how to wire up 2 batteries using diodes

[Colliemcc]

Hi
I am looking to wire up 2 batteries to my volvo 4.3GL.
The way I was thinking of doing it was for one battery to run the lights, sound system and VHF. And one to start the boat.
Could I wire it up using diodes. so the diode will be between the battery 1 & 2

Battery 1 will be the main battery and battery number 2 for the the lights etc. So the two batteries will charge all the time when engine running. but what ever is connected to battery 2 will not use any power from battery 1 and when the engine is off and stereo on it will only drain battery 2

Can this be done and does anyone what type of diodes I need to buy to do this

 

[Silvertip]

Re: how to wire up 2 batteries using diodes

Considering the losses in the diodes and the fact that you would need diodes rated higher than the alternator output, you would be advised to install a dual battery switch (OFF-1-BOTH-2). This elminates the diodes and provides way more versatility including the ability to start the engine from either or both batteries.. You do realize that the battery gets charged through the large Positive battery cable? Therefore current flows both ways on that cable. A diode is a one-way device so you would need to reroute the charing lead from the alternator to the junction of the two diodes. That may result in a malfunctioning voltage regulator. Better stick with what works.

 

[j_martin]

Re: how to wire up 2 batteries using diodes

I've done it with relays.
One way is to wire the accessory wire of an ignition switch, assuming it would be on when the switch is on, but off when cranking, to the coil of a relay. The contacts connect the batteries for charging. When the switch is off or when cranking, the relay would not be closed.

If you want the battery disconnected when in the accessory position, you can wire it so that the run circuit goes to the coil of a relay, and the accessory circuit feeds through contacts on that relay to the coil of a second relay that connects the batteries. When in accessory ->> disconnected. When starting --> disconnected. When running -->connected.

hope it helps
John

 

[seabob4]

Re: how to wire up 2 batteries using diodes

Have you ever heard of an Isolator/Combiner? Combines the batts for charging purposes when the engine is running, isolates them with the engine off.

And a dual batt switch is definitely a must...

 

[Boatist]

Re: how to wire up 2 batteries using diodes

Two diodes is called a battery isolator. You connect from the alternator output to each battery. The diodes would need to be able to handel the full current output of your alternator and a little more for saftey. The current only goes one way on this alternator output line. These high output diodes are pretty expensive.
For just a few dollars more you can buy a battery isolator which is just 2 diodes with a large heatsink to keep them cool.
There is a voltage drop acrossed the diodes of about .7 volts.
So if you have a one wire alternator your batteries will charge slower. Example if your voltage regulator is 14.8 volts then you will charge at 14.1 volts at each battery. In this case you would want to change your voltage regulator to a .7 volt higher regulator or instead use a (VSR) Voltage sensing relay. VSR charges the starting battery until it reaches a certain voltage like 13.8 volts then it kicks in the second battery and charges both.

If you have a 3 wire alternator then the battery isolator works perfect. It is the way I have my boat wired. Anytime the engine is running both battery are being charge and when you shut the engine off the batteries are isolated. The 3 wire alternator should have battery voltage feeding the Voltage regulator which feeds the field wire on your alternator an if the voltage at the battery is
.7 volts low it simply turns on more field current and the voltage at the battery is the same as before you installed the isolator. With a 1 wire alternator this does not work as all the voltage sensing is inside the alternator with no feed back from the battery.
There is no switch necessary for it to work. If you want a switch you can install one. I do not desire a switch.