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charging two 12v batteries in series

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Member - Posts: 6
Member spacespace
Joined: August 29, 2011
Location: United Kingdom
Posted: September 07, 2011 at 1:36 PM / IP Logged  
You have given this some thought oldspark!
I'm now quite some way out from the shore and didn't bring a paddle..
I think I prefer the first method but with a dpdt relay and fuse protection. I'm guessing that a single dpdt relay would remove any potential timing issues and the chances of a shorted battery would be less since both lines would have a much lower 'switch failure' rate (is that a valid term?) since they'd operate together. Not forgetting, of course, that I'll be sitting on top of any failures!
But this may all be moot anyway Oldspark since in my search for relays I found talk of a device which is used for what appears to be exactly what I need i.e. a series parallel switch. It seems that a number of military, heavy goods and even some cars were designed with 24v starting using 2 12v batteries charged in parallel. Need to do a bit more digging but if I can find a ready made device specifically designed to do the job then it simplifies things enormously for me (but knocks all the fun out of designing something). If you get a few minutes free sometime have a look - I'd appreciate your thoughts on whether I'm overlooking something.
Funny you should mention starting current as a friend is arguing quite strongly that the starter motor at 24v (cranking the engine at double speed) will draw the same current as it does at 12v 'because the batteries are in series'. I'm sure this is completely wrong since the work done in turning the load at double speed must be far greater and this can't be done for free. Once I find someone with a device capable of measuring such high currents I'm going to send him pics of the readings at 12 and 24.
Cheers once again and enjoy your trip.
Artificial intelligence is no match for natural stupidity
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: September 07, 2011 at 8:50 PM / IP Logged  
You don't need a paddle - you swim fine!
The truck devices may be physically big...
And I might see if I can test the 2xSPST, but that'll take a while to set up. Maybe next week (else next millennium lol!).
As before, 2 batteries in series means 2x the current if resistive - ie, V=IR hence I = V/R where R is constant, hence 2xV => 2xI (Amps).
But it's a motor. If it were a constant power device, then current would be halved. (ie, P=VI = 2V x I/2).
So the answer is somewhere between twice & half the current. (LOL!).   
Maybe I could test that too, especially since I have a reduction (geared) starter which probably reflects a bike starter more accurately, though that shouldn't effect much. (And I have a spare!) After all, theory is one thing, reality an other. Besides, even if it takes till next millennium, that's probably quicker than my old memory (brain) cells will react!
BTW - I just use a 50A shunt (Jaycar, ~$10) which is fine for short tests. I have measured 250A starters - the tests of up to 15 seconds don't allow the "50A" shunt to heat up enough to blow.
[ The shunt - a high wattage low resistor (1mV per Amp or something, ie, R=V/I = 1mV/1A = 1 milli-Ohm) - is placed in line (series) with the starter, then the voltage drop is measured with a multimeter (DMM). ]
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