I have added an electric motor kit to a recumbent trike. I'm using SLA batteries with a lithium ion 36v controller. Therefore the controller cuts out before the batteries are dead. I'm using 3 each 12v 10Ah batteries in series. I have added a 6v 5Ah in series for a 42 Volt battery. Now the system dies after 6 to 7 miles, compared to 17+ miles before. Would adding another 6v 5Ah battery in parallel to the other 6v 5Ah battery making a 6v 10Ah work? 12v 10Ah + 12v 10Ah + 12v 10Ah + 6v 10Ah in series = 42v 10Ah battery.
Yes - all batteries in a series-connected bank must have the same Ampere-hour rating, as all batteries are supplying the same current, regardless of their voltage.