What's the highest voltage on a 72V LiFePo charge?

[Floont]

Charger just failed and I have to get to a race at 5PM. I'm charging the 72v18AH battery now by raising the voltage on two 36v meanwell power supplies in series and making sure the amperage doesn't exceed 8 to 10 amps.

What's the highest voltage I should go to for a full charge on a 72V LiFePo charge?

EDIT: The first statement is true of my neighbor, since it is his Stealth Bomber. I just stated it as if it were mine for simplicity to get an answer quickly.

 

[sangesf]

88.8v is 3.7v per cell group....

 

[Floont]

88.8v is 3.7v per cell group....

Thanks!

 

[Degull]

Just for the sake of use newbies trying understand battery math, how did you come up with 88.8 volts?

 

[fechter]

Just for the sake of use newbies trying understand battery math, how did you come up with 88.8 volts?

72v pack is typicallly 24 series cells. 24 x 3.7v = 88.8v

 

[Floont]

It appears it saturates at 82volts.

 

[sangesf]

It appears it saturates at 82volts.

What does that mean?

 

[Gordo]

It appears it saturates at 82volts.

Then my guess is you are using 20 cells and you should quit at 75V? Because 20 cells of LiFePo4 top out at 82V, 4.1V per cell.

Edit; Wrong guess, it is a 24 cell.

 

[SamTexas]

Charger just failed and I have to get to a race at 5PM. I'm charging the 72v18AH battery now by raising the voltage on two 36v meanwell power supplies in series and making sure the amperage doesn't exceed 8 to 10 amps.

What's the highest voltage I should go to for a full charge on a 72V LiFePo charge?

WHAT IS a 72V LiFePo battery? Those multiple of 12 (6x12) are legacy from the lead-acid battery era, they have little (but potentially dangerous) meaning in the Lithium world.
How many cells in series in your pack? 22 (72.6V nominal, 79.2 max for A123), 23 (75.9V, 82.80), 24 (79.2, 86.4), etc...

So if your pack is 23s and you try to charge it to 86.4, well, good luck with that.

 

[neptronix]

Like fechter said, a 72v lifepo4 battery is 24 cells in series.

24 x 3.2 = 76.8v. ( regular lifepo4 )
24 x 3.3 = 79.2v. ( A123 etc )

No need for argument about this, labeling them as such is industry standard practice. The reason is due to the sag that occurs under load. Let's get back on topic now.

Floont, some lifepo4 is rated at 3.65v, thus you'd wanna charge to 87.6v more like it..

When you take them off the charger, they will gradually fall to a lower voltage of course; they will approach 3.2 or 3.3v/cell.

If you want to know more about your chemistry, a RC balancing charger or CBA would be the way to help you understand the voltage curves etc.

 

[Gordo]

There is no argument. Industry standard practice is a hangover from
LEAD 12V/6 cell=2V, nominal voltage when we all know it is a 13.2V battery. That is why the manufacturer's manual with my 20 TS cells calls it a 60V scoot and why fechter points out 72V is 24 X 3V. The fact that fully charged at rest they sit at 3.25V has no bearing on the nominal standard description. Floont's note on the saturated Voltage is what puzzles me?

 

[Floont]

What I mean by "saturates" is this: Since I did not have a charger, only Meanwell power supplies, I put my Meanwells in series and monitored the amperage using an ammeter and increased the voltage on each Meanwell using the front adjustment pot to maintain a good charging level. From 55v to 82v the behavior was the same; adjust the voltage slightly higher than the battery voltage and I watched the amperage slowly go from 10 amps down to <1amp as the battery charged. Once the battery reached 82v, the amperage would not fall below about 7.5amps. It maintained that current for over an hour and the battery pack started getting warm, To me, it's obvious that the pack could or would not take any more power, which is what I called "saturated".

BTW, the pack is from a neighbor's Stealth Bomber. He brought it over so I could fix it. It turns out his charger is dead. (It's charged now, so I consider it "fixed").

Here's some pictures:

 

[Gordo]

Floont, Pretty Please, how many series cells in the pack?

 

[icecube57]

yeah this is funny if its 22 the pack would be fully charged or saturated. 24s it wouldnt be fully charged at 82v unless some of the cells groups were going high. Is the BMS warm or the battery warm. Maybe the BMS is set to charge conservatively at a lower voltage and or maybe since there is no change in the voltage when you up it the bms has shut down the charger and is trying to bleed off excessive current in the high cells thus making the pack feel warm.

