[ltp] Measuring battery health (was: Suggestions for battery sources)

[Stefan Monnier]

> My T61 is 2.5 years old, and by that time I bought a noname 6 cells
> battery “for every day use”, saving my lenovo 6 and 9 cells for when I
> really need them (I keep them mid-charged, and use them from time to
> time). The noname one is always kept in my ThinkPad, using smapi
> controls to keep it between 30% and 70%.

> At that time, it's still pretty fine:

> corsac@hidalgo: acpitool -B
>   Battery #1     : present
>     Remaining capacity : 21550 mWh, 43.28%
>     Design capacity    : 56160 mWh
>     Last full capacity : 49790 mWh, 88.66% of design capacity
>     Capacity loss      : 11.34%

BTW, this reminds me of a question I've had for a while:

Is this 11.34% estimate reliable if the battery never reaches a full
charge (thanks for tp-smapi's setting of 70% in your case)?

More generally: how is the "last full capacity" computed?
My impression is that it can only be modified when:
- the battery reaches a full charge (in that case
  it can do last_full_capacity = remaining_capacity).
- the battery becomes completely empty (in which case it can do:
  last_full_capacity -= remaining_capacity; remaining_capacity = 0).

So unless there are other ways to adjust the estimate of
remaining_capacity and last_full_capacity, that means that
last_full_capacity will be too optimistic if you never charge the
battery fully (e.g. via tp-smapi settings) and never discharge it fully
either.  More specifically it will pretty much only notice that the
capacity has changed when the capacity gets below tp-smapi's max
threshold (in which case the battery gets accidentally fully charged).

Since such a use-case seems highly likely (after all, it's recommended
if you want to maximize your battery's lifetime), I'd guess that there
are other ways to adjust the estimate.  What are they?


        Stefan