Hello from Scotland, 
I have just acquired 2x 20.0s and now realise that I'll be needing a second set of batteries. I'm not yet ready for lipo. 
I see I can buy from Oset at £39.99, or from intamicro  https://www.intamicro.co.uk/p-653-rec10-12-yuasa-battery-12v-10ah.aspx 
for £24.99. probably elsewhere too. 
As I am buying two sets, the price difference is significant.  Anyone know a reason not to go with the cheaper supplier? 
Also, what is the perceived wisdom regarding off-bike charging? I was thinking of buying a socket like on the bike and simply attaching it to the batteries.  Can anyone point me to a source for the socket? 
Can I balance the current and future batteries by individually charging them using my newly created shorty charger cable? 
Can anyone recommend a handy volt meter that I can easily keep close to the batteries, to help me check them regularly? I'll need 4. 
I also note I'll have to buy lanyards for the bikes too.  I read previous posts about saving £25 with a DIY solution, has anyone any further/better advice to help?  
Many thanks, very impressed with the bikes. 
Cheers, 
CG