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: [email protected]
>>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'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
