Is it possiable to hook up battries in Parallel

I have two batteries, one hooked up to the engine and the other hooked to my accessories. The one battery on the engine is being charged by the engine. The other is not. My question is could I hook up the two batteries in Parallel so they are both being charged by the engine ?.Or would this be to much for the engine to handle and cause a problem. I have 2004 150 HPDI engine

It is possible to charge two batteries off of one engine. Take a look at Blue Sea Systems' products such as this one:

https://www.bluesea.com/products/7649/Mini_Add-A-Battery_Kit_-_65A

There are lots of good wiring diagrams on the Blue Sea site.

While its possible, it's much better to go through a battery switch. Or are you needing high amperage for starting?

If one of two batteries in parallel gets weak, it will take down the other one.

You could alternate (even/odd days or something) but this way always have a reliable backup.

You could still always start on #1 if #2 powers electronics and save them from V-Spikes from starting. Ammeter or voltage meter would tell you when to switch to #2, but never go to "Batteries OFF" while running.

Aircraft have an avionics switch which is 'OFF' for engine starting, cars used to turn radios off when key switch went to start position.
Art

absolutely correct on a bad battery taking out the second. best to use a battery switch. also, some electronics do not like the surge when starting, and it is best to not have them on. this can be eliminated with the battery switch.

I think the 150 HPDI might have an auxillary charging circuit that you can just connect to the 'house' battery. You may need to buy the cable for it if it isn't installed in your engine already. That would be the best way to do it. Failing that, a BEP, Blue seas etc VSR (voltage sensitive relay) is the next best option.

Thanks guys for your help. I think I will go with the Blue Seas set up .

Be sure and use the proper fuses you can never add too many fuses in a boat!

I have a 1991 Grady White Spirit 175 and they feed 'hot' from Battery Switch into a 40 Amp Circuit Breaker (near switch) ahead of main buss/fuse panel (not controlling engine/starting cables).

With Battery Select Switch in 'Both' position, you would in effect, have a parallel feed with amperage capacity combined from both batteries.
Art