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I have these brand new Li-Ion cells that are rated for 3000mAh and I'm trying to get their actual capacity. I have a discharger and its max current it can draw is 2A. However when I set that and run the discharging process it typically never exceeds the range of 1.6 to 1.8A.

In one example I measured on average 1.7A draw for 46 minutes. This was set to stop at 3V. In the first 30 seconds at 1.6A the voltage drops from 4.2V to 3.7V.

This seems terrible for the 3Ah rating. If I'm pulling 1.7A that means in theory it should last 1.8 hours right?

Just trying to confirm if I'm thinking about this correctly. The charger/discharger is an iMax B6AC.

edit:

The full product "name" is:

EBL 3.7V Li-ion Rechargeable Batteries 3000mAh 18J Lithium Battery

Although I can't seem to find much about it.

I will do some lower current draw samples eg. 0.5A 1.0A and see how it goes. I guess this battery is made for toys/torches(flash light) not sure what kind of operating current is normal there.

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  • \$\begingroup\$ This looks legit, what is the brand of the batteries? Can you post a link? \$\endgroup\$ – bobflux Mar 16 at 7:38
  • \$\begingroup\$ I bought it from the Bezo's online store it's called EBL 18J 3000mAh they have integrated chargers. Only reason I bought these is 18650 batteries seemed to have disappeared from the store... next time I will buy those good cells eg. LG MJ1 \$\endgroup\$ – Jacob David C. Cunningham Mar 16 at 7:43
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    \$\begingroup\$ Check the datasheet. It should have discharge curves at different discharge rates. \$\endgroup\$ – winny Mar 16 at 8:24
  • \$\begingroup\$ Haha... yeah data sheet. I think I would have to tear this battery apart to find any info about it. The manual itself doesn't say anything about discharge/C-rating. I'll just record different discharge rates and move on with my life/remember to buy from more reputable places. \$\endgroup\$ – Jacob David C. Cunningham Mar 16 at 8:29
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In theory yes.

However, batteries have internal impedance and testing is a fickle thing. What can happen is that manufacturers use a test that actually runs the battery for for 10 hours at 1/10th that current and then uses those numbers to back-calculate what the 1 hour amp-hour rating. Since the 10 hour run operates at lower current, the losses in the resistor are less.

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  • \$\begingroup\$ I see, I was not aware of the 10hr 1/10th capacity thing. Is it correct for me to discharge from 4.2V to 3.0V and assume that equals 100% capacity. That seems to be the case from what I've seen. And I don't know if you can say that it's linear eg. I can estimate/extrapolate from my 1.7A current draw how it would do with 3A(probably get worse). I don't want to say the batteries are bad if I can't confirm that they're bad... but I'm pretty sure they are bad. \$\endgroup\$ – Jacob David C. Cunningham Mar 16 at 7:38
  • \$\begingroup\$ 4.2V to 3.0V under no load. But if you are measuring under load it would be lower due to the internal resistance. Based on what I described it is in practice not linear. If you discharge at ever increasing rates higher than their actual test discharge, you will get increasingly shorter run times than the linear model would suggest. If you discharge at ever decreasing rates lower than their actual test discharge you will get increasingly longer run times than the linear model. \$\endgroup\$ – DKNguyen Mar 16 at 7:40
  • \$\begingroup\$ Yeah I can't seem to guarantee it. I wanted 2A but the discharger is stopping at 1.8A at most... not sure why/maybe it adjusts to the voltage it reads?... I think these batteries are bad... \$\endgroup\$ – Jacob David C. Cunningham Mar 16 at 7:45
  • \$\begingroup\$ @JacobDavidC.Cunningham More likely they are just lying labels. \$\endgroup\$ – DKNguyen Mar 16 at 7:46
  • \$\begingroup\$ Yeah... it sucks. It's hard to fight too. I got screwed in laptop replacement batteries 3x from 3 different sellers... reviews get removed oh well. Anyway thanks \$\endgroup\$ – Jacob David C. Cunningham Mar 16 at 7:47
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I cannot think of a sane 18650 cell that cannot output solid 2 ampere when fully charged.

