It looks like your flexvolt battery has similar rated wh to my B&S battery. How far can that battery get you if you are using some level of assist most of the time? I realize that range is dependent on a lot of individual factors, I'm only trying to get a general idea.I use 60V DeWalt tool batteries on my 52V bike. I use this adapter to make the connection:
I mounted the adapter to a base plate that snaps into the bike's Hailong battery mount. For now, there's even room for a bottle cage. I'm waiting for a good price on a second DeWalt battery to wire in parallel.
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The fully charged battery voltage is 62.8V but the max the bike's controller will accept is 62.4. I get around the problem by only charging to 95%. Most controllers will work with a voltage higher than rated but unless you can find the specs, It's a leap of faith to determine by experiment.
A bit of caution is necessary when using these tool batteries though. Most do not have a low voltage cutoff in the battery BMS. The cutoff is in the tool for some reason. You need to monitor the voltage or the battery could be damaged if over discharged.