How can I tell how much power is in my life cells?

[alpharalpha]

I have a cell log that shows me the voltage so for example can I tell the power by that ( 3.6v 3.2v 3.1v 2.9v etc?) I also have an analog ampmeter, have a digital one too that is a wattmeter ah meter etc but I don't trust it, was a cheap chinese one from ebay that's why I got the analog ampmeter which works great. So is there a way to tell by voltage (I'm thinking not but don't know) would buying a decent wattmeter work or do I need something like the Watts Up? I just want to know how much power is in each cell. BTW: I have one cell that only charges to 3.4v while the rest charge to 3.6 and stay there (last time I full charged a week before leaving and they all stayed 3.6 except that one. After riding 18 miles at 15-20mph all the other cells are at around 3.3v but that one is at 3,2v and I've tried balancing it, it just won't hold a charge over 3.4v. I wanted to see how far I could ride on my 30ah prismatic pack and since it was night and that cell log is useless to view while riding imho I rode it down to where my bms stopped, unfortunately it's a bleeding bms that stops at 2.0v and it's the first battery that's not fully charging now.) Well, that's kinda off track. Is there a formula using amps, voltage and milage to determine ah? say if I start out at 3.5v using 20amps for 18 miles and end up with 3.3v? (and using a 30ah cell) Any info is helpful and appreciated.

 

[bowlofsalad]

Would a watt meter such as the Watts Up tell me how many watt hours I have used?

Yes, you may also want to consider looking into a cycle analyst. A cycle analyst will automatically calculate watt hours per mile, which is the information you seem interested in. You can also use a cycle analyst to quickly and easily lower your maximum watt consumption which would extend your riding range.

To calculate watt hours per mile you need to measure both the watt hours and miles you traveled for that trip. Then, take the watt hours and divide them by the miles traveled. For example, say you consumed 500 watt hours over 20 miles, 500/20 = 25 watt hours per mile. With a watt meter and a 'bicycle computer', you can manually find watt hours per mile.

You may want to do a simple discharge test to determine overall capacity of your battery. https://www.youtube.com/watch?v=wP0MhOcPmQY Some portions of this video may help guide you in this task.

You get what you pay for.

 

[dogman dan]

A cheaper wattmeter will do what you want. Under 20 bucks for the blue one on ebay. It will tell you how many ah or wh was discharged on a test ride you do till 100% discharged, or to whatever voltage you wish to consider your stopping point.

Then on other rides later, you can see how many wh you have used so far, and do some quick mental math to know what's left.

A stand alone CA is the best wattmter, and if you have a controller compatible with it, a DP CA is the gold standard. Well worth the cash, I have two stand alones and one DP. I just like them that much.

That won't tell you how much is left in one cell that's weaker, but by watching the voltage on that one weak cell, you will know when to stop, and call it 100% of what your battery can do right now.

If easy, get rid of that weak cell of course.

 

[alpharalpha]

Yes, I am checking into replacing that battery; but it seems to me that the amount of ah between 3.4v and 3.6v = the amount of ah between 3.2v and 3.3v since I do a full charge before leaving and end up with this on arrival. So can anyone tell me how much ah (power) is in a life cell between 3.4v-3.6v and again between 3.3v-3.2v ?

 

[docnjoj]

Yes, I am checking into replacing that battery; but it seems to me that the amount of ah between 3.4v and 3.6v = the amount of ah between 3.2v and 3.3v since I do a full charge before leaving and end up with this on arrival. So can anyone tell me how much ah (power) is in a life cell between 3.4v-3.6v and again between 3.3v-3.2v ?

Unfortunately the voltage reading is not necessarily indicative of state of charge. The only accurate way to tell is to charge the battery fully, either ride the bike and use a Wattmeter to tell a/h or wh/hr used or, do this with a load on the battery, like a resistance, and see on the Wattmeter what happens when the BMS shuts off.
otherDoc

 

[dnmun]

the cell that only charges to 3.4V in your opinion may charge to a higher voltage if you increase the charger voltage slightly and leave it on the charger for longer. measure all cells while charging. if you measure them after you remove it from the charger you do not know how high it actually charged up to.

 

[John in CR]

alpharalpha,

The only was to know is to run a discharge test of your pack. Some lifepo4 cells can still have over 90% of their original capacity, but fairly quickly self discharge down to 3.4-3.35V. Those cells can be nearly impossible to balance charge with typical balancers, but that doesn't mean their not full or balanced. Just occasionally fill them all the way up with a single cell charge to make sure they're not really getting far out of whack. Do that before running your discharge test to find your pack's true capacity. Lifepo4's are tolerant of full discharge, so running a full charge all the way down until the first cell hits 2V or so to measure pack capacity doesn't risk hurting anything. When you do so, make note of that cell and any others that are well below 3V. Those are the ones to keep the closest eye on, since the capacity of your pack is only the capacity of it's weakest cell.

