take out the main batterie and ride with 2-3 extender battery...
You'd then have to have 2 extenders in your backpack (if you had a threesome....).
Everyone seems to handle backpack weight differently. For me, I'll ride with a chainsaw strapped on my back when I'm trail clearing and don't really mind it. But when I'm out to "play" and going for it I hate weight on my back. I bought a camelback a couple of weeks ago and with just 1 litre of water in there I hated it and actually rode home and dumped it. So 2kg's of batteries for some people will be a hindrance.