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Twin Batteries

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Paul Godfrey18/07/2009 18:45:50
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163 forum posts
292 photos
Its me once again, and so soon!!
 
I have had a wiring diagram produced for me by Dave Milbourne at Action Electronics (as I may have mentioned before), which uses a 12v 12ah battery to power everything - the rc equipment, lights, sound, smoke generator & fan.
 
My problem is that this size of battery is quite a lump, and in fact wont fit through the opening in the deck. Even if it did, it's a bit high for my liking (need to keep the weight in the boat low down as poss.).
 
So, I've decided I'll buy two 12v 7ah batteries, which are much narrower, and can be laid flat one next to the other.
 
My questions are: how do you conect the two batteries to maintain 12v (and not 24v), and will I need to disconnect both batteries to charge them (I ask this because one of the batteries will be slid up the front of the boat, and I dont really want to get it out when they need charging).
 
Many thanks, Paul
Peter Fitness18/07/2009 23:39:46
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510 forum posts
215 photos
Paul, to connect two batteries of the same voltage in such a way as to maintain that voltage, they should be connected in parallel, that is, +ve to +ve, and -ve to -ve. This still gives the same voltage, but doubles the capacity, in your case, 12v 14ah.
 
It is my understanding that you should charge each battery individually, but others may have more information on that. 
 
Peter. 
Dr John Booth19/07/2009 08:31:48
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90 forum posts
1 photos
Unless you are an electrical expert I would suggest that you follow the rule of batteries being all different and therefore should be treated as seperate units when charging.
 
John
Gordon Blanks19/07/2009 13:19:04
77 forum posts
13 photos
i have two 6v 4.5amp batteries in my boat connected together  for 6v x 8 amps  and i charged both  at the same time so far after 3 months no problems
Paul Godfrey19/07/2009 21:40:26
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163 forum posts
292 photos
Thanks for your replies - most helpful.
 
I suppose the thing to do is run down the batteries, and try a simultaneous charge, then test each battery with a meter.
 
In any case, I'm going to adopt the method for ballasting the front of the boat (which is somewhat inaccessable when the deck's in place) as recommended by Mountfleet, namely to use a 'tray' which can be slid up the front end of the boat on 'runners', and secured by a wing-nut fitted to a bolt bonded to the base-board. I can attach both batteries to this tray for easy removal (both for charging, and for easier transporting of the boat), and now knowing how to connect them, can make up a 'Y' lead for the positive terminals, and another for the negative terminals, with a single connector for each lead allowing easy removal from the power board.
 
Paul
Mike Davidson30/07/2009 21:15:45
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157 forum posts
9 photos

Paulyou could have all sorts of problems if you connected the two in parallel, like if one battery were to discharge, the other would try to charge it upbecause the internal resistance of the discharged one would be lower. As I see it, the best thing to do would be to carge each one individuallyHaving done that, I suggestyouconnect them up using the negative as a common ground or earthand connecting the positives through series diodes there are plenty of butch diodes available that can handle 7 to 10 amps forward current. this would ensure that you always had the correct polarity available to your motor and electronics.   If one battery were to take the lion's share of providing power to the motors and go flat, you would have the other available as a spare in this case, the second battery would not charge the first because there would be a reverse polatrity diode acting as a non- return valve in the way, but it would be OK to provide power to the motors and ESC's.  If access is a problem to awkwardly placed batteries, my suggestion again would be to install a charging harness at the same time with a nice convenient connection socket for your chargerjust keep a weather eye open for corrosive discharge from a charging battery, in other words, ventilate.
        Good Sailing.........................................................Mike Davidson
Paul Godfrey30/07/2009 22:12:57
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163 forum posts
292 photos
Thanks Mike
 
I need all the help I can get when it comes to electrics - hence the wiring diagram kindly produced for me by Action!
 
Regards, Paul

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