Once you know pack capacity, then as stated by others, you simply need a way to measure what's been used since your last full charge to know your remaining capacity. The CA3 even has a battery gauge to tell you this. I set my CA3 up with a 20% lower than actual capacity for my battery for a nice conservative "fuel gauge" and a healthy reserve when I go too far. I ran out of juice once in 2009 and again in 2010. Then I started using CA's and haven't run out since.

 

[dogman dan]

Exactly, if it's not easy to replace that weaker cell, just live with it, check it and top it up periodically.

Then using a wattmeter , find out what your real world capacity of the pack is. At less than 20 bucks, it's not like you can't afford the cheaper watt meters.

Very handy items, among other things, you can find out what rate your charger really runs at, helping you to know when to stop charging when in a hurry and needing only a partial recharge to finish a trip.

Also a must have, if you are making any kind of adjustments to your chargers trim pots.

Here it is for 14 bucks.


http://www.ebay.com/itm/RC-Boat-Heli-Watt-Meter-Battery-Power-Analyzer-Digital-LCD-Display-DC-60V-100A-/310969767011?pt=LH_DefaultDomain_0&hash=item48673e1463

 

[Ykick]

$4 + change difference I'd buy this power meter instead:

http://www.ebay.com/itm/G-T-Power-LCD-Display-RC-Watt-Meter-and-Power-Analyzer-130-Amps-130Amps-/310802824855?pt=LH_DefaultDomain_0&hash=item485d4abe97

Backlight and 130A instead of 100A max.

 

[dnmun]

i prefer the GT and it is half the cost of the watts up.

there is no evidence there is anything wrong with the cell he says only charges to 3.4V. he has not measured cell voltages while charging so we have no idea if his charger is able to fully charge the pack or not. if the BMS was malfunctioning and shorted on that channel then the cell would drain down much faster than the others and it would be obvious then if the BMS was causing the cell to drop to 3.4V like that.

he has to be able to fully charge and balance the pack to do the discharge or it is meaningless.

 

[alpharalpha]

I have not been using the bms for charging, instead just running the charger directly to the pack's pos and neg, perhaps this caused the problem as it's the #1 cell (neg) if so I can easily solder the charger wires to the bms. The bulk charger shows a full charge, a multimeter shows a full charge and when I use a since cell charger it shows a full charge even though it quickly goes to 3.4v the others all stay at 3.6. I have one of those cheap ebay wattmeters, just went and dug it out, didn't trust it's readings and it keeps rotating so it's hard while riding to read, I got an analog ampmeter and it worked perfectly (30amps--1c--at startup and then between 15-20amps. I'll hook the digital one up again to test it out. I think it says I can use it to test one cell if I use aux' power, I see what looks like a 2s jst plug (3 prong) do I just get a 1s jst and hook it to the pack or something else?

 

[7circle]

...Is there a formula using amps, voltage and milage to determine ah? say if I start out at 3.5v using 20amps for 18 miles and end up with 3.3v? (and using a 30ah cell) Any info is helpful and appreciated.

I think you want Energy capacity in Watt.hours (Wh)

that's power for duration...

each cell is supplying 3.5V x 20A = 70W
But LiFePO4 ramp down there volts after first 5% to a very stable nominal of 3.2V
And temp and load effect this voltage before reaching 90% Depth of dicharge.

If you know how long for ...say 15min (1/4 h) would give 70 x 1/4 = 17.5 Wh out of cell for 18miles.
OR
If you know the speed ... say 36mph over 18miles takes 1/2 hour so 70W x 0.5h = 35Wh per cell

so W.h = V x I x miles / mph

V is either per cell or whole pack.

So 12 cell pack is about 36V so 35Wh per cell x 12 is 360 Wh from the pack for the ride.

If the cells are 30Ah the hold 3.2V x 30 Ah = 64Wh per cell

So 35 Wh from 64 wh cell is a bit over half so 54% SOC or 46% DOD

The problem is the error of the measurement/estimate you make in guessing Amps volts speed time ....

A fast measuring meter like CA will track the Wh as things are varying.
A slow device measuring every second may miss the peaks and cause error.
Some use analogue accumulator ... but out of fashion now... all things must be apps. :wink:

I have not been using the bms for charging, instead just running the charger directly to the pack's pos and neg, perhaps this caused the problem

A poor connection on the cell terminal will cause heat from I^2.R for R being bad connection.
The end cell gets hot ... chemicals get excited .. degrade internals ... cell wont hold nice end charge and may have higher leakage.

