how to measure the amp-hours (Ah) of a lifepo4 battery

[Snookoo]

Hi all!
I recently purchased a 48V 20Ah lifepo4 and a 1000W 48V kit. It performs very well but the battery is low way too early. I get about maybe less than 20 kms or so.
I have the strong suspicion that the lifepo4 (I got it from ebay) does supply much less Ah than the 20 Ah stated.

Now I want to measure the real Ah of the battery to see if I'm right and eventually notify the seller...

Does anyone know the easiest way to do measure that?

I thought like this:
1. find out how much watts the motor sucks in idle running (should be 69 Watts at 48V according to that 48V1000W motor test table I read)
2. see how long the motor runs with one fully charged battery at full speed with wheel in air (= idle run)
3. Then calculating the Ah:  "hours  * watts  =  watthours"  and  "watthours / volt = amp-hours"

Ok?

Doet the fact that my battery has 52 V on full charge and about 45V at cutoff distort that calculation much?

Thanks for help!

PS: Of course I know that a lifepo4 battery needs to break in and needs many cycles to unfolds its full capacity but I think 20km instead of the stated 60-80 is way out of range.....

[Mel in HI]

Draining a Lifepo4 battery fully in the first few cycles is a sure fire way to diminish it's capacity if not make it fail entirely. 

[Snookoo]

hey mel in hi! there's a BMS on the battery and a voltage cutoff in the controller that prevents the batt from full discharge.

[gagnonjf]

That does not sound right. I also got a LiFePo4 from eBay. Mine is from Ping. After a full charge the voltage is around 59v When I ride  at full power it goes down to 49 then 48 then 47 and so on. I fully drained it once after 40 KM at full power but the battery was brand new so not broken in yet. The voltage dropped quickly from 40 something to 30 volts and the BMS shut everything down in an instant. I suspect the range should be around 50KM after a few more cycles. If you do the math. I consume around 1000 watt when I ride. It goes to 1500 uphill and 800, 900 when cruzing so I think an average of 1000 is fair. My speed is about 40km/h (I'm a big guy). 40km in an hour at 1000 watt is 1kw that's what the battery will supply. 20 a/h at 48v is 960 watts. So my battery is delivering around 1 kw and it's stated capacity is 1kw ... if you're getting 20km and you consume around 1000 when riding then your getting 50% capacity out of your battery.  Most LiFePo4 manufacturer state that you should get close to 100% after 5 cycles. So if you're not there send it back.

[biohazardman]

I get 32km from my 10ah battery with a combined bike and rider weight of 260lbs.  Your battery is not performing as it should even for a new one.  Go for a ride with a voltmeter connected and keep an eye on things.  Stop and check individual battery voltages as well as for heating of batts, wires and or connections.  I thought I had a battery problem recently and it turned out to be a connection problem. Heat can tell you allot of things.  One needs to be careful where one gets the lifepo4 batteries as all are definitely not equal.  Ping has a good reputation as does Zane at Foxpower.  I have 1700 miles on my pack from Foxpower and its just a little better than when it was new. Good luck in tracking the problem down.

[Snookoo]

thanks for your answers.
I got me a speedometer and actually I get 34 km, not 20. Which still is way too little for a 20Ah batt.

I've measured the amps with a multimeter (at full speed with wheel in air), it's 1,45 ampere. Then I measured the time how long the motor runs with wheel in air with a fully charged batt. It went for exactly 9,1833 hours.
That's 1,48 A * 9,1833 h = 13,59 Ah.
So that's definitely not a 20Ah battery.

I bought the battery from cammy_cc from ebay. I hope I can solve this problem with him.
I'll let you know how it turns out...

Greetings
Nicolai

[Snookoo]

Hi again... Well this is how it turned out:
The seller stated that he accidently sent the wrong battery (15ah) and finally after several eMails to and from I got a 40% refund from original price. (payed 424,32 EU and got back 171,56 EU). So for 252,76 EU it's really a bang for the buck in the end.

offtopic concerning 500W/1000W I have to say that I think 500W version is a better deal than the 1000W version as it doesn't eat so much current and thus will give bigger range (with 48V 15Ah my 1000w lasts just about 35km now with quiet some pedaling but going top speed mostly). So now I went for a 48V 350W nine continental kit which is probably better for this battery. Also it's much smaller and lighter than the goldenmotor.
Still I love my goldenmotor and I might get a new real 48V 20Ah battery for it, still not sure if I really should go for a kinda expensive ping battery or try another cheap ebay seller.... i'll see....
greetings
nicolai

[e-lmer]

From my experience in model aircraft I can refute your statement.

If you increase the voltage, you reduce the amperage required to get
the same power.

If you are drawing 500 watts from a 500Watt 36V system, you have
(loosely) 500/36 is about 14 amps.

If you are drawing 500Watts from a 48V system, you get
500/48 closer to 10.5 amps.

The only real difference is the weight of the system.  The 48V motor
is a few pounds more, and the 48V batter can be more that 10 lbs more.

Having said this, I did stay with the 36V setup just cause when it was
Lead Acid it was a whole extra battery at $40 bucks and 12 lbs.
That and nearly twice the price made me think.