Battery Response Time

[Just]

Hi all,

Is there a battery parameter named 'Battery Response Time'?

Accidentally I replace several LiFePo4 cells in my battery with the RC cells (used in the airplane and other toys modeling) and found that a response time (acceleration) of my bicycle increased twice or even more! It just became a rocket!!!!!!!!!!!!! 

My guess is so that these batteries have very good response time.  Their voltage drops very quickly (during acceleration), but they are recovering very quickly as well.

So, how can I know a battery response time before purchasing them? I went over several datasheets of the cells from several manufacturers, but did not found such parameter there...

Any comments? 

Thank you!

[Morgen 3Eman]

Hi Dmitryl,

As a long time acceleration  junkie, I think what you are experiencing is more current supplied to the motor. Ain't it great? 

Someone who understands these things will probably talk about battery "C".  I need to do my reading on this, as I am quite ignorant of its importance.   It has to do with a batteries  ability to discharge at high current rates. 

TTFN,
Dennis

[Just]

"Someone who understands these things will probably talk about battery "C"" - bad mood?

Surely it's "C" parameter, which is commonly used to represent the maximum discharge/charge rates - one of the most important parameters in the battery characterization! These cells have 30-40C rates!

Anyway, I wonder why a battery response time is not included in the battery specifications.

[Morgen 3Eman]

Hi Dmitryl,

No bad mood, I just don't like folks who talk like they understand things they do not, and that includes me.   The web needs less bloviating, and more helpful knowledge sharing, in my humble opinion.  Sorry if I annoyed you, but you asked the question, and it is in my nature to be helpful when I can. Even when that is only pointing where to look for information.  Thus my reference to "C"

You misunderstand my meaning.  I know what "C" represents, I just don't feel I understand it well enough to share my knowledge , or offer  specific instruction in any valuable way.  Apparently, you feel you understand "C" well enough. 

However, what you are calling "response time" is a ramification of a battery having a higher discharge rate.  If you have a battery that can discharge 100 Amps at full throttle, and replace it with a battery that can only discharge  10 Amps at full throttle,  you will reduce the rate at which your bike can accelerate. 

TTFN,
Dennis

[truly_bent]

The RC guys use LiPo battery cells. I believe they run them without battery management systems (BMS). LiPo's usually have much higher "C" or discharge rate than LiFeP04 cells. As you say, the LiPo's have 30C and 40C discharge rates, compared to LiFeP04 rates of around 2C. This means that LiPo's can produce much more current (amps) for a much shorter time.

For example, a 10AH LiFeP04 battery at 2C will provide 20A nominal. The same 10AH LiPo battery at 30C will happily provide 300A, if you can use it. If you're running a MPII, rated at 30A, there would be a noticeable boost in performance - not so much as the 300A figure would imply, but probably better than 50%.

The controller is rated at something like 45A. If not restricted, and under heavy load, the motor may draw as much as that, although I've noticed my motor seldom draws more than 30A, even climbing hills.

The downside to running LiPo cells, without BMS, is the potential for fireworks. Those RC boys keep a careful eye on their cell voltages, which is pretty much what a BMS does.

So you see, it's not a "battery response time" parameter, but a "current available" parameter, which we call "C".

Jeff

Dennis; what's "bloviating"?

[Just]

Morgen 3Eman: "I just don't like folks who talk like they understand things they do not" - hmm... and me don't like folks who think that others do not understand nothing... Let's not get involved into personality discussions...  Okay?

[Just]

Thank you Jeff! Now I understand this phenomena better. Thank you!

[Bikemad]

As you say, the LiPo's have 30C and 40C discharge rates, compared to LiFeP04 rates of around 2C.

Jeff, it really depends on what batteries you are comparing. I posted some details almost 16 months ago comparing two similar capacity LiPo and LiFePO₄ packs, and the discharge rates were (and still are) identical.
I have just noticed that the price for the LiPo pack is still exactly the same today as it was then, but the cost of the LiFePO4 pack has dramatically decreased to just 63% of what it cost 16 months ago.

Fortunately for us, battery technology continues to improve, and some of the more recent nano-tech packs are now rated for 65C continuous discharge and 130C bursts:

I find it hard to believe that the small 5Ah pack shown above is actually able to supply 300Amps continuously, albeit for around one minute, or a mere 650Amps for just a few seconds!
It can even be recharged it at a rate of 8C, which equates to a charging current of 40 Amps!!

Isn't technology wonderful?

Alan
 

[Morgen 3Eman]

Hi Jeff,

I think of it as a nice way of saying BS'ing.  But Webster says:

bloviating : present participle of blo·vi·ate
Verb:   Talk at length, esp. in an inflated or empty way.
More info »Merriam-Webster - The Free Dictionary

TTFN,
Dennis

And yeah, as I noted, I'm guilty too.  

But I'm still very good looking on the internet....Really, I am....

[truly_bent]

650 Amps.
Just think of the possibilities:

   1) Drag racing electric motorcycles
   2) Design a mini plasma cutter
   3) Tack weld the Queen Mary to dry-dock
   4) ...

The list is endless.