I went with 36v of SLA's for 8 months. They were fine for about 1/2 of the amp usage. That is normal for lead batteries. So your 12 amp SLA battery pack is really only worth about 6 ah. A 10ah lithium battery can deliver about 8ah usable.
Around here we use the rule of thumb that goes like this. Ah x V = watt hours. 25 Wh per mile average.
So a 36v pack times 10 ah is 360 watt hours divided by 25 = 14.4 miles on average. 36v*10ah=360/25=14.4 miles.
Now I tend to use a bit more watt hours then others around here because in the city it is all about stop and go. Also, wind cold, dirt roads and any other efficiency dampening conditions will cost you miles. Your mileage will very.
A 12ah lead battery that can only deliver 6ah is really only a 8.6 mile battery, and, they will be warn out after a years time.
A lead battery charger will take 5 or 6 hours to charge. A lithium charger can go faster depending on your cells and bms and such. My 15ah LiFePO4 battery can be fully charged from lvc to full in 90 minutes if I want to use my fast charger. I am moving on to 12000 miles on my pack, they are showing signs of old age but still deliver 500+ watt hours even with the sag on the hills.
Still a good run though. Lead batteries would never be able to last like that.
Go with lithium.
Favorite Quote: "This is L.A., sugar. There is no 'over the top." --- Chris Erskine
Current build: Liahona w/ cheap front suspension and suspension seat post. Yescomusa 36v 800w generic front hub motor. 15ah Headway triangle mounted pack. Tronsung 30 amp,
Previous Build:1992 Trek Antelope 800 - Bone Crusher (no suspension) - Yescomusa 800 watt 36 volt front wheel kit. Don't do it! Get suspension!!!