I have a Razor E300S and another 24V scooter I am overvolting to 28V. My original ideal for charging would be to split the battery bank into 24V and 4V and charge those separately, but I think I have 2 solutions to leave those at 28V and charge them.
Solution 1: use my laboratory power supply which can put out up to 31.4V and has power limiting I can set. For a charger to work on SLA batteries, it has to be at least 10% over the nominal voltage of the battery. For example, for a single 12V battery, 13.2V is the minimum needed to charge it fully. You can get partial charging with less voltage but I am talking about a full charge here. So using the same "10% over rule", for a 28V setup, we would need at least 30.8V. Luckily I have that and a little bit more. Charging a 28V battery pack with 31.4V is roughly the same as charging a 12V battery pack with 13.5V. It will work but will be slower than a smart charger that can go 20% over the rated voltage of the battery.
Solution 2: I remember I have some power adapter that can output 32V. This might also work as a charger although I would first have to check the amperage output to make sure I do not exceed the maximum recommended charge current for the battery bank (which is usually about 30% of the Ah rating). For example, for 7Ah batteries, charging at more than 30% (2.1A) is NOT recommended. For a slow "overnight" charge, 10% (700mA in this example) is even better. I would also have to monitor the charge since it is not a smart charger and could possibly overcharge the batteries.
Since the lab PSU is kinda heavy, I plan on using that when at home and using the 32V charger when I am away (such as for an opportunity charge while miles away from home but with access to an AC outlet).
I will try these both and report back here but I welcome some comments and/or suggestions about this.
F.Y.I., the reason I selected 28V is cuz my scooter "complained" about 30V so I had to back it up to only 28V which works fine and gave me both more power and more speed. With the extra 4 volts, my 9 tooth front sprocket feels like an 8 tooth for acceleration but a 10 tooth for speed. Overvolting gave me the best of both worlds.