Despite having worked on drones for 10+ years I only recently learned of the existance of “ideal diodes” and how useful they are on vehicles that require multiple batteries so I thought I’d share in case others have also not heard of them.
Technically speaking an ideal diode is just a regular diode in that it only allows electricity to flow in one direction but what makes it special is that the forward voltage drop is extremely low. In a quick test I did on the ideal diodes shown above (provided to me by Running Electronics) the drop was something like 0.01V.
These particular ideal diodes can also handle relatively large current (I hear 100 Amps at 25V) which makes them very useful for vehicles which require a lot of batteries like the 100km DeSET mapping boat that I’m working on with AttracLab, Lighthouse and Shimane University.
We estimate the vehicle will require 2 or 3 kWh of batteries to travel 100km so we plan on using 6x Panasonic NKY580B02 e-bike batteries (each is 16Ah, 6S).
Without an ideal diode you would probably need to be very careful that the voltage of all 6 batteries was the same or else current from higher voltage batteries would flow (through the + terminal wires) to the lower voltage batteries which could potentially be quite dangerous. Keeping two battery’s voltages the same is manageable but with six it becomes far too easy to make a mistake. In this setup, each battery has a little extension wire with the ideal diode included to protect against any back flow.
A side note though is that it may be that these particular e-bike batteries already have ideal diodes built in. They certainly have short circuit protection which is another extremely important safety feature when you’re dealing with batteries of this size.
Personally I have been attaching these types of diodes on any aircraft where I use more than one battery. Even with two batteries it feels like a safety risk to not have any form of protection between two batteries at different voltage levels.
The diodes also allow you to swap batteries without having to power off the system - since you can have a dead and full battery on as you swap batteries one by one.
Ideals didoes are a great simple solution for combining battery packs like this!
“Ideal didoes” in terms of this application do not refer to a diode, but instead to an “ideal diode circuit”. The main component of an ideal diode circuit is a MOSFET which is how they achieve such low on resistance and mimic the theoretical “ideal diode”.
If you look up the datasheet of the main IC in your circuit you can see this in their application circuits. LTC4357
Looking at your PCB and circuit the MOSFET is on the other side of the PCB? Though some ideal diode controllers have internal mosfets.
I’m sure more people would use them if they knew about them and they were readily available. E.g. if HobbyKing (or similar) sold them with XT60 or XT90 connectors and/or incorporated directly into a Y-cable.
I’ve ordered a few components from mRo recently, and some of the documentation suggests they have a bit of a relationship with ArduPilot. I wonder if they could produce a high quality diode board with the requisite specs?
You make a great point about battery safety, and these high output Lithium batteries are nothing to trifle with!
Its been a while since I have designed/built a vehicle that used two batteries (back in 2017) - so I don’t remember exactly where I acquired them. I think I just bought a good amount on AliExpress and conducted some in house testing - they are pretty hard to find.
Another interesting resource I found is the following: https://auvidea.eu/pdb/. However, the board doesn’t seem to be in their shop. It would be very ideal for my use cases.
The smaller boards like Mauch’s are great for planes to give you enough time to land with a minimal backup battery. One ideal diode board I’ve made had that purpose.
A much larger one was made to prevent back feeding solar panels during night time operations
Depending on your batteries they may have internal protection modules. Those modules may protect against too large of charge and discharge currents that can occur when you plug-in packs of different voltages.
It would be really nice if Hex (or anyone) made a compact circuit board that included an Ideal Diode array, had current/voltage measuring, and communicated to the autopilot via the power port or CAN.
Do those mosfets have enough heat sinking and stitching on the pcb for high currents? Or because they are running at such a low duty cycle it doesn’t matter so much?
They do generate a lot of heat at high currents the board is not stitched through but It is a 2oz pour so the heat dissipation is quite good I added a couple of small heatsinks which helps, I tested them on my VTOL in a 20 min hover on 12s at 45A ambient temp was around 25c the temps were well within limits
Nice testing! Was that with airflow such as from props or while enclosed?
The heat sinks make my informed (barely) enough to be dangerous self happier. The mosfets just looked a little small to do the full current without heatsinking. And of course 2oz copper helps!
