Why is my ebike controller reducing voltage to the motor??

[pvdm]

Hi All
I am totally new to ebikes and have searched for a straight answer on this but as yet have been unsuccessful.
BIKE: Montego city bike with brushed, 200W, geared front hub motor and 24V Panasonic 11.4ish Amp Hour Li-ion battery
PROBLEM: We live in a hilly area and when climbing hills (after about 30-60 sec) the assistance drops significantly until you stop the motor and restart. After the restart, it will have full power for about the same amount of time and then drop again. I have put a multimeter on the power wires going to the motor from the controller and the voltage is dropping from a steady 26ish volts to around 16. Battery voltage is stable and good at around 26V.

I imagine it might be some sort of current limit to protect the battery or controller, but can someone confirm that this is the case please?

Also, what can be done about this? I would ideally like it to stop reducing the assistance like this as its a real pain when trying to climb our hill home, but will that kill the battery by drawing too much current?

I am happy to change controllers if this will fix the problem, just need a few ideas on what to do first.
I have attached some pics of the controller if that helps.

Thanks a lot in advance!!!

 

[neptronix]

Your battery's voltage is sagging in response to the load put on it. This is not the fault of a controller - it is a fault of the battery's age, perhaps temperature, poor design from the factory, or some combination of the 3 factors that make for low performance under load.

How old is this bike?

 

[pvdm]

I dont think it is sagging battery voltage as I have had the multimeter on the battery and also on the motor leads while riding. The battery voltage sits very steady and always above 24V. When the problem occurs, the voltage going to the motor instantly changes from about 26V to about 16V. After restart it goes back to 26V until the error occurs again. The bike is new. About 1 month old. It has always done this. We bought it online and the dealer is not helping at all.

 

[hjns]

I don't know what the problem is, as I have no experience with Brushed motors / controllers.

However, if you want to change controllers, you will need to find another controller for Brushed motors. And I think that it won't even improve power very much as I think that you will mainly be limited by your motor and the total power/heat it can handle. But maybe you can tell us more about your expectations?

Usually we would look at the following:

• Weight of your bike
• Your weight + weight of any desired cargo
• Desired top speed on a flat road
• Desired cruising speed
• Desired top speed while climbing a mountain (specify the inclination in %)
• Desired cruising speed while climbing a mountain
• Desired range in your situation (including all hills that you need to climb on one battery charge)
• Your main limitations (budget, DIY skills, availability of a TIG welder, local laws, etc)

 

[John in CR]

Changing the voltage is how the controller controls speed. It switches it on and off very quickly (PWM - Pulse Width Modulation), so the motor "sees" a lower average voltage. The result you are seeing is that your system isn't powerful enough for the hills you are asking it to climb, so as it starts to bog down it has to prevent the current from skyrocketing. The only means a controller has of doing that is by limiting the width of the "ON" pulse. Because yours is a brushed motor, you are able to see and measure that RMS voltage, since it is DC current. Our brushless motors work similarly, but the alternating polarity of the pulses make measuring the voltage our motors "see" more difficult.

Solution- If your motor can handle more power, then you could increase the current with a simple controller modification or a new more powerful controller. Before doing that, like HJNS suggests you should evaluate how hot your system gets while tackling the steepest hills it currently allows.

John

 

[pvdm]

Thank you John and Hank for your replies.
The bike is about 25kgs, I am about 80kgs but my partner is only 50kgs and admittedly it has the problem more when I am riding for obvious reasons. We are relatively happy with how it rides when it has full power but more power would be great. We ride short distances of about 20km round trip for work but that includes a 300 m climb on the way home. Max would be a 15% climb. The motor is only ever warm at the end of the climb, as is the controller. Cannot feel any heat on the battery.

John, your explanation make perfect sence, thank you. What is the simple modification that can be done to the controller? Otherwise what should I look for in a new controller other than correct voltage and for a brushed motor?

 

[John in CR]

The controller calculates current based on the voltage drop across the shunt. By reducing the resistance of the shunt the controller thinks it is supplying less current that it really is. With the existing programmed limit of 12A, you can probably double your current without issue.

