I primarily have Milwaukee tools, you'll have to do your own legwork on the other brands.
I have 5.0ah lithium batteries and recently put in an order for 6.0ah since that's the best price/performance balance I've found, given the 8.0 and 12.0 battery issues are a lottery I don't want to play. Are 8 & 12 good batteries? I don't know, the 5 & 6 weight/size work well on power tools, I was looking at the 12 for their chainsaw, however there were "a lot" of comments on the Milwaukee site with chainsaw purchases that resulted in dead batteries, enough for me to stick with gas. Given how much current the saws draw it could be a spot-weld failure issue, that I don't know...but if it is that's a relatively easy work-around (solder them up, though that could be a safety issue).
If they aren't matched cells the battery will start to go further out of balance each discharge/charge cycle, continuing until the imbalance is "too much" and the charger either shows the battery as full or displays an error code. My observation is most cells are well-matched, some are a bit weaker and
some batteries fail early. To see how "in balance" they are remove the four screws holding the top and bottom of the case together and measure the voltage across each set of cells. The 18V batteries have 5 sets of 3 cells in parallel...let me see if I can find a video of how to measure voltage...this one is a bit long but shows how to measure voltage, how to use a high-speed balance board to charge the battery (so charging happens within a reasonable amount of time), and how to use a low-speed balance board to bring the weaker cells up to full charge. Note there are better balance boards out there but the ones he's using are under $5 each, so...
With that said, an R/C LiPo charger with internal balancing circuits could be used instead, requiring only a balance connector and alligator clips on the battery. The good ones are fast, though more expensive than what was used in the video.
In about August of 2023 when I was looking to buy a Milwaukee chainsaw there were about 1,500 posts, a lot of them describing how Milwaukee failed to honor the warranty even though they were purchased at Home Depot within six months prior and the person had the receipt (several people posted images of the receipts and denial). I see those posts have been removed from Milwaukee's site, but the remaining posts mostly talk about premature failure.
The odd thing is the chip on the internal board in the battery has a balancing feature, it seems it's not enabled. Balancing generates heat and extends charge time, but is necessary with lithium packs.
