Tool Platforms

[maarkmohamed]

Just curious regarding the different tool platforms like milwaukee, ego, dewalt, and so on, for some of there bigger tools they allow you to connect two batteries simultaneously for more power/run time. This is understood to be connecting two batteries in parallel however whats a little confusing is how are they allowing you to connect two different battery capacities and charge levels? Wouldnt this typically be a bit dangerous?

 

[Hillhater]

Not if the BMS & voltage protection is inside each individual pack ?

 

[maarkmohamed]

Not if the BMS & voltage protection is inside each individual pack ?

I would think the bms in this case would not allow the two batteries to connect

 

[amberwolf]

Something to note about any BMS that has separate charge and discharge ports (which might be the case with some toolpacks; I haven't opened any modern ones to check):

The body diode of the FETs used to switch charge or discharge off will still allow reverse current to flow regardless of the state of the port, unless there are additional diodes in the current path to prevent this (which is not typical in ebike BMSs, no idea what is typical in toolpacks)

So if you have two packs with separate C & D ports, and each battery is at a different voltage, and you wire the D ports together, the lower voltage pack will be charged thru it's D port even if it has shut that port off for whatever reason.

If it hasn't shut off it's D port, it'll also still charge thru it.

Same for connecting them up by their C ports. The higher voltage pack will discharge thru it's C port into the lower v pack's C port.


Also, if you permanently parallel the D ports, then because of the above neither pack can protect itself during charge from an overcharge if they are simultaneously being charged.

Similarly, if you permanently parallel the C ports, then because of the above neither pack can protect itself during discharge from an overdischarge if they are simultaneously being discharged.


If they have a common C & D port, then the BMS is designed to protect against any of the above scenarios, and they could safely be paralleled...but they should still be made the same voltage (or very close) first, so that there is no inrush current from one to the other that may be higher than the charge FET(s) on the BMS can handle. This can damage the FETs in a way that can short them "on" permanently, so the BMS can no longer control charge/discharge, and thus can't protect the pack anymore. Typically it's a silent failure so you don't even know this has happened, and cell damage can occur eventually if limits are exceeded.




Secondary note: Some toolpacks don't have a BMS in them at all, and rely on the tool or the charger to control pack usage. I have an old Black and Decker hedge trimmer that uses a Li pack that doesnt' have anything other than a temperature sensor in it, and that is wired to separate contacts on the connector. The tool has a board in it that monitors that temperature, and the pack voltage, and shuts off the tool if either exceeds the limits it has. The charger does the same. (oddly the tool can't monitor it's *own* temperature, so the motor can easily overheat and burn out...).

 

[maarkmohamed]

Something to note about any BMS that has separate charge and discharge ports (which might be the case with some toolpacks; I haven't opened any modern ones to check):

The body diode of the FETs used to switch charge or discharge off will still allow reverse current to flow regardless of the state of the port, unless there are additional diodes in the current path to prevent this (which is not typical in ebike BMSs, no idea what is typical in toolpacks)

So if you have two packs with separate C & D ports, and each battery is at a different voltage, and you wire the D ports together, the lower voltage pack will be charged thru it's D port even if it has shut that port off for whatever reason.

If it hasn't shut off it's D port, it'll also still charge thru it.

Same for connecting them up by their C ports. The higher voltage pack will discharge thru it's C port into the lower v pack's C port.


Also, if you permanently parallel the D ports, then because of the above neither pack can protect itself during charge from an overcharge if they are simultaneously being charged.

Similarly, if you permanently parallel the C ports, then because of the above neither pack can protect itself during discharge from an overdischarge if they are simultaneously being discharged.


If they have a common C & D port, then the BMS is designed to protect against any of the above scenarios, and they could safely be paralleled...but they should still be made the same voltage (or very close) first, so that there is no inrush current from one to the other that may be higher than the charge FET(s) on the BMS can handle. This can damage the FETs in a way that can short them "on" permanently, so the BMS can no longer control charge/discharge, and thus can't protect the pack anymore. Typically it's a silent failure so you don't even know this has happened, and cell damage can occur eventually if limits are exceeded.




Secondary note: Some toolpacks don't have a BMS in them at all, and rely on the tool or the charger to control pack usage. I have an old Black and Decker hedge trimmer that uses a Li pack that doesnt' have anything other than a temperature sensor in it, and that is wired to separate contacts on the connector. The tool has a board in it that monitors that temperature, and the pack voltage, and shuts off the tool if either exceeds the limits it has. The charger does the same. (oddly the tool can't monitor it's *own* temperature, so the motor can easily overheat and burn out...).

With that being said, what do you think they are doing with there bms in the modern day tools? As i mentioned they allow inserting multiple different voltage and AH packs together in parallel

 

[amberwolf]

With that being said, what do you think they are doing with there bms in the modern day tools? As i mentioned they allow inserting multiple different voltage and AH packs together in parallel

I don't know. I've never used a tool system that allows multiple batteries of even the same kind, much less different ones; I know they exist but without direct access to the entire system (tool and batteries) to disassemble and test I couldn't tell you anything about their insides and how they work.

 

[lnanek]

Messing with my Dremel charger, I know I can get 3 different voltages out of the four contacts depending on which I connect and which I measure. So I'm pretty sure the charger, at least, is designed to support multiple different voltage packs. No clue about the packs themselves. Entirely possible it's similar and a 12V 3S pack actually has ways to use just 1 or 2 of the 3 cells as well. Like if the balance wires could also be used as discharge wires, essentially, in ebike terms. I haven't seen a tool with multiple slots, but doesn't seem difficult to design one that could just use the same number of cells from each slot ignoring any extra cells in a mismatch case if that's true.

 

[lnanek]

Interesting charger schematic off Amazon:

https://a.co/d/iJFR5Yx

So you can see how there's a lot of slots for the bottom of the battery and they connect to a different number of cells.

 

[amberwolf]

The link goes to a battery case, not a charger;
"for Dewalt DCB200 18V 20V Li-ion Battery Plastic Case 3.0Ah 4.0Ah DCB201,DCB203,DCB204 Li-Ion Battery Cover-Parts "
https://www.amazon.com/dp/B07WMRVPZL
https://m.media-amazon.com/images/I/61lLkRO4ZjL.__AC_SX300_SY300_QL70_ML2_.jpg

I would guess from the design of the plastics/etc that the diagram is for how to wire the cells in the battery pack to the connector on the main board, and to the battery meter board.

Then there are five large contacts that are actually externally accessible, presumably allowing access to the main + and - (end cells) with two of them, leaving three for other things. Can't see the traces on the board clearly enough to tell if any go to other cells or not.