Okay..heres what a expert says about LIFEPO4 cell LVC...
I emailed ping battery....asking him if 2.1 LVC for each cell is safe and advisable. heres his response :
The low voltage cut off is cell based. When any cell voltage reaches 2.1v, the BMS will cut off the power. Voltage of LiFePO4 drops very rapidly when reaching 2.5v when it’s being discharged. So, you can also look it as 2.5v. So, the total will be around 2.5x16=40v. You cannot look it as 2.1x16, because when one cell is 2.1v, the rest cells could still have 2.5v or higher voltage. If all the cells can reach 2.1v at the same time, we wouldn’t need the BMS. Actually it’s hard to see 2.1v or 2.5v at the moment of cut off, because voltage will rise back to be close to its nominal voltage as soon as the discharge is stopped. You just need to set the low voltage cut-off of the controller to be around 40v. 33.6v is just ideal on the condition that all the cells are 100% matched and 100% balanced. It’s impossible. Best regards, Ping So according to him...40 volt LVC....is proper....not 33.6 or 46 or 45.... this is for a 48 volt lifepo4. |
And that's fine for "HIS" batteries. But while lifepo4 batteries are all simular they are not exactly the same manufacture to manufacture. Therefore, it is best to follow the recomendations of your manufacture and not a general recomendation. I prefer to error well on the safe side and set my alarm for 2.8v or 44.8v for my 48v pack. Many may consider this as a waste of capasity but I feel comfortable with it and I feel I have the extra capasity to spare. I run a ThunderSky 48v 20ah pack. Bob
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Originally Posted by dumbass
(Post 12507586)
And that's fine for "HIS" batteries. But while lifepo4 batteries are all simular they are not exactly the same manufacture to manufacture. Therefore, it is best to follow the recomendations of your manufacture and not a general recomendation. I prefer to error well on the safe side and set my alarm for 2.8v or 44.8v for my 48v pack. Many may consider this as a waste of capasity but I feel comfortable with it and I feel I have the extra capasity to spare. I run a ThunderSky 48v 20ah pack. Bob
but according to Ping..the BMS does not accomplish this... either way, I think a LVC of 40 volts should be safe, and allow much of the battery capacity to be utilized... having a LVC of 44- 46 , seems like you may be leaving useful energy in the battery , that would equate to several more miles of distance covered , without harming the batteries. |
You didn't read what I wrote earlier...
Forget 2.1 or 2.5... There is VERY LITTLE energy left in a cell once it reaches 2.8v unloaded. You get MAYBE, AT MOST a HALF of an AH once a cell reaches that plateau. That equates to about a mile... Look at ANY discharge curve of just about EVERY LiFePo4 and you'll see what I mean.. The "worst case" scenario of lifepo4 after 2.8v is .25ah and "best case" is .75ah. Constantly discharging at 2.1-2.5 cutoff WILL SIGNIFICANTLY lower your usable cycles. To repeat what I stated in another thread.. I have OVER A THOUSAND cycles on my 36v 15ah battery and my LVC I set at 2.8v per cell and to THIS DAY. I still get 14 AH out of that battery setting the LVC at 2.8 per and that's ~92% of "full AH rating" of my battery, even after all of those cycles.. My numbers dont lie... You seem to think that there is a significant amount of energy between 2.8v and 2.1v.. It just ain't true. |
FYI, I purchased a 48v 30ah battery from the guy you bought your kit/battery..
How long did it take for you to get the battery? What LVC does he say the BMS is set at? |
Originally Posted by Sangesf
(Post 12508499)
FYI, I purchased a 48v 30ah battery from the guy you bought your kit/battery..
How long did it take for you to get the battery? What LVC does he say the BMS is set at? the battery takes a bit longer to recieve, then the kit..because of customs... I just emailedf him..and asked him what the BMS is set at for the 48 volt/20 A/h kits... |
Originally Posted by Sangesf
(Post 12508473)
You didn't read what I wrote earlier...
You seem to think that there is a significant amount of energy between 2.8v and 2.1v.. It just ain't true. you dont think that would eqaute to a few more miles in distance travelled ? Right now, my BMS is cutting out at about 18 miles travelled...If I could reset it to cutoff at 40 volts, instead of 46 volts , you dont think that would result in a few more miles of distance, while still giving adeqaute protection to the lifepo4 ? |
Originally Posted by sunnyday
(Post 12508533)
well thats .7 volts x 16 cells...
you dont think that would eqaute to a few more miles in distance travelled ? Right now, my BMS is cutting out at about 18 miles travelled...If I could reset it to cutoff at 40 volts, instead of 46 volts , you dont think that would result in a few more miles of distance, while still giving adeqaute protection to the lifepo4 ? That 53wh/mile.. That's horrific... I get at WORST 30wh/mile and my bike with me on it weighs, over 350lbs.. Something is wrong.. Do you have a watt meter to test how many AHs you're getting? I get 21 miles off of 36v 15ah. FORGET volts.. .7v per cell at 2.8v unloaded goes FAST. |
Originally Posted by Sangesf
(Post 12508771)
18 miles on a 48v 20ah battery?
