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Relay control with arduino

Hey everyone 

I am making a circut to power my APRS from 2 batterypacks. one batterypack is backup incase the other batterypack runs out. my APRS takes 12V and it works with the battery pack with a transistor to give it exact 12v or else it will be damaged. anyway I am tryna setup a relay with the arduino that can switch the power from one battery pack to another one. what relay should i use and anyother components i need with it, but i have to keep it 12v to the aprs cant go above it, and can you give me the link that is available in radioshack cause thats were i will go to get it. i need to finish this project and in a hurry. :S

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These guys are basically correct on all that they have said about the batteries.

Two packs that are the same type and capacity can be directly wired paralell  [ plus to plus and minus to minus ]

Battery packs that are different could produce eddy currents that are better avoided.

You could use a relay, held energised by the primary battery and when it drops out would switch to the backup battery.

I might be more prone to wire both battery packs "almost" in paralell. By that I mean place a Schottky diode in series with each. Those diodes have a low forward voltage drop, typically about 0.2 volts across them. (0.15 to 0.3 range ). Anyway, whichever battery is the highest, it will supply current to the circuitry, and nothing will flow either to or from the lower voltage battery.


Not that I disagree with the Schottky diode tip, it still sounds odd to me that the problem should be Eddy currents. To my knowledge eddy currents are current inducted in a (solid?) conductor when exposed to an alternating magnetic field. And I do not see where magnetic fields come into the picture when coupling batteries in parallel.

Yes, I probably used an incorrect word. (Even when I wrote it, I considered that I had not used the right word, but I did not correct myself. I am not sure if there is a proper word for such current flow unless we identified it with ground loop current.) Anyway, whatever you wish to call it, there will be current flow from one battery to another if there are no blocking diodes or other method of limiting it.

If the two batteries are in paralell with zero resistance in the circuit, then even at a low voltage difference, the current could spike quite high (at the limit of current flow within the batteries). Now as it happens the current cannot spike extremely high because batteries do have their own internal resistance to flow, but they could damage certain types of batteries such as lithium-polymer which are quite finicky about their charging, and can burst open or even catch fire if charged too rapidly.

Someone said it did not hurt as the current would simply charge the other battery and there would be no loss, as you can use it later. While somewhat correct, this is not totally true, as part of the energy will be converted to heat which will dissipate into the environment.

If you're trying to use the Arduino to trigger the relay you'll want one with a 5v coil voltage and a 2n222 transistor, at the very least, but I don't think Radioshack has any 5v coil relays in anything but SPST and you'd need an SPDT to do what you're talking about.  CW is that what bird has suggested here is probably the better option-you should get a longer battery life out of the two in parallel than by sequentially using them.  It is definitely more simple and elegant.  Using battery packs in sequence has the added problems of electromechanical fault (which is more unpredictable at altitude than solid-state,) extra programming steps and therefor processor usage, charge dissipation of the unused cells, the problem of triggering on a low battery, the "leftover" charge on the first battery not getting used to full potential and just the fact that the more things that can go wrong the likelier they are too.

because the project I am doing these is no way for me to recharge them. I am doing the weather balloon project. so need backup battery pack for the tracking system.

that paralleling the batteries should work and be easier than constructing a circuit to switch between the packs.

I hope someone that is more knowledgeable will chime in on this topic.

I have read (not I'm 100% sure) that when paralleling the batteries, if just one battery is slightly less charged, high currents will be flowing from the other batteries to that cell because they try to charge that battery up.

In general paralleling battery packs is not a good idea, at least without some sort of monitoring mechanism.

That could happen but if you think about it you'd have to have two very different kinds of cells.  To charge a cell you have to put something like 130% of the current that the battery is capable of producing through it, so if one pack was 3300mah and one was 2200, yeah that would happen, but so what*-even if that is what happened, it is "charge" that you'd be getting back over the long haul-after all it's going into a battery that will supply power eventually, not disappearing. That said, I am neither an engineer nor a battery expert, but I've never experienced problems with running batteries in parallel and I've done it experimentally several times.



*Don't do this by the way.  You could start a fire because it would probably indicate mismatches in battery chemistry.

Why not hook the batteries up in parallel, watch the voltage level, and recharge them when need be?