It is definitely preferable to bring a spare battery pack or more, but it can be costly.
Note that matched battery packs are universally recognized as performing better, something that is measurably better on most bikes.
I have seen lead acid used as auxiliary packs on a bike.
Very heavy, but can have a useful charge life.
I rode someone's bike that had them mounted very low on the rear.
A friend suggests low mounting is actually counterproductive for control.
Even lead acid should be purchased from the same lot at the same time.
This is not the same as a matched factory set though.
Nicad or NIMH is probably very cost effective because besides the weight benefits, charging time is far better than lead acid.
It should be possible to plug in anywhere you are for a period of time that power is available and make a substantial difference.
Make sure you have a multi-plug connector and an extension cord..
Often there is surplus power and people are happy to let you plug in, if there is an outlet.
Lithium ion and lithium phosphate are considered superior by most people, but matched and regulated sets are even more beneficial, as is quality of the charger.
These are the most costly battery sets currently.
Some people still consider nimh superior, especially when a small set is being pushed hard.
On any, the quality of the charger is not an area to skimp.
A good fast charger may cost as much as the controller or battery pack.
Slow charging on lead and nicad/nimh is much less problematic.
A nice option at times.
But you will want the option of fast charging available in most cases.
I personally prefer to pay to share generator power with someone.
This allows charging in the evening, when solar is nil.
Last year though, I used someone's power when they ran it in the afternoon only and that was pretty adequate by itself.
It is an exceptionally efficient system though.
Even with the lightest of electric bikes, I was pretty much beaten to death with the rough ride, due to the extra weight, I think.
Wider tires are really needed and a suspension is looking real good to me right now.
The seat is sprung, but low quality, so I will be looking for a better seat, cost permitting.
Mine also came with few speeds, so more speeds would make a huge difference.
If you get one without a hubgear, I recommend changing over to one.