Depends on the weight you will carry, and how rough the ride is.
The best rack is steel welded to the frame IMO, but that does not mean you cannot use alu. Look for alu racks specifically designed for cargo panniers for the strong ones. The typical Blackburn rack is fine for up to 15 pounds of battery, but if you want to put 15 pounds of battery in each pannier, it needs to be better for sure.
Don't carry 20 pounds of battery on top of a rear rack, at all, unless it is a cargo bike with a rack your girl can ride on.
If you are not worried about the looks, you can make a very strong rack with a seatpost type rack. Add home made legs to it with some kind of sturdy metal. 1/2 inch electric conduit for one example, bolting them to the beam on the rack.
Look for this type seatpost rack, with 2 or 4 bolts to attach to the seat post. not the one with a quick release lever.
https://www.walmart.com/ip/Bicycle-Bike-Alloy-Seatpost-Mount-Rear-Rack-Carrier/111640752