Tuesday, Sep 19, 2006 at 22:39
Your reaction reflects a typical situation of tradesmen I have encountered her in
Australia! You must have been sick in year 12 Physics ... ahh sorry you don't learn about Peuckerts Law here in
Australia in High School ...
So may be here a little lesson:
The usable current from a
battery depends very much on how fast energy is taken from the
battery. Slower energy _Affordable_Storage_Drawers.aspx usually mean more usable capacity from the
battery. Energy draw is usually indicated by current because a
battery's voltage is relatively fixed by the
battery design. Energy draw is called power and is the product of voltage and current. Energy capacity is a product of power and time.
The capacity gained by reducing energy draw or current drain in a parallel configuration versus a series configuration can be determined by evaluating Peukert's Formula T = C / In where T is how long you can drain current I from a
battery that has a capacity C and an internal resistance characteristic n. For the case of two six volt batteries in series versus two 12 volt batteries in parallel, the comparison is when the current changes by a factor of two (when voltage doubles, current halves and vice versa for the same amount of power). The formula would be
Tp - Ts = (C / In) - (C / (2I)n) = C(2n - 1) / (2n In) = Tp ( (2n - 1) / 2n )
{subscript p for parallel and s for serial, serial has twice the current of parallel, the LCD (2I)n which is where the 2n comes from in the Tp term to be able to subtract the fractions, percent change divides both sides by Tp and then multiplies by 100 - check the algebra yourself and let me know if you think you see an error!}
So percent change from the low current to high current times is (2n - 1) 2n . When this is calculated, the range for typical batteries means drain times will be from 71% (worst case worth purchasing where n=1.25) to 65% (best case, n=1.05) changed from parallel to serial. In other words, doubling current will reduce time of draw from 65% to 75%. For the 40 amp hours available (discharge to 80%) at a 5 amp load, the parallel configuration could provide 8 hours of usable
battery while the serial configuration would provide maybe 6 hours. This would be balanced by the batteries in series probably have a bit more capacity to start with.
In above example the serial configuration of the same C20 capacity gives me less useable output ...
Well I tried to explain it 4 times .. now I give up and do my electrics myself anyway ..
have fun
gmd
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