general way to tell power used/left in pack by voltages?

[alpharalpha]

I am fairly new to life cells--just getting it into my head that most of the power is in the 3.2v range. Is there a fairly basic way to estimate how much power is used and/or left by looking at the voltages before and after? For example: I do an 18 mile ride one way using a 27v x 30ah pack which goes around 18mph. From a dead stop it takes 30 amps, while accelerating 15 amps, then finally at full speed 10 amps. I do a full charge before leaving ea cell at 3.6v and when I'm done I'm at 3.3v. Any simple way to guesstimate how much I'm using based on that? Doesn't have to be specific. Thanks.

 

[r3volved]

This thread probably has the info you're looking for: http://endless-sphere.com/forums/viewtopic.php?f=3&t=69431

 

[cycborg]

Voltage is not particularly useful on its own for LiFePO4 since the voltage vs. state-of-charge curve is so flat over most of its range. Voltage can be a good SOC estimator for other chemistries if you know what you're doing, but for LiFePO4 you really need a wattmeter.

 

[teklektik]

The two accepted practices for determining state of charge are open circuit voltage and expended amphours (not watthours). The Cycle Analyst V3 (and beta CA V2 code) use both techniques - each as appropriate for the situation.

Voltage measurement can be effective for most chemistries but it only works with 'open circuit' voltage. The difficulty with open circuit voltage is that is not available underway - (obviously) - since the battery is loaded and not resting with an open circuit. That said, if you know the battery internal resistance you can work back to an estimate of the open circuit voltage from the instantaneous current while underway, but this is beyond the capabilities of a common voltmeter. The CA continuously calculates the IR from voltage and current data and uses it in just such a manner to get an estimate of open circuit voltage even when it would not be available or valid from direct voltmeter measurement.

There is a brief discussion and some sample SoC curves in section "2.5 State of Charge Display" of the CA V3 Unofficial Guide. The curves are not detailed but do illustrate the difficulty with LiFePo4 SoC curves that cyborg referenced. In point of fact, the CA actually handles LiFePo4 a bit differently than other chemistries for exactly that reason.

 

[dogman dan]

Best way to guess lifepo4 is if you repeat a ride often enough, and ride it the same speed often enough.

Then by experience, you can ride similarly on other rides, and guess based on mileage alone.

But why bother guessing, when a watt meter is less than 20 bucks? Sure, a CA is much much nicer, but if you only need to guess occasionally just get a cheap blue watt meter from Ebay. Then when you do a full discharge and find out your capacity, the watt meter will tell you exactly when you are half way.

This watt meter does not have to be handlebar mounted, especially if you don't use it every ride. I put sticky Velcro on one, and when I used it, I could just stick it to the battery box, or the frame.

 

[alpharalpha]

I have one of those blue eBay wattmeters and a plug in point in my handlebar bag to hook it up. I found it useless while riding as it rotates the different readings and then eventually stops; if I turn the bike off and start it back up it comes back on. I ended up buying an analog ampmeter which worked perfectly. I really don't want to do a full discharge but if I ever want something more exact I will and use the wattmeter, though I've read the specs on my hipower cells and they seem pretty accurate. I think etrike is correct about the 3.2v-3.4v being where most of the power is because if if 3.4v is a starting point and 3.2v the end, I do a full charge and end up at 3.3v and dogman dan had once told me that my wilderness energy brush motor would use around 30wh a mile that equals about half my pack's capacity. I have been wanting to stop topping off the charge as I know it isn't good for the cells so now next time I'll stop at 3.5v and see where I end up and after that 3.4v. Wherever the cells stop at 3.25v. Do you think that charging up to 3.6v is doing much damage to my cells? They bulk charge at 32.8v & 3.2 amps and it takes maybe 6 hours to charge, of course it slows down towards the end. Next time I do a ride I'll keep track but it does seem that I'm using probably a little under half my pack. Once I came back from a ride, did a few in town rides--more stop and do so more 30 amps--then one night tried riding back a different way, probably 10 miles longer and the bms shut it down, that was about 50 miles total on the pack's charge, so I'd guess since my packs probably about 750wh and the ride I do is a long country ride about averaging .3c that my bike is running more like 15wh a mile since 50 miles depleted the pack 15x50=750. Yeah this is making sense now to me as I have some slight inclines and full stops on my 18 mile ride I'd say it's more like 20wh a mile (20x18=360x2=720) and that goes along with the 3.3v 50%ish I end up with between 3.2v & 3.4v a little more since I top them off. Does this seem about right?

 

[alpharalpha]

Do you think that 3.45 v would be good for long term storage?

 

[cycborg]

Do you think that 3.45 v would be good for long term storage?

That sounds a little high for storage - that's still on the steep part of the SOC curve. 3.25-3.30 should be safe.

 

[dnmun]

3.45V is usually full charge for a used lifepo4 cell after it sags off the charger. what was wrong with using the wattmeter like everyone else does?

 

[alpharalpha]

Would I use a 2s jst-xh to read just one cell with the cheapo wattmeter or some other kind of plug? So for now I serialed 2 cells together and am using my book light as the load. Both cells were close enough to 3.5v so before turning on the load the wattmeter read:

6.94v
6.05vm
4.5wp
0.65ap
0.1wh
0.022ah

During load:

6.6v (and falling)
5.89vm
4.2wp
0.63ap
0.7wh (and rising)
0.108ah (and rising)

Both cells at 3.39v

-----------------------------------

Just went back and looked and cells are ea at 3.4v but I've always had a problem with that wattmeter stopping reading after awhile so when I went and looked just now it had stopped and I had to disconnect it from the cells and then reconnect and it started working again like it always does. Now the cells are at 3.32v and the readings are:

6.57v
5.84vm
4.3wp
0.66ap
0.9wh
0.139ah

(oh and it says 5a and 3w)

The specs on the cells say they have an energy density of 95.7wh and a specific energy of 83.5wh

I've been putting this off so it's about time. The wattmeter hasn't shut off again so maybe it'll behave now.

Thanks,

 

[dnmun]

why are you using only one or two cells? where is your battery? the wattmeter will not work below about 9V.

 

[alpharalpha]

I was just getting a few cells ready for storage. It was suggested that I discharge the cells then charge them back up to know how much energy is at what voltage. But I got those cells down to 3,31v and then they stayed there for a long time so I'm figuring that is around where most of the energy is at and stopped there, charged them all back to 3.3v and am going to store them like that. They are stored at the factory at 3.32v (and sent that way) so I think that's 70% if I remember the specs right.

 

[dnmun]

the formation charge present as it arrives from the factory is not related to a 'storage charge' voltage.