Limiting motor current

[electricmaile]

I'd like to know if there is a way to change the current limiting of a motor controller. I'm guessing that it's probably done in firmware, so there would be no easy way to change it (like changing the value of a resistor), but maybe I'm wrong. I'd like to LOWER the maximum current that the controller can supply.

The reason is that I'm carrying a small 36v 4AH Li-ion battery as a backup for when my main battery 36v 12AH battery runs out of charge. I'm a bit concerned about pulling too much current out of the smaller battery. The controller I have seems to limit peak current to about 20A, which is OK with the larger battery. The motor is nominally 350W, but of course at peak load it's really drawing closer to 800W. I think that would be straining the 4AH battery a bit so I'd like to be able to drop the current limit to maybe 8A when the smaller battery is in use. I believe the rule of thumb is that you can shorten battery life if you pull more than about 2C from a Li-ion battery.

I'm not looking to ride fast (obviously), just to get a little motor assist for the hills. 8A is enough for that.

I can control current via the thumb throttle of course, so I can manually limit the current draw, but it would be nice to have an automatic current limiter I could switch in when going to the smaller battery.

 

[999zip999]

How about paralleling the packs.

 

[Jon NCal]

It's possible to limit current through a Cycle Analyst v3, or programmable controller with 3 speed switch, but assuming you don't use the backup pack much it might not be worth it, and just to go easy on the throttle instead.

The BMS in the pack also has a limit.

 

[Alan B]

To protect the battery you want to limit battery current, not motor current.

Current limiting merely restricts the throttle, so you can do it by hand, or you could use another device such as the Cycle Analyst, as was mentioned.

 

[dogman dan]

Or you could get really sophisticated, and just not twist the throttle all the way when using the small pack. At least for now you can do that, but it's really easy to let the throttle creep up when using that method.

I assume you have a throttle, since most pas systems have multiple power settings. If your bike has that, just use the lower power settings when you are riding the small pack. Keep it below 15 mph on the flat, and you will draw less than 300w.

 

[Matt Gruber]

since my pack is only 2p, 3ah, i use my controller on low, AND a throttle stop, which has a screw that i can adjust on the fly. still have to watch hills and even wind, however.
using a 15a fuse.
187 cycles so far.

 

[electricmaile]

Thanks for the suggestions. Looks like either a mechanical or an electronic limiter on the thumb throttle is the easiest way to do this. I think the throttle signal may be a simple 0-4v level, so that could easily be reduced to 0-2v. I'll have to play around with that.

The Cycle Analyst looks interesting, but overkill (and expensive) for this application. I have a similar unit which reads out voltage, power current etc but doesn't offer any control. It's a lot cheaper though! See http://www.ebay.com/itm/Digital-LCD-Watt-Meter-Battery-Voltage-Current-Power-Analyzer-Tester-60V-100A-RC-/281826900593?hash=item419e313a71:g:NZoAAOSwl9BWHPwg . There are other similar units on eBay in teh $10-$20 range. The main drawback is that the current shunt is inside the unit, so you have to use a long length of heavy gauge wire if you want to mount it up on the bars where you can see it, or mount it inline down by the battery pack if you just want to record data to look at later.

 

[Alan B]

The inductance of long wires between the battery and the controller should be kept to a minimum. This inductance causes voltage spikes in the controller that can cause FET failure.

It is best use a remote shunt or magnetic sensor to measure system current and keep the battery to controller line short and low impedance.

The throttle signal is generally about 0.5V to 4V. Reducing the low end voltage will increase the deadband at zero on the throttle. Reducing the high end will reduce top speed. It will not reduce motor current in general.

Maximum motor current is experienced in the low to middle speeds. At low speed the motor current is much higher than the battery current. At high speed the back EMF from the motor limits battery and motor currents, and battery and motor currents are nearly equal.

 

[bobc]

In the ideal world motor current limit is handy, it limits heating in motor and inverter. If the motor's running slow the motor current can be several times higher than the battery current; i.e. you can blow things up without a battery current limit noticing....