That 53wh/mile.. That's horrific... I get at WORST 30wh/mile and my bike with me on it weighs, over 350lbs.. Something is wrong.. Do you have a watt meter to test how many AHs you're getting? I get 21 miles off of 36v 15ah. FORGET volts.. .7v per cell at 2.8v unloaded goes FAST. is your riding location fairly flat ? |
I have a inline wattmeter installed on the bike....so I should look at my A/h readings ?
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53 watt hours per mile? Wow. That sounds like the usage pattern of a wannabe motorcycle, rather than a hybrid vehicle/ebike with substantial human contribution. I'm around 14 watt hours per mile on my trike, even in hilly terrain, as I pedal a lot. And its very much worth having a battery sufficiently large so that you never have to run it to cutoff of the BMS. Much easier on the cells.
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Originally Posted by sunnyday
(Post 12508533)
well thats .7 volts x 16 cells...
you dont think that would eqaute to a few more miles in distance travelled ? Right now, my BMS is cutting out at about 18 miles travelled...If I could reset it to cutoff at 40 volts, instead of 46 volts , you dont think that would result in a few more miles of distance, while still giving adeqaute protection to the lifepo4 ? |
Sunnyday,
I have some (maybe) good news for you... According to the manufacturer of the battery you (and I) bought, the 20ah is NOT 20ah, it's 21Ah! (Supposedly, he uses 3ah Cylindrical cells in an 8p configuration) His BMS is set (again supposedly) for 2.05v per cell which is too low, however MY controller is set for 2.6v, so I'm safe for sure. Have you tested out "full" range or AH yet? (P.S. Make SURE you don't drain past 2.6v per cell or 41.6v for pack and see how many AH you get) When I get mine (36v), I will test it out that day (and prolly night too!) (I will run about 10, 10mile "runs" (at full throttle) to "break-in" the pack and then run my pack from full (39.6v or 3.3v/cell) to "empty" (32.4v or 2.7v/cell) and let you know how close the manufacturer's specs are to actual. |
Originally Posted by Sangesf
(Post 12537607)
Sunnyday,
I have some (maybe) good news for you... According to the manufacturer of the battery you (and I) bought, the 20ah is NOT 20ah, it's 21Ah! (Supposedly, he uses 3ah Cylindrical cells in an 8p configuration) His BMS is set (again supposedly) for 2.05v per cell which is too low, however MY controller is set for 2.6v, so I'm safe for sure. Have you tested out "full" range or AH yet? (P.S. Make SURE you don't drain past 2.6v per cell or 41.6v for pack and see how many AH you get) When I get mine (36v), I will test it out that day (and prolly night too!) (I will run about 10, 10mile "runs" (at full throttle) to "break-in" the pack and then run my pack from full (39.6v or 3.3v/cell) to "empty" (32.4v or 2.7v/cell) and let you know how close the manufacturer's specs are to actual. |
AH when your resting voltage is at about 44v. (which should be very close to LVC and I would seriously suggest you take it easy on the battery or pedal home from there.
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Originally Posted by Sangesf
(Post 12538369)
AH when your resting voltage is at about 44v. (which should be very close to LVC and I would seriously suggest you take it easy on the battery or pedal home from there.
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Resting voltage of 44v would be 2.8 per cell and figured under load, that would hit his LVC on the controller or very close to it, since his cells are relatively balanced.
He will figure it out.. He'll lose top end speed and torque and the power level lights on his throttle will flicker... He'll realize that he's close to LVC. Also when he gets past 18 or so AH, he'll notice it. |
Originally Posted by Sangesf
(Post 12539607)
Resting voltage of 44v would be 2.8 per cell and figured under load, that would hit his LVC on the controller or very close to it, since his cells are relatively balanced.
He will figure it out.. He'll lose top end speed and torque and the power level lights on his throttle will flicker... He'll realize that he's close to LVC. Also when he gets past 18 or so AH, he'll notice it. |
His pack is only a month old and only has 30 cycles on it at most..
His cells should be pretty well balanced at this point. If his pack was a year or more old or had hundreds of cycles, I would give him different advice. This is the best time for him to test full AH capacity... Even if he has an errant cell group, even dropping it to 2.0v once is not going to be detrimental. I've had a cell group (on a 36v 15ah) go to 4.0v once and another group drop to 1.7v once, and my pack STILL holds a 90-95% capacity after 2.5 years. |
Originally Posted by Sangesf
(Post 12540231)
His pack is only a month old and only has 30 cycles on it at most..
