2V batteries with 4 bolt connetion between batteries - how to connect the cabel?

[Roil]

Yes, it is, and it does suggest that you need to make sure that all of your 24 volt utilization equipment can take that high a voltage or else provide a way to disconnect it when the charging voltage is too high.
Nominal 12 volt equipment intended for use in a vehicle will be designed for a wider voltage range. Equipment intended for use at 12 volts AC or DC in a fixed location may not have that same tolerance. Especially when, like some lighting equipment, it is designed to be fed by a regulated DC supply rather than directly from a battery.

I have a 12V system, so the battry is 2 strings of 6V. I will keep my 12V equipment on the old batterybank untill the new one is up and kicking. Anyway it's like you say - they do survive 15V