luvmytoolz said:
I can't find a rating anywhere specifically for the SYS 50 brushless motor, but I think it would be a fair assumption to be around the same rating or certainly no more than, the TS60K which is rated at max 1500w given the comparative battery life claimed.
This is well within spec for any half decent universal input DC PSU that will also cover bursts and higher loads.
If it was that easy and economical, then I think every cordless tool manufacturer would offer these alternatives. However, Oliver is correct when he writes it is difficult. The confusion could be in thinking the 5AH rating of the 18V batteries means this is its working discharge rate. The rating means the battery can be discharged at a rate of 5 Amps for one hour. In normal use, the discharge rate is many times higher and this reduces the available energy in the battery. If the discharge rate is doubled to 10A, the battery will have about 30 minutes of life. Double the discharge rate again to 20A, and the life drops to 15 minutes. This concept is well known in the RC model community where flight times for large battery packs are 10-15 minutes.
Using your example of 1500W and dual 18V 5AH batteries, the full load on the batteries will depend on the configuration. I don't know if the dual 18V batteries are in series for 36V or parallel for 18V. If the batteries are series at 36V, the demand on the batteries is about 42A. If the batteries are parallel at 18V, the demand is about 84A. I don't know of any "half decent universal input DC PSU" that can handle the sustained demand at these levels.
To make the design more difficult, the cabling between the power supply and the battery adapter must be large enough to handle the full current. As memory serves, the conductor size will be 6AWG or 8AWG, which is starting to resemble inexpensive automotive jumper cables. This problem is minimized in when using the batteries in the tools because of the solid connections and short paths.
Edit: To expand on the discharge rate, I used to fly RC helicopters (Align TRex 600) that used a large 6S 3.3AH 30C battery. The battery voltage was about 22V and the 30C means it could safely discharge at 30 times the rating, or continuously at 90A. This discharge rate would drain the battery in about two minutes; however in normal use the electric motor was drawing about 20A from the battery and I was realizing a flight time of about ten minutes.