I wanted to share my experience with charging an oddball 28V scooter setup. I chose 28V since 30V on my Razor E-300S did not work well so I backed it up to 28V using 12V + 12V + 4V. However, charging a 28V battery bank with a charger designed for a 24V battery bank will not work so I use a different method.
I have something called a laboratory power supply unit (lab PSU for short). It can be dialed in from 0V to 31.4V (it is rated 0-30V). The power range can be set anywhere between 0 and 3A. Although 31.4V is enough to charge a 28V bank, it will do so rather slowly as this is barely 10% over the nominal voltage of 28V. This is roughly equivalent to charging a 12V battery at only 13.45V. It will work but will be slow. Generally speaking, to get decent speed when charging AGM batteries, it is good to charge at up to about 20% to 24% over the battery bank voltage (such as charging a 12V battery at up to 14.4V to 14.8V max). However, my lab PSU cannot go this high so I had to use a helper power supply.
What I did was put a 9.5V 3.3A power supply in series with my lab PSU, thus having an additive effect of raising the maximum combined charge voltage to 31.4+9.5 = 40.9V. That is way more than enough for a 28V bank. So what I would normally do is set the maximum voltage on the lab PSU to 24.1V (since 24.1V + 9.5V = 33.6V which is 20% over 28V). Then I limit the current on the lab PSU depending on how much time I have to charge. For example, if I am waiting to ride the scooter again soon, I can blast them with the full 3 amps. However if it is the last ride of the day I can give them a slow charge overnight at 1A or less.
Generally speaking, it is not recommended to put power supplies in series like this, however it actually works (although it has a few minor quirks). I asked an "expert" on power supplies about this and he told me it is generally not a good idea but if it work then great.
The beauty of this system is I can control the charge rate very finely, unlike a smartcharger which usually only gives you a few "coarse" settings such as 2A, 6A, 12A... I generally like to charge at 1.5A or less cuz the lab PSU gets kinda hot using more than that, however a small fan can easily fix that.
An alternate method for this would be to charge the 4V battery separately (on the lab PSU) and just use the normal 24V charger for the pair of 12V batteries.
If anyone would like more details about this just leave a comment or question and I will be happy to try to answer it.