So many factors invovled...

for example...I weigh 260 lbs and my bike electric weighs another 50 lbs { without the 48 volt lipo] ....for my scenario...it would not be logical to use four SLA batteries because of their weight....


Nicds/Nimhs are better technology then SLA batterys..BUT...you may find that nicds/nimhs with enough A/H and voltage will be almost as much in cost as lifepo4 battery ! ..

so IMHO...there are mainly two choices..

SLA
--

or

lifepo4..

did you know you can get 48 volt -20/aH lifepo4 off ebay, brand new and shipped to your door for around $550 ? ...

so if you onyl need 36 volt and 10-15 A/H...you may be able to get them off ebay for near $400 .

IMHO...lifepo4 batterys/technology will continue to drop in price, to the point that in a few years, SLA batterys for electric bikes will become largely illogical.

If I went with 4 SLA...in series to try and match the 20 lb lifepo4 I have, the total battery weight of the SLA would be 60- 80 lbs...compared to 20lbs...

there are many advantages to reducing battery weight...including less stress on the spokes/ brakes/ rims , etc...



If I were you, and you are pleased with SLA for your current needs, I would stick with them..and wait another 1-2 years when you need to replace them again..by that time, I believe you will be able to get a lifepo4 battery for your needs , within the $200-$ 300 price range....