# Is there a safe method to revive dead Li-ion batteries?

My dad has an electric leaf blower with a huge 56V battery pack. It broke so he let me dismantle it and salvage the Li-ion batteries inside. There were 16 18650 cells I got, but the voltage on all of them were about 1V. They had been critically overdrawn. I really want to keep these since they are high quality and worth like $80. I read a bunch of stuff online about recharging these guys and I've seen a ton of different answers. Some answers say that it can be extremely dangerous and to just buy new ones. Other answers talk about boosting it for recharging like there's no risk in it. Some answers say if they have been depleted recently then it should be fine (mine are also recent). Some answers say if the battery is low quality, then don't do it (mine are high quality). One answer which I found particularly interesting is that charging it slowly at <100 mA which "will reform the chemistry on the plates, but if you charge too fast it will cause uneven forming of the plates and WILL cause dendrites." which are dangerous. I don't know what is true and what is not. Can I have some clarification? Although this last answer has unknown credibility, I like that it had a chemical explanation of the dangers. So, can I recharge them at low current? I plan on using a charging module with 1 A max draw, but connected to all batteries in parallel, so the current is distributed and <100 mA per cell. I am also going to closely monitor them with an IR thermometer to make sure nothing gets too hot. • Best advice : don’t mess with Li-ion unless you know what you are doing OR have a competant person to guide you. Relying on the internet is not a good strategy considering what can go wrong.... – Solar Mike Oct 6 '18 at 23:19 • yep, Lithium Cells do catch fire and/or develop explosive gases when being chaged incorrectly. Other than that "one answer I found" is not a good reference – use the fact that we all live in (1990s style) cyperspace and with these hypertext pages you can have hyperlinks to hyperlink to other hypertext pages (which contain the statements you refer to). Citing your sources makes contextualization possible! In general, fully agreeing to Solar Mike there. If they've been discharged to 1V, there's something fundamentally wrong with how they were operated, and all bets are off. – Marcus Müller Oct 6 '18 at 23:25 ## 2 Answers They may be worthless now, but if you put series R’s to each cell to supply 200mW charge max from 1 to 3.8V or 4Vmax , you might recover$8 worth charge left or more if recently dead. You can compute the R value right?? If not V^2/P=R or 9/.2= 45 Ohms... choose 50. 1/4W. X16 pcs Then monitor voltage drop or Vbat rise vs time and record.

Then discharge and do same. Then safely toss any that are <10% of rated capacity only use match ones. If V does not rise. Then damaged but at least no more than they wont get hot.

Thermal sensing is advised in future use. And don’t drain lower than 3V and don’t use more than 1C rates on these if in the unlikeliest event that they return to life.

• With a 1A current limiting, it would distribute to 62.5mA to each cell. I just tried wiring them up to the charger and I'm getting 3.6V from my multimeter at the moment which = 225mw. So if I'm fine with 225mW, no resistors needed? – Ryan Oct 6 '18 at 23:42
• also a bit confused of which value to use for voltage. Measuring from the charging module only measures >4V, but after connecting the charging module to the batteries, the whole circuit reads 3.6V. Which do I use? – Ryan Oct 6 '18 at 23:43
• Only put batteries with same voltage before putting them in parallel. If the voltage rose too quickly to 3.6V then they have excess ESR. With no individual R current limit, there is no protection fro. A dendrite short on 1 cell at 4W with temp rise and risks – Tony Stewart Sunnyskyguy EE75 Oct 6 '18 at 23:50
• I see, thank you. I actually started charging about 2 hours ago and the voltage has slowly risen since then. – Ryan Oct 6 '18 at 23:53
• In my experience 1V is too low. They will probably never get up to full voltage. – Bruce Abbott Oct 7 '18 at 0:56

All you need to do is to apply a charge-discharge cycle and measure the cell capacity, each cell individually.

Either you need to design a charge-discharge system by yourself, get a power supply with software-controllable voltage and current limit, a controllable load, and a simple data acquisition system to measure voltage, current, and calculate the total input-output charge. Then run few cycles while checking battery's ESR, and see if cell capacity is worth anything.

Or you need to buy a dedicated semiprofessional-grade diagnostic charger that can do a full cell evaluation, something like Hitec RC charger, or similar.

Again, you better try to evaluate all your cells individually, running them in parallel is EXTREMELY UNWISE IDEA, worst of the worst, stop it. Before running any experiments, please read ALL articles from Battery University. Monitoring temperature of 18650-sze cell at 100 mA charge won't reveal any useful information, they will be cold.

• Cold until they self short if they fail, then you need a safe evacuation plan ... – Tony Stewart Sunnyskyguy EE75 Oct 7 '18 at 3:30