Managing high power battery pack voltages

Question

I have a few upcoming projects that will require some higher power battery applications where voltage swing needs to be limited. Both will be powering electric motors.

The first on will be ~1.3-1.5 kW, 24 V motor (7s21700 battery). The second will be a 10 kW, 72 V (20s21700 battery) motor. So the 4.2-2.5 V swing per cell is going to put me outside my acceptable voltage range on both projects.

I could limit the charge/discharge voltages but that would just suck because I don’t want to lose that storage. So the obvious theoretical answer is a buck/boost converter.

Practically speaking though I have never designed a 150 A buck/boost converter. I played around with some sims without much hope and was met with a rather unreasonable set of components (do they exist? Sure, but I would like to not have to sell a kidney to build this thing).

I have searched around for a few night but I haven’t found anything of real value. So thats the background.

Questions:

A: Am I missing some pretty obvious solution here?

B: If not, is a DC/DC converter of that size feasible (space for cooling isn’t a big issue)?

C: If it is feasible, any pointers? It's definitely going to be trickier than a little 2-3 A one.

One last thing: the reason I need the voltage to be pretty constant is because these will be used in marine applications where variation in power output directly affects the handling.

Answer 1

You set up your design so you do not need to regulate high power items. For example, motors are not picky about power quality so designing a full blown regulator is a huge waste of money, time, effort, and efficiency.

So if you can run the motor directly off the power source you do that.

If you need more control over the motor then you use a motor driver. So if you want a motor to run at constant speed even though battery voltages vary, you use a motor driver to control the speed.

If you look into motor driver designs, you will see they are essentially specialized strip down regulators. They do not provide a nice clean, high quality output as a regulator but the motor doesn't need that. Because the motor driver does not need to burdened with all the requirements of a regulator, it can be simpler, more efficient, and handle higher power. One other concession is motor drivers 99.99% of the time only ever step down voltage from a DC source. This increases simplicity and efficiency.

Do not boost voltage for motors. Do everything you can to set things up so you don't have to. Every bit of effort you invest into that at the start will more than pay off for itself in the future. Use slightly higher voltage batteries and step down if need be. Or always run at below 100% duty with enough headroom so you can increase the duty cycle to maintain performance as the battery voltage drops.

In addition, motor drivers are not regulators. They are used to control and commutate (if necessary) the motor. They are not designed to let you run a motor off an inappropriately high supply. This is due to the fact their outputs are not as clean as a regulator. The supply voltage on the other side of the motor driver is still very visible to the motor, unlike with a regulator. So don't use run a 24V motor off a 48V battery by perpetually running the driver at at 50% duty cycle.

Speaking of setting up your design to find your way out of complexity: For higher power levels, you would try and choose higher voltage motors so that the currents you need to deal with are less. So the first thing you should try and do is find higher voltage motors to reduce the currents.

Your hands might be tied in a few odd cases, but try really really hard at the start to set things up so you don't have all that extra complexity later on. You will hate yourself if you don't.

Answer 2

One last thing: the reason I need the voltage to be pretty constant is because these will be used in marine applications where variation in power output directly affects the handling.

This doesn't really make sense. If you need the power and speed to be constant, then the way to approach this is to use a motor and a motor controller that can supply the necessary torque at the necessary speed over the full range of battery voltages.

Answer 3

A: Am I missing some pretty obvious solution here?

Yes you are - and the fact that you are makes me very concerned about your safety. Please don't take this as criticism, but it's something you'd only miss if you're a relative novice at electronics. Which is fine, because everyone starts somewhere. But novices at construction don't start by building suspension bridges over thousand-foot drops, and novices to electronics don't start by building high-power electronics for exactly the same reason. So before you do anything with building your own stuff, please think very carefully about what you're doing. At the very least when you're testing it, make sure you have an immediately accessible emergency-stop button wired in the battery line, and several dry-powder fire extinguishers suitable for electrical fires also immediately accessible. And let off one fire extinguisher as a practise run so that you can react without hesitation if things turn bad.

All that said...