What goes wrong if you cannot set 2A current:

  1. You hit the total dissipated power limit of the imax B6AC. Not sure about B6AC, but the similar B6 model can discharge batteries at no more than 5W, meaning ~1.4A for a single li-ion cell. The fact you get 1.7-1.8A shows that a lot of the voltage of the cell is lost to wires, see below.

  2. You use wires/clamps/connectors inadequate for 2A. You lose ~1V in the wires and if you pull 2A, the discharger simply cuts off because it thinks the battery is fully discharged. Running the discharge process at lower current gives some results, but they are compromised by the early cutoff.

  3. Your li-ion cell is not up to its label.

Repeat the test (both full charge and discharge) with sane cables - 0.75 or 1.00 sq. mm or their awg equivalent, no more than 50cm in total.

If the cell still cannot deliver its 3000mAh +/- 10%, at 2A discharge current, consider it low quality and use it accordingly.

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    \$\begingroup\$ You have different battery layouts, electrode thickness, electrolyte content, insulation thickness. 18650 is merely a specification for physical size. Some batteries are designed for high energy density and low discharge rate, some are designed for high discharge rate at the tradeoff of lower energy density. This has to do with the construction of the battery itself. You can have 18650 rated at 25A as well as 1A. \$\endgroup\$ – Damien Mar 16 at 9:21
  • \$\begingroup\$ the voltage lost to cell could be something, I am using these 24gauge braided wires to charge with granted I read up on those and they can support like almost 30Amps. I think I won't be able to hit 3A with the discharger since the display/buttons won't go past 2A. I could make something to pull 3A but yeah. Since I can't seem to find any data on this battery/does not just say "1C" or something... I'm inclined to believe current draw is not its strong suite. \$\endgroup\$ – Jacob David C. Cunningham Mar 16 at 9:30
  • \$\begingroup\$ @Damien indeed. But I am yet to see a cell in 18650 form factor rated at less than 1C for a maximal discharge rate. \$\endgroup\$ – fraxinus Mar 16 at 10:04
  • \$\begingroup\$ @JacobDavidC.Cunningham 24 AWG is like 0.25 mm2 - it won't melt at 2A, but one cannot rely on voltage readings unless using balance wires for the voltage feedback. And 30A will melt 24AWG copper for sure. Go for at least 17AWG for the B6/B6AC full capabilities (5 ampere, that is). \$\endgroup\$ – fraxinus Mar 16 at 10:09
  • \$\begingroup\$ Alright thanks for the tip. I did double it up at least but yeah I wasn't sure if that was affecting my efforts using the relatively thin wire. \$\endgroup\$ – Jacob David C. Cunningham Mar 16 at 15:04
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Rated capacity != Usable Capacity.

Different chemistry have different rates of usable capacity. That is, if you over-discharge a Li-ion to pull the rated capacity, you would kill the battery, so a Li-ion usable capacity is actually lower than a lead-acid at the same rated capacity.

Here is more detail and here

In addition, you have other effects like internal resistance, self-discharge, temperature, age, quality, rate of charge and chemistry efficiency that can have significant effects.

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  • \$\begingroup\$ Well this is all good information. I think I bought this particular battery for the wrong purpose. Unfortunately seems like it's hard to source Li-ion cells on Amazon right now. I will look else where in the future. I will also keep in mind the C discharge thing, I will also consider over spec-ing a battery for constant current vs. burst current/full capacity. \$\endgroup\$ – Jacob David C. Cunningham Mar 16 at 9:06
  • \$\begingroup\$ It's definitely a good idea to over-spec the battery, double would be safe. keep in mind the batteries lose efficiency over time, can become unbalanced, and so forth. \$\endgroup\$ – Damien Mar 16 at 9:12

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