Stupid Voltage Question

[honk]

This may be the forum's dumbest question but can someone explain to me why I cannot establish a 12 volt supply for accessories by simply making a circuit from one of the two batteries, independent of the rest of the vehicle's 24 volt system?

More clearly, couldn't I use one of the painless fuseblocks, wiring it's input directly to one battery for both pos and neg connections, and then use it's output circuits as a 12 volt supply with no further ado?

As some of you know, the BJ40 which I bought from Wayne had large connectors in the bib that were for supplying electricity to start airplanes or something. One of those was a 24 volt output, the other a 12 volt output. I've removed both and in doing that I noticed that they were each wired very simply. The 12 volt one had it's positive connection at the RH battery with it's ground being the common ground to the frame under that same battery. That's all there was - no converter, nothing but heavy wire and simple connections. So why can't I reestablish that 12 volt output on a smaller scale and never bother with a voltage converter at all?

 

[Greg_B]

There are others much more electrical savvy then I on this board, and perhaps they will step up and explain why, however in the meantime:

Your charge system does not care where the 28V goes when charging. When two similar (same make, model, manufacture time) are used in series, both will accept the charge equally (well, almost and close enough for us). When you tap off one of the batteries taking 12v's, the charge system does not care, however for some reason the battery you are taking the tap off will undercharge. Again, the charge system is pumping out 28V, so the excess charge goes to the other battery. The undercharged battery will sulphate, and the overcharged battery will cook. You will be replacing both batteries quite regularly.

I recall hearing a number of years ago that even with no taps, old LandCruiser salt wisdom was to switch the batteries from side to side, every oil change, as even with no taps the charge is not totaly equal. Switching side to side every oil change assisted in getting longer battery life.

This is one reason it is recommended to replace the batteries in a matching set, when one fails. The characterisitcs of the two batteries will not be the same, and chances are one will cook, and the other sulphate. I believe this has something to do with internal resistances being different.

hth's

gb

 

[Tapage]

Jumm I have 2 Batt setup in my HJ-60 ( 12V ) in paralel off course .. but one of this are ZL I can't perform a switch betwen both.

Let me explain it. Front of my TLC in the right batt the grownd pole is at right and the positive at left, the other batt at left side heve the grownd pole at left ( near to fender ) and the positive at right.

uff .. ! complicated ..