One of Dependency of battery charging/discharging heat losses on the current.

Joule heating power is equal to $I^2R$, where $R$ is the Cell internal resistance.

Cell's useful power is roughly $V_{avg}I$, where $V_{avg}$ is the average Cell terminal voltage over the course of charge or discharge.

Therefore, the proportion of Joule heating losses is $I^2R/V_{avg}I = IR/V_{avg}$.

Depending on the specific cell chemistry and type (see Energy density—power density tradeoff in batteries), these losses can be about 1-2% of the cell energy capacity at 1C charging rate (in non-thermally-modulated cells).

The linear dependency of heat efficiency on the discharging current can be clearly seen on this picture:

Picture from [1]

References

[1] Enabling fast charging - Battery thermal considerations (2017)