Just picked up the EGO 56V mower. There were two versions. The $500 model had a "45-minute" 5.0-Ah battery. I have been working a lot of overtime, so I splurged and treated myself to the $550 version with the 7.5-Ah pack, rated for "60 minutes" (I only needed the 45 minute battery), but...mostly I paid extra to get the "self propelled" wheel drive, which the $500 model does not have. Well worth it, whether gas or electric.
Battery pack teardowns have shown that the EGO brand is using top-quality name-brand cells, but...more importantly, they have active cooling on their packs. The cells are internally arranged to spread them out, and there are air-channels to allow air to flow through the pack. The string-trimmer or mower has an integral air-fan that pulls air through the pack when it is being used. Its not heat-triggered. If the device is on, the fan is running. Also, each individual cell has a thin thermal absorption sleeve, similar material as the Allcell blocks that cells are inserted in.
I have recently learned that keeping the pack heat down, and also not charging to 100%, are the two biggest factors in making a "frequent use" battery pack last as many months as possible.
Leave the battery pack low all week (if you mow once a week), then, charge to 80% just before the mow. If you want, you can charge to 80% (4.0V per cell) the night before, and then charge to 90% (4.1V per cell) just before the mow. This is the biggest reason to get a very large battery pack. you can charge to 90% and still complete the job on one charge. This alone is a great reason the pay the extra $50 for the larger 7.5-Ah battery. Doing this can double or triple the life of the pack.
I can now verify that the large 7.5-Ah pack will fit on the string trimmer, and the smaller 2.5-Ah pack will fit on the mower. (There is also a 2.0-Ah, and 5.0-Ah pack available.
The Echo brand cordless tools also have a 14S / 52V pack system (called 58V), but they do not use air-cooling with a fan. Their pack is physically smaller because the cells are in a tight cube...bad for heat, and long life is compromised.
edit: just mowed, very happy with power and grass cutting ability. Mowed at lowest speed and 50% speed. Fastest speed to too fast (jogging speed), and lowest speed could be slower. I prefer for the device to be designed so 50% speed to be normal walking speed, then...you actually have more usable options. Happy with it, the speed adjustment range is a minor annoyance. The cutting blades are very quiet, and the wheel-drive is louder than the cutting blade motor. Overall noise much quieter than gas mower it replaces, but making the wheel-drive quieter would be an easy improvement by the engineers at EGO.
The rear axle is solid, connecting both wheels (no differential so tight powered U-turns are awkward). And on one side it has a combination 90-degree gearbox and reduction (similar to an angle-grinder head). The wheel-drive motor is a small vertical cylinder. It sounds like one of those plastic 24V electric cars for 3-year-olds, 98% certain its brushed.
9/10, would buy again...