Dual Deep Cycle Batteries

[DoubleG]

I have 2 batteries on my boat now and I want to add a 3rd. Right now I have a starting battery that only has the motor attached to it and a deep cycle with everything else. I want to add another deep cycle and put a switch between the 2 deep cycles so if 1 dies I can switch over to the other one.

I bought a switch and on the back of the switch there's a terminal for the hot leads for both batteries and the hot lead that would got to he fuse panel or whatever you're powering. Where do the negative leads go??? The directions that came with the switch are horrible.

I forgot to mention that I have a 90hp outboard and an aluminum boat ( not sure if it matters ).

If you have any other suggestions like different switch or way to run it that would be great too.

Thanks!!

Edited April 9, 2015 by DoubleG

[skipper19]

1 st question...how do you charge the deep cycle you have now? From the 90 outboard or on shore?

[bout time]

The negatives go straight to your ground (other battery, engine ground, etc) they don't go to the switch. The switch operates much as a househild light switch it just completes the "HOT" side of the circuit. Depending on your switch you can normally run it as Battery 1, Battery 2, or Both. Make sure you pay attention to what battery runs a float switch bilge for example if equipped

Mike

'Bout Time

Edited April 9, 2015 by bout time

[DoubleG]

Skipper - yes I have an onboard charger that I was planning on keeping attached to Battery 1.

Mike - does it matter which battery's negative post I use? So if I'm understanding this I can run the positive wire from my fuse panel to the switch and the Neg wire to the battery neg post on Battery 1. The positive wires from both deep cycle batteries to the switch. Sound right?

[bout time]

As long as all your battery ground posts are commoned together and return to the main engine ground it doesnt matter. this can be done by either running a jumper from engine ground to battery ground 1 and from battery ground 1 to 2 and from 2 to 3 or from running a jumper from the engine ground to a common buss bar and then running jumpers off the buss bar to each individual battery grounds.  

 

As far as the switch itself. there should be posts on the back labeled 1 and 2 and usually a center post not lableled. The post 1 and 2 will go to each battery and the center post (some have a wire) would go to the "HOT" side of the motor/starter

 

found this on another site. this may help.

The use of a switch and wiring like this is recommend with dual battery installations. A brief explanation of the operation of the switch in this circuit follows.

In the OFF position, the two batteries are disconnected from all loads. The OFF position is used when the boat is being stored or otherwise not in use. It prevents any drain from the batteries. This can be useful if a circuit has been accidently left on, say a cabin lighting circuit or similar drain. Such a load can completely discharge a battery in a day or two, leading to an unanticipated dead battery situation when you return to your boat. On some switches a key lock is provided, allowing the switch to locked in the off position. The provides another level of security in preventing the boat from being used when left in storage or unattended.

In the 1 position, all loads and charging currents are connected to the No. 1 battery (the PORT battery in the illustration). Starting current for the engine starter motor is supplied by the PORT battery. When the engine is running, surplus current developed by its charging circuit will flowing into the PORT battery. Current to lighting and other loads will flow from the PORT battery. The STDB battery is completely isolated and has no load current, nor does it receive any charging current.

In the 2 position, all loads and charging currents are connected to the No. 2 battery (the STBD battery in the illustration). Now it supplies current required by starting and running, and it receives all the charging current from the engine. The PORT battery is totally isolated.

In the BOTH position, the two batteries are connected in parallel. This has a number of implications. Unless the batteries have exactly the same state of charge, the combined voltage to the two batteries in parallel will sag to a voltage somewhat lower than the highest battery's terminal voltage. Current from the higher voltage battery will flow into the lower voltage battery and begin charging it. As long as the state of charge in one battery is higher than the other, the lower battery is more of a load than a source of power. Eventually, the batteries will reach an equilibrium, and they will both have the same terminal voltage. At that point they will both tend to supply current to loads that are attached to them, and they will both receive charging current furnished by the engine.

It would seem like operating in the BOTH position would be beneficial, but that is not always the case. Even though the batteries will eventually rise or fall to the same terminal voltage when connected together, they will not necessarily become exactly the same. A battery (or any source of electrical engery) can be thought of as having an internal resistance. The lower this internal resistance the greater the current it can supply. The internal resistance will also affect how the battery absorbs charging current. Even though they are connected in parallel, it is possible that they will supply unequal currents to the loads, and it is also possible that they will accept unequal currents from an the engine charging source.

If the batteries are significantly different in their age, their type of construction, and their state of charge, this unequal distribution of current can be more significant. To describe the situation in the simplest of terms, when two batteries are connected in parallel, they will probably tend to behave more like the weakest battery of the two than the strongest.

Paralleling the batteries can come in handy in some situations. For example, both batteries may be discharged to a point where neither alone can provide enough current to crank the starter motor, but combined in parallel they can turn the engine over.

If one battery is fully charged and the other is totally discharged, connecting them in parallel (by using the BOTH position) can cause very high currents to flow between the batteries. Extreme heat can be generated by the sudden charging of the discharged battery. Use caution in this situation. It is better to recondition a discharged battery by slowly re-charging it with an AC-operated battery charger.

The arrangement of the contacts of the typical OFF-1-2-BOTH permits the operation of the switch in the range of 1-2-BOTH without ever disconnecting the batteries from the load or the outboard charging circuit. This is important, as it is possible to cause damage to the charging circuit if the battery is disconnected while the engine is running. By choosing the path of rotation of the switch, it is possible to change from 1 to 2 without moving through the OFF position.

 

Hope this helps

Edited April 9, 2015 by bout time

[DoubleG]

OK I think I got it.  Take the outboard out of the equation, that's on a stand alone starting battery.  I will disconnect the onboard charger.  How does this look?  I don't plan on using the both position.  The main reason for this is when I run down the first deep cycle I can switch to the 2nd and keep using my trolling motor.

 

Edited April 10, 2015 by DoubleG

[jimski2]

There are two types of battery switched. One is a simple on and off with a choice of single or both battery operation . The other has an alternator disconnect circuit to protect your alternator diodes that will burn out when you change the battery while your engine is running in case you do this. The alternator senses a low voltage condition while switching and sends a high surge of power that the diodes can not handle.

Sent from my iPhone using Lake Ontario United mobile app

[skipper19]

Looks ok double G. You can still hook your on board charger up. Not sure if you have a double bank or single charger. Single charger I presume, and you can hook your charger direct to the batteries, positive on positive and negative on either battery. Turn your switch to both and then you will charge both batteries simultaneously. After charging switch to off.

You could use both batteries while on the Troller and you should have double the amp hours for twice the time before run down. On the other hand if you did like you say, used one battery at a time, then you would have an emergency 12 volt supply to get you home in case the big motor misbehaved.

I assume your switch has a 1,2,both and off position.

Mark

[DoubleG]

Mark - the onboard charger I'm using actually runs off of the outboard's alternator so I'm not sure if I can keep it on. It senses when the main battery is at full charge and switches the alternator over to charge the deep cycle battery. I bought the one for 1 battery because that's all I had at the time.

http://m.basspro.com/Minn-Kota-Onboard-Alternator-Chargers/product/95618/

[skipper19]

Ok, yeah the alternator charger should be alright to charge either battery. I see it's only 10 amps so it's best probably to charge one battery at a time from your 90.

[DoubleG]

Cool, thanks for all of your help!!!!