battery capacity?

[ebike-infected]

I have 10 new 5s 4200mah lifepo zippy batteries 16,5v nominal voltage. They're wired to make 8400mah ~80V battery.

Yesterday i was riding 40 km with it, CA said starting voltage was about 83,5V (few hours after full recharge with an icharger) Towards the end of the ride the voltage started to drop, and was at about 77 volts when i came home, it would further drop under 70V when under heavy load on the last straight before base. When i came home i measured the invidual cell voltages with celllog and >80% were showing low and a handful were showing REALLY low. (2,4V or something like that) Handful were still over 3 volts. I immediately parallel-charged them with 1 amp until they were all showing over 3 volts, after that, i charged them with 10 amps until they were 3,2V and after that 20 amps until full. All cells charged okay it seems. CA now reads 83,2V and i recharged the pack about 12h ago.

Can someone explain to me why this happened? By CA readings, i should still have had half of the juice to go by?
How is possible that my pack became so depleted, even when CA said i had only used 4,2 Ah of the (theoretically) available 8,4 Ah?
My strategy was to never use more than 6-7Ah from the battery, and so leave a safe margin and never overdrain the battery but i somehow managed to do it anyway.

Also, can someone explain to me why the invidual cell voltages seem to get unbalanced when the pack is really low or hot off the charger? Once the batteries sit for a while or i drain them a bit by driving, they get very close to same voltage 3,33 - 3,36, most steady at 3,35. Off the charger the voltage may differ as much as ,3Volts. Is this normal? What is acceptable voltage difference? I have balance charged the batteries invidually a few times (recharged the whole set i think 5 or 6 times) The cell voltages and their differencies were pretty much the same whether i parallel-charge or balance charge...

 

[dogman dan]

Well, the fact that your CA showed exactly half the expected capacity says something. Sure it's reading correctly? Ever compared the AH it showed with what the charger shows in mha before? My first guess is that the CA is off by half.

But lets back up just a bit, and tell me exactly what you have. 25s? Five sets of paired packs. Fully charged that should be 105v. 84v would be 20s lipo charged to 4.2v.

20s 8.4 ah lipo would be stretched very far to go as much as 40k distance at 40 kph. So no suprise you ran out. It should never have been possible to do it at any decent speed on 4.2 ah.

As they go off the cliff, the tiny differences between one cell and another can cause one cell to go over the edge much sooner than the others, resulting in the big unblalance at the bottom. 73v resting should have been a safe stopping point for a 20s lipo pack. But you haven't weeded out the bogus packs from your HK order yet, eh?

 

[dogman dan]

Reading back, 5s 16.5v lipo makes no sense. 5s is 18.5. Typo? EDIT No, I just cant read.

 

[Jeremy Harris]

My money is on the common problem of the CA not having been set up for the shunt resistance correctly, so misreading current and hence capacity, as dogman rightly says. This happens so frequently that it's almost worth putting a sticker on CA displays when they ship out saying "Have you set Rshunt correctly?"

 

[wesnewell]

Reading back, 5s 16.5v lipo makes no sense. 5s is 18.5. Typo?

He has 5s lifep04 packs, not lico packs. I assume these.
http://www.hobbyking.com/hobbyking/store/__14071__ZIPPY_Flightmax_4200mAh_5S1P_30C_LiFePo4_Pack.html

 

[wesnewell]

Starting voltage should have been over 90V. I have you running 5x5 for 25s. 25*3.65=91.25v. Did you check the pack with a voltmeter? 83.5V puts you barely over nominal charge at 3.34V per cell.

 

[ebike-infected]

yes wesnewell got it, thats what i have for batteries, 4200mah 5s LiFePo cells, says on the package; 16,5Volts.

I dont know if i should tweak the charger settings then?
what kind of charger-settings do you people use for these LiFePo cells?

My iCharger has a preset for charging 5s lifepo cells which i have used, and it levels at 18V, and then slowly dropping amperage until it beeps for ready.
After that, all cells' voltages slowly balance out and drop down to about 3,35V. (*5 = 16,75V)
If i try to recharge again at this point, it will add maybe 50-150 units of power, the voltage rising really fast, and then soon beeps ready again.

The whole pack right off the charger with max voltage shows about 85V on both my e-meter and my CA.

Oh, and CA shows, -12W and crazy regeneration stats when idle?
I have to check on the rshunt thingie... thanks for the tip, ill get right on it.

 

[wesnewell]

Well, I've never used lifep04 , but everything I've read says fully charged is about 3.65v per cell. Checking my charger set for lifep04 shows it's set to 3.60v per cell as default. Pack voltage dropping from 18 down to 16.75 doesn't sound normal to me, but wait for some info from people that actually use it. Checking this is interesting.
http://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery#Specifications
Cell voltage = min. discharge voltage = 2.8 V. Working voltage = 3.0 V – 3.3 V. Max. charge voltage = 3.6 V.

 

[Gregory]

What sort of amps are you pulling during the ride? If you're trying for 10C on those cells, they won't like it. They aren't reported to be as strong as the label may suggest.

Also people talk about breaking in LiFePO4 packs for the first few uses. Then they gain capacity and stay better balalced. So it may perk up over the next few cycles...

 

[ebike-infected]

[/quote]

Well, I've never used lifep04 , but everything I've read says fully charged is about 3.65v per cell. Checking my charger set for lifep04 shows it's set to 3.60v per cell as default. Pack voltage dropping from 18 down to 16.75 doesn't sound normal to me, but wait for some info from people that actually use it. Checking this is interesting.
http://en.wikipedia.org/wiki/Lithium_iron_phosphate_battery#Specifications
Cell voltage = min. discharge voltage = 2.8 V. Working voltage = 3.0 V – 3.3 V. Max. charge voltage = 3.6 V.