Charge without bms ... no chance for shunt fuction to balance cell voltages ...

$$$+time into bin save $ + time

 

[alpharalpha]

I've read people saying they go down after charged but my hipower ones stay right around 3.6 even after a week; thing is the company went out of business here in the usa and I fried one of my cells; there's one company I found that has a few left. I asked them to fully charge them and let them sit and after a week they were at 3.52v 3.52v 3.55v and then after a month they were 3.46 3.49 3.52
Do you think these cells are ok? Before asking them to fully charge them they were at 3.32 just like the ones I originally purchased.

 

[dogman dan]

Hell yeah, those sound fine to me. Holding the charge that good is definitely a healthy cell.

 

[dnmun]

no way to know how badly they were overcharged if he charged with no BMS. there is no evidence he has actually correctly measured cell voltages while charging or has analyzed the BMS to see if the channel is shorted.

 

[dogman dan]

I didn't say the cell was like new. It could have significant wear and tear on it. Rethinking, it could still be unusable.

But holding a charge indicates to me it's likely still usable. May not last so long, but right now, usable IMO. It's quite possible though, that it will hold a charge, but not be able to stand any kind of load on discharge. It's real world c rate could be shit now.

 

[alpharalpha]

The cells are coming from a dealer that has some remaining stock so I assume they are new and just have been sitting on the shelf for awhile. He charged them again for me and the next day they were at 3.62 3.66 3.68. Seems ok. Can they be charged without cells puffing/damaged? For all I know they did secure them but who knows?

My damaged cell's resting voltage is 3.31v and if I put a single cell charger it will go to 3.62v, bigggest problem is that when bulk charging all the rest go to 3.65v while it stays around 3.27v and then when I'm riding that cell runs down first. For example last 20 mile ride I pretty much just motored the whole way and like a block or two from my place that cell got down to 2.2v while the rest were still 3.3v

My setup pulls 30amps (1c) from dead start, then 15amps (0.5) while accelerating, and evens out to 10amps (0.3) so my pack doesn't drop as quickly (the yellowish line is 1c and that's around 3.2v, so at 0.3 I stay well above that).View attachment Discharge Curve.bmp

I have to make a trip before getting these new batteries anyways so am going to charge that single damaged cell up to 3.6v right before leaving and see what happens.

 

[dnmun]

if the dealer is charging to such high voltage that they have 3.68V on them after sitting for several days then that would account for the dead cell you have. the BMS should balance the cells to 3.60V to prevent damage.

 

[alpharalpha]

The dead cell I have was drop shipped directly from the manufacturer and was great, I underdischarged it; unfortunately the company went out of business and finding a replacement has been difficult. The three I'm buying now are the last an ev dealer has old new stock and he just charged ea at a time, they aren't a pack or anything. After sitting over the weekend they are at 3.58 3.63 3.64 I'm going to take the chance and get them, they seem ok

 

[dnmun]

ok, but they should not be charged much past the 3.60V even though the full charge voltage is considered to be 3.65V. i thought maybe the guy who sold them just piled on the volts. he did not need to charge them that high in voltage but it is not too big a problem if they don't get pushed over the 3.9-4V level. at 4.2-4.3V they will be somewhat damaged imo.

 

[alpharalpha]

Strange, after my last ride I single charged ea cell which were all around 3.32v and on a 1amp charger it took about 10+ hours to get them up to 3.65v, except for the damaged cell which went from 3.30v to 3.65 in about 1/2 that time but by next morning is back down to 3.35v. I'm going to buy the replacement cells but have one trip to make before that so can see what's going on.

 

[dnmun]

there is no evidence there is anything wrong with any of the cells. this guy is a kook and has no clue about batteries.

you have to charge the battery up to full charge and then discharge it into a dummy load to measure capacity. cell voltage has nothing to do with it.

 

[alpharalpha]

One of the things I liked when I bought these cells is that they came in numbered sets so internal resistance was not an issue. But I ran them with bms only hooked up for charging wanted to see how far I could go on the pack, I forgot that the voltage would go lower under load (and while my cell log gives an accurate reading I can't see it while riding.) I ran the pack down and cell #1 where the neg load is at is the one that is now damaged.

The question I had was what typical resting voltage is for lifepo4 cells but my post got moved to this earlier one I had about power in the cells.

 

[dnmun]

that is how most people ruin their battery. discharging with no BMS to protect it.

 

[Nobuo]

that is how most people ruin their battery. discharging with no BMS to protect it.

+1