"Maximum Continuous Discharge Current" for BMS for Power Tool Battery Pack?

[YoshiMoshi]

I keep seeing elsewhere online that the BMS does not limit the maximum continuous discharge current of the battery pack, but the controller does. Is this accurate? What is the "controller", and where does it reside? Is it on the same board as the BMS?

In my case for a power tool battery pack, does the controller reside in the power tool?

I thought of the maximum continuous discharge current rating as the following:

• The BMS has a rated maximum continuous discharge current. This rating is the maximum continuous discharge current that can be pulled of the battery pack, regardless of the load.
• The power tool has a rated maximum current draw. This may be less or more than the maximum continuous discharge current of the BMS. If the power tool is capable of drawing more current, it will be limtied to the maximum continuous discharge current of the BMS.
• The gauges of the wires connected to the battery connector within the power tool can be used as a reference to the maximum current draw of the power tool, likely with a safety margin. The maximum current draw of the power tool is often considered propitery and not known.

Thanks for any help in understanding the maximum continuous discharge current rating for BMSs.

 

[fechter]

The BMS's job is to protect the cells. Most will have a maximum current rating that will trip and turn off the pack if it tries to go above this. Sort of like a resettable circuit breaker. The tool has the "controller" inside and normally should never go above the tripping point of the BMS. The BMS will also trip if any cell goes too high or too low on voltage, and most have a temperature sensor that will also trip if the pack gets too hot.
Without documentation, the only way to determine the tripping point would be to measure it by increasing the current until it trips.

 

[YoshiMoshi]

The BMS's job is to protect the cells. Most will have a maximum current rating that will trip and turn off the pack if it tries to go above this. Sort of like a resettable circuit breaker. The tool has the "controller" inside and normally should never go above the tripping point of the BMS. The BMS will also trip if any cell goes too high or too low on voltage, and most have a temperature sensor that will also trip if the pack gets too hot.
Without documentation, the only way to determine the tripping point would be to measure it by increasing the current until it trips.

Thanks for the reply. So the "maximum continuous discharge" rating of the BMS is the maximum current rating that will trip and turn off the pack, if it stays at that current rating for longer than "instantaneously"?

BMS - Limits current to the maximum continuous discharge current comming out of the battery pack.
Controller - Limits current going to load, this should be under the BMS maximum continuous discharge current, but it can be above it and would result in the battery pack turning off by the BMS

Is this the correct understanding? Is there a picture you could point me to of what a controller looks like physically within a power tool?

I appreciate the help in understanding this concept!

 

[Papa]

I think you should identify the tool packs you refer, especially if used outside manufactures' intent.

I use Ego 58V 10Ah packs, and I'm told that pulling juice directly from said Ego packs (for purposes unrelated to the tools they are designed for), bypasses the BMS. I've not confirmed the information yet, so I'm cautious and carefully monitor the loads I place on them.

 

[fechter]

I think you got it. Tool configurations vary widely, but if you take one apart, there should be some kind of circuit board connected to the trigger. Sometimes it's all inside the trigger assembly.