Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
BMS problem (need second opinion)
#1
I have been working on a replacement battery for a lawnmower (28.6V @ 6A) 

I have built the battery (7S 7P) and everything works fine. I put the battery in the case and tested the voltages for and I am at 28.64V. 

So all good. 

Put the battery in the mower and test it, the mower starts fine and cuts my front lawn with no problems. 

Take the battery out of the case and add BMS to it (25A rated) - https://www.aliexpress.com/item/40005410...web201603_

Put everything back together and then test out the mower. 

Mower starts up, then dies. 

BMS seems to reset after 2 sec, and then the process repeats over again. 

I am thinking it is a bad BMS, but maybe I do not know enough about BMS' and purchased the wrong one/style/model.   

Any help would be appreciated.
Reply
#2
I think your power ratings for your lawnmover might be wrong. 28.6 volts x 6 amps = 171.6 watts. The bms is 28.6 volts x 25 amps = 715 watts (but it peaks at 28.6 volts x 60 amps = 1716 watts). It can handle the peak but only for a few seconds.

171.6 watts seems too small for an electric lawnmover, when I google it mention anywhere from 900 to 1500 watts for most lawnmovers. If you have a dc wattmeter, you might be able to use that to get an accurate wattage for your lawnmover.
Reply
#3
Perhaps the battery is "6Ah" vs. "6A".
Reply
#4
As said you have a surge current on the system and the BMS cant take it.

If you replaced the stuff: What type of battery did you have before? What type of BMS?
NOTE! My links supplied in this message may be affiliated with Ebay and by clicking on them you agree on the terms.
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
Upcoming: 14S 18650~30kWh
Reply
#5
Agree, sounds like the BMS doesn't like the motor current. Going to be big surges on start-up with that type of use.
Running off solar, DIY & electronics fan :-)
Reply
#6
(08-27-2020, 07:35 AM)daromer Wrote: As said you have a surge current on the system and the BMS cant take it.

If you replaced the stuff: What type of battery did you have before? What type of BMS?

The battery was lead-acid, with no BMS installed. 

The previous owner left it outside in winter and it froze the battery. 

I had the 18650 cells, so they gave me the mower for free. 

I have not taken it apart to read the motor requirements but found online something about it being a 6A motor. 

This is the owners manual for the mower - https://www.manualslib.com/manual/820725...e=1#manual

(08-27-2020, 03:08 AM)gauss163 Wrote: Perhaps the battery is "6Ah" vs. "6A".

The original lead-acid battery was 10Ah battery @ 24V.
Reply
#7
The easiest thing to do is to disconnect and run the device directly on the battery and add a clamp-meter to check its total/max current.

Its safe to do that for testing without bms. Discharge is not as critical as charge in this sense. (Basic use-case in RC).

When you know the max current you can go back and get a BMS that will work

Note that you should not use the original charger with this unless the voltage on that charger fit the battery voltage you have and you trust the bms to cut if something happens.
NOTE! My links supplied in this message may be affiliated with Ebay and by clicking on them you agree on the terms.
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
Upcoming: 14S 18650~30kWh
Reply
#8
(08-27-2020, 02:51 AM)jonyjoe505 Wrote: I think your power ratings for your lawnmover might be wrong. 28.6 volts x 6 amps = 171.6 watts. The bms is 28.6 volts x 25 amps = 715 watts (but it peaks at 28.6 volts x 60 amps = 1716 watts). It can handle the peak but only for a few seconds.

171.6 watts seems too small for an electric lawnmover, when I google it mention anywhere from 900 to 1500 watts for most lawnmovers. If you have a dc wattmeter, you might be able to use that to get an accurate wattage for your lawnmover.
It could be that I have some bad information (finding any info on this has been a real issue). 

I do not have a dc wattmeter at this time (and my multimeter stopped working yesterday as well). 

Here is what I was thinking (not sure if this would work or not, this is my first small battery project). 

What if I was to bypass the BMS for the discharge of the battery and just use the BMS for charging the battery? This way the battery keeps balanced but I am also able to run the mower without any problems. 

The other option would be to pick up a lager Amp rated BMS (maybe something in the 60/70A range)

(08-27-2020, 06:51 PM)daromer Wrote: The easiest thing to do is to disconnect and run the device directly on the battery and add a clamp-meter to check its total/max current.

Its safe to do that for testing without bms. Discharge is not as critical as charge in this sense. (Basic use-case in RC).

When you know the max current you can go back and get a BMS that will work

Note that you should not use the original charger with this unless the voltage on that charger fit the battery voltage you have and you trust the bms to cut if something happens.
No clamp meter at the moment (need to get a new meter, and am looking at the clamp meters as well). 

The system did not come with a charger (using a benchtop power supply to charge right now). 28.6V @ 2A. 

Trusting the BMS right now (this is a BMS I had laying around that I thought would work for this application). 

This is my first working project. Most of the other things I have done have been just to see if something works or not, never put together something that is supposed to work at the end of the project (I just like to do this for fun).
Reply
#9
Just going by the "marketing" values stated in the manual:
24V * 10Ah SLA battery => 240Wh energy
runtime of 50(~60)min => 288W or 12A motor draw

The BMS has a continuous discharge current of 25A, and peak discharge of 60A. Peak is probably allowed for only a second or two.

A motor typically needs 2x ~ 3x the current during startup, and lawn mowers typically need several seconds to spin up the heavy blade.
So the likely situation is that the motor starts up drawing in the region of 25A~60A, but then the BMS cuts off after the time allowed for peak discharge is exceeded.

Second Daromer re clamp meter, but it might be difficult to get good accurate reading for startup current.

Possible solution besides a bigger BMS:  motor speed controller.
No actual experience, but something like this should do.  Might require additional cooling.
https://www.aliexpress.com/item/4001014716980.html



edit: Doh! simpler calculation:  10Ah battery / (50/60)h => 12A
Modular PowerShelf using 3D printed packs.  60kWh and growing.
https://secondlifestorage.com/showthread.php?tid=6458
Reply
#10
On further thought, I think the motor speed controller could be awesome. There are of course some losses in the circuitry, but you can probably reduce the motor speed to 80% or so and still get a good enough cutting performance.
Also, did you know that you can sharpen the blades with a file? It's probably dinged up pretty bad.
Modular PowerShelf using 3D printed packs.  60kWh and growing.
https://secondlifestorage.com/showthread.php?tid=6458
Reply


Forum Jump:


Users browsing this thread: 1 Guest(s)