The metal used for shunts is relatively high resistance, so I like to modify them by wrapping a portion of the shunt length using a thin strand of copper and then solder it in place to effectively make that portion of the length near zero resistance.
In your case I'd try wrapping a little less than half the length and see how it works. First, check along the throttle wire for a potentiometer or other device limiting your throttle. A lot of the low power bikes have something external like that to limit them artificially and meet the laws for different markets with just a simple change.

Assuming a mod is needed. Here's yours with some copper drawn on it.


Here's one I did, before and after solder.

 

[dogman dan]

Brushed controllers are pretty cheap, and easy to install. You might just try a slightly stronger one if after the shunt mod, it still results in this voltage drop. Not too strong though, say 15 amps. I had a brushed controller that would overheat and shut off climbing a really steep hill, but in my case it was a complete shut off.

But to really get up steeper hills, you need new, stronger, everything. 1000w not 200w. But your battery won't like 1000w.

 

[pvdm]

Fantastic, thank you John. I'll give that a try and monitor motor and controller heat after to make sure it does not overheat significantly. Your absolutely right, there is a pentiometer on the handlebars which is designed to reduce the power (in case 200w was too wild for you) but I always have this on max (i.e. supplying the max voltage to the motor) as I like to live on the edge. . Thanks again for your help. I'll let you know how it goes.

Thanks Dogman, I will look into a 15 amp controller if this does not solve the problem. Your right, my next project bike will be at least 500w.

 

[hjns]

When you do the shunt mod, just make sure not to overdo it. I overdid it with another controller, and ended up bricking it.

As John said, start with winding a copper wire around the first 1/3 of the shunt, and fill it with some solder. Then check out if you like the performance increase. You can always go back and add more copper/solder if you need more. Removing any solder is much much harder.

 

[John in CR]

PVDM,
The governor up on the bars may be hindering operation even at 0. I'd bypass it completely first. Then also look around for extra wires or jumpers that could also be telling the controller to restrict operation. IOW try things that are easily reversible first, because reducing the shunt resistance in that manner isn't easily reversed.

 

[Gregory]

Before I’d suggest a shunt mod I’d install a watt meter, which can be had from $20, just to give me an idea of what the bike is doing, particularly up those hills.
And will give you a before and after comparison.

Adding to the shunt is definitely worth a try, but as said, be conservative, little bit at a time or either the controller or battery will find their limits.
Be warned a brushed controller that blows FETs can suddenly go full speed so a kill switch is a very good idea (even if it’s just an ebrake).

 

[pvdm]

Many thanks everyone for your constructive comments. I have done the shunt mod as described and it has fixed the problem! It seems to have a little more power also. Motor and controller temps are fine, only just warm. I had already done the mod before I saw the two last posts but thank you anyway guys. I'll keep you posted on how long it lasts.
All the best, Pier

 

[pvdm]

Hi again everyone
So the shunt mod has been working like a charm for a few weeks and my partner was so happy with the new performance and the fact that it was running at full power constantly. Thanks to everyone who helped with this solution!
Unfortunately we seem to have another problem which may or may not be related to the increased current running through the system after the shunt mod.

BIKE: as per first post: Montego city bike with 200W brushed hub motor and panasonic 24V ~11.4Ah Li ion battery.

PROBLEM: When we connect the charger to the battery pack the charger does not charge the battery now. Its LED used to turn from green to red when charging, but now it is constantly on green and does not seem to be supplying voltage to the battery. Battery voltage is 25.5V and under load is 24.5V. Charger output is 29.6V. When I connect the charger to the battery and measure the battery voltage at the controller it does not increase (remains at 25.5V).

QUESTION: Is this simply a dead charger? I understand there is normally an integrated BMS in the panasonic batteries, so is it possible that we have hit the current limit imposed by the BMS and it has somehow shut the pack down??

Any ideas would be much appreciated!!
Many thanks in advance, Pier