Custom Battery for Item, To Much Current Dangerous?

[YoshiMoshi]

So let's say I want to make a custom battery pack for a power tool. I can look at what cells the manufacturer puts in their official battery packs, and see what the highest continuous discharge current is within their battery line, by analysing which cells are in their various batteries of different capacities and additions that are added over time. Let's go with 45 A. Now we can assume (hopefully) the manufacturer would not sell a battery pack whose maximum continuous discharge current is to high for their tools, due to liability reasons. Now I can go and make my own custom battery pack whose maximum continuous discharge current is much higher than 45 A. Let's say 70 A or higher, especially with the tables cells.

Now the power tool is designed with a maximum continuous discharge current in mind.
1) The insulation and garage of the wires. I've seen custom insulation, sometimes, no temperature rating, and googling the markings on it leads to nothing.
2) How much current the MOSFETs can handle
3) The maximum overall continuous discharge current of the tool that it can draw.

What is the weakest link? Can I assume the manufacturer wouldn't put in a brushless motor that can draw 70 A continuous but whose internal wiring only supports 50 A continuous? Some of these tools were made prior to tables cells, and properly were never designed to have 70 A running through it continuously. But the motor may be able to draw it.

What should one due when building a custom pack for a device whose maximum continuous discharge is unknown? Is there some risk in making a battery pack that is capable of putting out "to much" current for what the device was designed to handle, but will still draw it?

Also I've heard that the BLDC controller IC limits current. But looking through a datasheet I see nothing as far as maximum current or maximum power. Only maximum voltage. I see sections about a current comparator circuit and a sensing resistor, but no maximum current. I don't know if I'm not looking for the right thing? Some data sheets are rather large. I was expecting a maximum current or maximum power at the start of the datasheet with all the characteristics listed in tables, but there's nothing on current.

 

[bananu7]

What should one due when building a custom pack for a device whose maximum continuous discharge is unknown? Is there some risk in making a battery pack that is capable of putting out "to much" current for what the device was designed to handle, but will still draw it?

Not really. Utilizing battery's internal resistance as an implicit current limiter would be an extremely bad engineering practice. In virtually all cases I'd expect the tool to work correctly with a theoretically perfect battery (infinitely powerful, r = 0, Z = 0).

Also I've heard that the BLDC controller IC limits current. But looking through a datasheet I see nothing as far as maximum current or maximum power. Only maximum voltage. I see sections about a current comparator circuit and a sensing resistor, but no maximum current. I don't know if I'm not looking for the right thing? Some data sheets are rather large. I was expecting a maximum current or maximum power at the start of the datasheet with all the characteristics listed in tables, but there's nothing on current.

What datasheet?

 

[YoshiMoshi]

Not really. Utilizing battery's internal resistance as an implicit current limiter would be an extremely bad engineering practice. In virtually all cases I'd expect the tool to work correctly with a theoretically perfect battery (infinitely powerful, r = 0, Z = 0).


What datasheet?

Im looking at a datasheet that states that the over current protection threshold can be calculated via this equation from the datasheet

"For the OCP circuit to function, select the PWM DutyCycle = 0 to 99.9%; 100% should be avoided. The
following SPI registers must be configured as PWMSkip = 0, OCDisable = 0, ISnsHiGain = 0 / 1 (GAIN = 4
/ 10) with DAC1: DAC2 selection of 0 to 1024. The OCP threshold is calculated with

[1.2 × DAC / GAIN / 1024 / Rsense]"

I can find Rsende on the PCB but am unsure how to find the Gain which is 4 or 10 (but I'm not sure which one to use) or DAC (which I assume is digital to analog) and the 1024 is 2 to the power of 10.

 

[docw009]

I would think the tool controller limits the amps to protect the motor. Just like I could do a 70A battery (but I won't) for my ebike, but the bike controller peaks at 20A.

 

[glennb]

You'd hope so, but at least for cheap tools I'm pretty sure they dodge the issue by simply using motors whose stall current is marginally higher than the BMS' over-current tripping point. I.e. the tools are entirely reliant on BMS protection.

Tool BMS could be argued to be a partial exception to the rule that the BMS' job is exclusively to protect cells, due to having to also protect upstream components.

It's why identical BMS are used on tool batteries with different capacities and capabilities. You'll find the same 30A BMS on both a vendor's 4Ah and 8Ah batteries, for example, despite the 8Ah having much greater current delivery capability. (I say "much greater", rather than "double", due to thermal limitations of pack construction materials and techniques.)

Easy to test either way, just measure current whilst progressively loading the tool toward stall, first with BMS functional and then with BMS bupassed.

 

[bananu7]

I'd have assumed that in the era of variable speed control in almost every power tool, that would get routed to at least a rudimentary controller with overcurrent protection but yeah, I guess the cheap stuff is... cheap.

 

[YoshiMoshi]

I see that good engineering would dictate tool design to assume you have an infinitely strong battery so that it's not the limiting factor, so as battery technology improves, higher continuous current draw cells, larger capacity, you can release new battery packs, but still have the new battery packs not overpower your older tools that may not need the extra continuous draw.

I see that I cannot really get the maximum current from the datasheet of the the BLDC controller, it requires me to read values within the registers.

Another limiting factor is wire gauge and insulation, but in my case it's custom insulation without a temperature rating.

The MOSFETs for my power tool are rated for 209 A, so not really helpful. It's no more than that.

The BLDC motor is a custom motor so I cannot find the datasheet.

Is there anything else I can look at to determine the maximum current draw of a power tool?

I might just have to test to see what the maximum continuous draw is, and create a rig, than once I have this value make a battery pack that can support it?

 

[bananu7]

Is there anything else I can look at to determine the maximum current draw of a power tool?

Hook it to a lab PSU instead of a battery and slowly increase the allowed current.