Batteries

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Dave Hayfield
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Re: Batteries

Postby Dave Hayfield » April 13th, 2016, 12:45 pm

Rob, the reason there is only a small current flow when a flat battery is paralled with a fully charged one of the same type is that there must be a significantly higher voltage applied to any battery to charge it. In this particular case the flat battery is only capable of responding to its nominal voltage, this has nothing to do with amps, which is the same as the fully charged battery. The only way an excessive current will flow is if the flat battery is faulty, ie one cell or more are short circuit.
Oh well.....let's add to the confusion !!!!
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Rob Buckley
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Re: Batteries

Postby Rob Buckley » April 13th, 2016, 2:07 pm

Dave,
That's exactly right, and why I did the test to have some real numbers to show why it's a myth that a full battery will charge a flat one if you just connect them together.
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Re: Batteries

Postby Tony Collins 1073 » April 13th, 2016, 8:21 pm

Ohm's Law tells us that when connected together, the stronger battery will discharge through the weaker battery.

No 2 batteries will have identical internal resistances, the stonger will always discharge through the weaker.

PERIOD............... Unless Mr Ohm got it wrong.

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Re: Batteries

Postby Rob Buckley » April 13th, 2016, 8:27 pm

It does, just very very slowly.
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Re: Batteries

Postby Dave Hayfield » April 13th, 2016, 10:28 pm

Hi Tony, if you use Ohms Law to explain this quandary your facts do not compute. V=IR where V is the voltage, I is the current and R is the resistance. The nominal voltage levels of each battery are the same so there will be no or insignificant little current flow presuming the internal resistance of the cells are the same, which in a perfect world they will be) Q.E.D.
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Re: Batteries

Postby Tony Collins 1073 » April 13th, 2016, 10:58 pm

How can the battery voltages be the same if one is flat and the other is full. The flat one does not suddenly reach the same
as the full one as soon as they are connected. The voltage of any flat battery that is subject to a higher voltage supply will
only increase gradually as it charges up. As the voltage increases the current flow drops. That is what occurs when we
use a battery charger on a 12v car battery for example.
An increase of potential energy in a circuit causes a charge to move from a lower to a higher potential (ie. voltage). Note the difference between potential energy and potential.

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Re: Batteries

Postby Bob Thompson1894 » April 14th, 2016, 9:46 am

[quote="Dave Hayfield" The only way an excessive current will flow is if the flat battery is faulty, ie one cell or more are short circuit.
[/quote]
Surely the only advantage of using two batteries is redundancy if one goes faulty? Therefore it is pointless having two batteries without some means of isolation?

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Re: Batteries

Postby Dave Hayfield » April 14th, 2016, 10:13 am

Hi Tony, put it like this....if you have two jars of the same capacity, ie. the same voltage capacity, one full of water and the other empty and then put a tube to syphon water from one to the other. Nothing will happen even though they are both of the same capacity. What will get things moving is to raise the level of the full jar, ie creating a rise in 'voltage' levels, and this will allow the water or voltage to flow. Very simple example and the electrical explanation is obviously a little more complex but the basic science is there.
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Rob Buckley
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Re: Batteries

Postby Rob Buckley » April 14th, 2016, 12:09 pm

Bob Thompson1894 wrote:Surely the only advantage of using two batteries is redundancy if one goes faulty? Therefore it is pointless having two batteries without some means of isolation?


A diode would protect against a direct short circuit inside a battery, but not an open circuit (battery comes unsoldered etc.). Having two batteries would protect you against the failure in both cases.

When introducing something to protect against a potential failure, you need to weigh up the likelihood and severity of the feared event against new failures that can be introduced by what you're fitting (in this case the diode itself failing either short or open circuit, or it becoming unsoldered/breaking)

You don't need to do a full failure analysis for your models, but just think about what it is you want to achieve, and try and have an open mind about old wives tales like batteries cross-charging.
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Re: Batteries

Postby Tony Collins 1073 » April 14th, 2016, 1:38 pm

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Re: Batteries

Postby Dave Hayfield » April 15th, 2016, 12:45 pm

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Edgar Skipsey
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Re: Batteries

Postby Edgar Skipsey » April 15th, 2016, 9:44 pm

Thank you all folks I appreciate your comments. I just goes to show there's a lot off ways to skin a rabbit!!
From CG calculations I will need a lot of weight up front. Having checked that the Servos and Receiver will cope with 6.6 voltage and having a suitable charger, I have decided to try 2500 mAh A123 batteries in parallel. This with twin parallel in each side - preferably Shockley -diodes. (where can I get them, I cant find any PNPN diodes?). Unfortunately I don't use JR or Spekrum receivers so can't go the powersafe route. This set up with 4 batteries should give me up to 300gms up front which I am sure will be less than I need, plus double safety all round. Now the A123 batteries are just about the diameter of the cylinders of the rotary engine in this aeroplane - I wonder!!

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Re: Batteries

Postby Dave Hayfield » April 15th, 2016, 10:10 pm

Hi Edgar, Google CPC who are a components supplier who you can order from on line. There will be quite a selection of diodes so you will have to select according to your reuirements.
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Re: Batteries

Postby Tony Collins 1073 » April 15th, 2016, 11:19 pm

Hi Edgar, CPC no longer stock the diode you require.
Radio Spares do. This axial Schottky diode which is rated at 45v 8 AMP will do you just fine.
Solder the diode in the positive lead of the battery and cover with heatshrink.
Make sure the end of the diode with the ring on it is facing AWAY from the battery positive terminal.
Tony.

http://uk.rs-online.com/web/p/rectifier ... s/7815616/

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Re: Batteries

Postby Dave Hayfield » April 17th, 2016, 6:05 pm

Just in case you have a Maplin store near you they have a Scottky diode ....3 amp rated with 80 amp surge which will do the job for you.....stock number N91CA at 36 pence. Sorry about the no stock at CPC...they didn't tell me !!!
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Steve Perry
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Re: Batteries

Postby Steve Perry » April 17th, 2016, 8:55 pm

For belt and braces, use them in pairs



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