Replacing lithium cells  - Page 1
Forum section: electronics / beginners

author: NOON Design
author_rank: Contributor
timestamp: October 20, 2023, 04:54:41 am
content: I'm very experienced with some aspects of electronics repair but a noob when it comes to the care and feeding of lithium batteries. I recently picked up a decent quality Bosch stick vacuum that works well, apart from only giving a few minutes before hitting low battery cutoff. It's got about 25v worth of 'standard' lithium cells (18650?)Can I identify a weak cell and just replace that one or do I need to replace all of them?Any recommendations for sourcing decent quality cells at a good price in Australia?Anything else I should know?

author: BlackICE
author_rank: Regular Contributor
timestamp: October 20, 2023, 06:25:23 am
content: If you know what you are doing, buy quality replacement cells and are careful it may be safe. If you aren't then you potentially have a fire hazard! The cells have have be balanced in charge and capacity and the BMS working correctly. Also you have to have a way to weld the new batteries in place. Soldering them can be problematic.Even commercial sold products have been known to catch fire and even burn down bulidings!

author: Mpegger
author_rank: Contributor
timestamp: October 20, 2023, 06:48:07 am
content: When dealing with lithium-ion batteries, very bad things can happen when individual cells go bad in a pack, or in the not so bad case, the internal protection circuit of the pack just prevents the pack from being used at all (dead pack). Seeing as how the pack is still usable but barely has any run time, its more then likely that the entire pack would need to be replaced, not just 1 individual cell.I don't know about Australia, but here in the States theres a number of reputable vendors who deal with used battery packs for harvesting li-ion cells at reasonable prices. Like old server hardware, when companies get new equipment, perfectly working used, and sometimes practically new equipment, like battery packs, get sold off for pennies on the dollar by the pallet. So you can get great deals on individual li-ion cells if you're willing to go that route.My last piece of advice if your gonna try replacing the cells yourself,do not try to solder directly to the cells. Li-ion cells are VERY sensitive to high heat and a soldering iron, even a low wattage one, is more then enough to cause thermal runaway, which leads to fire or explosion. Can it be done? Yes. But be aware that you are taking a big risk doing so. Spot welding cells is the way, or get cells with tabs built in.If you cannot reuse the original pack case, you could always just use some 18650 battery holders (don't forget a balance/protection circuit) and make your own "pack" to power the device. It might not be pretty or look OEM, but at least you have a working device. Heck, you can go overboard and make a pack with 2 or 3 times the capacity if you want.

author: tunk
author_rank: Super Contributor
timestamp: October 20, 2023, 11:02:27 am
content: Open the battery and measure the voltage of all cells.If one (or two ..) is lower, it could have (slightly)higher self-discharge than the others, having beendischarged over the years. If that's the case, you couldtry to charge only that cell(s) to the same level as theother cells. If all have the same voltage, the batterymost likely is dead.Replacing one (or a few) cells is difficult as you willhave to match manufacturer, model number, actualcapacity, internal resistance, etc.

author: wraper
author_rank: Unknown
timestamp: October 20, 2023, 11:09:37 am
content: You need to replace all of the cells. Even if you buy exactly the same type, mixing old and new cells won't give satisfactory result. Note that BMS may disable the battery once you disconnect the cells.