The thing you're missing is PWM. Unless you have some super specific requirements, no-one ever varies the voltage of a high-power device. Instead you use PWM to switch the power on and off very rapidly, and the inertia of whatever you're driving will smooth this out. It's way easier to switch things on and off than it is to vary the voltage, so this is how pretty much all high-power equipment works. (One super specific place where you might not use this is audio, but even there a so-called "Class D" audio amplifier uses exactly this principle.) For electric motors you typically have an H-bridge driver anyway, so it is very easy to PWM-vary the on/off signals to the H-bridge without needing any extra electronics.

As far as "constant" goes, PWM is the most reliable, consistent and accurate system you'll get. Losses are minimal, so two electric motors driven by PWM with the same duty cycle and fed from the same battery will both output exactly the same power (within a tolerance that's well below any matching tolerances for your motors or props).

There is no reason at all why you should build one of these though, and I strongly suggest you don't. There are many, many motor control modules out there. Commercial ones have all sorts of useful features in there such as smooth starting to limit starting currents, protection against shorting/overcurrents, and of course stopping the PWM switching from jamming all radio and phone reception in the area (which is a genuine problem). These are all things you need, so buy one off the shelf.

By all means play with PWM control on a remote-control car. That's a great project to get you going. Don't mess with 72V/150A until you REALLY know what you're doing, because it'll kill you if you look at it wrong.

Answer 4

Your 24V motor will almost certainly work fine if you supply it with a 10S battery (25-42V) and make sure the effective voltage you apply to it never exceeds 24V. This means for instance that your PWM duty cycle will never go above ~60% at full charge, and will reach almost 100% at 25V. If this is a BLDC, you need to choose a controller that supports the entire battery voltage range.

The same idea applies to the 72V motor, but I'd be more careful here: because of both higher voltage and higher power, it may not be possible to simply supply it with a higher voltage. The type of motor will matter a lot in this case. Some BLDCs support field weakening, where high phase currents result in a drop in \$K_T\$, which achieves target RPM with a lower voltage. On other motors this will not be possible, as you'd hit either the current limit of windings, or the de-magnetization limit of permanent magnets. In that case, you'll need to pick a motor which can deliver the required power/torque at 50V, and still supports 72V.

Finally, note that Li-ion batteries have only about 25% charge left when they hit 3.3V, so you could choose to work in the range of 4.2-3.3V per cell, which reduces the variation in battery voltage by half. This will also be beneficial for the lifetime of the batteries.

Answer 5

Anyone about to say "using COTS products isn't really EE" needs to build a successful electric vehicle that does a practical job moving humans, and then tell me how there's no EE in that.

You don't. You buy a Curtis controller COTS.

Or a Zilla if this thing is a locomotive.

What you're talking about is just the stuff that EV home brewers were doing 25 years ago, and golf cart guys have been doing for 50. They use, yeah, PWM controllers (a form of "buck" if you want to look at it like that) to regulate current to DC motors.

Pretty easy stuff.

If you want an AC solution, there are great gobs of them in "shake the box kits" (easily assembled) also intended for home-brew EVs, that marry an AC motor to a compatible AC motor controller. These controllers can do the whole bag of tricks, including regenerative braking.

While you're at it, don't shellac together some sort of battery charging. Use more COTS equipment to use J1772 type charging, which has all the safety bells and whistles, including smart GFCI, and "no drive-away while plugged in".

DON'T reinvent the wheel. It's a waste of talent, and only leads to frustration. On any project, take the direct route to "win". If you want to refine later, have at. But win first.

By the way, the motor always runs well below battery voltage.

Your problem is that in your early prototyping, you've been "hot-shot" connecting the motor to battery, and I'd guess you've been dampening the mechanical shocks of doing so by using a hydraulic transmission (i.e. "the water"). That was a design mistake, because you've now married yourself to delivering that voltage.

So either refactor for a motor with lower voltage, or bump the battery voltage considerably so a worst-case sagged battery is still above motor spec. The PWM controller is simply going to fatten up the PWM slightly to compensate for sagging battery voltage, in other words: bucking.