10 basic questions about batteries and matching to motors

[egosphere]

I have a motor/controller combo that's rated 48V 750W 25A (it's the BBS02).

Q1: Why don't these numbers match up? 48V * 25A = 1,200W =/= 750W. What is the physical meaning of each of these numbers? Does the number relate to the motor, controller, or both and how?

I have a battery pack that is a 13S4P combination of Panasonic NCR18650PF cells. Each cell is rated 3.6V, 2.9Ah, 2-3C (anyone confirm these ratings?). So the nominal voltage of the pack is 46.8V and capacity is 11.6Ah.

Q2: The voltage rating of the motor is 48V but the nominal voltage of the pack is 46.8, which means that more often than not the pack will be below 48V. What is the implication of this mismatch? Will the motor be (slightly) underpowered? I see some people put 3.6V cells in 14S configurations to get 50.4V -- will such packs lead to greater power being outputted from the motor? Or will these nominal-50.4V packs just work for a longer time before being cut-off for having too low voltage?

The discharge rating, I read, is given by multiplying capacity by C-rating. That'd be 23.2-34.8A.

Q3: Why is this a range?

Q4: As the Ah capacity of the battery decreases while it's being used, does the discharge current get correspondingly worse and worse until the pack cannot supply the current that the controller needs?

Q5: Suppose the discharge rating is at the lower end, 23.2A. The controller is rated for 25A. What is the implication of this mismatch?

Q6: The controller can require even more than 25A if, e.g., it's starting up a hill, right? What happens then? Can it kill the battery if it doesn't support this amount of draw?

Q7: What is the physical meaning of C-rating? It must have units of 1/hr to make the discharge rating equation work. Why do we use C-ratings? Most places just rate the discharge current of the cell. If you google this cell you find that it's rated for max continuous discharge of 10A but 10A/2.9Ah = 3.45 1/hr, which doesn't agree with 2-3C. What's up with that?

I understand why voltage imbalance between the parallel parts is undesirable because it may lead for premature cutoff. But I don't understand why it happens.

Q8: If one parallel part has higher voltage than another won't it charge the other back up toward equilibrium? Or is this something that the BMS specifically prevents in order not to cycle the cells and it does this through fancy circuitry that is beyond simple understanding? If so, why can't the BMS also prevent imbalance from happening to begin with? How does one make sure that the cells are balanced?

Finally, I want to know how to monitor my battery.

Q9: How can I check that the battery pack has the characteristics it is rated for with a multimeter? I can charge it and measure the voltage with the multimeter. What number should I look for? It will be higher than 46.8 when fully charged right? Then I can run the motor and check the current running through right? What number should I look for if I'm running the wheel in the air? 25A? How can I check the heavy-load current? How do I check the Ah capacity rating?

Q10: How can I check that the pack consists of high quality, authentic Panasonic cells? It's inside one of those fancy frame mounted containers with the USB output etc.

Thank you for the help!

 

[teslanv]

First piece of advice: Ask one question at a time. :wink:

Secondly, Motors are typically rated for "Continuous" wattage. So that "750W" BBS02 motor should be able to handle 750W continuously, or MORE for short periods of time.

If you want to know the MAXIMUM wattage of a system, multiply the battery's Maximum voltage (for 14S Lithium, that would be 58.8V) and the maximum current of the controller (25A) = 1470W. Now of course you WILL have voltage drop from your battery if you are loading it with 25A of current, so realistically, it more like 54V x 25A = 1350W of power, when the battery is fully charged.

As for the discharging process, the controller can demand a constant amperage from the battery. however the voltage of the battery will gradually reduce, until it hits the Low Voltage Cut-off (LVC) at which point the BMS or the controller should cut off power to protect the battery.

A higher voltage of battery will provide more power, assuming it can output the current demanded by the controller. Some batteries are more capable of outputting higher current than others.

 

[dnmun]

13S of lithium ion charges to 54.6V but the nominal voltage ratings are holdover from when people used 12V SLA batteries in the past. that is also why the controller had a LVC to protect from over discharging the lead acid batteries in the past. no need for them now with batteries having a BMS to protect them from over discharge.

 

[999zip999]

Parallel cells act as one cell but only as strong as it's weakest cell ( simple answer ) or the cell with most I.R. and lease capacity.
What's the lvc of your controller ? As you could use a 24v controller with that motor.
Panasonic cells as all cells depends on your manufacturer and the builders Q.Q. ect.
Where did you get your pack ?

 

[wesnewell]

You don't match batteries to motors. You match batteries to the motors controller. The controller is what draws power from the battery, not the motor. The motor could be 10W or 10,000W and the amount of power draw from the battery still depends on the controller.

 

[wesnewell]

1. The watt rating of both the motor and controller are the max watt rating for continuous operation. It has nothing to do with the actual max watt rating either can deliver. For the controller, that's determined by the voltage times the max amp rating of the controller. For instance, a controller with a 20A max rating at 50V is 1000W. the same controller at 100V would be 2000W. For the motor, there is no limit, except the point where you burn it up. You can exceed the max watt rating of most motors by 5-10 times for shorter periods of time.
2. Voltage ratings on motors are pretty much meaningless within reason. The only thing relevant is the watt rating. And that's only relevant for continuous operation. Don't worry about it.
3. Controllers have a low voltage cutoff (LVC) and some have a high voltage cutoff (HVC). They also have high voltage limits determined by the caps and fets used. Typical 48V controllers have a LVC of 42V and are limited to 63V max by the caps used.
4. As battery voltage drops, so does the max power that can be drawn from it. VA=W, so lower either voltage or amps and power drops.
5. None. Nominal C rate remains constant during battery discharge. If the battery is rated fro a 30A draw, you can draw 30A until it's empty. You just get less watts because of voltage drop.
6. The controller can not draw more amps than the max rating. If you need more power, you'll have to pedal harder, modify the controller for more amps, or get another controller.
7. For all practical purposes it determines the max power rating. Multiply C rating times AH rating to get max amp rating. Voltage doesn't matter, A 5ah 20C pack is rated at 100A at 1s or 24s. Voltage imbalance can't happen between paralleled cells, only cells in series.
8. Yes. That's why they won't imbalance. Lot's of different ways to monitor a battery pack. While riding I use a voltmeter for rc lipo.
9. For your pack type a simple voltmeter will work fine. If your controller has a LVC of 42V it will cutoff before you over discharge the pack.
10. Open it up and look. And that won't tell you if they're authentic. If the pack performs as expected, I wouldn't worry about it.

 

[John in CR]

Unless you're confident that you'll be satisfied with performance of a fit cyclist, invest in more powerful batteries, motor, and controller than you need initially, and get a programmable controller capable of handling a wide range of voltages such as 36-72. The less stressed your system, the more efficient and durable it will be, and it gives you the flexibility to improve your performance for little or no cost. With good shopping, and don't be afraid to buy used items from established forum members, it's not hard to get a flexible system capable of greater performance than you imagine. If you're going with a hubmotor, then get one with the smallest diameter wheel you can live with.