Lithium battery question

twilloug

10 mW
Joined
Dec 15, 2019
Messages
21
I have an older Scoot-e scooter. 24v scooter. I wad having issues with motor so i decided to buy a new 500w motor off of ebay as well as a controller. Also purchased a new throttle. Now for the question...the battery is a 7s4p 24v battery from china. It measures 27.5 volts, so it is charged. However when i connect directly to motor, nothing happens. Is my battery not big enough for motor?
 
Good lithium batteries have a BMS board that limits the maximum current the battery can deliver. A standstill motor has a huge inrush current at startup and that might trigger the BMS to disconnect the battery. The controller will drive the motor with a PWM signal. It rapidly turns the power on and off so that the motor gets a lower average voltage and will consume a lower current. It might be worth connecting the controller and throttle to it to see if you can engage the motor with that. It's likely you will need to connect some brake contacts as well before the controller will start engaging the motor. The controller should also limit the maximum current demand and might keep the battery BMS happy with that. Hard to tell without testing.
 
Thanks for the quick reply. I have two throttles, 2 controllers and one motor. I actually sent the first motor back to seller thinking the motor was bad. They tested and it worked. None of the combinations of throttle, controller, motor worked. I even tried start the rear wheel spinning to see if that worked. The battery does have bms on it.

The controller also had a connection on it that needed to be " completed" for it to function. I did this as well for both controllers i have and still no go.

Would a battery that reads 27v be bad? Bms does not look bad. Ways of testing battery?

With the 7s4p battery, is my motor pulling too many amps kicking in the bms. Not sure how that system works. How do i know max current vattery can provide? Controllers are 30 amp.

Thx for your help.
 
Fully charged, a 7s battery should be around 29 volts. At 27 volts it is near the low voltage cutoff. Is that as high as you can get it to charge?
 
What kind of motor do you have? Normally a motor for this kind of use is a 3 phase motor, and connecting a battery directly to any of the phases will only lead to a quickly fried motor, not a spinning motor.
 
Very likely it is the battery. Even if it could charge to full, it may still be old enough to not hold voltage when a load is put on it. Then the bms shuts off instantly.

Do look hard though, at the problem that is the cause of 90% of it won't go problems, the wiring and connectors. Look particularly for connectors that have backed out of the housings, or are not secured in the housings. When you plug in, the contact moves and never connects. Looks like it, but it isn't.
 
You could take 2 12V 35W halogene spots and connect them in serie.
If you connect them to the battery, it should form a good cheap load to see if the battery is in healthy condition.
35W should give a load of 3 Amps. 50W halogene spots would form a load of 4.2 Amps.
With that amount of current, an unloaded motor should at least start to rotate.
Just make sure both lamps are the same power. Don't mix a 35w lamp with a 50W lamp.
 
Another test would be to measure the battery voltage and at the same time apply the load to see if the voltage is dropping.
 
I guess I'm having a little trouble following along... Has the motor ran at all? Even during controller self testing?

A lot of speculation and guessing would be eased if you would be kind enough to give the particulars of your systems components...
(battery, controller, motor and throttle... links, makes and models, ETC.) And more pertinent suggestions can be offered.


Regards,
T.C.
 
So digging in to the battery a little further and it appears the bms might be the issue. Found a black spot on the board. It appears the max current for bms is 15 amps. Thinking the old motor on the scooter drew more than this. The new controller that i have is 30 amp, so i need a replacement bms. Any ideas best one to get. HY P7c25 is the current one.
 

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twilloug said:
So digging in to the battery a little further and it appears the bms might be the issue. Found a black spot on the board. It appears the max current for bms is 20amps. Thinking the old motor on the scooter drew more than this. The new controller that i have is 30 amp, so i need a replacement bms. Any ideas best one to get
I decided not to use a BMS because i don't trust them. And as far as a smart bluetooth BMS, too much money for a 7S 24V Scooter. Doubt there is even a smart bluetooth BMS for a 24V Scooter. So, not too many choices as most 24V BMS' are $15 at most.

As far as the best BMS ... "You get what you pay!" so don't buy a cheap $15 Chinese BMS, but maybe that's all that's available. If i had a 24V Scooter (assume that's what you have) would buy this one ... https://vruzend.com/product/24v-7s-battery-management-system-bms/ ... as my preference if it was for my grandson. From a supplier that stands behind their product ... even though it's made in China with India go-between and maybe the same thing at one-third the price from Aliexpress. FWIW a 20A controller should be sufficient (JMO) unless the rider is abusive and then a 30A may not be rugged enuf to take excessive physical punishment..

