Two controllers, one battery

[squared]

I've searched the threads but don't see anything similar. I'm going to run two 48v motors (TDZ2B mid-drive, Bafang front hub) and would like to use my Greenworks batteries which are ~80V. Thinking that two in series should result in a voltage drop across the controllers within "safe" limits. Of course, this assumes that both remain in the circuit simultaneously. Has anyone done this...are my assumptions correct?

 

[Chalo]

I've searched the threads but don't see anything similar. I'm going to run two 48v motors (TDZ2B mid-drive, Bafang front hub) and would like to use my Greenworks batteries which are ~80V. Thinking that two in series should result in a voltage drop across the controllers within "safe" limits. Of course, this assumes that both remain in the circuit simultaneously. Has anyone done this...are my assumptions correct?

As long as the battery is good to support the sum total maximum current of both controllers, there's no problem in principle. Your cables and plugs also need to be sufficient to carry the current, and it's harder on the battery to deliver double the previously anticipated power. But technically, it should work just fine.

EDIT: This post is incorrect for the kind of voltage mixing described above. It's true if the battery voltage is compatible with each controller being used.

I generally recommend against combining a hub motor and a mid drive, because two hub motors do the same job cheaper, better, and more reliably. But given that your mid drive is the least reliable thing short of something totally homemade, having a reliable backup could only help.

 

[squared]

As long as the battery is good to support the sum total maximum current of both controllers, there's no problem in principle. Your cables and plugs also need to be sufficient to carry the current, and it's harder on the battery to deliver double the previously anticipated power. But technically, it should work just fine.

Thanks, that's what I'm thinking, I just didn't know if there would be a condition where one could potentially be in an "off" state and the other get all 80v. Brief transients might be ok... I'm new to the e-bike thing, but not to electronics

 

[E-HP]

Thinking that two in series should result in a voltage drop across the controllers within "safe" limits.

Two what in series? The controllers??

 

[E-HP]

Thanks, that's what I'm thinking, I just didn't know if there would be a condition where one could potentially be in an "off" state and the other get all 80v. Brief transients might be ok... I'm new to the e-bike thing, but not to electronics

You can’t do that.

 

[squared]

Two what in series? The controllers??

Good point... I guess technically it's both motors in series with their respective controllers being connected to that.

 

[squared]

You can’t do that.

Any chance you could show your work?

 

[E-HP]

If you aren’t new to electronics then Im sure you’ll figure it out. Chalo misread your post.

 

[Chalo]

Thanks, that's what I'm thinking, I just didn't know if there would be a condition where one could potentially be in an "off" state and the other get all 80v. Brief transients might be ok... I'm new to the e-bike thing, but not to electronics

Oh, I'm sorry, you can't run 48V controllers on 80V unless they're specifically designed to do that. I thought your question was only about running two motors on a single battery of correct voltage. You can't series motor controllers to reduce voltage to each one, and full voltage is too high for what you have.

 

[squared]

Oh, I'm sorry, you can't run 48V controllers on 80V unless they're specifically designed to do that. I thought your question was only about running two motors on a single battery of correct voltage. You can't series motor controllers to reduce voltage to each one, and full voltage is too high for what you have.

Ok. Not really understanding why kirchoff's voltage law doesn't apply in this case with both in series.

 

[squared]

If you aren’t new to electronics then Im sure you’ll figure it out. Chalo misread your post.

thanks, that was really helpful

 

[E-HP]

Ok. Not really understanding why kirchoff's voltage law doesn't apply in this case with both in series.

It always applies, but we’re not talking about fixed resistors here, are we?

 

[Chalo]

Ok. Not really understanding why kirchoff's voltage law doesn't apply in this case with both in series.

Controllers aren't steady loads that draw power in sync with each other. You can put two incandescent light bulbs or two space heaters or even two brushed DC motors in series and they'll divide the voltage, but BLDC controllers aren't like that.

