lets complicate the two battery switch

[williampmccarthy]

What I would like is to have position 1 the start battery, position 2 to be two house batteries in parallel ( stereo, lights, inverter, etc).

I've been reading and if I use the simple "1-2-both-off " switch :

1. if I run my house battery down very low and put the switch to both the voltage from the full start battery will rapidly discharge to the house batteries possibly causing an explosion

2. If I switch to off while engine running the alternator will be damaged

3. That the alternator only works well if both battery banks are of similar size and make up..ie one start battery and one or two deep cycle are unbalanced and the charging of both banks together will be hard on the batteries and alternator.

this link is a dialog of a vehicle wiring, but references boat wiring

http://www.pirate4x4.com/tech/billavista/dual_battery/my_dual_battery_setup.htm

I find some products on the net and on this sight for several different setups, but can some one please give me a little more insight on the issue.

I've also been reading that my standard alternator on my 5.0L merc will probably not be sufficient to keep all three batteries charged.

I'm pretty sure I've already damaged my alternator with the current switch...at least that's what autozone says.

much thanks for any advise, products, diagrams, specs....Bill

 

[bruceb58]

Re: lets complicate the two battery switch

1. if I run my house battery down very low and put the switch to both the voltage from the full start battery will rapidly discharge to the house batteries possibly causing an explosion

No...won't happen. Have you ever heard of a battery exploding when you hook a jumper cable from one car to a car with a dead battery? Batteries do have an internal resistance so its not like you get infinite current. There is resistance in the battery and in the cable. You would normally not switch to both until the engine is started right? Once you start and flip to both, the alternator will pretty much make up most of the current to charge the low battery assuming a 50 amp alternator.

3. That the alternator only works well if both battery banks are of similar size and make up..ie one start battery and one or two deep cycle are unbalanced and the charging of both banks together will be hard on the batteries and alternator.

This is not true either. The alternator has no idea what is connected to it. It is basically a constant voltage...or at least tries to be... voltage source and will deliver enough current to get its output voltage to the proper level.

I've also been reading that my standard alternator on my 5.0L merc will probably not be sufficient to keep all three batteries charged.

This all depends on how much you discharge your batterries and how long you have to charge them.

 

[Boatist]

Re: lets complicate the two battery switch

I agree with everything bruce said.

To be clear what we are talking about it would help to know more about your engine.
I am assumeing it a V8 5.0L I/0 Mercruser but it would help to know the year.
A lot of the answers depend on your engine's charging system. I/O have bigger alterrnators than outboards and later years have bigger alternators than older models.
If you motor is a Mercruser 5.0L I/O 1990 or later I think you will likely have a alternator of at least 60 amps. So what I say is assuming that.

1. Yes if you have two discharged house batteries and a charged starting battery and you switch to both the starting battery will try to charge the house batteries. This can produce gasses out of the house battery but for it to explode you would need a spark. Most batteries should be in a battery box to help prevent any spark from getting to the batteries gasses. So very unlikely it would explode but it is rapidely discharging your starting battery.
If you have a good battery switch that is a make before break type then better to leave it on battery 1 and start the engine then switch to both or all but do not go thru off setting as this will blow the alternator on many boats.
With a large alternator of 40 amps or more it will produce enough current to charge both house batteries and the starting battery.
The alternator will put out about 14.4 to 14.8 volts to charge the batteries then each of the battery and it's internal resistence will determine how much current it takes.
The starting battery will likely charge up quickly.
A large discharged house battery may take 12 hours to be fully charged so unless your on a really long trip unlikely your house batteries will fully charge while running the motors. Batteries wired in parallel should be the same type and age. The reason is when not being charge the weaker battery will draw a charge from the stronger battery. IF the batteries are the same type and age then they should be pretty equal.

2 Yes if your turn thru off with the engine running and your switch does not have a properly wired field disconnect circuit then it will blow the alternator on older motors. Newer motors with a alternator with an internal regulator might not be damaged. Better to never switch thru off.

I myself would never charge batteries thru a battery switch. It just too much going back and turning the switch and too much chance for error that will cost you money.

