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Author Topic: 12 volt tap  (Read 886 times)
uemjg
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« on: January 22, 2013, 05:30:07 PM »

Can i tap the positive from one starter battery and the negative from the other starter battery for a "balanced" 12 volt source?
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Lee Bradley
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« Reply #1 on: January 22, 2013, 05:33:25 PM »

The only out come I see is 24 volts or 0 volts.  More information please.
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Ace
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« Reply #2 on: January 22, 2013, 08:11:14 PM »

A volt meter will and can teach you alot! Just play around with it and you can find what your needs are!
Cant hurt too much playing!



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Ace Rossi
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« Reply #3 on: January 22, 2013, 09:42:35 PM »

Lee has it dead on.  Here's why --

Battery has + and - terminals.  Electric current flows from one to other.  If you don't have connection from one to other on same battery no current flows so you'd have nothing to balance.

Batteries can be connected 2 ways, series and parallel.  In series, the + of the first battery connects to the - of the next battery until you run out of batteries and the voltage is the number of batteries times the battery voltage.  You take power to loads from unconnected terminal of each end battery.  If batteries are much different in voltage, unfortunate things happen.

In parallel, you connect all + together and all - together, voltage is that of battery type and current is number of batteries times battery rated current (amps).  Connect much different batteries and unfortunate things happen.  Connect to load as if just one big battery.

So, if your scenario has load connected to + of battery A, - of battery A connected to + of battery B and -m of battery B to load they are connected in series and you get 24 volts (blowing your circuits) -- not balanced 12V.

edward
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« Reply #4 on: January 23, 2013, 03:44:20 AM »

You can tap the positive and negative of the same battery and get 12v.  The draw of 12v power from that battery will put it out of balance with the other battery and the pair will not charge equally.  The cure is to use a battery equalizer like the Vanner, which supplies 12 volt from a center tap between the two batteries and chassis ground and draws equally from both batteries to do so, keeping them equal in charge.

The exception to this rule of thumb is that when you draw equal 12v power from both batteries at the same time, they remain equalized.  This is how some 12v headlights on 24v buses are set up.  With a four headlight system one pair of headlights draws from the 0 - 12v battery and the other pair of headlights draws from the 12v - 24v battery.

Brian
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Len Silva
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« Reply #5 on: January 23, 2013, 05:15:24 AM »

It is theoretically possible but not practical.  If you look at my sketch, it will work but LOAD B must be completely isolated from chassis ground.  That is  impossible with most typical automotive accessories, radios, motors, lights etc. as they are internally grounded and use the chassis for return.

It would require that everything on the LOAD B side be a two wire circuit.  Imagine the confusion five years down the road when you or someone else is trying to do some work on this circuit.
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« Reply #6 on: January 23, 2013, 05:40:02 AM »

It is theoretically possible but not practical.  If you look at my sketch, it will work but LOAD B must be completely isolated from chassis ground.  That is  impossible with most typical automotive accessories, radios, motors, lights etc. as they are internally grounded and use the chassis for return.

It would require that everything on the LOAD B side be a two wire circuit.  Imagine the confusion five years down the road when you or someone else is trying to do some work on this circuit.

Might not be practical for much of anything beyond headlights, but some MCI buses came with that headlight system as stock in the early 1980's, according to those who know.  In that case the wiring is reflected in the relevant schematic in the manual, I would assume.   Your sketch and warning is 100% correct.  In the case of my bus virtually all 12v loads are two wire, I used a "home-run" ground system and most everything is mounted in wood so there is no inherent chassis ground.  Mind you, I power them from a Vanner and ground is tied to chassis ground at the bus bar.  My headlights (1980 MC-5C) originally had 24 volt bulbs but there is now an MCI-sourced conversion box on each that converts 24 volts to 12 volts.  It works really poorly and I need to figure out how to fix it or eliminate it.  Splitting the headlights as I described is definitely an option, since I feel the total load is getting to be fairly high on the Vanner what with other possible loads as well, and another option is going back to 24 volt bulbs.

Brian
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1980 MCI MC-5C, 8V-71T from a M-110 self propelled howitzer
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robertglines1
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« Reply #7 on: January 23, 2013, 07:05:41 AM »

Downside: if load (12 volt) is significant the batteries will be tempted to not be charged equally.  May cause problems.  answer equalizer with 12 volt tap. Most use vanner.  or just check inside your elect box for a 12volt bus bar(it might already be split) I was thinking my Mci 8 had one.  Bob
« Last Edit: January 23, 2013, 07:11:49 AM by robertglines1 » Logged

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