After balance charging every pack invidually and after 10 hours idling the battery shows 85,6V (/5 = 17.12V /5 = 3,42V average per cell)
I read conflicting info on the web about lifepo max charge, estimates varying between 3,6V 3,65V 3,7V and even 4,1V (which feels too high to me! as i also read that @4,1V the lifepo cell will start to get damaged) I dont know how much difference in capacity between a cell charged to 3,6 and a cell charged to 3,7V...

What sort of amps are you pulling during the ride? If you're trying for 10C on those cells, they won't like it. They aren't reported to be as strong as the label may suggest.

CA says max drawn amperage a few digits under 30amps... But that cant be trusted b4 i calibrate rshunt. The battery label says 30C so i definitely should be within safe margins as far as i understand. If the CA is off by half, i could theorize that then the max amperage drawn was max double that which CA shows so ~60A ... 30C for 8400mah battery is 8,4*30 = about 250 amps. Drawing 60 amps from the pack would be around 7C so it should very easily within recommended limits(?)

Also people talk about breaking in LiFePO4 packs for the first few uses. Then they gain capacity and stay better balalced. So it may perk up over the next few cycles...

Hope So!
Anyone have an educated quess on the rshunt value for 50Amp 72V crystalyte controller? tried googling it and contacting manufacturer, but they are on summer holidays...

 

[dogman dan]

HA! Knew I was missing something. It's lifepo4 :lol: Gotta start reading the cesspool till the coffee kicks in.

 

[NeilP]

OK, tell us this..
What is your controller
What is the shunt value set in your CA.

if your controller is a known one, we may be able to guesstimate the shunt value, and compare with what you have set.
OK so we won't get it to 1% accuracy , but at least with a guesstimate we can make a start

 

[ebike-infected]

OK, tell us this..
What is your controller
What is the shunt value set in your CA.

Controller = Crystalyte 50Amp 72V (min 55V)
CA rshunt 1.601

I just took a 10km ride, CA shows i spent 1,335 Ah / 122,02Wh
icharger showed 14774 watts charged after the ride.

 

[NeilP]

I guess from controllers I have from Xlyte, that yours will have 4 shunts,
Ideally need to know how many shunts you have. Better still if you have an external reference then compare the two currnet results


That shunt figure have does seem a bit high,

Did that CA come with the controller ..from same supplier? and did they say it was setup ready to go, or did you buy them seperate

 

[ebike-infected]

i have no idea about the shunts.

CA came with controller. Bought them at the same time, same shop, but no there was no speak of calibration etc.

 

[NeilP]

One other bit of info that may be of help..how many FET controller? 12? 18?

if it was 18 FET I would guess at 1.00 as a better guesstimate for a shunt setting..each shunt being about 4mOhm

 

[ebike-infected]

One other bit of info that may be of help..how many FET controller? 12? 18?

if it was 18 FET I would guess at 1.00 as a better guesstimate for a shunt setting..each shunt being about 4mOhm

It is 18 FETs

So then, i will try to set rshunt to 1.0 then and take a ride and compare results.
Cant do it now though, thunderstorm started!

 

[NeilP]

not far off here too witht eh weather...you in UK too?

 

[NeilP]

not far off here too with the weather, ...you in UK too?

 

[ebike-infected]

No man, i'm from Finland but it seems its quite stormy weather across most europe now. Today too, really thick moist still air everywhere, like always before a bigger thunderstorm.. wish it would already rain again! Well, just came back from a ride, did a bit forest trails too. Here is data:

23km ride, CA shows 3,638Ah 289,75Wh 12,6Wh/km
icharger 20961

It seems to me, to be much closer now considering i ran the battery empty with a 40km ride before. But i should be able to calibrate still closer... But how do i calculate it from this data? Nothing in it seems to compare nicely at first sight?

I suppose icharger is showing mAh put into battery... And when i parallel charge, my battery is 16,5V 42Ah. When i discharge the battery is 82,5V 8,4Ah ...So, i should multiply CA Ah by 5 or divide icharger Ah by 5 to compare.. Right?

So lets see 20961/5 = 4192,2 and 5*3,638=18,19 ...That is fairly close! only 10% off ...So, to further fine tune the calibration i need to change rshunt value to ,9 since it was 1,0 ...I think

Do you concur?

 

[NeilP]

Ah OK, Finland..not been there. Put your location in your profile, under your photo, makes life a lot easier for people replying to you with regard to suppliers etc.


I have not had time to look at your calculations, but yes...see what the charger puts in and compare to what the CA says you put out.

If you had a large scale external ammeter, you could put that in series with your battery, and compare amp readings between the ammeter and the CA and adjust further from there. or do like a lot of us do..buy a second CA..the external shunt Stand alone CA..it comes with a pre wired shunt that is calibrated to the CA...It can them be used on your charger when bulk charging, to check other devices or whatever.

 

[nieles]

if your icharger has a foam cutting function you can use this for calibrating your CA

more info in this thread:
http://endless-sphere.com/forums/viewtopic.php?f=2&t=12316

 

[nicobie]

It doesn't matter how you connect your batteries when charging. The Ah shown on the CA after a ride should match the Ah shown on the charger after charging the battery if you have your shunt value right. Just change values until you get it right. It's not hard. After a couple of tries you'll get a feel for it.