What is the capacity in Wh of a 3000mAh Lion battery?
Volt x Amp isn’t the answer I think because the safe voltage of a cell ranges between 2.5 and 4.2.
When a manufacturer states 3000mAh is that between the safe range? Or when totally depleted and damaged, just to be able to advertise a high number?
I’m aware that 3000mAh also depends on how many amps you pull, but even then can a cell deliver say 2A if it gets close to 2.5v, even if it’s spec states 10A continuous?
Is there some rule of thumb to calculate the real world Wh?
You don’t calculate the energy, you measure it. But often multiplying the electric charge (Ah) with the nominal voltage (usually defined as 3.6 or 3.7V) gives you a good approximation.
As Henk stated, you have to measure it at whatever specific draw level you expect to be operating at. If the cell states “up to continuous 10A discharge”, then it will sustain up to 10A discharge continuously, thus maintaining that capacity. Most of the time though, that’s the rating of a protection circuit, for the longevity and safety of the cell/battery in question.
A data sheet for the cell will probably state what cut-off voltage was used and what the current was to measure the capacity. They usually use a cut-off of 2.5V or higher and tend to understate.
Brands that rewrap and add protection might overstate the capacity slightly and the maximum continuous discharge rate.
In the context of promoting a 4000mAh 18650 cell that’s bundled in with a flashlight that has a power bank feature, it can be a bit misleading if the cell has had a protection circuit added or the flashlight has a cut-off of around 3.2V. The power bank may not get anywhere near the 4000 mAh stated.
And then you also need to consider the efficiency when stepping up the voltage to 5V, 9V, 12, 15V etc when USB-C PD is used. This relates to another question that has already been answered. The effective Wh of a battery may change depending on the voltage and current used. Power bank manufacturers may state the Wh at 5V/3A and include that in small text on the power bank or in a user manual.
Excellent observation! Since the voltage is variable across the discharge, naively multiplying capacity with voltage does not yield an accurate measurement.
However, if you view this product as the area of a rectangle with mAh and V as side lengths, it naturally generalizes to the area under the voltage-vs-mAh discharge curve, i.e., the integral of voltage against mAh-capacity.
In the graph above, the red area is the true total energy in mWh, while the blue area is the naive product of nominal voltage and mAh-capacity. In most cases, the latter is a decent enough approximation of the former.
Of course, the true total energy in practice also varies inversely with discharge current: the greater the current, the greater the voltage sag and the earlier the cutoff.