In the end (24V Scooter) you may just decide to take your chances and buy one of the following 4 choices ... https://www.amazon.com/Protection-Charging-Controller-Balancing-Function/dp/B07DMTSZQ5 ... Good Luck !
 
The motor is a Currie Technologies 24v 500w motor xyd-6b. Rated current 28amps. Controller rated for 30 amps.

That is why i think i need a 30amp bms in lieu of the 15amp bms that appears to have failed.

I will take a look at the shortcuts you sent.
 
Just confirmed the motor works. Briefly direct connect battery to motor and it works. So pretty sure bms is problem. So now i need to replace bms.
 
Are you sure your cells can handle a 30 Amps discharge rate.
I seem to remember it's a 4P which means only 4 cells in parallel.
Assuming the cells are 2500mAh. If you pull 20A they will be empty after a 30 min. ride.
 
twilloug said:
Just confirmed the motor works. Briefly direct connect battery to motor and it works. So pretty sure bms is problem. So now i need to replace bms.
You possibly should have started another thread as your question is about a 24V BMS ... not a lithium battery. Your post could give one the impression the purpose of your scooter is for very aggressive use with your 30A controller ... (but only 500W motor rated at 28 amps).

How fast is fast enuf? Does your DIY 7S4P battery have Sony VTC6 20A high drain cells for aggressive scooting if that's your intention? What brand cells and rated amperage (e.g. 2.75A, 5A, 10A, 20A) of your 7S4P 18650 Li-ion battery?

Getting back to this inexpensive BMS ... https://www.amazon.com/Protection-Charging-Controller-Balancing-Function/dp/B07DMTSZQ5 ... do you believe the following and if so does it require you to leave the charger plugged in after the green light (fully charged) comes on ... and if so for how long before unplugging your CCCV charger? ...
  • BALANCING FUNCTION: This battery protection module has a equilibrium function to keep each cell in balance and can extend the service life of battery.
Are you planning to adjust the charger cut-off voltage to 4.1V instead of fully charging?
 
The charger charges the battery to 29.4 volts. Not concerned about speed just that the scooter runs for awhile. Right now i have a new motor controller and throttle. Battery is fairly new as well ...ebay purchase.

If the 7s4p is not big enough to operate the scooter, can i buy a 2nd battery and put it in parallel to get more run time?
 
Sorry for all the questions but i think i am missing something. I believe the existing bms failed because it was only rated for 15 amps. So if i am going to replace it, wouldnt i want a 30 amp bms to match the 28 amp rated motor and 30 amp controller? I see a lot of 20 amp bms but wouldnt that put me in the same boat? Thanks for all your help.
 
A 20 Amps BMS will limit the current to 20 Amps.
It usually does this by switching off the battery output voltage when there is 2 much current asked.
Sometimes you need to connect a charger to the battery to bring a BMS that went into current protection back to life.
So, you don't want it to go into current protection during a ride.

Connecting 2 batteries in parallel is possible but a bit tricky. The battery voltages lowers when a battery discharges. You want both batteries at the same voltage before you connect them in parallel or you can get nasty sparks and you will lower your battery life.

You could provide a switch and connect the second battery when the first is empty. It all depends upon how often you will need the higher capacity.

It's also important that you check if it's safe to draw 20 - 30 amps from your battery. The fact the BMS was only rated 4 15A could mean you shouldn't load your cell much higher. Just check the brand and type of your battery cells and see how many cells are connected in parallel. Check the datasheet of your cells and you know what your safe discharge rate is. Multiply this with your number of parallel cells and you have your total allowed discharge rate.
 
twilloug said:
Wouldnt i want a 30 amp bms to match the 28 amp rated motor and 30 amp controller?
I see a lot of 20 amp bms but wouldnt that put me in the same boat?
Not if you have a reliable 20A BMS and healthy battery.
Are you sure your battery is AOK?
BMS and controller don't need to be rated at more than your 28A 500W motor.
 
obcd said:
A 20 Amps BMS will limit the current to 20 Amps.
It usually does this by switching off the battery output voltage when there is 2 much current asked.
Some are like switches or fuses, no protection just fail.

 
eMark said:
twilloug said:
Wouldnt i want a 30 amp bms to match the 28 amp rated motor and 30 amp controller?
I see a lot of 20 amp bms but wouldnt that put me in the same boat?
Not if you have a reliable 20A BMS and healthy battery.
Are you sure your battery is AOK?
BMS and controller don't need to be rated at more than your 28A 500W motor.
Dead wrong!

