I  mounted a second battery on my e-biked V-Rex today. I'm getting ready to do a short - week or two long - tour at the end of this month and I'd always planned to do e-bike tours with two batteries. My argument for doing that instead of using a single larger battery is that I don't want to carry the extra battery weight when I'm not touring and having two batteries means that having problems with one of them won't stop my tour, but just reduce the distance I can ride in one day. On my first e-bike tour I discovered another advantage. I was able to do two days of riding when I was unable to find a place to stay and had to wild camp, and was not able to charge batteries until the following night.

Two days ago, I was careless about having the V-Rex's battery fully charged when I went for an unusually long and steep ride. The battery was at about 1/3 of its full capacity. That might have been barely enough, but I'd set a fairly high cut-off voltage to protect the battery from being damaged by being run down too far and I was climbing steeply, about twice as steeply as my usual climbing on my daily rides. I actually was lucky in that the battery cut out just as I finished the last of the of the first set of hills that were too steep for me to climb without granny gears. I was able to continue on with no assist and no granny gears, mostly down, to the Blue Ridge Parkway and then ride the Parkway down to the next exit. Then I called my wife and asked her to meet me as I pushed my bike up the long, and too steep to climb without granny gears, road up the ridge between the Parkway and Asheville. 

On that ride I was actually carrying a spare battery with me, but it was an old battery that I damaged years ago. I was carrying it in a seat back bag on the V-Rex to check on the effect of the extra weight carried up relatively high. That extra weight noticeably hurt the low speed handling.  Yesterday I tried carrying that dead battery in a small pannier on my under seat rack and found that I really didn't notice it was there. Today, I took the battery from my Rans Fusion bike, put it in that pannier and discovered that the cable connecting the battery to the motor was a little too short to reach the new battery. Then I took the  longer version of that cable from my Fusion and that worked.

 I turned the battery from the Fusion on and tested that the V-Rex display had power using the new cabling. Finally, I went for a test ride using the battery in the pannier. 

Using this setup, I am able to switch batteries simply by moving the red and black wires from the motor between the corresponding wires from the batteries. I don't expect to have to do that very often, but it is nice to know that I can do it quickly and easily if I need to. It is also good that these batteries have switches that disconnect them from their power connectors so I don't have to worry about accidentally shorting those red and black wires together when changing batteries. 

That looks pretty clever, Mark. What’s the voltage and weight of your batteries? 

I’d like to carry a second battery but may not be able to get the exact same one as my original, so I’m wondering how I can mix and match. Not an electronical person so your setup is intriguing.

For most e-bikes, the only things that needs to match are the battery electrical connectors - to the motor and the charger- and the battery voltage.  It is simple to make (or pay someone with soldering skills to make) or buy,  short cables that adapt those connectors. 

After I did my two battery setup, I realized that I could have done it with two different kinds of batteries as long as I could match/adapt the battery and charging connectors.  The opensource firmware on that system I have on the V-Rex will actually let me use batteries with different voltages by changing a setting which is easily accessible on the display, but I wouldn't want to have to do that while wild camping.

The folks I bought the motor/controller/display on my V-Rex from, electrifybike.com  , are in the US and sell a wide variety of e-bike conversion kits , chargers,  batteries, and adapter cables. They might be able to help you. 

Thanks. That’s a question I’ve had for a while.