I am thinking if the bike comes with SLA batteries of 10Ah, I could cheaply make a NiMH pack with 2Ah (bunch of AA cells) to boost performance. I am thinking of connecting it parallel to the SLA batteries (+ to +, - to -). I am thinking this will slow the discharge rate on the SLA and also reduce the depth of discharge, causing the SLA batteries to have much a longer life cycle.
Never tried it, and not sure what sla is, but I don't think it'll reduce the depth of discharge. Might slow it down, though.
It's difficult to mix different type of batteries. The lead acid battery has a nominal voltage of 2.1 V per cell, and an end voltage of 1.75 V per cell. In a typical 6 cell battery, this means 12.6 V charged to 10.5 V discharged. For a 10 cell NiMH pack, the values are 12 V -10 V. The ranges are close, but you would either need to undercharge your primary SLA battery, or you would be overcharging your NiMH batteries to 12.6 V (about 5% over). This may progressively shorten the life of the NiMH batteries. You might consider charging the packs separately, and then connecting (via a switch) the NiMH pack in parallel after the SLA has been run down a little. Of course, the best option would be to use a second SLA battery. If the charging circuit is smart, then it should be able to charge both SLA batteries in parallel. You could also use them in parallel. If you are in a sunny area, you could even tie in a PV array designed for 12.6 V parallel applications.
