How do we properly measure AH for LIFEPO4

[recumbent]

I've had the same LIFEPO4 battery now for over a month, ride 30 KMs per day average, but never reach the 12 AH rating according to my CA meter. It's plenty of range and rarely reach the end, unless i try.

I realize there are many variables, except one BIG ONE, and that is the BMS cut-out.
Is the power still left in the batteries part of the 12AH's? because my BMS cuts out at about 40 volts.

And no i never manually ballanced my cells yet. Although i will open-up the shrink wrap and check the cells when things slow down a bit, like winter.

Should i open the battery pack and manually balance my cells earier?

 

[Ypedal]

http://www.ypedal.com/Lbd.htm


:wink:

I would open it up and install a pigtail of wires to every cell junction to you can monitor the cell voltages externally..

 

[dogman dan]

I would think if your pack is supposed to have 12 ah, that is a number for what the battery could potentially deliver, if all cells were fully charged, and discharged perfectly. The actuall performance could easily be substatially less, especially if imbalanced. And some will need to be left when the BMS cuts off, and again, that cut off point could vary depending on how evenly it discharges. I've got no cycleanalyst, but my ping would theoretically go 25 miles or more, but the furthest I've actually gone is 18. A lot of that has to do with the way I ride, but my expected 25 mile range takes that into account. I think the rate of discharge on an ebike is not ideal for even discharge. This is the stuff Don Harmon is always pointing out about prismatic cells. Maybe we could wring more out of our lithium if we only went 5 mph, but I won't be doing that.

 

[Link]

Actually, good cells are rated conservatively, meaning you'll get a bit MORE than what they're rated for.

I'd check the cells if I were you. Could cause problems if they aren't balanced properly.

 

[magudaman]

my lifepo4 A123's seem to put out pretty close to what i would suspect for ah. Sounds like the batteries you got might have been under rated

 

[recumbent]

I would open it up and install a pigtail of wires to every cell junction to you can monitor the cell voltages externally..

Spoken like a true "battery guru" I will graciously take your advise and get out my soldering iron and bundle of wire one rainy day.
Thanks, It's after all, a relatively big investment.

 

[dogman dan]

Ok I speculated on the answer to one of the questions. We seem to agree an unbalanced pack could cause this problem. Link says the battery specs should be conservative on good cells, and I can agree with that. But does anybody know if when the bms cuts out on a 10 ah battery it has used all it can deliver of the 10 ah, or is the battery expected to deliver only the majority of the amp hours and reserve some of that 10 ah to prevent reversal? I'm having trouble phrasing this. Is a 10 ah battery really larger that 10 so it can deliver 10 without damage? We all know not to expect 10 ah out of a 10 ah lead battery so do we only expect 9.5 or something out of a 10 in Lifepo4? Now you guys got me curious. I bought a big one and have lots of extra capacity so it's just something I'm wondering about now.

 

[Ypedal]

Cell reversal is a " NIMH thing " .. not really applicable to lithium.

Cells are rated at the factory, but as we all know , take that with a grain of salt.

A " Pack " is only as good as it's weakest cell.. so if the BMS is working properly, it's set so that it will cut out before the cell is 100 % empty ( 2.0v per cell for most LiFePo4 of the chinese brand X variety )

LiFe has a very flat discharge curve, so if the bms cuts out at 8, 9 or 10 ah, take that as your " usable capacity " and if it gets worse over time.. the pack needs to be balllanced so that the low cells are charged up to their max level, then the whole pack will provide more ah's !

 

[recumbent]

"Dogman" made a good point about usable capacity. And Ypedal's answer.

This means that my 12ah pack only has a usable capacity of approximatly 10ah.

Be forwarned everybody, 12ah=10ah and your range should be about 45 kms.

 

[Deepkimchi]

Think my 12ah cells from FalconEV were getting 11.2 - 11.5 ah. Would worry if it were 9-10 ah.

DK

 

[Ypedal]

"Dogman" made a good point about usable capacity. And Ypedal's answer.

This means that my 12ah pack only has a usable capacity of approximatly 10ah.

Be forwarned everybody, 12ah=10ah and your range should be about 45 kms.

Lol.. not exactly like that. If the pack is currently well ballanced and all cells stay in the same voltage range all the way to a min. of 2.0v and the CA meter says you used 10ah.. then that's your usable capacity.

If you start to have one cell or more hit LVC under load when you hit the throttle, then consider this the point to put the feet on the pedals.

Not the best idea to drain ANY battery pack 100 % frequently, imo.

 

[dogman dan]

Amen to that, I did a ride to cutoff after my pack was broken in just to know where I stood without having a wattmeter.
When I made it 18 miles I knew I bought the right sise battery since my normal ride is no more than 14 miles to a place to charge. That should put me on the road to actually get the thousand cycles, if not more. Unless I crash again and smash it, LOL.

 

[recumbent]

If you start to have one cell or more hit LVC under load when you hit the throttle, then consider this the point to put the feet on the pedals.

Not the best idea to drain ANY battery pack 100 % frequently, imo.

You didn't approve of my interpretation? :x I was just pissed at the Chinese battery bullshit system, 12ah should mean 12ah period.

As for the one cell going low under acceleration... This is what kicks the LVC off and the whole system shuts down, with little warning.

8) Thanks for clearing it up for us Ypedal. I know i should check the cells, but i thought this was done at the time of purchase according to the owner. They didn't send them until they were checked.

 

[Johnbear]

---

You didn't approve of my interpretation? :x I was just pissed at the Chinese battery bullshit system, 12ah should mean 12ah period.

I agree with this 100%

 

[Battboy]

There are many factors at play here. To try to give a simple answer is just not possible. Some things that have been said are true and some are dependent on the manufacturer of the cells. In any event, we don't recommend people try to squeeze the full amp hours out of any LiFePO4 battery pack. That is why you have a VMS or BMS to protect the pack. My advice is to invest in more amps than you expect to use by 10% at least. This way you will extend the life of your battery pack and not have to worry about the whole issue. Just my two kw.

Don Harmon

 

[dogman dan]

Exactly right Don. I hope to never see my pack drained to cutoff again. I bought what I hoped would be twice what I needed, and it turned out I have about a 30% reserve. Hopefully that will be enough with cheapo prismatic cells. I hope to build a motorcycle some day with your cells.

 

[Battboy]

Dogman - Since you seem to grasp the point I will give you a simple formula to use with all LiFePO4, although some cells are higher quality, it should hold true for them all:

You really want to spec the pack around a 3C draw and 2C recharge. Doing so increases the efficiency. Its not a large increase, but a 3C draw means 20 mins of full power. Lowering the C rate increases increases the pack operation time, so the lower the C rate, the better. You never really need full power, just enough power to combat wind resistance, so there is a balancing point that is in effect. Most people will lean towards a less expensive pack that gives optimal performance but lacks in range. We just have to let them do what they can.

Best,

Don Harmon