Battery & BMS Wiring Help

[CDMaso]

I'm converting an old lawnmower from gas to electric. Made tremendous progress, but on Monday I received my batteries and BMS and fear caused progress to grind to a halt...

Here is a pic of my monster:
View attachment bluemonster.pdf

The battery pack is 48v 100ah comprised of 16 individual 3.7v 100ah packs. Here is a pic of one of the battery packs:
View attachment IMG_2015.pdf

The BMS is a 16S100A

Below is the diagram that was provided with the BMS:
View attachment 48V BMS connecting drawing.pdf

The diagram shows the B- and P-/Output- (on the right and left side, but no wires, just holes in the circuit board), and all the connections to the battery terminals (B-, B1+, B2+, B3+, ..., B16+/Output+)

I really appreciate any help in answering the following questions:

1) I understand that all the "pig-tails" that will connect at the bottom of the BMS will connect to the appropriate battery terminals (as is detailed in the diagram). Are these the only wires that will connect to the battery pack?

2) What should be connected to the B- and P-/Output- on the BMS (left and right side)?

3) I need to buy a charger for this battery pack, and would appreciate any recommendations. Since it's a lawnmower for personal use, it does not need to charge quickly (days would be fine).

4) Where do I connect the charger (e.g. directly to the battery or BMS)?

 

[CDMaso]

So, the company I got the batteries and BMS from posted a video to youtube showing me how to wire up the batteries and BMS. Got it all done yesterday and tested today with no problems.

Now I'm preparing to wire up the other components in the circuit to test the motor.

Couple more questions, and hopefully I'll get some responses

1) Is there a "best practice" I should follow when building the circuit? Meaning, should I check the components individually first, or just build the circuit and turn it on? I figure there is a process I should follow to prevent damaging anything if it a component is bad or if the circuit isn't designed correctly...

2) My circuit design is (from Battery + around to -), on/off switch, 150amp fuse, 600A 48v Solenoid, ME1004 (motor), shunt (for the multimeter), and back to Battery -. Any issues with the order of the components?

3) When I tested the 48v Battery it measured ~50v... Is there a chance that the 48v Solenoid will weld if the batteries are putting out 50v?

Thanks in advance for the help...

 

[amberwolf]

I'm assuming by "solenoid" you mean "contactor" or "relay", as a solenoid by itself doesn't directly switch anything, it's just a rod (or other shape) moved by a magnetic field generated by a control coil.

It's not really the voltage that causes the welding of the relay, but rather the switching under load causing arcing across the contacts from current flow. Since it's a 600A unit, it's probably not going to fail from current flow, depending on the load the motor sees when you disconnect it.

But the voltage, if exceeding the rating of the contacts, may continue arcing across them even after tehy're fully open, so the circuit can't be broken and either the contacts fail or potentailly a fire starts.

You'd have to look up the spec sheet for the relay to see what the absolute max voltage is allowable across it, and what it's guaranteed to break contact for.

BTW, if your battery pack is made from LiFePO4 cells, then the full charge voltage (the point where a BMS balances cells) is around 58v, for the BMS to do balancing (3.6-3.65v/cell x 16s). 51-52v is probably around the average voltage it might be at (16s x 3.2v). 50v is on the low side, more toward empty.

If it is typical Li-Ion, then full charge is 16s x 4.2v = 67.2v.

 

[amberwolf]

1) Is there a "best practice" I should follow when building the circuit? Meaning, should I check the components individually first, or just build the circuit and turn it on? I figure there is a process I should follow to prevent damaging anything if it a component is bad or if the circuit isn't designed correctly...

Not much to test really, other than verifying that when you switch the relay that it reads continuity across it's output contacts.

2) My circuit design is (from Battery + around to -), on/off switch, 150amp fuse, 600A 48v Solenoid, ME1004 (motor), shunt (for the multimeter), and back to Battery -. Any issues with the order of the components?

Well, there's a bit of complexity for your switch and relay that you don't indicate there.

The relay should have four (at least) connection points. Two smaller ones are for the coil. This coil has a specific voltage that it needs across it to cause the relay to close it's contacts. What is that voltage listed as? If it is not within the range of the battery's full to empty, then either it won't operate or it'll overheat the coil.

This coil is wired with one end to the switch, and the other to battery negative. The other side of the switch goes to battery positive. (or to the fuse, which then goes to battery positive).

Then the two large connection points of the relay are wired so that one of them goes to the fuse, and the other goes to the positive pole of the motor (the other motor pole goes to the shunt and thru that on to battery negative).

 

[CDMaso]

Thanks for the replies. The contactor is rated 48v 600a. I just checked the voltage of the battery pack and it was 52.4V (and each cell individually was 3.28v).

I completed all the wiring and it worked... the motor was drawing ~20amps. However, after about 15-30 seconds the BMS was getting pretty hot, and it started to smoke a little. The heat was on the B- side and appears the smoke was coming from that side as well. The wires and the batteries were normal temp.

How hot should the BMS get when under load? I expect that when I engage the blades the draw will be between 40-80amps, so I'm concerned there is an issue with the BMS.

 

[amberwolf]

You should verify that you've got the motor/etc hooked up to the output side of the BMS, not the input (charge), if they're separate.