Any BMS given a max current rating, should only be used where the maximum possible current will be **significantly** lower.

The safety margin should be 80/20% with even a top notch BMS costing hundreds. Just like wiring gauge, switches, DC converters, fuses etc

Cheap Chinese stuff is often wildly "optimistic" in its ratings, sometimes 50% is required.

Another approach is to just use the BMS for voltage sensing, use a fuse for OCP.


 
If the controller is rated for 28A, you would need at least a 30A BMS. A 30A unit is supposed to handle 30A all day and will trip when the current goes somewhat over 30A (this should be listed in the specifications). If the BMS is rated for way more, like 50A, it won't be a problem. The controller will keep the current below 28A unless something shorts out, in which case the BMS should trip.

The cells in your battery may not be happy with 28A though. They will sag a lot and overheat if they are not rated for that much current. A larger pack may be needed.

You could place another pack in parallel but it gets tricky maintaining the proper protections with the BMS.
 
fechter said:
If the controller is rated for 28A, you would need at least a 30A BMS. A 30A unit is supposed to handle 30A all day and will trip when the current goes somewhat over 30A (this should be listed in the specifications). If the BMS is rated for way more, like 50A, it won't be a problem. The controller will keep the current below 28A unless something shorts out, in which case the BMS should trip.
His controller is 30A while motor rating is 28A. He may only have needed a reliable 20A controller and the "best" 20A BMS for his 28A 500W motor. His old defective 15A BMS is a HY P7c25 c may be similar or same as this one (now $8.43) from Ali ... https://www.aliexpress.com/item/32848328139.html ... Wouldn't a true 20A BMS and 20A controller be sufficient for his 28A 500W motor, but water over the dam already having a 30A controller? If Aliexpress (China) actually overrates its BMSs then, "YES" it's best he bought a questionable 30A rated BMS and 30A rated controller. It's possible most all AliExpress BMSs are overrated (has a 7S 24V battery for powering his 28A 500W motor) ...
twilloug said:
Currie Technologies 24v 500w motor xyd-6b. Rated current 28amps. Controller rated for 30 amps.
So, if we follow john's logic than maybe even the 28A 500W motor is over-rated (depending on who's doing the test.
All twilloug wanted to know is what reliable (best) BMS you guys would recommend for his application. My two recommendations didn't get any flack from john... which was a refreshing change :D or possibly he didn't see it.

john61ct impressed us with what he knows/thinks is good info for twilloug to consider before deciding the best BMS for his ebiking application. Thanks to john's explicit post twilloug and others are now even more informed when it comes to choosing the best BMS for their ebiking application ...
john61ct said:
Any BMS given a max current rating, should only be used where the maximum possible current will be **significantly** lower.

The safety margin should be 80/20% with even a top notch BMS costing hundreds. Just like wiring gauge, switches, DC converters, fuses etc

Cheap Chinese stuff is often wildly "optimistic" in its ratings, sometimes 50% is required.

Another approach is to just use the BMS for voltage sensing, use a fuse for OCP.
Sounds like john61ct and fechter are both in agreement that you need get a 30A BMS as you already have a 30A controller for your 7S 24V battery, and get their wise rationale. What about this 30A for your 7S 24V batt toery for $14.40 from Ali ... https://www.aliexpress.com/item/32879269241.html?spm=a2g0o.detail.1000014.9.18752c52XtxN3D&gps-id=pcDetailBottomMoreOtherSeller&scm=1007.13338.128125.0&scm_id=1007.13338.128125.0&scm-url=1007.13338.128125.0&pvid=4210cce2-8dae-4041-b27a-c7705b4050ce ... they may have even a better "best" suggestion.
 
The motor rating is irrelevant in this case. It only determines whether the motor will get hot or not. The controller current limit is what counts.

For something like that I would be looking for a 45A or more BMS. They are quite inexpensive if you avoid the Bluetooth ones. Under $20 shipped.

FWIW, I am running a 45A BMS on my Sur-ron secondary pack at nearly double the current rating. I beefed up the copper in spots and modified the current shunt, but otherwise the stock unit.
 
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