# Most efficient method to switch between 2 battery system for EV?

#### TheDroneGuy

Joined Nov 22, 2019
4
Hello all!

I am currently conducting my thesis research. My aim is to determine the most efficient way to combine high power density batteries with high energy density batteries in an hybrid system for powering electric aircraft. The idea is by using two different types of batteries which are optimise for take-off and cruise phase, it is possible to decrease the overall weight of the system by 10-25% without affecting power capability. (I have attached the hypothesis for more detail

"Review of Electrically Powered Propulsion for Aircraft" - R. Bolam, Y. Vagapov​

For proof of concept, I am designing such power management and distribution system for a VTOL drone. The requirements are:
6s (22.2V)
8000mAh
180 A for take off
20 A for cruise
<1.2kg

The Two solutions are:
Choose one or the other
If current required is <20, then use battery A (High energy density)
If current required is >20, then use battery B (high power density)
or Add one to the other
If current required is <20, only use battery A
If current required is >20, use 20A from battery A and the rest from battery B

The problems:
What would be the best solution, considering weight as a crucial limiting factor?
What would be the best way to design such power circuit?
Can it be scaled? The idea is to use this solution on system that operates at 100V + and 1000A+

I am asking the help of the community and posting a new thread since I couldn't find anything to support my problem from previous thread. My problem requires a lot more power than usual, and a close attention to the weight of all components. Any support is greatly appreciated, and any support is a step toward a better greener electric world

Friendly,
Gabriel

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#### Dodgydave

Joined Jun 22, 2012
11,302
If your intention is to reduce weight, then just use the lighter batteries, otherwise if not then Mosfets will connect the extra battery in parallel, or Solid state Relay.

#### TheDroneGuy

Joined Nov 22, 2019
4

If I could just use lighter batteries I would trust me! But I wasn't clear, my bad. The problem with battery is that the more energy you store (so the lighter they are), the harder it is to release the energy. So the idea behind the thesis is to have a double battery system where each part is optimised for its respective mission phase. So when not much power (release of energy) is needed, it uses the battery (A) which stores a lot more energy. And when a lot of power is needed, it uses the battery (B) that is good at giving out power. Doing so can overall decrease the weight of the battery.

Solid states relay can be quite expensive and heavy, but power Mosfets seems like the way to go! Will look more into it, thanks!

But I'm still wondering if its easier to just switch between batteries than combining both and current limiting one...

#### KeithWalker

Joined Jul 10, 2017
3,091

If I could just use lighter batteries I would trust me! But I wasn't clear, my bad. The problem with battery is that the more energy you store (so the lighter they are), the harder it is to release the energy. So the idea behind the thesis is to have a double battery system where each part is optimised for its respective mission phase. So when not much power (release of energy) is needed, it uses the battery (A) which stores a lot more energy. And when a lot of power is needed, it uses the battery (B) that is good at giving out power. Doing so can overall decrease the weight of the battery.

Solid states relay can be quite expensive and heavy, but power Mosfets seems like the way to go! Will look more into it, thanks!

But I'm still wondering if its easier to just switch between batteries than combining both and current limiting one...
It would not be a good idea to connect one battery in parallel with the other part way through the flight. The one with the highest voltage would dump current into the other one. The least complex solution would be to monitor the current and switch between battery packs using power MOS FETs.

#### TheDroneGuy

Joined Nov 22, 2019
4
It would not be a good idea to connect one battery in parallel with the other part way through the flight. The one with the highest voltage would dump current into the other one. The least complex solution would be to monitor the current and switch between battery packs using power MOS FETs.
Check, thanks for the reply! I was considering using a CMOS circuit configuration with power FETs for the switching. Would that be a good approach?

#### KeithWalker

Joined Jul 10, 2017
3,091
Check, thanks for the reply! I was considering using a CMOS circuit configuration with power FETs for the switching. Would that be a good approach?
Yes, that would be a good approach. Just be careful that the measuring and control circuit is immune to power switching spikes and digital and RF interference or you may get some unexpected and costly results. Make sure you have sufficient hysteresis in the current detection circuit so that it does not jitter between the two states.

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