Adding second battery

I am adding a second battery to my boat. Do you to have to run a seperate ground to the engine for it, or can you run both battery grounds to a bus bar and then to the engine.

Thanks,
Scott

Re: Adding second battery

What's the other battery for? If it's a totally separate circuit not to be charged by the engine, like for a trolling motor, no need to tie the grounds together unless you're in salt water in which case wire the (-) terminals together.

If it's for accessories and will be charged by the engine, consider an isolater switch to allow you to run your accessories from the 2nd battery only, avoiding the risk of draining both batteries, and switching over for charging.

How many amps in your engine's charging circuit?

Re: Adding second battery

The second battery is for accessories and a backup. I was going to use a switch so I could charge both while under way. I might add an isolater or combiner down the road as money permits. I don't know how many amps my charging unit draws, how can I figure that out. It is a 1984 mercury outboard 60 hp 3 cylinder. I don't really want to run a second ground back to the engine and would rather tie both batteries together at a bus bar than back to the engine. It is used in the Chesapeake.

Re: Adding second battery

If you intend to use the second battery for starting duty, both cables need to be the same size as those on the primary starting battery. Grounding at the buss bar is not an optioin in that scenario and it applies whether or not you use a battery switch.

Re: Adding second battery

Ground Your Secondary Battery To the Primary Battery's Negative terminal that is already wired to your outboard. Use the switch to use battery #1 or 2 for cranking/charging.

If this is an outboard engine it will charge two batteries at once very slowly you would be better off cranking with one and charging the weak one.

Develop a system when you go out:
IE When you leave in the morning be on battery #1, crank your engine then head off to fish/swim or whatever. When you get there shut down the engine and switch to battery #2 to troll/play the radio or whatever. If it goes dead #1 will be there to crank you and you can re-charge #2 as you ride around.

If you do such a system it might be wise to charge both with a shore charger when you bring the boat back in. This will ensure that your batteries are always full when you leave as you don't want to have two weak batteries.

Re: Adding second battery

Thanks everyone,
Right now the starting battery ground goes to the engine. The existing negative bus bar goes to that battery. So if I add a battery, and ground it to the starting battery, and also send the neg. bus bar ground to the starting battery, is that okay, or do I have to increase the wire size from the starting battery to the engine, since it now carries 2 batteries and the accessory ground from the bus bar. I have very minimum accessories. I am sorry for all the questions but this part has always confused me. On every diagram I look at it just seems to show everything grounded to the engine, and it doesn't make sense to me to have all these ground wires going back to the outboard, as opposed to just one.

Thanks,
Scott

Re: Adding second battery

No wire size change necessary. The current (Amperage) requirements of your engine and accessories haven't changed. Don't quite understand your 'bus bar' explanation, but just run the cable from the negative terminal of battery #2 to the negative terminal of battery #1 and leave the existing connections to negative of #1 intact.

Many electrical schematics don't show WHERE the connection is made, just to what circuit path. Many times it's left to the reader to determine what best practices are in reference to where a high current device should be connected.


Good Luck!

Re: Adding second battery

You'd need a manual to find out your motor's alternator output, but a 3-cyl of that vintage is unlikely to be more than 6, possibly 9 amps. It'll restore the power you used in cranking the engine pretty fast, but after anchoring for a couple hours with the stereo on or whatever, you'll have to burn a lot of gas at full throttle to fully recharge it.

Make your 2nd battery a Marine Deepcycle with the most Reserve Capacity you can afford. Unless you have a refrigerator or something, it'll power your accessories a long long time. Then put an external charger on it soon as you get home, bring it back up to 100% for storage till the outing.

Re: Adding second battery

1989 23' cuddy w/ shore power, ac/dc fridge, etc...


similar question, if I may, I have 2 batterys and the switch 1, 2 ALL or OFF. How should I use this? like the first reply, 1 to start and run and 2 to run the accessories. Then back on 1 to start and run again?

I also have many wires connected to both batteries. Tired to clean them up when I replaced the batteries recently. I install one for starting and a larger deep cycling for accessories. I hope this was right.

When and what charges when running the enigine, then one I am switched to or both? same question for shore power, I switch to AC and turn on Battery Charger, does that charge only the battery the switch is on or both??

Thanks in advance

Re: Adding second battery

Some accessory circuits need to be live all the time - the switch should only connect and/or disconnect batteries from the engine, for cranking and/or charging, and for "switched" accessories - those that only get power when the keyswitch is "on". So, switch on "ALL" while cranking hooks the batteries together like you would with jumper cables. On "ALL" while running splits the charging current between the two batteries. Don't switch to "NONE" while the engine's running - risk of damage to engine electrical system.

There should also be an accessory fuse panel connected directly to the deepcycle for accessories that need to be powered regardless of the keyswitch (bilge pumps for example).

Your onboard shore-powered charger - depends on how it is installed and connected - I'd want a 2-bank charger that would charge both batteries independently.

Many wires connected to the batteries - it might take some time & patience to figure out exactly what's connected where - but I highly recommend doing so.

Re: Adding second battery

That does make the most sence. I realize we do need some "unswitched, live" accessories, like the bilge and some lights. I will see about the charger and over the winter trace out all wires and label them. This will make it easier to diagnose any future problems.

One more question. If I do have a single charger, would I assume it is charging both batteries, regardless of the switch postion??

Re: Adding second battery

No.......unless the charger has two sets of cables, each connected to one of the batteries. In that case, set the switch to anything but "ALL". I'm not certain this would cause damage - I think probably it wouldn't be good for the batteries or the charger.

If it has one set of cables connected to one battery - it will only charge that battery unless the switch is set to "ALL", in which case it will charge both.

Setting the switch to "ALL" combines the two batteries - effectively turns them into one big battery - so you can't charge one without charging the other, but you can't drain one without draining the other, either. Combined, charging (and draining) should take twice as long (give or take) as separately.

Hope that's not hopelessly confusing - best check back to see if anybody found any mistakes in my descriptions - it's not unlikely -

Re: Adding second battery

No, it is now very clear to me. I will check once it stops raining here in NY!!

thanks a ton.

Re: Adding second battery

Problem solved. When I replaced the batteries, I connected both battery charging wires to one battery.....

thanks for all the explaining...