The AGC as well as the TSC are high-current tools and will not accept lower capacity/current batteries.
Officially, they require either the 10-cell/2-row classic batteries (so 4.2 an up) or the new 4.0 high power packs with 21700 cells.
Mine will actually start with the 3.1 5-cell packs and - for very light work like knife sharpening - it is fine. But that is about it as far as the 5-cell packs go.
This tool eats batteries for breakfast. When doing something serious, I usually have all 5 of my 5.2 packs lined up and change them after 10 minutes or so. It would be battery abuse of the first order to use it with the 3.1 packs for any heavier work. It would literally damage the 3.1 batteries from a too-fast discharge. Be happy it did not agree to (destroy) your 2.6 packs.
mrB said:
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Also my PSC doesn’t like the new 4.0ah batteries. It’s fine with 5.2 and the 3.0 compact batteries.
Consider to warranty it. The PSC absolutely does support the 4.0 packs. Something must be amiss with yours. It even works with with the old 10.8V/12V packs. Is kinda handy as those are the lightest packs for it.