What percentage of power taken from a batteries pack is going to the load ?
Why are your batteries getting so hot so fast , here is a possible answer
Very suprising !
Was testing some very high capacity NiHm "D" cells 12-15 A/H from china and little socked at 12-18 milliohms readings
averaging about 15 milliohms.
If when assembled into a 48 volts pack (40 cells) would give 600 millohms add another 10% for interconnections
and we are up to 660 milliohms or .66 ohms .Doing a 36 volts or 24 volts pack , the same math applied .
Taking a 48 volts 500 watts motor has a internal resistance of 4.6 ohms .
When the power pack is driving the motor at standart 500 watts load the motor is only getting 87% of
the power coming out of the battery the remaining 13% is going to heat the batteries up .
Now picture yourself accelerating and the load goes up to 1500 peak power load impedance dropped to .833 ohms
[i]Now the percentage to motor is 55% and battery internal loss is 45% so effectively you are using 1/2 the power
coming out of the batteries and the other half to cook your batteries .[/i]Just thoughts that it may be of interest , the low impedance of cell is not only beneficial for peak power surge
but will also prevent damage to the batteries by heat generation .
As mentioned before impedance is king even before amp/hour or volts ,since you can have all the volts you want , but if the internal impedance is hight the net result is cooked batteries and little power to load
The math is the same regardless of batteries chemistries , just the number of cells are differents , ounce you have a impedance reading of a cell the rest is very clear as to efficiency , peak power etc..
Do not understand the tremedous emphasis on amp/hr , cost , weight , size , # cycles etc ... and the complete disregards on impedance measurement , it is the whole key to efficient power transfer both charging and discharging .
Just my two cents worth ! ! !
So you have a controller that is lossless? Where do I get a controller that puts 100% of the power coming out of the batteries into the motor?
You forgot the controller takes some of that power as well. And with the caps in the controller, they take some of the brunt of the quick current surges... helping your batteries out. 13% is a calculated number, not measured.
Most batteries (not lifepo) do better when warm. Sure, there's power lost in heating them, but did you know that power output actually increases with temperature? Go look at a battery curve for some of the batteries you mentioned. The racers preheat their batteries... especially in the electrathon races. We cooked our batteries and went 35miles on 60V 18Ah pack.
Your calculations are somewhat misleading. The CONTROLLER is only getting 87% of the power, then you have another loss from controller. Then loss in the motor. It would be good to calculate the battery power, then measure it, measure the controller power, motor power and draw an actual real world curve... not some calculations on a spreadsheet.
Your calcs aren't real world, but they are things to consider. Lower resistance cells are better.
1986 Honda VFR Conversion
As I have stated in other post the impedance of a cell is a good indicator of the cells overall performance but it does not answer the questions most of the members here are asking. "How far can I go?" to answer this question you must still rely on Amp Hours.
When comparing batteries of the same type, i.e. Lead Acid, Ni Cad, Nm Hi, or Lithium, The impedance for a given cell and brand regardless of type is what it is. In other words, if I have 3 12-volt 12AH Type-X, Brand-Y batteries and I want to know how far I can go on 12AH verses 20AH impedance does not answer this question. Generally speaking the impedance for Type-X, Brand-Y will be very close and the real difference is the AH. If I can go 12 mile on 12AH then I should be able to go 20 miles on 20AH given the type and brand are the same.
If I had a battery pack (A) with a total impedance of 100 milliohms and a battery pack (B) with a total impedance of 500 milliohms which pack will go the farthest? You can not answer that question because impedance is a performance measurement not a measurement of capacity. It could be that the A will go the farthest or B depending on the capacity of the pack not the impedance. I hope this answers why we look at AH more than impedance.
Grandpa Chas S.
Thank for mentioning the controller , you are absolutly correct , should have mentioned it ,reason that I did not .
I was thinking of on off state where a switch would connect the batteries to the motor .Have not done much research on controller efficient , would like to believe that it is in the high 90 % whether it is 92% or 98% will depend on the wattage of the unit .
Have difficulty picturing a 75% efficiency controller driving 500 watts or higher watts load .
As far as I known controller work in PWM mode , fairly efficient ! no analog mode .
The main point I am trying to convey ,if only the same amount of attention is paid to impedance or resistance of cell as amp/hour and voltage, then we would not have the failure rate of batteries that is so commmonly mentioned on all the web sites .Heat is the number 1 reson for failure in batteries , controller or whatever , and heat or loss is a direct result and function of resistance X current ............
HEAT & LOSS & INEFFICIENCY RELATED DIRECTLY to IMPEDANCE
Hello Grandpa Chas S.
A very good point ! , I am not saying to disregard A/H , just that the impedance is just as important if not more !!!
Let use those numbers for example at 500 watt 48 volts you current is 10.41 A ( let assume no loss in the controller )the batteries loss with 100 milliohm is 10.85 W ( I* X R) or about 2& of the load , very respectable figure .In the other case of 500 milliohms is 54.18 W or over 10% of the load .
Now increase the current for acceleration and the number become 97.53 watts with the 100 milliohms compared to 487.65 watts for 500 milliohms pack .where would you rather dissipated the extra 400 watts in your motor or your batteries ?.
In real life the number would not be as bad , but you get the idea .
I always think of 1000 volts 1,000 amp batteries pack capable of 1,000,000 wh a fantastic power pack for light or running a cell phone or radio for the next century , but useless to power a car unless the impedance allow it .This is a extreme example to make a point .There is tremendous amount of work being done to lower impedance of cells .Yokogawa recently can out with AC impedance meter specially for batteries measurement capable of sweeping frequencies from 100 to over 1MH , they getting serious money ( over $12,000 last time I heard)
Have not seem Yokogawa barking up the wrong tree often , has anyone ? .
Why is the A123 Cells so popular ? , surely not for it high A/H ,measly 2.3 A/H ,but because of its extremely low impedance for a LifePo4.
HEAT & LOSS & INEFFICIENCY RELATED DIRECTLY to IMPEDANCE
They all seem to hover around 98% efficiency regardless of size.
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