I routinely use 3x 17Ah SLA at around 45 pounds for CrazyBike2, which is a pretty heavy bike by itself at around 80 pounds, plus a 15-20 pound motor+gearbox, and got about 30-35 Wh/mile out of it. It's front suspension only, hardtail. It took that weight without breaking the frame, but definitely breaking rear wheels on potholes and such, too often to deal with easily.
I'd say that 4x 20Ah batteries on any normal bike frame is pushing it, especially if you end up mounting them A) higher up in the frame and/or B) distributed any other way than 2 in front and 2 in back.
I tried mine in a few ways in back before moving them to the central frame, and it basically made the bike truly rideable to put them there instead of all in back. If I had had 4 of them (I do now) I'd've probably put two on the front end (not on the wheel) as low down above the fork crown as I could, so it would clear the fork/wheel and be fixed to the frame. Then two in back in front of the cargo pods (like I started with), or just two where they are right now in the central frame. Then I'd have better bike performance when loaded with cargo in back, and a lot less broken rear wheels.
20Ah batteries are also fairly large, and finding places to put them on a bike frame is challenging at best.
As others have mentioned, keep Peukert's Law in mind, that the faster you pull the power out of the batteries the less power you will get. If you actually use the potential that 1kw motor has, a lot, you'll not have a lot of range even with big SLAs, and with small ones you'll have much less than you expect, since it's not a linearly shrinking range vs smaller batteries with the same power draw from the motor.
You won't know for sure what Wh/mile rating you'll have with the bike until you've built it, but you can guesstimate based on it's weight, gearing, desired speeds, and terrain, with some of the various calculators around the web. The higher that rating ends up, the less range you'll get.
Weight is a big factor when it comes to starting from a stop or climbing any slopes (even small ones, especialy if they are long). At some point, having bigger heavier batteries is going to have *worse* performance than smaller lighter ones.
FWIW, I don't much enjoy most bikes that have more battery weight than the rest of the vehicle weight. My first successful ebike (meaning it was actually useful) was DayGlo Avenger, with 3x 12Ah batteries plus a couple radiator fan motors and metal baskets and stuff, and it was quite heavy. I've forgotten exactly how much it weighed, but I think that the motor+battery system all told was at least as heavy as the bike itself.
CrazyBike2, my next successful ebike, is 150+ pounds, with at least 1/3 of that being the motor+battery system. It performs a lot better than DGA did, but it would be far better with something other than 45 pounds of SLA on it.
Lithium is fairly expensive. NiMH and NiCd less so, but still significantly more than SLA. If you just want to get the bike going, SLA is a good learning system if for no other reason than that they are cheap and they are pretty good at taking abuse as you learn.
) and thought it weighed 34lbs, we thought it was pretty nice.
