Al, Your point is well taken and I understood this. However, for the most part a rig is not going to be drawing much current, in receive most of the time. Little affect on the charge/discharge of the lower battery. This is why in a previous post I said depends on the load if wanting to connect across the lower battery.
I think the solution to this topic came from you and others long ago. Use a 24-12 converter. They are about the same price as a battery. However, for the typical 50 W rig I would have no problem connecting across the bottom battery. The batteries get lots more abuse from the vehicle. A 1 amp draw 99% of the time is little compared to the typical draw from the other stuff. If drawing the currents you are talking about, again connecting to the lower battery would be a problem. 73, ron, n9ee/r >From: Al Wolfe <[EMAIL PROTECTED]> >Date: 2008/03/29 Sat AM 01:07:34 CDT >To: Repeater-Builder@yahoogroups.com >Subject: [Repeater-Builder] Re: 24 vdc to 12 vdc > >Ron, et al, > A corollary of Kirchhoff's law says that all currents are equal in a >series circuit and all voltages are equal in a parallel circuit. Tapping a >load across the lower battery represents a parallel circuit of the bottom >battery and the load (radio) using 12 volts, and this parallel circuit is in >series with the upper battery. If I supply 10 amps from the 24 volt charging >circuit the upper battery receives 10 amps. However, if the load (radio) >draws 5 amps the bottom battery can only get the other 5 amps to charge it. >Hence the application of Kirchhoff's law of currents equal in a series >circuit. There is no way for the bottom battery ever to be equally charged >with the top battery as the radio has drawn away some of the current from >the bottom battery. This current can never be replaced in this >configuration. > >If the charging current drops to 5 amps then the top battery get 5 amps >and the bottom battery gets none. If the charging device is off then the >bottom battery continues to supply 5 amps to the load (for a while!) > >If this condition continues very long the terminal voltage of the bottom >battery will be depressed towards 12 volts or lower but the 24 volt charging >device doesn't know this and will attempt to apply full charging voltage >(probably 28 volts or so) to the two seriesed batteries. If the depressed >voltage on the bottom battery is down to 12 volts (typical) the top battery >will get 16 volts, not a healthy situation! It only get worse as the >condition continues. > >Now there are solutions. One can switch batteries (top with bottom) >every few hours but who would actually do this? Or one could put a phantom >load across the top battery, something that draws about the same current as >the radio but this would be rather wasteful of energy and tricky to make >work. > >Many 12 to 14 volt linear power supply circuits (Astron or ARRL >handbook) can be adapted to convert the 24 - 28 volt source down to 12 - 14 >volts just by injecting the +24 volts into the power supply after the main >rectifier diodes. But here again the power supply will use up about as much >power as the load. (Kirchhoff's law again) > >If conserving power is important then a switching supply would be in >order as some of them are more than 90 % efficient. > >73, >Al, K9SI > >> Re: 24 vdc to 12 vdc >> Posted by: "Ron Wright" [EMAIL PROTECTED] n9eerptr >> Date: Tue Mar 25, 2008 5:04 am ((PDT)) > >>Depending on the load connecting across one battery, the one connected to >>ground or the lower of the 2 12 batteries, will work. I >would not do is >>load is heavy because I am sure the charging system is for both batteries >>and draining one much more than the other >could upset things. > >>Kirchhoff's current law says the sum of the currents will be zero. >>Kirchhoff's voltage law says sum of voltages will be zero. Not sure >why >>revelant here, but I am sure Kirchhoff had something else to do with >>voltage and current sources. Would like to know. > >>73, ron, n9ee/r > >>>From: Al Wolfe <[EMAIL PROTECTED]> >>>Date: 2008/03/24 Mon PM 04:36:54 CDT >>>To: Repeater-Builder@yahoogroups.com >>>Subject: [Repeater-Builder] Re: 24 vdc to 12 vdc > >> >>>>This is a very common issue in aircraft. The most obvious question is >>>>does your jeep use 2 each 12 volt batteries? If so, simply connect >>>>your radio across one of 'em. >> >>>This is done all the time but is a very, very bad idea. Ever hear of >>>Kirchhoff'<wbr>s law? Check it out. It's a very quick way to ruin two >>>batteries. >> >>>Get a real converter. The switching kind are much more efficient than the >>>linear voltage dropping kind. >> >>>Al, K9SI > > > > Ron Wright, N9EE 727-376-6575 MICRO COMPUTER CONCEPTS Owner 146.64 repeater Tampa Bay, FL No tone, all are welcome.
