Resetable fuse (Poly fuse) for lithium pouch cell - Page 1
Forum section: electronics / projects

author: Wilson__
author_rank: Frequent Contributor
timestamp: April 27, 2025, 11:39:19 pm
content: For 'extra safety' would it be good to put a resetable fuse in series with Lithium pouch cell of (a) 140 mAh (b) 1000 mAh respectively? Checked and many open source circuit does not have fuse, so it is 'extra safety'.Littlefuse 0603L050 resistance is 0.1 to 0.68 ohm.For a slow charging rate (about 0.5C) of 70mA and 400mA. Charger datasheet states charging will end when current drops to 10%, 7mA and 40mA respectively. Volt drop across resetable fuse is 5 mV and 27 mV.Charger datasheet states cell floating voltage is 4.2 +/- 42mV. So, the 27mV drop on the resetable fuse will only leave the cell very slightly 'non-full', right?Many thanks

author: mikerj
author_rank: Super Contributor
timestamp: April 28, 2025, 12:09:38 pm
content: The common protection circuits used with pouch cells such as the DW01A include over-current and over/under-voltage protection. If you have a raw well without one of these protection circuits than I would advise adding one.

author: Wilson__
author_rank: Frequent Contributor
timestamp: April 29, 2025, 07:31:58 am
content: Many thanks. I have the normal electronic over-current and over/under-voltage protection.I am considering "extra safe" protection, work on different principle, based on simple physics, if the resistance does NOT affect the end-of-charging detection mechanism of the charger

author: mikerj
author_rank: Super Contributor
timestamp: April 29, 2025, 09:21:00 am
content: A small amount of series resistance from a polyfuse shouldn't affect the end of charge detection to any significant degree as it's a constant voltage phase and current is low.

author: Wilson__
author_rank: Frequent Contributor
timestamp: April 29, 2025, 11:56:31 pm
content: Many thanks. I believe ending is fine as threshold detection current is 10% of fast charge mode (2 hours charge rate is not fast but that is the wording on datasheet)I wonder if the charger chip may got confused during the switching threshold from CC (constant high charging current) to CV (constant voltage) mode.Without the fuse resistance, at the threshold and switching from CC to CV, the charger chip will see no change on the "detected" voltage .With fuse, charger chip will see a drop in "cell voltage" ( actual cell voltage plus fuse resistance drop) as charging current changed away from high charge current.

author: thm_w
author_rank: Super Contributor
timestamp: April 30, 2025, 12:49:49 am
content: Quote from: Wilson__ on April 27, 2025, 11:39:19 pm27mV undercharge is nothing. Its going to be 95%+ full, but sure, slightly less full than having no voltage drop.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 30, 2025, 07:27:43 am
content: Polyfuse doesn't make a lot of sense IMHO. You should have the proper protection circuit which acts as e-fuse, so the polyfuse should never trigger - so you would not need the self-recovery feature either.Fusing is valuable to protect against fire in the case that your primary protection circuit (MOSFET based switch probably) dies - and it will die as a short circuit, so your product is doomed to be replaced anyway. In that case, good old normal fuse is better specified in every respect than a polyfuse, and you don't need the self-recovery feature.tldr: use normal fuse.

author: Wilson__
author_rank: Frequent Contributor
timestamp: April 30, 2025, 11:04:59 am
content: Many thanks. That is correct. Old normal fuse in SMD package will do

author: artag
author_rank: Super Contributor
timestamp: May 01, 2025, 04:31:58 pm
content: I've put a polyfuse on a recent design because it's wirelessly charged, and since Apple screwed up the Qi ecosystem with their non-compatible chartgers its become very common for these systems to get warm, perhaps excessively so.The idea of the polyfuse is therefore to reduce the current and/or the charging voltage endpoint if the battery/coil system is getting hot. You could consider it a PTC rather than a polyfuse .. they're pretty much the same thing but those speficied for use as a polyfuse seem to have lower cold resistances.I'm interested to hear if other people think this is a good/bad/ineffective idea.

author: thm_w
author_rank: Super Contributor
timestamp: May 01, 2025, 10:07:43 pm
content: Quote from: artag on May 01, 2025, 04:31:58 pmI think its a good idea for charge limiting, or other non-critical rate limiting (LED brightness maybe), if you don't want to rely on MCU to regulate it.The issue is if its used in critical areas, you'd want to do a full environmental test on the product and it behaves as expected. eg if you say the device will run up to 75C, then your polyfuse can already be 50% of the trip point, same idea at low temperatures. It can easily bite you.