Yeah, just the weight of 36v 20 ah of lead is about the max for bikes to carry. Try to rig one on each side of a rack, pannier style, so you only have one up high on the bike, on top. Or maybe one in the triangle of the bike.
If you can afford it, 36v 20 ah of lithium is so light and small, it carries easily on rear racks, or in the triangle of many bikes.
Figure on about double the range from lithium, because lead has so much peukerts effect. If you use lead, stop when its only half empty, or it will die in a few months. You will need new lead every spring even if you are very careful.
Still, lead can work ok if you only need about 10 miles of range. Same size lithium lasts 2-3 years easy, and can go 20 -30 miles easy.