 

[squared]

It always applies, but we’re not talking about fixed resistors here, are we?

hence my question to show your work...

 

[squared]

Controllers aren't steady loads that draw power in sync with each other. You can put two incandescent light bulbs or two space heaters or even two brushed DC motors in series and they'll divide the voltage, but BLDC controllers aren't like that.

ok

 

[E-HP]

Any chance you could show your work?

If you are confident about your own work and calcs, then just go for it. I’m not using a keyboard, so maybe you can get Chalo to show his work.

 

[squared]

If you are confident about your own work and calcs, then just go for it. I’m not using a keyboard, so maybe you can get Chalo to show his work.

Okay... yes I'm lazy and just looking for someone to explain it to me vice having to do the work myself...but I thought that was what the internet was for

 

[squared]

I've spent quite a bit of time reading about BLDC controllers, hall effect sensors, back EMF sensing sinusoidal, trapezoidal control methods...yada yada yada. TDZ2B firmware modification, the hardware and freeware to do that and so forth. Just trying to leverage on existing experience to come up with a basic running system that I can play with. I'm old, so time is short.

 

[E-HP]

Using the internet to find your solution, get a buck converter with sufficient current capability to run both controllers at their rated voltage. You can get one with a metal enclosure and find a spot to mount it.

https://www.aliexpress.us/item/3256806155255931.html?src=google&pdp_npi=4%40dis%21USD%2133.50%2120.77%21%21%21%21%21%40%2112000036823128056%21ppc%21%21%21&src=google&albch=shopping&acnt=708-803-3821&isdl=y&slnk=&plac=&mtctp=&albbt=Google_7_shopping&aff_platform=google&aff_short_key=UneMJZVf&gclsrc=aw.ds&albagn=888888&ds_e_adid=&ds_e_matchtype=&ds_e_device=c&ds_e_network=x&ds_e_product_group_id=&ds_e_product_id=en3256806155255931&ds_e_product_merchant_id=467024831&ds_e_product_country=US&ds_e_product_language=en&ds_e_product_channel=online&ds_e_product_store_id=&ds_url_v=2&albcp=20542171673&albag=&isSmbAutoCall=false&needSmbHouyi=false&gad_source=1&gclid=EAIaIQobChMIsM6m9e3SiwMVOJDuAR36PSEwEAQYASABEgIRDfD_BwE&gatewayAdapt=glo2usa

 

[squared]

Using the internet to find your solution, get a buck converter with sufficient current capability to run both controllers at their rated voltage. You can get one with a metal enclosure and find a spot to mount it.

https://www.aliexpress.us/item/3256806155255931.html?src=google&pdp_npi=4%40dis%21USD%2133.50%2120.77%21%21%21%21%21%40%2112000036823128056%21ppc%21%21%21&src=google&albch=shopping&acnt=708-803-3821&isdl=y&slnk=&plac=&mtctp=&albbt=Google_7_shopping&aff_platform=google&aff_short_key=UneMJZVf&gclsrc=aw.ds&albagn=888888&ds_e_adid=&ds_e_matchtype=&ds_e_device=c&ds_e_network=x&ds_e_product_group_id=&ds_e_product_id=en3256806155255931&ds_e_product_merchant_id=467024831&ds_e_product_country=US&ds_e_product_language=en&ds_e_product_channel=online&ds_e_product_store_id=&ds_url_v=2&albcp=20542171673&albag=&isSmbAutoCall=false&needSmbHouyi=false&gad_source=1&gclid=EAIaIQobChMIsM6m9e3SiwMVOJDuAR36PSEwEAQYASABEgIRDfD_BwE&gatewayAdapt=glo2usa

Thank you, I did go down that path as well in my thought process and looked at some buck controllers from China. I didn't see that one in particular and most I found I figured that they wouldn't be able to withstand 10-25 amps for any length of time. Plus the efficiency is an issue as well and while they are better than a basic voltage controller, wasted battery energy on voltage control is wasted energy that's not going to the wheels. So...I thought that maybe the controllers would have enough resistance to be able to operate in series. I have not found schematics for the one's used in Tongsheng or Bafang controllers but did look at some BLDC examples from the web and thought there might be enough resistance from the controller electronics to support my theory.

