Distance verification question cuz I can't believe my eyes

[e-beach]

Today was a good day because I got my e-bike back on the road with my new 36v 15ah Headway LiFePo4 pack.

The wiring harness is not fully complete, and I need to get my aluminum side covers, but it was ready enough for a test ride. So I took an nice ride along the beach in 78 degree weather. Perfect beach weather. The views and the scantly dressed pretty girls are always nice to see. There was a slight breeze in my face going and at my back heading home. All-in-all a good day.

Now, I have to say that because of all the beach traffic on the bike paths I was not able to go very fast for about 12 to13 miles of the ride. Just too many people on the bike path. So most of that 12 - 13 miles I wasn't pulling much more then 10 amps. A lot of the time I was pulling between 4 and 8 amps. My motor seems very efficient at its highist rpm's so when I go above 15mph or so on the flats I still was still only pulling 7-10 amps. When I got home and calculated that I went 21.51 miles the confusion started to set in. :?

Anyway here are the numbers that I need help with:

12 cell, 36v, 15ah LiFePo4 pack.
I started with a voltage of 39.9 under no load.
I road 21.51 miles.
the last .2 miles was up a 10 percent grade and I was pulling 30 amps.
The voltage under the 30 amp load was 34.4 volts.

My bms may have a low voltage cut off of 2.05 volts if I am reading the technical index correctly.
It reads: "Cell Over Discharge VPOC 1.95~2.05 V"

I guess the BMS won't cutoff until the cells get down to almost 2 volts each.
But, my controller low voltage cutoff is 28 volts. That would leave the cells at about 2.33 volts each at cutoff.

All this considered, here is my question:

If I can go 21.51 miles on the 5.5 volts drained from my pack, and have 6.4 (under load) volts left before my controller cuts out, could I then, given the same riding conditions, go another 25 miles?

5.5 volts / 21.51 miles = .2556 volts per mile.
With 6.4 volts remaining before controller cutoff then: 25 miles X .2556 volt per mile = 6.39 volts.

Can it be true the this pack can go 46 miles????

If you want to see all my BMS specs they are here:
http://stores.headway-headquarters....bms,-signalab/Detail.bok?category=BMS/PCM/PCB

Any help would be appreciated!!! Thanks!!!

PS. itchynackers, did you 48 miles the other day? :lol:

http://endless-sphere.com/forums/viewtopic.php?f=3&t=42206

 

[TylerDurden]

Voltage drop is not linear. A better method of estimating range is measuring Wh consumed.

http://www.ebikes.ca/drainbrain.shtml


If you don't have an Wh meter on the bike, you can use a "kill-a-watt" type of meter on your charger's AC cord.

http://www.amazon.com/P3-International-P4460-Electricity-Monitor/dp/B000RGF29Q/ref=pd_cp_hi_0

The Wh you put back into the battery can tell you (approximately) your average energy consumption.


Your pack has roughly 540Wh capacity. You don't want to drain it all the way, so say you can use 430Wh.

At 200W (5.5A), ~15mph... you can go 2.15hrs... 32.25mi.

You can also ride the bike until LVC and get a direct measurement.

 

[Rassy]

You have confused me by relating distance remaining to voltage remaining. LiFePO4 has a very flat discharge curve when looking at voltage, and at the end drops off fast.

With LiFePO4 you need an AH meter (just a little more complex than a simple voltage meter). A 15AH battery should provide about 15AH before running out of juice. In other words, if you used a constant 10 amps you could ride for an hour and a half before you ran out of juice, or about 15 miles. Since you don't use a constant number of amps, you need an AH meter as a fuel gauge.

I see Tyler has provided the same info in a slightly (better?) different way. Just ask some questions if we confuse you.

 

[Russell]

Your basic idea about trying to calculate the remaining capacity from the battery voltage is possible however to do it you'd need detailed information on the battery pack or perhaps detailed data from historical rides you've done. For example here are some plots for generic LiFePO4 cells;




You stated your pack voltage hit 34.4V or 2.87V/cell under a 2C (30A) load so if your pack was made of the cells represented in this graph you could look at the 2C discharge plot and see that the voltage would hit 2.87V at just over 60% of the rated capacity. At about 90% of rated capacity the voltage under a heavy load will dip significantly and it's generally a good idea not to exceed 80% so in this example you might have about 7 to 10 miles of range left.

Guessing isn't the best way to go so if you really want to know your range get yourself an inline wattmeter, they're available for as little as $25, that displays the actual energy used.

-R

 

[Russell]

If you don't have an Wh meter on the bike, you can use a "kill-a-watt" type of meter on your charger's AC cord.

With a Killawatt meter you would have to guesstimate the efficiency of the charger (perhaps 80%). If an inline wattmeter is not carried on the bike then simply install one betwen the charger and the battery, that'll give you closer to actual power consumption.

-R