I took your advice and the Sidepower SE60/180 12V is on the way. Thank you!
Someone skilled will install the tunnel for me, and I will do the wiring.
The idea is to put the additional battery as close to the bow thruster as possible., to keep the thick cables as short as possible.
The manufacturer recommends a switch that goes to the plus cable. It can be manual or automatic, automatic is quite expensive, around 500 euros. if switch is manual it needs to be easily accessible which means I'll have to put it somewhere away from the bow thruster which spoils my idea of short cables.
How is it with you?
Next thing how big should the extra battery be?
And the last question.
How to connect to the existing electrical system?
I currently have, engine battery and house battery (2x140Ah), two power sources, shore power (2 outputs) and an alternator with a Victron fed splitter (2 outputs).
At first I thought of just connecting the house battery to the thruster battery with a 25 mm2 cable. There will never be a big difference in voltage between these batteries, so there will never be a big current (I would check this in reality).
The disadvantage is that the thruster battery would be discharged as the home battery is discharged. This could be prevented by adding charging relay in series, so that these two batteries would be connected only when charging.
Second option is to put a DC-DC charger, but this is a much more expensive option.
The third option is to replace the splitter and shorepšower charger with models with three outlets, this is the most expensive option.
I listen to your ideas and suggestion.