MS390 Battery Select switch at panel

[danac]

Hi all - new MS390 owner with an electrical panel question. I have two start batteries (twin engine, one per engine) both wired to the DC side of the panel. At the top of the panel I can select ONE of the two batteries to power the panel with a lockout lever preventing both from being selected at the same time. (I have attempted to upload and attach a pic). My question is around the lockout lever - is there a safety issue with drawing from both batteries simultaneously, or is this just to prevent the user from inadvertently draining both batteries? Being that they are both AGM start batteries neither are really appropriate to power the house electricals for long and I would have thought it would make more sense to pull from both.

 

[Gdavid]

Hi all - new MS390 owner with an electrical panel question. I have two start batteries (twin engine, one per engine) both wired to the DC side of the panel. At the top of the panel I can select ONE of the two batteries to power the panel with a lockout lever preventing both from being selected at the same time. (I have attempted to upload and attach a pic). My question is around the lockout lever - is there a safety issue with drawing from both batteries simultaneously, or is this just to prevent the user from inadvertently draining both batteries? Being that they are both AGM start batteries neither are really appropriate to power the house electricals for long and I would have thought it would make more sense to pull from both.

Yes, there is a safety reason for the lockout. The reason there is a lockout at the panel is to prevent current from flowing from one battery to the other battery (or the starter of its corresponding engine) through the relatively small DC wires that feed that panel. There is likely switch in the engine room that allows you to parallel the two batteries, and the electrical cable that joins the batteries to this switch are much larger, shorter in length and therefore suited to support starting one engine off of the other engine's battery. I hope this makes sense.

Assuming that there is such a switch and it is in the closed position, it would not matter which circuit is turned on at the top of your DC Circuit Panel, as they are connected in the engine room. If there is no such combining switch and battery cables, it would be pretty straightforward to add.

 

[Gdavid]

Being that they are both AGM start batteries neither are really appropriate to power the house electricals for long and I would have thought it would make more sense to pull from both.

AGM are actually well suited for house batteries, for a standard lead acid battery, they have thin plates to maximize surface area but discharging them to any significant degree can degrade them. AGM's don't have this deficiency, they may not be large enough to support your house loads, but it really depends on how you are using the boat as well as the capacity of the batteries.

 

[danac]

Yes! This makes perfect sense now. There is indeed a combiner solenoid exactly as you have described between the two batteries. Although I have not used it, the switch at the helm seems to be labeled "PAR Start" which I assume is short for parallel.

Thank you for your insight!