more than 2-3 times.The proper fix is to replace the old Ni-Cd cells with new ones.

author: Kim Christensen
author_rank: Super Contributor
timestamp: January 20, 2024, 03:54:01 pm
content: Quote from: loop123 on January 20, 2024, 06:30:39 amIs the GST60A24-P1J the original adapter? Sometimes the manufacturer relied on the old wallwart to be the current limiter. (They spec'd the wallwart to output only 0.1C)In that case, the wallwart output, battery, and switched load are all in parallel with no additional circuitry. So it would be up to the user to know they cannot run the unit plugged in if the batteries are uncharged or bad. If you've plugged your aftermarket adapter into such a device, and it has shorted cells, that would explain it.More modern equipment would switch the battery out of circuit and provide enough power to run the device while at the same time charging the battery via a built in charge circuit. In that case, the unit would always work when plugged in if properly designed.What is the open circuit voltage on each of these 7.2V NiCad packs? Is it around 7.2V or higher ?