Battery Connections

[blewett_john]

I'm sure this has been beat to death but . . .

I've searched battery connections, house batteries and engine start battery via an isolator and three position switch. Most show the engine starter coming off the common (both) position on the three position switch. Some show the engine starter coming off the engine battery position on the switch.

Which is best? If the starter comes off the common position doesn't that use both battery banks to start the engine? If the starter comes off the engine battery position can you still use the house battery bank to start the engine?

I'm a mechanical engineer, electrical stuff makes my head spin.

Thanks!

[JK n Smitty]

IMO, the best set up is a large primary bank and a small reserve bank. For instance my primary bank consists of 4 6v golf cart batteries wired in series/parallel for a 12 volt bank with 460 Ah and my reserve bank is a group 24 battery. I use my primary for everything and only use the reserve occasionally to test it or if I drained the primary too much (hasn't happened yet, knock on wood). The primary bank is wired to the #1 position and the reserved is wired to the #2. I just leave it on #1 all the time.

The starter is wired to the common so that it can be used in any of the positions except off. But I would probably never use the "both" setting because if I was going to use #2 it would be because the primary bank is depleted. For this same reason I would not wire the starter battery to bank #2 position. You would have to use the both to start with bank #1. You could get a different switch or a series of switches to make a dedicated starter battery but there really is no need to have such a complicated system.

Another thing to consider is the alternator wiring. I believe that the typical setup is to have that go to the common of the 1/2/both/off switch. This can be bad for several reasons. You only get the alternator charge to the selected bank. If you change the switch to get the charge to a depleted bank after starting you could damage your alternator depending on the switch you have. Instead I ran my alternator directly to the primary bank and installed an automatic charge relay between the primary bank and the reserve bank.

Here is a really good write up on the 1/2/both/off switch.

Good luck.

Jesse

[Maine Sail]

Originally Posted by blewett_john

I'm sure this has been beat to death but . . .

I've searched battery connections, house batteries and engine start battery via an isolator and three position switch. Most show the engine starter coming off the common (both) position on the three position switch. Some show the engine starter coming off the engine battery position on the switch.

Which is best? If the starter comes off the common position doesn't that use both battery banks to start the engine? If the starter comes off the engine battery position can you still use the house battery bank to start the engine?

I'm a mechanical engineer, electrical stuff makes my head spin.

Thanks!

There is no "both" lug on a 1/2/Both switch. The lug you refer to is called the "C" post or the output/On post.. What ever position you select on the switch determines which bank the "C" post is connected to.... Set it to #1 and the "C" post is connected to #1 etc....

[Stu Jackson]

Had you searched on "1-2-B switch" you would have found these, posted multiple times:

Basic Battery Wiring Diagrams This is a very good basic primer for boat system wiring: Basic Battery Wiring Diagrams

This is another very good basic primer for boat system wiring: The 1-2-B Switch by Maine Sail (brings together a lot of what this subject is all about)
1/BOTH/2/OFF Switches Thoughts & Musings - SailboatOwners.com

[Ex-Calif]

Originally Posted by blewett_john

Which is best? If the starter comes off the common position doesn't that use both battery banks to start the engine? If the starter comes off the engine battery position can you still use the house battery bank to start the engine?

I'm a mechanical engineer, electrical stuff makes my head spin.

Thanks!

To expand a little on MaineSails response -

A and B are designed as "input" lugs

C is the output lug.

House and Start batteries are connected to A & B respectively.

If Starter cable is connected to C - either "A" or "B" or "Both" banks can supply power to the starter depending on switch setting

If the starter cable is ganged on post B - the switch position is irrelevant as the starter is unswitched and permanently connected to the start battery.

The house battery cannot be used to supply current on it's own (position A) but would be paralleled in when switch is set to both.

Depending on what you want the switching to do this could be an OK setup, albeit not a common one.

[ontherocks83]

Originally Posted by JK n Smitty

IMO, the best set up is a large primary bank and a small reserve bank. For instance my primary bank consists of 4 6v golf cart batteries wired in series/parallel for a 12 volt bank with 460 Ah and my reserve bank is a group 24 battery. I use my primary for everything and only use the reserve occasionally to test it or if I drained the primary too much (hasn't happened yet, knock on wood). The primary bank is wired to the #1 position and the reserved is wired to the #2. I just leave it on #1 all the time.

The starter is wired to the common so that it can be used in any of the positions except off. But I would probably never use the "both" setting because if I was going to use #2 it would be because the primary bank is depleted. For this same reason I would not wire the starter battery to bank #2 position. You would have to use the both to start with bank #1. You could get a different switch or a series of switches to make a dedicated starter battery but there really is no need to have such a complicated system.

Another thing to consider is the alternator wiring. I believe that the typical setup is to have that go to the common of the 1/2/both/off switch. This can be bad for several reasons. You only get the alternator charge to the selected bank. If you change the switch to get the charge to a depleted bank after starting you could damage your alternator depending on the switch you have. Instead I ran my alternator directly to the primary bank and installed an automatic charge relay between the primary bank and the reserve bank.

Here is a really good write up on the 1/2/both/off switch.

Good luck.

Jesse

This is the exact set up I have also. What I really like about it is as long as you don't deplete your house bank you never have to bother moving the switch. Only in an "emergency" that your house bank has shorted or been depleted do you need to switch to your reserve battery. Other then that you don't have to worry about it.