Hi @all,
short question:
we're having some serious trouble with super capacitors (3F) leaking acid after ~2 years of operation in the field.
Is there anybody out there who had a similar problem (and maybe a solution)?
not so short question and further details:
We have a building control PLC that runs Linux OS. The use case for this device is to run 24/7, however, when power fails, we need to shut down and store some data on the flash. To achieve this, we have an on board ups that buffers power for ~15 s after power fail is detected.
The ups consists of four super caps, 3 F each, in series, giving us a voltage of 10 V fully charged. The caps are charged with constant current until charge end voltage is reached, then we keep the voltage constant. The caps also have a voltage balancing circuit.
Charging and balancing circuit is verified to work as expected, keeping the voltage across each capacitor within tolerance.
The device is in the field for ~3 years now. When doing maintenance work at a customer, our men discovered that on >75% of the controllers, the capacitors have leaked acid onto the pcb. On some boards, the acid already damaged traces and vias (see attached images). I seems that this starts after ~2 years operation.
I've done some search on the net, and the only occurrence of such a case I've found is this one here:
https://www.eevblog.com/forum/testgear/fluke-189-with-leaking-surface-mount-coin-cell/225/. Of course, Fluke replaces failing meters...
We've also contacted our distributor, which actually admitted that there is a problem (called "wetting"). However this only occurred the first time a year ago or so, and that nobody really knows what causes it to happen. He also told us that this is not a manufacturer specific problem but industry wide.
We have asked other vendors and distributors and -what a surprise- none of them ever heard of this.
So we don't know who to believe...
- Did anybody of you have a similar problem? Maybe even know what causes leaking?
- We've been told that reducing temperature and voltage will increase lifetime significantly. But our problem is not EOL because of capacitance reduction but because of acid leaking. Any ideas on that?
- Any other Ideas?
Thank you for your help!