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Old Feb 02, 2013, 04:00 AM
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┼tvidaberg, Sweden
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Switchover circuit from LiPo to LiPo

I have been searching the webb for a good circuit for a switchover so I can have one LiPo that feeds the groundstation and if that one is going down for what ever reason it should switch over to the backup LiPo.

I thought that there must be a lot of this on the webb and in a way there are. But, they are mostly for AC to batt and different voltage and so on. Not any nice circuit that one can use. The ones on this site is using a servo and that's not an option for me.

I guess that some analog components and perhaps a relay is what is needed.
* When main battery voltage < or = 10,5 volt or so it should switch.
* It should have some delay like a second or so for a sudden drop that isn't permanent. Probably not an issue but anyway...
* It must not switch back after the switch is done.

Is there someone out there that has the answer to this?
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Old Feb 02, 2013, 04:18 AM
Stuart
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Originally Posted by Pukin_Dog View Post
Is there someone out there that has the answer to this?
Yes, its a lot more reliable to just use a bigger lipo or to parallel them up, see the thread below;

http://www.rcgroups.com/forums/showthread.php?t=1735666
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Old Feb 02, 2013, 06:40 AM
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Yes, I have read that already. It talks about parallel instead but that doesn't answer my question. I search for a switchover circuit.
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Old Feb 02, 2013, 10:23 PM
We want... Information!
Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
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Here is a circuit that should do what you want. It will switch over when the battery voltage drops below cutoff (set for 10V by R1/R2) for more than about 1.5 seconds. It must be turned on initially by pressing the 'ARM' button, and will not automatically switch back on when the original battery is reconnected. It will take a few milliseconds for the relay contacts to switch over, so you should make sure that the load can handle a short disconnection (add an extra reservoir capacitor if load voltage drops too much during switchover).

The TL431 provides an accurate cutoff point, and C1 produces the delay before switching off. You may think this circuit is overly complicated, but all the parts are necessary to get the features you want. You could just use a DPDT relay with the diode and push-button, and a resistor or Zener diode in place of T1 to set the cutoff point (wire the extra relay contact in series with it to disconnect the coil on cutoff) but the delay time would be short and the relay might be sensitive to vibration.

One thing my circuit does not address is standby power consumption. If the current required to hold the relay closed is too much then it could be replaced with a low power 'solid state' SPDT relay of appropriate rating, or a couple of FETs with suitable gate drivers.
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Old Feb 03, 2013, 07:16 AM
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Yes! This is what I'm looking for. Sweet!

One question though, the switch S3, that is a momentary switch I guess?

This is nice for my groundstation if any failure with main power should occour or battery gets to low when I'm to far away to get home before it quits completely. Parallel batteries wount help here...

Thanks Bruce!
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Old Feb 03, 2013, 07:50 AM
Stuart
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Originally Posted by Pukin_Dog View Post
Parallel batteries wount help here...
Why not ?
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Old Feb 03, 2013, 09:11 AM
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I think like this:
If I use one ore two batteries in parallel and is far away when the low batt alarm on the gs i activated, then it might not be enough to get me back. If there is a single or a parallel doesn't matter then. It might help if one battery is failing for some other reason but that depends on what type of failure. I don't see the gs battery level in my goggles.
If I use this switch instead then I hear the alarm and know that something has happened to the main battery and that then reserve is on. Then I can turn around directly. Also I don't have to use matching pair of batteries for main and reserve.

For the batteries that is in the plane it's another story, there I almost always use parallel batteries. I also see the battery level realtime there. If one of the batteries fail for some reason then I have a problem. But still it is probably more chans of noticing that something is wrong when I can see Volt and Amp in realtime, and then turn back before it's to late.
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Old Feb 03, 2013, 10:04 AM
Stuart
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Originally Posted by Pukin_Dog View Post
If I use one ore two batteries in parallel and is far away when the low batt alarm on the gs i activated, then it might not be enough to get me back.
So instead of all those electronics to detect when one Lipo is flat and switch over, why not use the same electronics to warn you when the one (larger) battery is only 1/2 or 3/4 flat ?

Not difficult to do, and avoids adding the extra points of failure that are the all the wires switches and relays that are involved in a battery switchover setup.
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Old Feb 03, 2013, 03:51 PM
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That's one way to do it. One thing that don't take in account is if one cell in the main battery breaks down for some reason or something like that. That is of course unlikely but still...

When it comes to the electronics that can fail, well these are a handful of components compared to all the ones that are in all the other equipment that also can fail so that's not an issue really.
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Old Feb 03, 2013, 06:38 PM
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Just get a cheap lipo alarm that plugs into your balance plug. You can set the alarm up to 3.8v per cell which gives you about 50% battery left, and it checks each cell.
http://flyduino.net/1S-8S-Dual-Speak...Lipo-Checker_1
They are cheap on eBay.
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Old Feb 03, 2013, 07:43 PM
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I have some of those to and I use them.
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Old Feb 03, 2013, 09:00 PM
FPV in Hawaii
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I have some of those to and I use them.
So why won't that meet your needs?
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Old Feb 03, 2013, 11:08 PM
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Because of what I have stated previously.
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Old Feb 03, 2013, 11:08 PM
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Iskess is right. Upping alarm voltage to warn when you have 50% capacity left is exactly the same as hearing an alarm on switchover to reserve battery. Exactly. The same. Which is one reason why people don't use a more complicated setup.

It also automatically takes into account one cell going bad on either the "main" or "reserve" battery since that will also cause a voltage drop which will trigger the alarm.

Paralleling two batteries of different capacities is also a non-issue. What you shouldn't do is paralleling batteries of different cell counts.
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Old Feb 04, 2013, 02:12 AM
Stuart
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Originally Posted by Pukin_Dog View Post
When it comes to the electronics that can fail, well these are a handful of components compared to all the ones that are in all the other equipment that also can fail so that's not an issue really.
Your so wrong ...........
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