Hi guys, Just wanted to run this idea by some people before I implement it. I'm Teching 2 MoVI's on a feature right now running 6 simultaneous LiPo's at any given moment. We're shooting in a variety of locations and often times I wish I could ninja swap a battery without docking, or powering anything down. I've my knowledge serves correct, this could be done with parallel wiring i.e. having a Y splitter, one end with a battery, and the other reserved for a second battery when hot swapping. The only downside I can think of is having an exposed lead that has the potential to short out. Has anyone experimented with this?
There are some other risks, and it would take an EE to really figure out. The parallel connection could cause the weaker battery to “pull down” the stronger one a bit when they both come online, but I doubt it’d be for more than a second or two, so it’d probably be OK. You could easily make a dummy plug that you could plug into the exposed lead to protect it when you remove the battery, too. But I just got my MōVI M5, so I can’t profess to know anything about their personality and what other gremlins might leap out of the tall grass.
The technical term you're looking for Steve is "in-rush current." And it would definitely happen when the voltage of the discharged battery is less than the charged battery. Some assumptions: 1. The internal resistance of the two batteries is around 16 milliOhms each, so total of 32 milliOhms, that's 0.032 ohms. 2. The charged battery is 16.8 volts and the discharged battery is, oh, 15 volts (allowing for it be higher than normal for a copter flight battery) -- so that's 1.8 volts difference. 3. Ohms law (Volts = Current * Resistance, or, rewriting that, Current = Volts/Resistance). [EDIT] 4. So we have Amps = 1.8/0.032. Then the in-rush current will be 56.25 Amps, or 101.25 Watts. However, I suspect the batteries will equilibrate pretty quickly, so I don't think that current would last very long -- but it's not to be ignored. I'd still be a bit cautious about doing this and try and minimize the time both batteries are connected to each other. Chris: Let us know if this warning label applies (found it in Consumer Reports on their back "funnies" page). Actually, I think it reads almost like poetry and it certainly supports the fundamental truth we should have in mind when dealing with LiPos: Prevent occurrence Bang! Andy.
Andy: lol I was praising, in a forum, a Chinese Gopro gimbal manufacturer, for updating the software. They then thanked me for "the propaganda".
Gary: We'll be watching your future posts for propaganda! I also realized (ok, after a glass of good Chianti) that my calculation is wrong because the two LiPo's would be in parallel, not series, so the resistance would thus be 8 milliOhms. Thus the calculation is current = 1.8/0.008, so the in-rush current would be 225 Amps. Hmmm. Anyone with superior knowledge please double check this as that seems like "rather a large current" and significantly increases the need to prevent occurrence Bang. Andy.
I was unaware that there is such a thing as “good Chianti”, but let’s not get distracted. I always run dual LiPo batteries in parallel on my copter, and connect my two 6S 8000mAh 25C LiPos one at a time. However, they’re always carefully charged to the same voltage level. The issue here could be that the two batteries are not only at different voltage levels, but there is a current draw from the MōVI. On my copter, the system’s obviously at rest with a minimal current draw when I plug in the flight batteries.
Does the Movi draw too much current to implement a switch that will swap from one battery to another? Therefore there is no time where the two batterys are connected? Maybe the switch over would cause the Movi to power off? Just some ideas...
Sam, much discussion has taken place here (and elsewhere), and my read of that is switches are a bad idea. There’s no “perfect” switch that maintains current levels to the board while making a perfect cutover. There are active devices that could do that, but not a mechanical switch. And the high current potential of a LiPo makes building an active device expensive.