Motor controller help

[Fordulike]

Could someone please confirm or correct me on a couple of points, reference the motor controller.
1) With a 24v battery, 24v motor and 20 amp motor controller, would that mean that the motor receives 480 watts of power maximum?
2) The voltage of a li-ion battery starts off at just over 29v when freshly charged and drops as it is used. Is the motor controller designed to provide a constant 24v to the motor from the battery, even when fresh, and until the battery provides less than 24v or reaches it's cutoff point?

 

[NRG]

Hi, the motor will draw 480watts peak power...in reality it will be more with a freshly charged battery due to the higher volts.

The controller will feed whatever voltage the battery is at to the motor as long as it's within the controllers maximum and minimum voltage levels.

Usually theres a low voltage cutoff programmed at some level to provide a degree of protection for the battery and the high voltage protection will be set to protect the battery from blowing the controller....if your lucky....many controllers simply have a maximum rating and no high voltage protection....

 

[Fordulike]

So, going on the facts that my li-ion battery feeds 29.6v to the motor when fresh, i can assume that a 24v motor is capable of handling a little more than this top voltage?
The reason i ask, is that when the current battery does finally not provide acceptable performance, i want to wire in a LiFePO4 battery.
I will get Ping to make up a 10 cell 20 AH which will produce 32v when fresh.
I'm hoping this will allow me to utilize most of the batteries capacity before it drops below the motor controllers lower cutoff.
Do you think this would work and is a sensible idea, or should i stick with an 8 cell (25.6)?

 

[NRG]

I ran a 24v Cohnis motor from a 7s3p Lipo RC pack with no issues at 25.9v nominal and 29.4 peak. I don't think you'll have an issue at 32v.

However, there are a few things you need to be aware of. The high voltage limit, if there's no protection circuit, is going to be determined by the maximum ratings of the power Mosfets used to drive the motor, the voltage rating of the electrolytic capacitors used and the voltage tolerance of the control circuit voltage regulator, typically set to 5v.

Using the 24v controller I have, it uses IRF3205 Mosfets, rated for 55v @ 110A (!) So 32v would be no problem....Electrolytic caps come in a range of voltage ratings, typically these are 6, 10, 16, 25, 35, 50, 63 and 100v+ ....there are other voltage ratings available but these are typical values...I very much doubt yours has 25v caps as this is too close to the voltage rating and I would suspect, like mine, it has 50v caps or maybe 63v. So, I don't think there will be an issue but it may be best to open up the controller and check.

The voltage regulator for the control circuits is a little more tricky as it all depends on how and what is used to provide the regulated voltage. On my controller a variable LM317 regulator is used. It has a maximum differential voltage rating (input minus output) of 40v, so is just OK with a battery voltage of 32v...other LM317 variants have a 60v differential rating.

I'm sorry if this is over technical, so bottom line is: The motor can handle a bit of extra volts and therefore go a bit faster, the controller should be able to handle a few more volts (32v is possible) but be careful and if you can open the controller up have have a look at the parts then you will have a better feel for what's possible.

 

[NRG]

One other thing, raising the voltage too high could mean the low voltage cutoff in the controller no longer provides any battery pack protection (as its too low for the new pack) so you will be reliant on the Ping BMS to provide cutoff.

BTW LiFePO4 nominal voltage is 3.3v so at 8 cells you have 26.4v with a 28.8 peak voltage, you would need 9cells to get to 32v....and at 10cells you would have 36v peak...

 

[Fordulike]

BTW LiFePO4 nominal voltage is 3.3v so at 8 cells you have 26.4v with a 28.8 peak voltage, you would need 9cells to get to 32v....and at 10cells you would have 36v peak...

Ooooooh, thanks for the information NRG and no it's not too technical.
I mistakenly thought LiFePO4s were 3.2v nominal. So they're not too dissimilar after all.
Maybe i'm barking up the wrong tree and please correct me if i'm wrong.
What would be better is to stick with 8 cell LiFePO4s but go for a 30AH battery.
Ping sells these standard on his website.
Then i would have the extra capacity to use on very long trips without the worry of the voltage dropping below the controller cutoff???

 

[flecc]

Yes, the 8 cell LiFePO4 is almost the same nominal voltage as a standard 7 cell li-ion/li-polymer.

Most controllers would have no trouble with 32 volts, but some have quite strict upper voltage limits, for example the eZee controller only allows 6% above the normal maximum charged battery voltage before signalling over-volt and cutting out.
.

 

[NRG]

Yes, agree with Flecc. 30Ah will give you the range, the voltage will hold for longer and the stress on the cells would be less on shorter trips. It may still be worth a look at the controller internals (if you're as inquisitive as me) but if undecided maybe it's best to leave alone and stick with 8cells.