Need Help With Battery Conversion Project

Thread Starter

mesutben97

Joined Feb 28, 2023
7
I was trying to upgrade the battery pack of a Black Decker 12V cordless vacuum from NiMh to Li-Ion. I assumed a 3S25A BMS together with three cells would be fine. But when I connect the new battery pack, it only runs half a second then power is cut. When I connect battery without BMSi it runs fine. Interestingly when I try this BMS on a 12V cordless drill, it runs fine. Then I tried the battery pack from the drill on the vacuum, it again runs fine. What could be the issue?

Another related question; attached photo side by side two cheap 12V cordless drill boads, is there any protection on them and what is the difference. 24V label is fake, they are just 12V.

Thanks in advance.
 

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crutschow

Joined Mar 14, 2008
34,397
Sounds like the vacuum cleaner may be drawing enough current to trigger the current-limit on the BMS.
Can you measure its current draw?
 

Thread Starter

mesutben97

Joined Feb 28, 2023
7
Sounds like the vacuum cleaner may be drawing enough current to trigger the current-limit on the BMS.
Can you measure its current draw?
Thanks crutschow. I tried and blew up the fuse of my 10A multimeter :). But I read somewhere that the vacuum' current rating is 20A; so that's how I choose 25A BMS. I also tried it with 4S40A BMS (18V) with no success. But why does the drill's board work on the vacuum; should I assume that it does not have current protection?
By the way, I'm attaching the BMS photo that I'm using for this project.
 

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crutschow

Joined Mar 14, 2008
34,397
I read somewhere that the vacuum' current rating is 20A
Its start-up current may be considerable more than that which triggers the current limit.
Motors can look like a near short at startup.
One solution would be to use a delay circuit to momentarily (perhaps a second) add a 2 ohm (or so) resistor in series with the motor during startup.
why does the drill's board work on the vacuum; should I assume that it does not have current protection?
That would be my guess.
The designers of the drill battery know that they don't need to protect against excessive current, so did not add a battery current limit.
 
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Thread Starter

mesutben97

Joined Feb 28, 2023
7
Its start-up current may be considerable more than that which triggers the current limit.
Motors can look like a near short at startup.
One solution would be to use a delay circuit to momentarily (perhaps a second) add a 2 ohm (or so) resistor in series with the motor during startup.
That would be my guess.
The designers of the drill battery know that they don't need to protect against excessive current, so did not add a battery current limit.
Thanks a lot. That sounds like an easy solution, will try. But it's still frustrating that, vacuum draws more current than a drill, both having a 12V DC motor.
 

Juhahoo

Joined Jun 3, 2019
302
Disable the current sense by shorting the sense resistors, if problem goes away, its the current limit. If not then I would measure the voltage in the battery when the unit is started, might be too low voltage and the unit is cut off. Depending on your Li-Ion cells, they might not be able to supply enough current and the voltage will drop a lot. You might need an oscilloscope to measure fast transients.
 

Thread Starter

mesutben97

Joined Feb 28, 2023
7
Disable the current sense by shorting the sense resistors, if problem goes away, its the current limit. If not then I would measure the voltage in the battery when the unit is started, might be too low voltage and the unit is cut off. Depending on your Li-Ion cells, they might not be able to supply enough current and the voltage will drop a lot. You might need an oscilloscope to measure fast transients.
I don't think the issue is battery realated because they can feed the motor without the BMS. I assume it would be these two resistors marked with red?
 

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Juhahoo

Joined Jun 3, 2019
302
Methinks you would have a new problem: smoked driver transistors!
Current is sensed over the resistors and compared towards the limit. When voltage reach a threshold i.e. current flow rises output is shut. When current sense is shorted sense voltage is zero and switches remain open. They don't burn.
 

Juhahoo

Joined Jun 3, 2019
302
I don't think the issue is battery realated because they can feed the motor without the BMS. I assume it would be these two resistors marked with red?
Those red marked resistors measure the current flow from the negative side and they will shutt off the load switch MOSFETs if overcurrent is detected. So if the motor will exceed the current at startup it triggers the overcurrent protection.
 

Alec_t

Joined Sep 17, 2013
14,312
Current is sensed over the resistors and compared towards the limit. When voltage reach a threshold i.e. current flow rises output is shut
Precisely. But if the sensing resistor is shorted out as suggested then a huge current would flow and the output would not shut off until the magic smoke escaped.
 

Juhahoo

Joined Jun 3, 2019
302
Precisely. But if the sensing resistor is shorted out as suggested then a huge current would flow and the output would not shut off until the magic smoke escaped.
2,5 milliohm resistor (2 x 5milliohm) have much of nothing to resist the current flow in anyway. Those resistors are there ONLY to measure current flow, they have nothing to do to resist the current flow to directly protect the mosfets. Actually smaller the overall resistance less power losses thats why they parallel MOSFETS.
 
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