 

[squared]

For $35 delivered I'm going to give it a shot as a science project to see how it works. I'll do the experiment and report the results here on the website for other users. That fan probably burns a lot of dB's but we will see..

Edit: Maybe I can duct the fan to the TDZ2b motor for cooling

 

[Chalo]

I have not found schematics for the one's used in Tongsheng or Bafang controllers but did look at some BLDC examples from the web and thought there might be enough resistance from the controller electronics to support my theory.

Understand that loads placed in series will by definition pass the same current at the same time. No matter how much you smooth and filter incoming power, the two different controllers aren't going to demand the same amount of current at the same time, plus there's pulse width modulation in there which presents a huge wild card in terms of when the controllers are actually "on" vs "off". If either load is switched off, then both are off, and the states can change thousands of times per second.

To use the hydraulic analogy, two BLDC controllers in series aren't like two rice fields receiving the same flow of irrigation water one following the other. They're more like two different dish washing stations, where the workload isn't the same and the faucets and sprayers will never be synchronized with each other moment by moment.

 

[E-HP]

For $35 delivered I'm going to give it a shot as a science project to see how it works. I'll do the experiment and report the results here on the website for other users. That fan probably burns a lot of dB's but we will see..

Edit: Maybe I can duct the fan to the TDZ2b motor for cooling

I can't vouch for the quality of that unit; it just turned up in a search. Personally, I'd choose something with a case, unless you have some other type of enclosure. It would be fine for testing with though. Looking forward to see how it works out.

 

[squared]

Understand that loads placed in series will by definition pass the same current at the same time. No matter how much you smooth and filter incoming power, the two different controllers aren't going to demand the same amount of current at the same time, plus there's pulse width modulation in there which presents a huge wild card in terms of when the controllers are actually "on" vs "off". If either load is switched off, then both are off, and the states can change thousands of times per second.

To use the hydraulic analogy, two BLDC controllers in series aren't like two rice fields receiving the same flow of irrigation water one following the other. They're more like two different dish washing stations, where the workload isn't the same and the faucets and sprayers will never be synchronized with each other moment by moment.

I see what your saying. With both in series if power is switched off by one, then the other gets no current. I thought only the motor was PWM switched according to rotor position based on the hall sensors. There will also be some phase shifting of the signal due to the combined effects of the other controller that could induce some unexpected results in the system as a whole.

 

[amberwolf]

To use the hydraulic analogy, two BLDC controllers in series aren't like two rice fields receiving the same flow of irrigation water one following the other. They're more like two different dish washing stations, where the workload isn't the same and the faucets and sprayers will never be synchronized with each other moment by moment.

To use that analogy, you would be using the drain of the first washing station to provide the water for the second. The second wouldn't be able to do much with what was coming from the first (because of the pressure drop), and if the first was off, providing no drainwater, the second doesn't have any to do any work.

In the controller sitaution, very roughly (without using any electrical simulations or math): *if* current were able to flow correctly thru the system (it probably won't) to allow the seriesed controllers to work, then whenever one of the controllers has more load on it and appears as a lower resistance to the source, it has a lower voltage drop across it, which means the rest of the voltage is across the other controller with less or no load on it...if that is enough, it will damage that controller. When that controller is damaged enough it will prevent current flow thru it which will stop the other controller from working...or the damage to the first failing controlelr will cause it to be a short circuit long enough to cause full voltage of the pack across the other controller, damaging it, too.

(assuming the controllers aren't designed to handle the full voltage of the battery in the first place....if they are designed to handle that full voltage, there is no reason to do anything other than just connect each one to the battery normally).