If a Li-Ion cell has 3000mAh capacity shouldn't I be able to pull 3A from it for 1 hour?

Question

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.

Answer 1

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.

Answer 2

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.

Answer 3

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.