LiFePo4 question.. 3.2v or 3.3v?

[sangesf]

I have always been curious...
Is it 3.2v per or 3.3v per?
I am always reading all the specs and most of the time I notice they say 3.2v per.
When I check all my batteries, they read at an avg or 3.3v per.
(I check at differ SOC)

Why is that?

 

[dumbass]

As I remember they refer to it as the "nominal" voltage. Which to me means normal average voltage. The voltage after charging will normally be much high. Most charge to 3.65-3.70. So it is very common to see a voltage about the nominal 3.2v until you actually put a load on the cell. Then it generally stabilize at 3.2v at rest. Of course it depends on the load applied.

Bob

 

[sangesf]

As I remember they refer to it as the "nominal" voltage. Which to me means normal average voltage. The voltage after charging will normally be much high. Most charge to 3.65-3.70. So it is very common to see a voltage about the nominal 3.2v until you actually put a load on the cell. Then it generally stabilize at 3.2v at rest. Of course it depends on the load applied.

Bob

That's what I mean...
I'll have ridden 10 miles out of my 40 mile range and my batteries will read 39.6 or 3.3v per.
I'm guessing, they mean, the minimum normal voltage at rest is 3.2, BUT can be higher...
Over 90% of my cells, after rest, read exactly 3.30v after a decent load has been put on them.
The rest of them read anywhere from 3.22 to 3.29v.

 

[TMaster]

The A123 cylindrical cells are nominal 3.3V and I think the A123 Prismatics are too. Everything else I've see is 3.2V. That may be because the A123 composition may be slightly different then all the others; but I'm not sure. Hope this helps when you are adding up the voltage to do your calculations. I usually use 3.55V as a peak voltage after charging my A123's. They charge to 3.6V, and level off after 30 min or an hour to around 3.55V. Cells that are older or more used will sit at 3.45V after charging.

 

[sangesf]

Im reading voltages and charging at the cell level. (no parallel groups)
I charge to 3.5 at 5a and then to 3.65 at .5a.
They are 15ah Lifepo4 cells.. (NOT A123)
I have 58 of them...
After fully charging them using the above formula, and then putting a 5ah load on them (5a for one hour) here are the results after resting for 30s to 5m.
47 of them read 3.3xv per.
8 of them read 3.26v per.
2 of them read 3.24v per.
Last one reads 3.22v.

These have been used in many different configurations, in terms of. voltage and AH.
They all have a capacity of 14.5ah to 15.2ah

 

[TMaster]

Those voltage values seem fine from what I know, they are all closely matched except the last 3 compared to the majority of them. Check your brand specs and see what the peak voltage is and not to go over it any. Over charging cells is not good for them. I'm guessing the last 3 cells are lower in capacity? If they are, and you want to be nit-picky dump the last 3 cells and replace them with better ones, or just run it like it is for a while and check again later. I usually don't go by the voltages when I am picking cells. I go by the capacity of each one, then match them into the packs if needed. You need a RC charger if you dont have one, they show how many mAh is going in and out of the cell.

If you are living on the edge draining your packs almost to the LVC, then you may have problems because the weaker or lower capacity cells may get damaged by running out before the better cells. Seeing your cells are 500-700mAh in difference thats pretty bad, I would be sure I had at least 900mAh left in my total bike capacity when I shutoff the bike, not to damage the lower capacity cells.

 

[sangesf]

The cells have been tested from 3.6 to 2.9, that's how I test.
In regards to the cells that are lower in voltage, you are correct, however I got those voltages from testing every cell INDIVIDUALLY, with the above Load (5a - C/3) for an hour. They are not in any of my packs at the moment, I know to keep all the cells as close to possible....

I was just curious as to why most vendors say 3.2v nominal, yet 99% of my packs from vpower/ping/etc have all read 3.3v per cell through most of the DOD.

My plan is to make/use two 48v 30ah packs for my electric chopper.
My current setup using two 36v 20ah packs with 36v(48v capable) 250w motors gets me between 80-100+ miles, and I'm sure you can see where I'm going with this setup...

 

[TMaster]

More votlage is great, go for it! Wow 80-100 miles thats a long way!!!

 

[sangesf]

More votlage is great, go for it! Wow 80-100 miles thats a long way!!!

I'm hoping to break the 200mile distance barrier with the 48v batts. Should do it... More voltage IS good AND more Ah at the same time should help...
My ultimate goal would be a 250 mile at 20mph.. As soon as I'm ready to start testing, I'll start a new thread.

 

[Gordo]

As I remember they refer to it as the "nominal" voltage. Which to me means normal average voltage. The voltage after charging will normally be much high. Most charge to 3.65-3.70. So it is very common to see a voltage about the nominal 3.2v until you actually put a load on the cell. Then it generally stabilize at 3.2v at rest. Of course it depends on the load applied. Bob

I think you will find that like automotive batteries being 6V or 12V the LiFePO4 "nominal voltage" is 3V. A 20S LiFePO4 pack and controller are described as 60V but the charge does not stop until 72V. Nominal does not mean normal, here is he definition;
"Nominal means 'named'. So a nominal voltage is the 'named' voltage." For example, the nominal voltage of an automotive system is 12V. The resting voltage of the battery in the system is 12.6-12.8V depending on the battery chemistry. Slightly higher acid concentration in a glass-matt battery has higer voltage.

 

[AndyH]

I don't know if this is absolutely accurate from an engineering perspective, but have found this definition in a number of papers so believe it's at least fairly close... Nominal voltage may be the cell voltage at 50% SoC.