REFLECTOR: [c-a] Re: COZY: Re: 24 volt versus 12

[Dave Philipsen]

I agree with Ron.  Another way to look at it is this:  All batteries (or 
cells for that matter) have an internal resistance.  In most cases that 
resistance is fairly low.  If a battery had infinite internal resistance 
it would never charge as it would never present a load to the charger.  
In order for multiple batteries (or multi-cell batteries for that 
matter) to charge evenly, the internal resistance of each battery (or 
cell) must be pretty close to the same.  Otherwise, the batteries (or 
cells) will not charge evenly.  If you wire two of the same type12-volt 
batteries in series and then put a load across one of them, the load 
will make the battery appear to the charger to have a lower internal 
resistance.  Thus, if you are charging the batteries with, say 28-volts, 
perhaps you may measure a voltage of 13 volts across one of the 
batteries (the one with the load on it) and 15 volts across the other 
one.  That would result in uneven charging.

If you want to look at it from a current standpoint, if you break any 
electrical circuit and measure the current (amperage) at any given point 
it will be the same as at any other point.  That means that if you 
charge two 12-volt batteries in series with 50 amps of current, each 
battery is getting 50 amps of current at approximately 12-14 volts.  If 
one of the batteries has a load across it (let's say your avionics bus) 
then it will be sharing that 50 amps with other stuff.  Thus, the other 
battery (unloaded one) is getting more of the charge.



Ron Breukers wrote:
> The problem is that the current you draw - even if it is generated by the
> alternator- goes THROUGH the first battery. Thus the problem remains the
> same if you are charging or not. In both cases the fully charged battery
> gets an overcharge. The only case where such a setup would be permitted was
> if the current draw of the 12 Volt circuit is very small compared to the
> capacity of the battery- which is not the case if you want to run the
> avionics of the battery.
>
>  
>
> Ron
>
>  
>
> From: canard-aviators at yahoogroups.com
> [mailto:canard-aviators at yahoogroups.com] On Behalf Of U-Nick
> Sent: Saturday, January 27, 2007 22:44
> To: Cozy builder group; canard
> Subject: [c-a] Re: COZY: Re: 24 volt versus 12
>
>  
>
> I received a lot of emails on this subject, most saying the problem is
> unequal charging of the batteries due to the draining of ONE of the
> batteries.... That makes sense to me.
>
> So here is another question....
>
> Most electrical systems keep a higher voltage across the battery (when
> charging) then the battery voltage. If there is any electrical drain,
> is made up by the alternator. So IF I tap off power from one of the
> batteries, I wouldnt really be drawing power from the battery (the
> power would actually come from the alt as it is making up for system
> loads).
>
> I could see depleting one battery IF I had a load on it when the
> engine is not running, but I dont see how I would be drawing the
> battery down if the alternator is making the amps as quickly as I am
> using them...???
>
> Is Joe right?
>
> On 1/27/07, Joe Toop <joecozy at telusplanet.net
> <mailto:joecozy%40telusplanet.net> > wrote:
>   
>> Hi, My 2005 GMC is factory wired for 24 volts from 2 series batteries for
>> starting with some electrical device on the connection between the
>> batteries. The truck runs on one battery. There is a warning "if charging
>> the batteries charge each separately". It also says both batteries should
>> be replaced at the same time. Joe
>>     
>
> Nick Ugolini
>
>  
>
>
>
> [Non-text portions of this message have been removed]
>
>
>
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-- 
Dave Philipsen
Velocity STD-FG
N83DP