How do I safely revive a 3.7V Li-Polymer battery? - Page 1
Forum section: electronics / repair

author: shadow_mist1
author_rank: Newbie
timestamp: December 17, 2020, 09:09:18 am
content: Hello guys, I've recently been given an old HP Slate 7 tablet which I want to boot up to grab some data off it but the battery is currently reading at about 1.05V. I know these types of batteries need to be charged at a steady current until it has reached a certain voltage (CC) and then it needs to be charged at a steady voltage (CV) but I'm kind of unsure of the current to charge it at. I want to make sure I charge it correctly using a variable laboratory power supply to prevent potential explosions and fires but I can't seem to find a charging sheet, if anyone could give me some advice that would be much appreciated thanks.

author: sleemanj
author_rank: Super Contributor
timestamp: December 17, 2020, 09:22:51 am
content: I'd probably go with C/100 until you get to about 2.7v then I'd go to C/10. If I was less patient I'd start with C/50.Where C is 3500 in your case, so start at 35mA charge current until you get to 2.7v and then 350mA.NB: by "get to" I mean the cell voltage, not the charging voltage, stop charging and then check voltageNB2: Set voltage limit to 4.2v

author: shadow_mist1
author_rank: Newbie
timestamp: December 17, 2020, 09:33:04 am
content: Thanks for the advice, I'll probably charge it up manually until I can get the tablet to charge it up the rest of the way just to be safe

author: Ian.M
author_rank: Super Contributor
timestamp: December 17, 2020, 10:18:15 am
content: You *cant*SAFELYrevive it as there is a significant risk that the LiPO cell has deteriorated to the point that its internal leakage will cause self-heating, possibly puffing, and a lesser risk of fire if you recharge it. It may fail immediately or a few charge cycles later no matter whether it is being charged, is under load or is just sitting there.Therefore, I would strongly recommend charging it out of the device, in a fireproof enclosure on a fire resistant surface, and if you intend to keep it, checking that its self-discharge rate isn't excessive and its capacity is at least 50% of nominal before refitting it.

author: amyk
author_rank: Super Contributor
timestamp: December 17, 2020, 10:21:53 pm
content: If it is a single cell that hasn't gotten into reverse polarity, and only self-discharged to that low voltage, it could be fine.

author: edpalmer42
author_rank: Super Contributor
timestamp: December 18, 2020, 12:27:28 am
content: If you only want to grab data off it, why not just unplug the battery and power the tablet with the AC adapter? I've got an old Android tablet that I use that way since the battery puffed up. The only thing you lose when you unplug it is the clock.

author: Ian.M
author_rank: Super Contributor
timestamp: December 18, 2020, 02:59:16 am
content: Quote from: amyk on December 17, 2020, 10:21:53 pmI agree the cell *COULD* be fine, but don't bet your house and family's lives on it!A colleague's household has had two totally different devices powered by single LiPO pouch cells fail due to 'puffed' batteries so far this month, one broke its screen, and the other fortunately just popped the back off. I don't have any data on how far the cells were previously discharged. Puffing is almost invariably associated with self-heating, and if a cell puffs fairly rapidly, (as opposed to slowly puffing due to old age), you've got no way of knowing how close it is/was to thermal runaway and a possible fire.*PLEASE* charge safely, and don't store the abused device (with its LiPO) or the abused LiPO near flammable objects or materials, or where puffing may go unnoticed.A further note on charging:Monitor the cell temperature- if the exterior goes over 65°C during charging the cell is almost certainly unsafe.Although you can simply keep a very close eye on it and frequently check for heating by touch, that's a PITA if its in a fireproof enclosure, and if its out on your bench, tethers you to tasks you can perform in the near area without getting distracted and forgetting to check. Its therefore preferable to automate recording if it has overheated. You can use any sort of thermometer that has a max hold function or data logging and can be kept in good thermal contact with the pack, or a small sliver of natural beeswax (which has a melting point range of 62°C to 64°C) on top of the cell as a temperature indicator, or an irreversible temperature indicating label with a suitable range e.g. this 60°C to 77 °C one:RS 286-030.

author: tunk
author_rank: Super Contributor
timestamp: December 18, 2020, 12:24:15 pm
content: To be on the safe side, you could use an even lower charge current.A simple way to do this is to use a resistor in series with the cell:if you want 1mA, use a 1k resistor and set your PSU to 2.0V. Whenthe cell voltage e.g. reaches 1.5V, increase PSU to 2.5V, and so on.When it's a bit above 3V, you could stop charging and monitor thecell voltage over e.g. day. If it holds charge, then it may be good.