Battery buffered power supply

I’m about to swap the steppers on my bench mill (beefed up Sieg SX3) for ODrives and KEDA 63-64 motors. I can’t quite get a grasp on what kind of power I’d need, but I suspect that my pair of 36V 10A (X+A and Y+Z) chinesium power supplies from eBay won’t cut it on their own. Combining them with three 12V starter batteries in series seems like a good option, but how? Would I need a diode to protect the PSU from overvoltage when braking? Would I need something like a current limiting resistor to prevent overloading the PSUs when charging?

I also have some 100V 15000 uF caps left over from a retired stereo project. Is that a viable alternative to batteries?

All axes are driven by ball screws with belt reduction, giving an effective 2mm travel per motor revolution. Max speed will be about 1000 rpm under load and hopefully 4-5000 rpm during rapids. Linear rails all round means minimal friction loss and possibly quite considerable braking currents.

The ODrive provides a connection for a load dump resistor. I’m not exactly sure how it’s programmed right now but you can use that resistor to help dissipate reverse current during braking. @madcowswe should be able to explain whether or not the brake resistor is setup to avoid over-voltage or not.

Batteries are fine, caps are probably better tbh. But just make sure you precharge them to the correct voltage through a resistor so you don’t blow up your power supply.

After a tiny bit of thinking and googling I don’t think my 15000 uF caps will do much good. At 36V, it won’t hold more than 10 Joules, which would be gone in a tenth of a second at 100W - if it were drained all the way down to zero. In contrast, three 12V lead acid batteries in series can support about a 100W constant draw per Ah, and roughly five times that for a few seconds, so even the tiniest snowblower batteries would be more than enough. The problem is that I don’t have a clue how to make that work with my 36V PSU, as batteries generally need higher voltages to charge.

It doesn’t help that I’m eternally confused trying to figure out how much power I really need…

For overvoltage protection I think I’ll just use the braking resistor already built into the ODrive. There’s no real reason to mess around with extra circuitry just to recover the braking energy.

I haven’t done this myself, so these are all opinions at this point, but I would be careful about running either batteries or capacitors on your DC bus in combination with a power supply without a proper charge controller. When fully charged your batteries will have a voltage that is a few volts above your power supply which may or may not be bad. At some point the batteries will drain to the point where their voltage is less than what the power supply provides and they turn into a power sink rather than a source.

Look up bidirectional dc to dc converter. That is how you fix the problem of mismatching voltages.

the batteries will discharge to the supply voltage, but will not drop below that, so they won’t become a long-term energy sink.

if the power supply can’t supply the full current draw, the batteries will supply the extra (and drop in voltage a little bit and then recharge after the draw drops). This is sort of like a huge capacitor.

Hams have been doing this for decades, They put a battery between their power supply and their transmitters, which supply the peak currents needed for transmitting without needing a huge power supply.

This is one areas where different battery chemistries don’t all work. Lead-Acid batteries are very tolerant of abuse, and work well when ‘floated’ on a power supply like this. other technologies need more attention to make sure they stay within their voltage/current limits.

I agree that to some degree you can do as you describe, but at some point their voltage will drop enough to overload your power supply. Basically in the configuration where they are just hooked up directly to the bus your power supply limits still dictate the system power draw. The amount of time you can run over the power limits of your supply depend on how big of a mismatch you have in power delivery capability between the supply and your batteries.

As long as the power supply can supply the average power that’s being drawn, and the power supply can self-limit the output current, it will work.

As I said, people have been doing this for decades.

now, you don’t want to try to make the power supply the bare minimum size for the average power drawn, but a battery can easily handle power peaks, greatly easing the power supply requirements

the key is that the batteries can output a lot of power with only a small voltage drop, and charging is relative to the voltage difference.

Yeah, you are right about the voltage drop being small for a given amount of energy delivery.

I was equating the performance of a capacitive energy storage system with battery backed. They are really two different beasts. Capacitive energy is much more susceptible to voltage drop.

I was kinda expecting to find something about batteries and power supplies in the docs, considering it’s mentioned under Key Specs on the front page:

“Optional use of a battery means you can achieve very high peak power output with only a modest power supply.”

Maybe they’re just referring to the solution that’s so simple it’s not worth writing about :slight_smile:

Based on what I’ve gathered from a few hours of scattered reading, however, directly connecting a regulated power supply and battery of equal voltage shouldn’t work. Batteries need about a 10% higher voltage to charge at all, meaning the PSU voltage would have to drop those 10% under load for the batteries to start providing any power. By that time the PSU should already have thrown in the towel. Disconnecting and separately charging the batteries when the machine’s turned off is one option, but not very practical when my workshop only has power when I’m using it.

I suspect I’ll just start by hooking the mill table with a KEDA 63-64 up to the 10A PSU once I get it machined and assembled, and see what kind of performance I get. That will give me at least some idea of how much trouble I’ll need to get into with batteries and/or new power supplies. And to think that one of those power supplies runs my mill just fine with stepper motors…

Your understanding is slightly incorrect. Even if the batteries have the same voltage as your PSU they will provide some degree of power to the system. They just won’t recharge to full capacity if you don’t apply a high enough voltage.

That being said, I do agree that it would be best to start with just a power supply though. That will be the simplest setup. Once you gain confidence in that approach you can always try adding a few batteries to get a boost in power output. Just be careful about charging them with them still hooked up to the same circuit as your PSU.

I think we’re pretty much in agreement, I’m just being hopeless at arranging my mind into words again :slight_smile:

I’m thinking that a direct parallel connection at nominal voltage would work pretty much the way I want it to, where the batteries provide more and more power as PSU voltage drops under load, but they would take little or no charge. Then I’d need offline charging, which is impractial with my workshop situation. Inline charging would need more complex and expensive electronics, possibly to the point where I’d be better off getting a beefier power supply.

I’m sorry to make such a fuss with little to contribute, it’s just that I’m still on the fence here. A full ODrive setup for my mill could easily top €1k, and my stepper setup does a reasonably good job as it is.

It does not seem that you would have to disconnect your batteries at all. If you keep them in a circuit that pumps power to them below their nominal voltage, but not above that, you should be able to leave them in circuit and not charge them other than the power supply doing so as part of normal operation. I am speculating here and please correct me if I am wrong.

However I know that the 10% number comes from the fact that a batter’s nominal voltage is basically the middle of their operating range. You charge at 10% above nominal (the number on the tin) because a full battery has a voltage higher than nominal. So if you charged at the number on the tin, you wouldnt have a full battery.

However, the voltage for storing a lithium batttery should be at around 70% of full battery voltage, so if you have your system voltage set at that value, you should be able to run this setup for some time.