His cells should be pretty well balanced at this point. If his pack was a year or more old or had hundreds of cycles, I would give him different advice. This is the best time for him to test full AH capacity... Even if he has an errant cell group, even dropping it to 2.0v once is not going to be detrimental. I've had a cell group (on a 36v 15ah) go to 4.0v once and another group drop to 1.7v once, and my pack STILL holds a 90-95% capacity after 2.5 years. |
Here's something I came across today while looking for cell bleeders. I found it interesting and worth reading.
About Lithium Iron Phosphate Batteries A “battery” is made up of “cells”. Each cell is an individual unit that cannot be split into a lower BCU and on again to check that the contactor is operating correctly. Check the cells for corrosion orvoltage component. LFP cells have a nominal voltage of 3.2-3.4V. This is the voltage that the cells drop back to when at rest. They will stay around 3.2V until about 90% discharged when the voltage will begin to decrease until fully discharged at 2.5V. It is highly recommended to discharge less than 80% of the cells total Amp Hour capacity. This will help to maximize the cell life. LFP cells may be connected in “series” (+ to – to + etc) to obtain a higher nominal voltage. This creates a “battery”. They may also be connected in “parallel” (+ to +, - to -) to increase the Ah capacity. Under no circumstances should cells or a battery be short circuited, that is the + connected directly to the – to create a loop. This will damage the cells and most likely the operator also. Under no circumstances should the cell voltage be allowed to fall below 2.5V for a sustained period. Permanent damage will result. It is possible that this situation may occur if the battery is allowed to stand for a long time (ie. months). In this situation the BMS will not allow recharge because a cell is outside the safe range. To try and rectify the problem a small 4.5V 300mAh DC plug pack type power supply can be applied to individual cells one at a time until the cell voltages rise above 2.5V and the BMS will allow normal charging to commence. Care must be taken with the first few charges as the battery may be severely unbalanced. A visual check of the battery pack during charging should be made every 3 months. Switch off the other damage. http://www.evpower.com.au/IMG/pdf/BC...-8C-manual.pdf |
Yep..
That's about right... Just for fun(?), yesterday, I took one of my many extra (bad) lifepo4 cells (this particular one has screw posts on it) and took a buss bar and shorted it out to see what would happen... I'm talking a complete sustained short... It was fun to watch... The only thing that really happened was the copper buss bar got really hot and the cell puffed... I left it like that for 20 mins and at the end, the buss bar was too hot to touch and the battery was quite warm.. Lifepo4 cells are really "safe" compared to the other lithium chemistries.. BUT, DO NOT TRY THIS AT HOME! (I was in an empty 10 acre field near my house) |
But did you notice they say a cell will show about 3.2v for 90% of it's capasity but they recommend not normally exceeding 80% of capasity? This is basicly what I was saying. It is not adviseable to graw down lifepo4 cells to 2.0v or even 2.5v. In fact if they hold 3.2 for 90% of capasity they shouldn't even be drawn down past 3.2v. But we all know that under a load the voltage is going to drop below 3.2v. It's hard to figure but I am guessing they are talking about voltage at rest again. Which is a little stupid when talking about a draw down capasity.
Bob |
Also depends on the C rate (and appropriate voltage sag).
The best way is to figure out your max Ah available by going from full to LVC just ONCE. THEN, figure out 90% and never use more than that for the longest possible cell life. Like I said before... My best battery life so far has been the 15AH lifepo4 that I've had for 2.5 years and over 1000 cycles and I still have a usuable 14ah out of 15 or 93% capacity. ~15% less than 20% depth ~75% between 20% and 80% depth ~10% more than 80% (and 25% of that is to LVC of 2.5v - 2.7v) Also of note, my controller pulls 25a MAX (1.6c) and I usually cruise at about 10a (.7c) and my battery is rated for 1C continuous and 2C max. I also know, that once I start getting close to the 14ah, I can notice the slightly higher sag on the battery and that's when I stop using it. Now mind you, I almost never use 14ah, BUT I constantly use 13AH.. (For me, 14AH is about 19-20 miles and 13AH is 17-18 miles) Once I "break in" the new 36v 20ah, I will charge it fully and then once I get close to 19AH I will take it "easy" on the battery with a max amp pull of 5a (C/4) until I hit LVC. That will give me the max range I'm able to get. And will ride accordingly.. I will let you all know the "capacity" of this batt. (P.S. Sunnyday, have you tested your max range yet or total AH capacity of yours yet?) |
I just ordered 2 - 36v 20AH batts to extend my range, and on a side note, I replaced my back battery box with a new one of the same type and did NOT put the subwoofer back in and hooked up a smaller amp to just power the two tweeters and one midrange on my handlebars. The subwoofer was just too heavy for the box's material to hold it well.
WHEN (not if) I get my new diamond plate aluminum box, I will put the subwoofer back (along with the neon) because.. 1.) The diamond plate will hold the sub VERY securely. 2.) I think I will get much better bass out of it. |
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