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18650 Batteries

18650 BatteriesLithium-ion 18650 rechargeable batteries can store a lot of energy, but nowhere near what many eBay sellers are claiming. As of this writing, the maximum real capacity is 3600mAh - any claims over that are BS (e.g. 9900mah Etsair's).

I knew that when I bought my first 18650, but the SkyWolfEye "5000mAh" battery (eBay pic) with charger was dirt cheap. I then got what looked like a generic "2200mAh" because it was a realistic number (also cheap). I wouldn't have bought it had I known it was labeled UltraFire (most are fakes).

Wondering how to tell what the real capacity of my batteries was, I found that the weight of the battery is a good indicator. While somewhat informative, I wanted to know more after winning an auction for a couple of Cooligg 2500mAh batteries.

ZB2L3 Discharge Capacity Tester

Discharge TesterAfter reading Syonyk's post on the ZB2L3 battery capacity tester, I decided this inexpensive ($3.40 on eBay) tester could tell me what I wanted to know. It is currently mounted to a board along with a cheap charger base (bypassed electronics) so that I can test various cylindrical batteries. The tester seems to be reasonably accurate, but the preponderance of low readings has me thinking I should get at least one brand-name (e.g. Samsung) battery. The following results were done using a 20 ohm resistor* (~.2a load) and a 3.2v cutoff (+/- buttons before OK/start button)... Added a 75 ohm (~.05a load) column and started using a 4.17 start voltage (n*).

Brand/SizeRatingActual 20Ω/.2aActual 75Ω/.05a
SkyWolfEye50001127      
UltraFire22001308/1344*1323/1422*
Cooligg25002517      
403040500332/378*425/410*
402035250166204


*The stock resistor is 75 ohms which won't put much of a load on anything (~.05a @ 3.7v) - testing an 18650 would take days (...on the other hand, it would be good for testing small batteries with a low discharge/C rate, e.g. the bottom two in the chart). Currently you could get 5pcs of 5 ohm 10w Ceramic resistors for $1.19 delivered. At 3.7v that's a .74a load w/ 1 resistor, .15a w/ 5 in series and 1.48a w/ 2 in parallel. Lots of possibilities, 3w resistors is probably the minimum.
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