Thread got me wondering about the cost of lithium cells. Googling the latest 21700 cells, 4 to 5Ah capacity from Panasonic or similar it looks like the 5 cells you would need to make an 18V 4Ah battery pack will cost end users like us a little over £30 delivered, tax paid from Hong Kong.
That compares with about £50 street price of the corresponding battery pack for a major brand.
So there's not a great deal to be saved by trying to refurb a worn out pack at those prices.
If you buy in 100's then the price falls to nearer £3 per cell or £15 per 4Ah pack
Cordless tool battery performance tends to depend on the current rating, i.e. they have to be able to sustain a high discharge rate as well as sufficient capacity to provide the runtime The general rule of thumb is higher capacity cells have lower current ratings - reliable cells that have capacity and decent current ratings are more expensive.
This is why a lot of the clone batteries can end up failing - cheap cells are used and while the capacity is fine, they don't have a high enough current rating. They can cope for a while but the repeated high-discharge rate demands of (some) power tools eventually kills the cells. That said, they probably last as long as an OEM battery if used in tools that don't demand much current e.g. a jobsite radio, torch, smaller drills etc. This usage profile may go some way to explaining why some people say that they don't have a problem with clone batteries and others do - it's not a like-for-like comparison.
Makita batteries (>= 3Ah) don't use 21700 cells, they use 18650 cells. In order to enable them to provide both the run-time capacity and the high discharge rates, each battery pack has 10 cells arranged in 2 banks of 5.
e.g.
a 4Ah battery would use 10 x 2000mAh 3.6v 18650 cells with a current rating of, say, 25A
Bank 1 : 5 x 3.6v 2000mAh = 18v / 2Ah
Bank 2 : 5 x 3.6v 2000mAh = 18v / 2Ah
The two banks are connected in parallel to give a nominal 18v / 4Ah battery pack
What's currently stopping Makita producing batteries > 6Ah is that 18650 cells with >3000mAh capacity that are also capable of repeated high discharge rates are few and far between (and relatively expensive). In order to get more capacity, they'll need to move to something like 21700 cells and, while they tend to have larger capacities, they still have the same capacity/discharge rate tradeoff - e.g. something like the Samsung 50E 21700 has a 5000mAh capacity but it's only current rated at 10A.
