LiFePO4 voltages

[aseb]

Can anyone tell me the voltages for a 36V LiFePO4 battery, or where I can find them? I've looked but come up blank. I want to try sorting out an led meter to show Voltages for fully charged, Flat (cut off point), 10%, 20%, 30%,through to 90% battery left. (Not sure I'll use all points, perhaps just cut off, 10%, 20% 50% 100%).

How do the fully charged voltages and cut off/flat voltages compare with li-Ion and Lead batteries?

Thanks

 

[flecc]

LiFePO4 (lithium iron phosphate)

Cell voltage = min. discharge voltage = 2.8 V.
Working voltage = 3.0 V – 3.3 V.
Max. charge voltage = 3.6 V.

Approximate for other lithium types like Li-ion manganese

(Typically with 10 cells = 37 volts)

Cell Working voltage = 3.15 - 3.7 V.
Max. charge voltage = 4.2 volts

I think lead acid is similar to the li-ion figures.
.

 

[Grizzly Bear]

Can anyone tell me the voltages for a 36V LiFePO4 battery, or where I can find them? I've looked but come up blank. I want to try sorting out an led meter to show Voltages for fully charged, Flat (cut off point), 10%, 20%, 30%,through to 90% battery left. (Not sure I'll use all points, perhaps just cut off, 10%, 20% 50% 100%).

How do the fully charged voltages and cut off/flat voltages compare with li-Ion and Lead batteries?

Thanks

A fully charged "36 volt" LIFEPO4 battery should be 39 - 40 volts fully charged (after it has been allowed to settle off the charger). There is a sharp drop off in voltage initially, and then it stabalizes for most of the discharge, but when approaching a fully discharged state voltage will drop off very quickly.
This makes it difficult to determine the state of charge with voltage, it's best to use the Cycle Analyst which will accurately tell you the amps used.

Tony a "36 volt" LIFEPO4 battery has 12 groups of cells in series not 10

Griz

 

[onmebike]

I recently purchased a lifepo4 36v and noticed the slightly lower fully charged state as compared to a Li-ion.
Could upgrading to Lifepo4 without changing the controller lead to a slight mismatch with the controllers low voltage cut-off. 31,5 for Li-ion[10cells] and 33.6 for Lifepo4[12cells]?

 

[Grizzly Bear]

I recently purchased a lifepo4 36v and noticed the slightly lower fully charged state as compared to a Li-ion.
Could upgrading to Lifepo4 without changing the controller lead to a slight mismatch with the controllers low voltage cut-off. 31,5 for Li-ion[10cells] and 33.6 for Lifepo4[12cells]?

LIFEPO4 have a battery management systems, so no need to worry about the controller low voltage cutoff, buy a LIFEPO4 battery you will not regret it, unless you are very very unlucky, I've not had a dud yet(11 so far).

Griz

 

[aseb]

Thanks all, esp Flec.

The actual numbers are what was needed, so that the point at which an led illuminates is controlled (to 0.1V), so there is a visual indication of the battery state so fairly accurate figures are needed to give an accurate indication of the kind of range I should have left.

The original batteries are LiPolymer and I was wondering what the points that the existing led indication triggers, I was quite sure its indication would not be relevant to the new LiFePO4 battery (just confirming to make sure!)

The cut off for LiFePO4 is controlled by the BMS, so isn't a worry- the battery will cut out before damage is caused- the warning is so I get a warning before I have no power. As the figures are different to LiPolymer I need a new indicator and all those I've seen commercially produced are pre-set for Lead batteries. Some suppliers say they can customise the meters for you but they need the explicit figures for trigger points. One supplier who did reply says there is a minimum order of 50 units so I could look like a Christmas Tree. So adding a few leds triggered at specific voltage points will work out cheaper and shouldn't be too hard to produce using close tolerance resistors once I have the accurate voltages determined. And I can pick the points thay will be most helpful- say cut-off, 10%, 20%, 50%, 100% (but I'll think a little more about it).

Thanks!

 

[Grizzly Bear]

Thanks all, esp Flec.

The actual numbers are what was needed, so that the point at which an led illuminates is controlled (to 0.1V), so there is a visual indication of the battery state so fairly accurate figures are needed to give an accurate indication of the kind of range I should have left.

The original batteries are LiPolymer and I was wondering what the points that the existing led indication triggers, I was quite sure its indication would not be relevant to the new LiFePO4 battery (just confirming to make sure!)

The cut off for LiFePO4 is controlled by the BMS, so isn't a worry- the battery will cut out before damage is caused- the warning is so I get a warning before I have no power. As the figures are different to LiPolymer I need a new indicator and all those I've seen commercially produced are pre-set for Lead batteries. Some suppliers say they can customise the meters for you but they need the explicit figures for trigger points. One supplier who did reply says there is a minimum order of 50 units so I could look like a Christmas Tree. So adding a few leds triggered at specific voltage points will work out cheaper and shouldn't be too hard to produce using close tolerance resistors once I have the accurate voltages determined. And I can pick the points thay will be most helpful- say cut-off, 10%, 20%, 50%, 100% (but I'll think a little more about it).

