Normal for batteries to lose ~3% charge per week when idle?

[COBill]

I just got my first Festool, a CXS 18 on February 21.

Of course I charged both 4.0AH batteries to 100%.

Now, just ten days later, the spare battery reports a charge of 97% and the one attached to the tool reports 96%.

Of course all my other brand tool batteries only have simple bar graphs, but as an example my Makita batteries were last charged two years ago and they all still report fully charged.

Even if you assume the bar graphs show full for anything over 75% charge, that would mean if a loss of 3% per week were normal, my Festool batteries would report being down one "bar" in just about six weeks.

On the other hand, if it's just the drop from 100% to 95% or so, that's no big deal.

Update: Festool replied, apparently this type of drop is normal, though it seems excessive; perhaps it's because most other tool batteries aren't Bluetooth.

I'll update later whether it drops at the same rate or plateaus.

 

[squall_line]

Lithium Ion batteries don't like to be stored at 100%.

I have a small consumer drone that I play around with occasionally.  Its batteries are designed to self-discharge if left sitting, and hover around either 40% or 60% once they do that.  This is the optimal charge level for long-term storage.

Any other lithium ion battery that self discharges even 2-3% in only a week wouldn't surprise or concern me at all.

 

[mino]

... though it seems excessive; perhaps it's because most other tool batteries aren't Bluetooth.

I'll update later whether it drops at the same rate or plateaus.

As mentioned, read up a bit on how LiIon batteries work. What are the charge/discharge curves etc.

Further, at/close to "100%" charge the measuring accuracy is limited. The capacity/voltage graph is very very flat, so the battery "settling" a bit by 0.02 V or so can easily be enough of a voltage change to look like 3-5% capacity loss.

A chemical battery is not like a jug you "fill up" with water that has an exact capacity which is also easy to measure. There is a lot of non-linearities involved, not only in charge, discharge, measuring capacity but also in time from chemical "settling" of the battery after charge/discharge, the temperature response, etc. etc.

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If you want batteries to last you very long, with Festool that means decades, do not store them at 100%. For practical hobby use, the best approach is to store batteries with one to two "lights" on the capacity indicator. I .e. when they are between about 30-70% capacity. Also, rotate the packs. A pack that is on a shelf for a month will age faster than a pack that has seen some use once in a while.