Proper voltages from same source

Question

I'm building a small electromobile. I was planning on using a car alternator as the motor, however I'm now looking into buying a 48V 500W BLDC motor.

The problem is I'm not really sure how to manage the difference in voltage between what the motor requires and the rest of my circuitry.

I have a bunch of components that run at 12V as well as another bunch at 5V. In the scenario where I have a 12V battery I just use a buck converter to step-down the voltage for my 5V circuit and it all seems fine since it didn't draw anywhere near the 3A maximum rating of the converter.

Now my 12V circuit draws ~10A at full load. If I have a 48V battery I think I'll have trouble using a buck converter for that purpose. I was thinking I could tap into a point between the battery cells where the voltage will be 12V but that is probably dangerous, I'm pretty sure difference in voltage in the cells will upset the battery.

What would be the "proper" way to power my 48V motor and 12V as well as 5V circuit?

Answer 1

Use a 48v battery, and a buck down to 12v. Both are nice commonly used voltages, and there should be plenty of converters available to choose from in that power range.

If you were only going to use a few mA at 12v, then it may be permissible to tap into an intermediate battery connection, and rely on your charge cycle to rebalance the batteries, but 10A is rather too much unbalance to handle this way.

You did right in rejecting the car alternator as a motor, they are horribly inefficient. They are built down to a price, have plenty of air-cooling and an abundant non-metered energy input, so efficiency is the last thing on the designers' minds.

Answer 2

The proper way would be to use... a buck converter.

10A at 12V is only 120W. Assuming 80% efficiency, you're looking at around 3A out of the 48V, which is pretty manageable. You will need to design it carefully, a bit of simulation and prototyping, as you're looking at dumping around 25W of heat. A good design could take that lower, but plan for worse and give yourself some decent head room. Simulate it, find out what gets hot (and by how much) and make sure you design cooling accordingly.

Depending on your set up, there may be benefits from having a couple of 48V to 12V in strategic locations in the vehicle. That way you can take advantage of the lower currents at 48V, so that your volt drop is lower and losses can be managed better. If you do have multiple 48V to 12V, be careful to keep the grounds either the same, or completely isolated (take care with communication if you isolate some of the power rails!).

Don't forget the impact of driving from the 48V though. Your drive motor will draw a lot of current (10A or so at 500W, more during stall events or heavy loads), so that could mean a significant voltage drop on the battery pack at high loads. Also, motors can put a hell of a lot of noise on the 48V rails. So you'll need to make sure your DCDCs can cope with all of this noise and disruption. That should be manageable with suitable EMC design.