I charge both my batteries thru a battery isolator. It will charge all the batteries any time the motor is running. Just leave the switch on 1 and start the motor. On my boat I have a external voltage regulator so it senses the battery voltage and the output of the alternator will supply 14.4 volts to the batteries.
On engines with a internal voltage regulator the alternator will put out 14.4 volts to 14.8 volts but the battery isolator will use about .7 volts to bias the diodes so the batteries get about 13.7 to 14.1 volts. This means the batteries will take a little longer to reach full charge. Batteries will charge with any voltage above 12.6 volts but 13.8 to 14.8 volt is normal to charge.

another popular way to charge is with a VSR relay or ACR relay. These also charge automaticly, they let the starting battery charge until it reaches a normal charging voltage then connect the house batteries. Again no switches to turn. On boats with small alternators I do not think they work to good. What happends is the starting battery charges the the relay connects the house battery the load of the discharged house batteries pulls down the voltage of the starting battery and the relay drops back out then the voltage comes back up and the relay again connects the house battery. On boats with small alternator like many outboards this relay can sound like a busser as it picks and drops. With Big alternator it works very well.
Hope this helps some.

 

[flargin]

Re: lets complicate the two battery switch

William -- Welcome to the forums...

Boatist, great writeup. One point I personally think needs repeating:

I charge both my batteries thru a battery isolator. It will charge all the batteries any time the motor is running. Just leave the switch on 1 and start the motor.

One reason I like this model is that you directly charge both batteries without going through the switch. Thus if you forget to go from position 1 to position both, you still have the benefit of both batteries being charged...

Another reason to like this model... you never have to worry about damaging your alternator by turning the switch to off.

 

[bruceb58]

Re: lets complicate the two battery switch

I am a fan of the ACR. As Boatist says, they have a voltage threshold where they connect and disconnect. Most have some hysteresis built into them so they don't have the buzzing problem as he described. There is a delay to connect when voltage on either side of the ACR sees a voltage higher than 13V or so and a disconnect threshold if either side sees something lower than 9V. What basically happens though is that is only takes a few cycles to get the low battery up to the 9V threshold so that the ACR stays connected.

 

[williampmccarthy]

Re: lets complicate the two battery switch

SO much thanks for the quick replies. the boat is a 97 bayliner, but has a new 5.0L engine that a dealer put into it befor purchase. the alternator is 12v 70a....its in my hand right now. the auto parts store put it on a tester and says its shot. I probably put the system to off while running-didn't know about that problem

So it sounds like an exact replacement will give plenty of charging power?
and a smart switch that charges everything will be the simplest?

one last question the boat is currently wired with only one lead comming out of the current switch that powers everything...house and starter, should I rewire this so that the start battery is only wired to the starter and the house bank goes to everything els? I'd have to figure out how to do that befor I get started :{ also the smart switches I've seen from BEP marine on this site still have an off position....could that still harm my NEXT new alternator if put into off while running

Thanks again, happy boating, Bill

 

[williampmccarthy]

Re: lets complicate the two battery switch

sorry one more...

so ther is no problem having two batteris in constant parallel for the house power...but they should be same kind and age?

 

[Boatist]

Re: lets complicate the two battery switch

I do not know how much you know about alternator but it is cheaper to repair than replace. If you do your own work then one of two parts is likely the problem. Either the output diode block or the voltage regular. At a good alternator shop they will have both parts. Here both parts are about $10 to $15. If you have your engine factory manual then it will have a touble shooting sectiion for the alternator to help you find the bad parts. Some alternator shops will repair for you for about another $10 plus parts.
It you have it repaired at a shop tell them it a marine alternator and if they do not know what that means go to a different shop.

One of the reasons I use a battery isolator is it protects the alternator. For you to blow the alternator with a battery isolator you would have to disconnect both batteries at the same time with the motor runing. You do not really need a switch at all. Even if you have the switch and turn thru off the isolator will still connect the alternator to both batteries so it will not harm your alternator.

If you keep your switch then there are a lot of ways to wire. In this forum can get a lot of debate as to which is the best way.

The way I would wire the switch with a battery isolator is the house battery is connected to switch post 2 and all the high load like trolling motor, big stereos, refigerators are also wired to switch 2 or to the house batteries.
The starter is wired to the common post of the switch. All the low current iteams to run the boat are on post one of the switch. This is iteams like navigation lights, bilge blower, bilge pump can go post one or common, GPS, Depthfinder, VHF radio.

In this configuration there is no need to ever turn the switch unless battery one is dead and you want to try and start off the house battery.

Paralell batteries- should be the same type and age. Also note that they will run the load twice as long but the life is almost always less than a single battery. This because one battery will always be a little weaker and load the stronger battery. They will only last as long as the weakest battery.
Another option would be to wire two large 6volt batteries in series to provide the same power to the load. Wired this way this way a week battery has very little effect on the other 6 volt battery.

Good Luck and have fun.