And that the BMS is operating correctly, shutting off it's output, disconnecting the battery from the load, when a cell goes low, or current is too high. (sometiems disconnecting one of the cell sense wires will make the BMS think it's low, so that's one test you can do, otherwise you'll actually have to run it low).

AFAICR, B- is usually for the battery (cells) negative. If you have a C- it's probably for the charger input negative, and P- for power output negative.

It's also possible that the BMS is not actually what it's supposed to be, not able to handle even the 20A the motor is pulling. What is the spike current when the motor starts up?

Another possibility is the RF from the motor brush arcing is feeding back into the BMS and has damaged the FETs, so they are no longer working correctly and are high resistance. If you measure any voltage across the FETs from Source to Drain, while the motor is running, something is wrong with the BMS: either the FETs are not being turned on fully , or they're damaged, either one causing high resistance leading to heating.

 

[CDMaso]

Thanks Amberwolf...

I do have the motor connected to the BMS output (P-). Which is easy, because there is no C-... B- on one side and P- on the other.

WEll... in going to collect an answer to your second question (what is the spike current when the motor turns on, which was 30 amps) there were sparks, followed by a small fire in the BMS... :x

So, going to keep a positive attitude, but I think the BMS is shot.

I can't figure out how to attach pictures, so I've created a .pdf with all the pictures and explanations for each. Sorry for not being able to put them directly into the post... Really appreciate your feedback!

View attachment 48vBMS.pdf

 

[CDMaso]

Correction on my first post... I said my batteries were 3.7v 100ah... they are 3.2v 100ah...

 

[Dui ni shuo de dui]

Thanks Amberwolf...

I do have the motor connected to the BMS output (P-). Which is easy, because there is no C-... B- on one side and P- on the other.

WEll... in going to collect an answer to your second question (what is the spike current when the motor turns on, which was 30 amps) there were sparks, followed by a small fire in the BMS... :x

So, going to keep a positive attitude, but I think the BMS is shot.

I can't figure out how to attach pictures, so I've created a .pdf with all the pictures and explanations for each. Sorry for not being able to put them directly into the post... Really appreciate your feedback!

48vBMS.pdf

The BMS is not supposed to heat up during normal operation. The only case I've seen mine heat up was during balancing, but otherwise it never happened, even while running it at more than 200A continuous.

This unit might have been defective from the start, or maybe you did something wrong while wiring it.
In any case, you'll need to buy another one, try to get a "smart" BMS so you can check that everything is fine with the app on your phone, before switching the thing on.

Also, now that I think about it, it seems to me that you are running the motor directly without using any kind of throttle control. So it is possible that the motor ruined your BMS when you switched it on, or even when you stiched it off. When you switch on the motor, it will draw as much current as it physically can for a very short moment, which can be so brief that you cannot really record it with a cheap ampmeter. Also, when you switch the motor off, it still carries energy that needs to go somewhere. Usually there are freewheeling diodes in parallel to prevent this kind of damage due to freewheeling, but I don't think you have those in your setup. It could also be something like the motor creating a lot of parasitic noise which confused the BMS somehow. So maybe you should consider adding a controller, which would allow you to control the motor speed, or some kind of protection to prevent current and voltage spikes.

I'd go with the controller since it's easy and cheap. In which case, find a controller that has a slightly higher power rating than your motor, so that the controller won't heat too much. It will add a few losses but I guess you'll be fine with your gigantic 100Ah battery

 

[CDMaso]

Dui,

The company that provided the batteries and BMS felt that my solder job was for $hit, which was probably correct. Trying to connect a 2 AWG wire to the board was very difficult. They are sending me a new BMS (for free) with the wires already soldered. So, I'll give the new one a shot before I look at a "Smart BMS".

I was thinking about a controller this morning, and after reading your post, I'm more convinced it's a good idea. Any recommendations on a decent one for my application?

You are the second person to mention that the motor could cause issues. Amberwolf said "Another possibility is the RF from the motor brush arcing is feeding back into the BMS and has damaged the FETs, so they are no longer working correctly and are high resistance."

And you mention "When you switch on the motor, it will draw as much current as it physically can for a very short moment, which can be so brief that you cannot really record it with a cheap ampmeter. Also, when you switch the motor off, it still carries energy that needs to go somewhere. Usually there are freewheeling diodes in parallel to prevent this kind of damage due to freewheeling, but I don't think you have those in your setup."

You are correct, I don't have any "freewheeling diodes" in my setup. I do have a 150 amp fuse, but I bet you are going to tell me that will not protect the BMS from a very short startup spike.. Will a controller protect the BMS from both the startup spikes and the energy that is carried after shutting it off? Or are you suggesting I add both a controller and the "freewheeling diodes" you mention?

Thanks!

 

[Bas]

I think when you have a bluetooth BMS, you can set the battery On and Off by an app on your phone. So first shut the battery down, then connect your controller to your battery... No spark, I guess!

Ps: I love your soldering, it could have been me who made that! Except for me... I use about twice the amound of solder, a big strong tin ball.. 100gr tin for joining 4 big fat Kelly controller wires you can't even bend.. :lol: