Robs. Connecting 2x 6v-4.5 a/hr batts to give 12v will still only give you 4.5 Ahrs. DO NOT  use an 11 amo charger on the batteries os they will probly melt!   they need to be charged at a very low rate, say 0.5 amps, so best to get a charger especially for this sort of battery (SLA "gell" cell lead-acid…not a NiMh charger) . A lot of stockists sell one  coping with 6 or 12v for about £12. Maplins do a small plug`n`play charger for 6v batteries only fairly cheaply. Ashley

 Well said Ashley, What you need Robs is to look at the battery and read the Amp/Hour rating of the device, this gives you the preferred charging rate. Have a look at what people are selling, and look for a Constant Current charger rated at one tenth of the battery A/H rating. I have always worked on a ten hour charge rate. Your batteries are 4,500 mAmpere /hour. and a tenth of that is 450 milliampswhich I say is the ideal rate for a good healthy charge in ten hours. This will not damage your battery, quite the contrary in fact it will do them good. I have an automated charger that discharges thecells in the first cycle, then charges them at a fixed current for ten hours, or until a certainvoltage per cell has been reached, and I have had outstanding service from all my batteries.

          You did ask another question, which was about connecting your batteries or parallel. If you connect in series, each battery contributes its own voltage at the rated ampere /hour current, so your two six volt batteries will give you twelve volts at four and a half Ampere/hours. It id definitely NOT a good idea to connect them in parallel where theoretically, you will get six volts at nine Amp /Hours, because practically, if one cell discharges down to a low level, the remaining cells will try to charge it up, and it has been known for a cell to charge in reverse polaritymuch to the detriment of your poor motor because it won’t get anything like the required voltage to drive it, so do not connect any cells in parallel as it will not give you anything like you expect, and maybe a blown battery into the bargain….Mike     D