Keeping devices alive as the power supply is switched.

[Fulliautomatix]

Hi,

I have a number of devices (all 12volt, low current ~200ma) that are required to run in a remote location and powered by a pair of 12 volt batteries.

I have a separate arduino device that monitors the batteries, and when the current battery get's flat, it switches the devices to the standby battery.

The relay that switches from one battery to the other does the change-over in a few milliseconds (according to the published spec) for most of the devices this isn't a problem, they just carry on running. On of the devices is a bit more sensitive, when the batteries switch over the device thinks the power has been switched off and then on again, so it reboots itself.

My current thinking is that just strapping a reasonably large 12volt capacitor across the loads should keep them 'alive' for the few microseconds it takes to switch from one battery to the other.

Is this likely to work? Or will I need something more sophisticated?

Thanks

[DVDdoug]

A capacitor should work.    You may have to experiment with different capacitor values.   

[terryking228]

Hi,
Yes a capacitor seems like the usual answer.  As I used to teach , "Capacitors oppose a change in voltage"..  Still works the same 50 years later :-)

You can figure the approximate value from the old T = R * C equation.  R in ohms C in FARADS, T in seconds.

Rough guess: Equivalent R = E / I = 12/.2 = 60 ohms  Guess: try 1000 uF (Micro Farads)

Hmmm: 60 * .001000 Farads = 60 Milliseconds. MAYBE enough. So try that, then 10,000 uF if needed.

Let us know what works!

[janerlve]

What about using diodes to limit the voltage drop while switching?

[graynomad]

If you switch before battery gets too flat (and of course you would) then the diodes would work.

You do of course waste power when using diodes but this doesn't sound like a really low power application so that might not matter.

[Fulliautomatix]

ok, I'll experiment with the capacitor option.

Thanks everyone

[gpsmikey]

One consideration is just where you put the capacitor - if it is across the main 12v bus, then you will also be supplying power to all the other devices on that bus as well during the transition period.  You may also find (depending on the drop) that the capacitor is causing some arcing of the relay contacts.  My preference (since the other devices seem to handle the changeover correctly) would be to isolate the problem one with a diode feeding a capacitor which then feeds the problem device.  That would limit the capacitor to only supplying current to the problem one and not the others.  This assumes the problem one does not have a problem with the 0.7v drop through the diode - you need to see at what point the problem device decides it has "lost power" (is it at 10v or 3v or just where?)