How many cells do I actually need for a 5ah EGO pack rebuild?

The capacity thing is easier than it looks. Voltage is set by how many cells you have in series, and for a 56v EGO that's 14 in a row. Cells in parallel add capacity without changing voltage. So on your pack, the cells are grouped into pairs or triples sitting side by side, and each group is one 'step' in the 14-step chain. Total amp hours is just the capacity of one cell times how many are in each group. If the original cells were 2.5ah and you had two per group, you get 5ah. Swap in 3ah cells in the exact same layout and you end up with 6ah. You don't need fewer cells to get more capacity unless you drop down to one per group, and then your current draw per cell doubles, which most small cells won't like on a mower. So keep the same group count and just use higher capacity cells if you want more runtime.

On the 40 cells you counted, double check that number. 14 times 2 is 28, 14 times 3 is 42, so 40 is a wierd count for this platform. Some packs have a couple cells tucked in odd spots or you may have missed one under the shrink wrap. Worth recounting before you order anything. The little wires grouped in threes and fours, those are your balance or sense wires and the order absolutely matters. Each one taps a diferent point along the series chain so the board can see the voltage of every group. If you mix up the order, the board reads wrong voltages and will either refuse to run or worse. Before you desolder anything, take a photo and label each wire with tape and a marker. Then put each one back on the same spot of the same group it came from. One thing since you've got the spot welder coming: the old cells still hold a charge, so keep metal tools and loose nickle strips away from the terminals while you're in there. A wrench across the wrong two points will spark before you even feel it.

Done a couple of these packs now and the biggest headache was cell size, not wiring. Some of the higher capacity cells are a hair taller or fatter and the original holder just won't close right, so measure the old ones before you order. Also learned the hard way that bargain cells sag under mower load even if the label says the same capacity. My trimmer pack runs fine on cheaper ones but the mower pulls way harder. If you've got a meter, check the old cells before scrapping them, usally only a couple groups go bad and the rest are still decent for a smaller project.

One thing I'd add about the holders is that the cells are usually oriented negative-up or positive-up in an alternating pattern, and it is surprisingly easy to drop one in rotated 180 degrees when you are working quickly with a bunch of them laid out on the bench. I mark the top of each cup with a sharpie before I start. Also, before you commit to new cells, check that the failure is actualy the cells and not the board, because I once rebuilt a pack and it still cut out under load, which turned out to be a fried component on the protection circuit rather than anything I replaced.

Watching this one closely beacuse I'm mid rebuild on an old Greenworks pack myself, way smaller than yours so it's been slow going. One thing I keep going back and forth on, do you charge the new cells up before you weld them togather, or build the whole thing empty and let it charge as a unit? I've read mixed things and my gut says empty is safer but I honestly don't know. Also how are you getting the case back together without snapping the tabs? Mine are brittle as anything after years in the shed.