 

[fechter]

That charging behavior does not sound normal.
Make sure your voltmeter is reading accurately. Sometime a weak meter battery will throw off the readings.
If the meter is OK, I'd suggest trying to measure the individual cell voltages.
The charging current should drop to near zero at end of charge.

 

[Floont]

The battery is one large blue shrink-wrapped yellow plastic encapsulated "brick, so I can't count the cells. I now know after returning the bike to its rightful owner that the normal charge voltage is 88v. The battery settles down to 81 to 81v after its off the charger. Since the bike was not mine, I couldn't break the pack open to measure or count. But I think the above info is enough to go on to make an educated guess. (Both my VOM's have new batteries and are high quality).

The battery definitely is LiFePo... no balance taps. It weighs 23.5 pounds. And fully charged it propelled me smoothly across a pasture at 40MPH!

 

[999zip999]

Somebody must more info on that bike then that, like a owner's manual. How much did that bkie cost ?

 

[wineboyrider]

Same color wrapping that ping packs his batteries in....??

 

[The Mighty Volt]

LiFePo or LiPo??


a 72v LiFePo {I assume you mean LifePo4} has a NOMINAL charge of 3.3v times 24 is 79.2v, resting voltage, after a full charge.

The voltage at 24s, "hot" off the charger is 3.6 times 24 = 86.4v.

One flick of the throttle will bring that value down to 79.2v in a hurry.

A fully charged battery under full acceleration will drop to 72v or so, then rise back up as you come off the throttle.


NOW! for LiPo, the values are 3.7v for nominal and 4.1 or 4.2 for "Hot".

Hope that clears it up.

 

[Gordo]

"In measurement, a nominal value is often a value existing in name only[1]; it is assigned as a convenient designation rather than calculated by data analysis or following usual rounding methods. The use of nominal values can be based on de facto standards or some technical standards."
It is also referred to a base value. It is not the real value. The real value of any cell depends on it's current state of charge.
The nominal value, as defined by NEMA and the European Union, for LiFePo4 and other Lixxxx cells is 3V. When we follow this convention, we can all understand what is being discussed. 36V is 12 cells, 48V is 16 cells, 60V is 20 cells and 72V is 24 cells.

Using any other unconventional "nominal voltage" just leads to confusion, as this topic has shown very clearly.

 

[Floont]

...How much did that bike cost ?

$10,000 US.

 

[Tench]

My Bomber has a 24cell lifepo4 battery, the charger is set to cut out at 87.6v and the battery settles to 81v after the charger has cut. There are different Bomber battery packs though, i have seen 2 different types, one is just 24s and the other (earlier one) was 24s2p, dont know whether there are any with a lower cell count though? The bike pictured above has the seat mounting cut out so is later than mine so i doubt it will have the 2p pack.

Simon.

 

[sangesf]

...How much did that bike cost ?

$10,000 US.

TEN THOUSAND?!?

LOL... Is it made out of gold or something?
What's the AH of the pack?
I don't care what it's made of $10k for an electric bike (just like the OptiBikes) is just ridiculous...
That's for those to say.. "Look at me, I spent $10k on a bicycle".
I wanna be your friend and borrow 1/2 of that and make a bike 2 times faster with 10 times the range!

 

[Floont]

...How much did that bike cost ?

$10,000 US.

TEN THOUSAND?!?

LOL... Is it made out of gold or something?
What's the AH of the pack?
I don't care what it's made of $10k for an electric bike (just like the OptiBikes) is just ridiculous...
That's for those to say.. "Look at me, I spent $10k on a bicycle".
I wanna be your friend and borrow 1/2 of that and make a bike 2 times faster with 10 times the range!

1st of all, it is not mine. I did not pay $10K for it.
2nd: It's and impressive bike, custom frame, custom high torque motor, hydraulic disk brakes, motorcycle tires, a transmission instead of a derailleur, and other amenities.
3rd: the AH of the pack is 18AH and that's conservative. It lasted all day at the track with one charge. About 4 dozen people rode it all afternoon.
Lastly: It's probably got $5K worth of parts and $1K of labor into it, so $10K is steep, but so is paying for a Lamborghini.

 

[Gordo]

Floont,
It is a very impressive bike. I see nothing wrong with anyone who can afford it. For those who can't, go build as close as you think you can, then show up and race it.
I like it as I am trying to build something which will just roughly resemble the frame. I really like the drive chain coming from the swingarm pivot point. Perfect.