18650 capacity test - Page 1
Forum section: electronics / projects

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: July 13, 2019, 09:52:56 pm
content: I've got some random salvaged 18650's that I'd like to test for actual capacity, however I am not sure to what voltage they need to be charged prior to discharging them and at what current and to what voltage should they be discharged?Is there some kind of standard for 18650 capacity testing or does every manufacturer go by their own criteria?

author: sleemanj
author_rank: Super Contributor
timestamp: July 13, 2019, 10:54:55 pm
content: If they are used then even if the manufacturer had a published standard it wouldn'`t be very useful.Measure voltage discard any 0v. Charge them fully (using a proper charger, do the bulk charge slowly say 500ma, and preferably in a suitable explosion containmemt container, check after a few minutes to be sure it is accepting a charge), disconnect let sit for a week and remeasure, it shouldn't have dropped too much (testing self discharge), use a cc discharge at 1a and measure capacity down to 3v, decide if that meets your needs or it's too low capacity to be useful, you might go to 2.7v but diminishing returns.If it loses more than 0.2v in a week from 4.2v I'd say trash, if it's capacity is less than 600mAh not really worth it, and if it never completes charging obviously trash., but if it charges, holds, and has a decent capacity at 1a, all good, charge it back to a storage level and put it in your stash (suitable explosion containment stash of course!)

author: Audioguru
author_rank: Super Contributor
timestamp: July 14, 2019, 01:30:03 am
content: An older 18650 lithium-ion cell is charged at its recommended current (500mA is fine but might take hours) to 4.20V, then it is about70%fully charged. Then the charging current must be monitored and the charging must be stopped when the current has dropped to about 33mA. If its voltage has ever dropped lower that 3.0V to 3.2V then it is ruined.Newer Lithium-Ferrous is charged to a lower maximum voltage and newer Lithium-Graphene is charged to 4.35V.

author: sleemanj
author_rank: Super Contributor
timestamp: July 14, 2019, 01:43:34 am
content: QuoteI have quite a few cells with plenty of capacity (up to 2Ah) that were salvaged after having sat at 1v or even less for long LONG periods of time.As long as you take care when you initially charge and characterise, there's no problem. Deep discharge (at least as a one-time) on it's own really isn't as damaging as the old wive's tales might imply.

author: Siwastaja
author_rank: Super Contributor
timestamp: July 14, 2019, 02:35:26 pm
content: For random salvaged 18650 cells, assume:If initially <2.5V, discard the cellCharge at CC= about 600-700 mA, CV = 4.20V, stop charging when current tapered down to about 100mATo measure capacity, then discharge at about 1A max until V=2.80V.These assumptions are based on guesstimating the capacity as 2Ah, max charge rate as 1/3 C and max discharge rate as 1/2 C. Any unused cell from 2000's will do better, but it's better to derate because of aging.Finally, almost all 18650 have printed markings, you can actually find a datasheet for it. Knowing the original capacity helps as you can decide the state-of-health based on the percentage left. 2000mAh is a good condition for an originally 2200mAh cell, but quite bad for a 3000mAh cell.

author: SiliconWizard
author_rank: Super Contributor
timestamp: July 14, 2019, 03:04:19 pm
content: Additionally to what Siwastaja said:- If the cell embeds a protection circuit, the output voltage may read close to 0V when the cell is fully discharged (open circuit), not necessarily meaning that the cell is dead. So you can still try and charge them if in that case. If you don't know how long they have been stored fully discharged though, you can't assume anything. If it's been too long, they will most likely be dead.- While charging, if the state is unknown, especially if the cell is fully discharged, I would start with a lower charge current, something like C/10 or C/8. If the cell voltage hasn't significantly increased after 30min to 1hour, I'd consider the cell dead. If it has, then you could go for a higher charge current such as C/2 and let the charge cycle finish (CV when it reaches 4.2V, then stop when current drops down to C/10 or so...) This is basically what many integrated LiIon/LiPo charger ICs do, so if you're using a decent one that can "trickle charge" fully discharged cells, you may not need to do anything but try and charge the cells with it and let it handle charging errors.

author: tunk
author_rank: Super Contributor
timestamp: July 14, 2019, 03:28:38 pm
content: QuoteSame here. That said, you may want to charge them with maybe 10-50mA until they reach 3.0V.As for self discharge, I guess it depends on what you're going to use them for.

author: drummerdimitri
author_rank: Frequent Contributor
timestamp: July 14, 2019, 06:15:49 pm
content: Thanks for the replies.I've found the datasheets of most of the cells and will try to imitate the discharge characteristics of the OEM and compare the actual values with the original ones to check their usefulness before implementing them into projects.