1, 2, or Both?

[ajkolar]

I have a dual battery setup in my 5.7 OMC Cobra powered boat. When I bought it, it only came with one battery, but all cables were there and ready for a second. I bought and installed the second battery and have all accessories (radio, lights, etc...) connected to one, and the ingnition connected to the other. I usually only point the switch to 1 or 2, but never usually use the "both" option. Both batteries are deep cycle marine batteries, will leaving the switch on both hurt anything? I thought it might be hard on the alternator, or overload the cranking circuit or something, but if the option is there I would imagine it is made to be used? Just wanted to check first... <br /><br />I have run into a few situations when the accessory battery ran almost dead and it is very hard to get a good charge back into it until getting home and using an actual battery charger. If I can charge both batteries all the time I could avoid this situation. Thanks!

 

[Haut Medoc]

Re: 1, 2, or Both?

Keep it on both while running....I switch to 1 or 2 while docked or if Ishut down on the water & leave on the radio, lights etc. That always leaves you a fresh battery to re-start.....<br />I have one deep cycle & one starting battery ....JK

 

[KnottyBuoyz]

Re: 1, 2, or Both?

A few questions. Are both batteries exactly the same? Same capacity (amps) etc. Same type? Starting or deep cycle?<br /><br />If both are exactly the same you can run the boat in "both" but you'll loose the ability to have the second battery as an emergency backup. If they're not the same and the same nominal level of charge you'll end up damaging both.<br /><br />I just answered a similar question on another board so I'll copy and paste it here. Maybe it'll help.<br /><br />Two deep cycle batteries are probably not ideal for your boat. Ideally you would want a good marine starting battery (high cranking amps) just for starting your engine. The second battery, preferably a deep cycle, is intended for the constant draw items such as running lights and your stereo when your engine isn't running. The ideal setup is to have a "1,2,both" battery switch (which I believe is required by law as well as circuit protection, breaker/fuse close to the batteries) to disconnect, switch and combine your batteries. The normal position for starting/running would be in the "1" position so that your alternator can recharge your starting battery after startup. If you happen to flatten your starting battery position "2" would switch to your deep cycle for emergency starting. Position "both" is not normally used in this setup as it cross connects both batteries with differing nominal charges (not good). <br /><br />The drawback to this setup is that only your starting battery gets charged from the alternator while the engine is running. It usually doesn't take too long for the starting battery to recharge before the alternator stops charging. This leaves your second deep cycle battery being uncharged. Some boats will have a dual diode bridge that can charge two battery banks but they're not too common. You may be able to switch between battery banks while the engine is running but I'm not sure. It may damage your alternator. You'll have to check with your equipment manufacturer.<br /><br />This is where a battery isolator/combiner comes into play. It is wired in between the batteries and keeps them electrically separated while having the ability to detect which battery needs charging and routing charging current accordingly. There are a few drawbacks to isolators in that they have a fairly high voltage drop (somewhere around .7v) which means your batteries may never be fully charged properly. Battery combiners seem to be more efficient.<br /><br />Everything you ever wanted to know, and probably more, can be found here...<br /><br /> http://www.islandnet.com/robb/marine.html <br /><br />Generic diagrams here...<br /><br /> http://www.bcae1.com/battiso.htm <br /><br />Just a word of caution. If you go that route both batteries have to be the same nominal capacity and charge and would have to be run in the "both" position all the time, which eliminates the second battery as backup plan. If you have two batteries connected by the switch that are at different levels of charge the moment you select "both" you'll get a full amperage flow of current from the higher charged battery to the lower charged battery. This could lead to excessive off-gassing (boiling = loss of electrolite) as the second battery attempts to accept the charge (which it cannot). At bestthe battery will sulfinate and possibly de-laminate its plates. You'll be lucky if you get a years service from those batteries before they fail. Starting and deep cycle batteries have totally different charging requirements. An isolated starting batter would rarely ever see it taken down below 98% of full charge and deep cycles are designed to be taken down to 80% or lower and fully recharged, hence the name "deep cycle". If you mix the two types in a typical arrangement as discussed you have to isolate them or you'll find yourself replacing both of them every year.<br /><br />Rick

 

[ajkolar]

Re: 1, 2, or Both?

Wow, thanks Rick! I believe both batteries are deep cycle. However I have been using one as a starting battery and the other as the "deep cycle" accessory battery. Will it be ok to keep using the deep cycle battery as a starting battery until it needs to be replaced? I will probably just keep using 1 & 2 switch positions and not risk charging them both at the same time. Thanks again!

 

[Boatin Bob]

Re: 1, 2, or Both?

You should be fine to keep using the deep cycles for starting until you need to replace it. You should also be fine with running the switch at both as long as you understand that when in both if one battery should go bad (bad cell) it will pull the other one down to it's level unless you wire in an isolator or battery combiner. I ran with 2 deep cycles for years on a switch. A point of caution though is don't go switching the switch while the engine is running unless you have a "make before break" switch otherwise you can fry the regulator in the alternator.

 

[KnottyBuoyz]

Re: 1, 2, or Both?

Like Bob said. I did it that way for the first year before I figured out what was happening and fried both. What I described is ideal situation. If you experience some hard starting it's a sign of trouble. In both position you have no backup if you kill the batteries. The deep cycle aren't ideally suited to starting but will work. It shortens their service life by about half.<br /><br />Get Charlie Wing's book. Boatowner's Illustrated Electrical Handbook. Worth every penny.