# How to seamlessly switch between dc lipo power sources

I have a question which has been asked a few times on here with some very detailed replies ...which would be great, if I had any idea what they meant! I've been building my own RC FPV setup and groundstation and so far been ok but now stuck.

I want to switch between 2x 4000 mah Lipo 3 cell batteries with a SPDT On-On toggle switch. I am not sure what devices I will need to do this somewhat properly. I'm pretty sure I shouldn't be simply connecting both lipo's to either throw and the output into my ground station so I can toggle without dropping the flow.

I was hoping someone might be able to link me to the devices I should be connecting together so I can "seamlessly" switch between power sources. I can't read circuit diagrams too well and electrical jargon (still learning) ...so hoping for a more straightforward approach.

Thanks for any help.

Cheers

What is the current (Amps) your load (device being powered, base station in this case) is drawing? Or do you know how much power (Watts) and or how many volts, too?

The simplest thing to do is put a capacitor after the switch. The capacitor acts as a small voltage reservoir. The problem is that depending on the load, they usually discharge quickly. That being said you can get a large enough capacitor which will act as a voltage source (like a battery) for a few seconds while the switch is momentarily "open" when it's not connected to either contact, and should provide more than enough leeway to allow you to switch from battery to battery.

simulate this circuit – Schematic created using CircuitLab

Hopefully that's simple enough for you, if you have any questions about the connections, feel free to comment.

• That's definitely the simplest solution, if you don't mind a bunch of inrush current when changing to a new, higher-voltage battery. Some switches are designed for that, some not. I'd be tempted to use transistors. It's more complex to do it that way, but it handles the inrush much better. May 28, 2015 at 23:06
• Thanks for the simple diagram ...this is exactly what I was after :) May 29, 2015 at 15:46
• Thanks for the simple diagram ...this is exactly what I was after :) So if I don't know my total draw of the groundstation (it is very little - the biggest is a small monitor which draws 5w max) ...this by the sounds of it will be the only thing that dictates the size of the capacitor. Is there a device I can buy which has it all there and I just connect input/output so has a housing which I can fix to my station? I am trying to avoid actual components if possible since I'm rather new at all this. May 29, 2015 at 15:56
• @NickvanDiem Sorry to say, but no there isn't something like this "out of the box" that I'm aware of. Because it's such a simple circuit it probably can't be patented and there's no money in it. ;) 5W is a fair bit. Also if your circuit / batteries run at, say, 12v, be sure you get a cap that is rated for 16v just to be sure it wont fail / wear out quickly. The price difference is usually negligible. You will probably want a cap that is 10,000uF (micro Farads) just to be on the safe side. At 12v with a ~.42A load you should have .3 seconds of dischange time with the cap. Enough to switch them May 30, 2015 at 1:50
• Hi Korozjin. Thanks for the further infos. I didn't think 5W was much so clearly lots more for me to learn here. This appears to be EXACTLY what I am after and fits your suggestion in terms of cap? talkingelectronics.com/projects/CDU%20-%20with%20SPDT%20Sw/… May 30, 2015 at 9:13

I'm not entirely sure what your end goal is. Are you running your ground station on LiPo's? My first approach would be to run a ground station on 12V lead-acid, aka car battery, and leave the LiPo's for the flying thing.

Anyway, here's my attempt at a solution:

simulate this circuit – Schematic created using CircuitLab

It's a classic S-R latch between the two NAND's, that provides a definite on-off signal to both battery connectors. You could use NOR's instead if you happen to have those; search google images to see how.

The latch provides a fast switch time with no "bouncing" like a raw switch does. Depending on how the circuit actually works with your parts, you may actually need to slow down the one coming on to avoid connecting both batteries in parallel.

The connector itself needs a bit of explaining. The reverse-series connection of two FETs accounts for the parasitic diodes that you can't get away from. They're wired to turn on and off together, but the common reference for a control signal is not entirely predictable as to what voltage it is, hence the isolators O1,2. When the LED is on, the transistor conducts, and when it's off, it doesn't. The selective shorting of the zener diode turns the FETs off, or allows them to float on via R5,6 and a parasitic diode. The 5.1V zener diodes ensure that the gate never exceeds the rated voltage as specified in the FETs' datasheet. Feel free to change the zener to match your specific FETs.

When you first plug in a battery, its FETs float on and provide power immediately to the rest of the circuit. At this point, it's not guaranteed to select that one on its own unless you hold its select button. Or you could allow it to randomly choose one and reset itself until it chooses right.

Select 1,2 could be replaced with a toggle switch like you mentioned originally. There's no harm in holding a selection like that, as long as it's break-before-make. It gets really confused if you try to select both at the same time.