Thanks!

Did you read my post?

Griz

 

[wurly]

Rather than using led's i would recommend a power analyser meter like a Turnigy. Voltage isn't going to give you a real idea as to the capacity used or what is left in the pack, you really need to know the Watt/Hours used.
I bought mine for about £26 and use it when i'm on a long jaunt somewhere or to see about power i have used by monitoring the charge.

Picture shows how much power went into my pack after charging it. Or vice-versa, connect it between your battery and controller and see how power you consume from the pack.

Do you run your LiFePO4 packs down to LVC often Griz?
I'm a bit scared of doing that

 

[Grizzly Bear]

Rather than using led's i would recommend a power analyser meter like a Turnigy. Voltage isn't going to give you a real idea as to the capacity used or what is left in the pack, you really need to know the Watt/Hours used.
I bought mine for about £26 and use it when i'm on a long jaunt somewhere or to see about power i have used by monitoring the charge.

Picture shows how much power went into my pack after charging it. Or vice-versa, connect it between your battery and controller and see how power you consume from the pack.

Do you run your LiFePO4 packs down to LVC often Griz?
I'm a bit scared of doing that

I try to avoid that, I charge after every trip, But who am I to say, I'm building LIFEPO4 A123 M1 packs myself now, I must be thick!

Griz

 

[aseb]

Rather than using led's i would recommend a power analyser meter like a Turnigy. Voltage isn't going to give you a real idea as to the capacity used or what is left in the pack, you really need to know the Watt/Hours used.
I bought mine for about £26 and use it when i'm on a long jaunt somewhere or to see about power i have used by monitoring the charge.

Picture shows how much power went into my pack after charging it. Or vice-versa, connect it between your battery and controller and see how power you consume from the pack.

Do you run your LiFePO4 packs down to LVC often Griz?
I'm a bit scared of doing that

I looked at these meters a while ago but I'm sure the price was far higher- I think it was Cycle Analyst but with the speedo function, I'd forgotten about the others.

 

[jerrysimon]

I have built a small A123 pack (36v * 2.3mAh) and in the six months plus of using the pack most days, have found they don't need a balancing circuit. These cells are Nominal.: 3.3V, Max.: 3.6V, Min.: 2.0V. I have balance taps on the two 6s packs and have checked them quite a few times finding only a few mv difference between cells. By using 12 cells in series (measured 43.2v at full charge) the low voltage cut out in the controller cuts off at 31.5v which equates to 2.6v/cell well within tolerance of the cells. That said A123, M1 cells are extremly forgiving both of over charging, fast discharging and taking down to low voltages.

All the details of my build are here.

PS I can also thoroughly recommend that little Turnigy meter to find out what is going on when you are charging and discharging packs. The link above shows this and also a CellLog meter which plugs into the balance tap of each 6s pack and records the individual voltages of each cell (every 2 seconds) over a 30+ minute ride.

Regards

Jerry

 

[flecc]

Tony a "36 volt" LIFEPO4 battery has 12 groups of cells in series not 10

Griz

I know, read my post which clearly states 10 cells for other lithium types, not LiFePO4, and it's 37 volts nominal, not 36 for those other types.
.

 

[tangent]

I know, read my post which clearly states 10 cells for other lithium types, not LiFePO4, and it's 37 volts nominal, not 36 for those other types.
.

Are all 36V LiFePO4 chargers designed to charge 12 cells in series?

 

[flecc]

That I don't know offhand Tangent, but I'd assume so since as Griz says, they normally have 12 cells to make up for the lower cell voltages than other lithium types.

If you check a lithium iron phosphate charger's output voltage on an accurate meter and it's near to but no more than 43 volts, it's for 12 LiFePO4 cells.
.

 

[aseb]

Won a GT Power meter off eBay for about £18 inc carriage, less than a pure voltage display and this allows 2 inputs so should be ok with both battery types connected. I just need to work out which diodes to get to stop the batteries back feeding each other because they are different types.
Thanks for the pointer to these meters!

Added note: It looks like the 2nd input is not for load, but for memory retention- but info given isn't very explicit.

 

[tangent]

Thanks, I do not have a need for LiFePO4 at present, but I think the centre of gravity is moving in that direction - particularly for the lower capacity batteries made from A123 cells.

Just something to ask about on ebay when ordering a 36V charger!

 

[jerrysimon]

Again from my battery build post, I have been using this this charger to successfully charge the 12s pack for the last 6 months. Cost me £8 off ebay. I have two one at home and one at work.

The pack can also be spilt into tw 6s packs and balance charged but as I said does not need it except for the first charge when you build them.

Regards

Jerry