Battery pack doesn't balanced.

[hadoo]

Hellow,
I'm Working on a battery pack for my project. Is id Diver Propulsion Vehicle. After some try and errors, I found that i must use best quality products on the market. Therefore I selected original panasonic 18650 cells, and Reap System BMS http://www.reapsystems.co.uk. But after some charging and discharging cycles, cells become totally unbalanced. I tried to balanced them by the BMS, but was unsuccessful, I did it manually. But again after couple of charging-discharging cycles it becomes considerably unbalanced.
Anyone had this problem before. have I make any mistake?

- Hadoo

 

[hadoo]

Hello,
Does any one know the real efficiency of the E-bikes? I mean the efficiency of energy came out from batteries and the energy which Hull drag force and rolling resistance consumes.
I expected that it is about 80% because the motors are Brushless Dc, but some company revealed their test results that it is about 35%? any one can confirms?

- Hadoo

 

[heavymetalthunder]

I think you answered your own question. As there are too many variables to the equation there is no one answer.

 

[999zip999]

Panasonic cells ? What cells Panasonic makes more than one cell some good enough for ebikes. Where did they come from.

 

[999zip999]

Usually people never leave on charger for bms to have time to balance, maybe days for your battery to balance. Did start with a balance battery. Does your charger click back on after left on charger for balancing. Time on charger for balancing. Do you have the right charger ? Deep discharge imbalances the battery.

 

[amberwolf]

Does any one know the real efficiency of the E-bikes?

http://www.ebikes.ca/tools/simulator.html

 

[hadoo]

Dear Heavymetalthunder,
Yes I understand that there are a lots of parameters that reduces the efficiency. But as what I understand from http://lianinno.com/electric-motorcycle-optimization/ they has tested some models of E-bikes on the markets and discovered the the main source of low efficiency is the Electric motor!!!
Thats what confuses me.

 

[hadoo]

Dear 999zip999,
Yes I have the right charger. And because I selected one of the best BMS on the market i'm sure that there is no deep discharge. But some where on the net (http://lianinno.com/custom-lithium-ion-battery-pack/ ) I have discovered that the cells (even the best quality ones) are not equal on capacity and internal resistances, when we buy them from the market. Is it right?

 

[Gregory]

Is the voltage output from your charger high enough to start the BMS balancing?

 

[hadoo]

Yes, BMS report balancing. But the cells voltage may differ up to 0.2 v . and the Balancing current is just couple of hundreds mili volts.

 

[dogman dan]

Even the best cells may benefit from sorting before pack assembly, removing the more obvious high resistance cells.

You did not mention discharge rate, if you are hammering the pack with a high enough discharge rate it can easily result in a badly unbalanced pack every cycle.

 

[dogman dan]

They are pretty efficient at cruise speeds of 20 mph on the flat. (light load, high rpm) They are horribly inefficient when overloaded and going up too steep a hill. ( heavy load, low rpm)

This is all pretty clear when you play with that simulator awhile.

The endless variables encroach when you choose components unwisely, resulting in more time per trip spent in the inefficient operating rpm. The worst scenario is a small low wattage motor, high rpm winding, overloaded, and up steep hills with a 26" wheel.

In other words, The fat guy who doesn't pedal melts the 350 rpm 350w motor when he rides up a big hill on a regular bike.

As for the motorcycle place, sure, they can also choose optimal motors or not. And choosing wrong will result in inefficiency.

With bikes, sure, motors could be better, but not at this price point. And most e bike riders do pedal, aren't that fat, and don't ride in the mountains. So they never spend much time in that 35% efficiency rpm/high load situation. For them, an overall efficiency in the 70% ballpark works fine. In this case, improving efficiency to 90% doesn't result in an improvement that matters. For them, a small battery is still more than enough range. So saving 100 wh simply does not matter.

 

[amberwolf]

Yes I understand that there are a lots of parameters that reduces the efficiency.

Thats what confuses me.

http://www.ebikes.ca/tools/simulator.html

 

[hadoo]

which kind of sorting shall I do? would you please let me know more details?

 

[999zip999]

What Panasonic cells they make hundred different cells. It's takes a loooong time for bms to balance. Keep Longer on charger. Matching cells meaning matching capacity. Are all the water glasses the same size ? Capacity. Are they solder or spot welded ? Are you discharging to hard for the said Panasonic cells or to deep of discharge. NO deep discharge till fully balance. Deep discharge is bad for battery balance. What size pack ?

 

[hadoo]

it is 4p14s. yes I first fully charged them and then starts to use. I'm sure that I have not exceeding 1C discharge rate.

 

[999zip999]

WHAT BATTERY CELLS. model number. If you can't answer questions we have hard time going forward. Pencil and paper list voltage.
1. 4.15
2. 3.99
3. 3.98

14. 2.99 volt
Let's us see what you got. State of charge. So state it.

 

[hadoo]

Thanks a lot for your help
The battery model is GCR18650CC Panasonic.

Cells voltage (right now):
1- 4.16
2- 4.16
3- 4.13
4- 4.16
5- 4.16
6- 4.15
7- 4.16
8- 4.08
9- 4.08
10- 4.16
11- 4.16
12. 4.15
13- 4.16
14- 4.10

State of charge is: balancing.

 

[999zip999]

Bring up 3,8,9,14. To 4.16

 

[999zip999]

2,500mah 10a cell ? Just don't over charge a string of cells past 4.2v. What motor and controller ? State of charge is the voltage list 1thru 14.

 

[dogman dan]

What I meant by sort cells, was that before assembling a DIY pack, test each cell for internal resistance. Junk any obvious duds that are way different from the rest.

But IMO, your problem is nothing. If you NEED it balanced better, that would be because you are riding it till the bms clicks off each ride, which will result in a need to make your bms balance two or three times for the next ride. You might need to force the bms by doing some very shallow cycles to get the bms to fully balance it.

 

[izeman]

The battery model is GCR18650CC Panasonic.

where did you get that cell from? ihmo this is either a typo or fake. there is imho no GCR cell. it's CGR. and i don't know of CC cells. can you post a shopping link to it? datasheet? something like that to clearly identify the cell and it's capabilities.

if you buy genuine, unused cells with a high enough c-rate for you application, then they come at the very same voltage level - if they are good cells. you connect them to the bms, let them fully charge, and that's it. if you use the pack according to it's specs and don't let it discharge until it shuts off, there will be almost NO NEED to balance. and even if you would discharge very low, with high quality matched cells balancing would hardly be needed.

you didn't tell us how much amps you draw from the pack - max and sustained, and what the individual cell string's voltage was when it shut off.

 

[Eteck]

hadoo
Does any one know the real efficiency of the E-bikes?
I mean the efficiency of energy came out from batteries and the energy which Hull drag force and rolling resistance consumes.

WIth your definition of efficiency: 72% +- a significant amount depending on a range of factors.

I expected that it is about 80% because the motors are Brushless Dc

Electric motors by themselves are often around 80%. So that's usually going to be too high.

some company revealed their test results that it is about 35%? any one can confirms

By your definition 35% is way low.
If you are looking at TOTAL SYSTEM EFFICIENCY: producing and disposing the batteries, charging equipment losses, and a bunch of other miscellaneous losses, then the much larger macro system efficiency might indeed be that low.

For your efficiency question you need the motor efficiency, and data for these items:
http://www.engineeringtoolbox.com/rolling-friction-resistance-d_1303.html
http://www.engineeringtoolbox.com/drag-coefficient-d_627.html