Storage voltage for LiFePo4 cell?

I have no immediate use for my LiFePo4 (NOT A123) pack. I will be storing it for 6 months. I have already disconnected the "Battery Murdering Systems". What is the proper storage voltage for LiFePo4 cells? No, not LiCo cells (about 3.8V for storage).

LiFePO4 storage is about 60% so 3.0V

rick

I fully charge and disconnect the charger.. they sit at 3.2v

3.0 is a touchy number, some cells at 3.0v no load can be nearly empty .. not a good idea imo.

If the pack is made of foil cells in parallel ( ping ) .. i'd highly recommend fully charging, put a load on it for a minute to drop them from 3.5v+ to 3.3 or less and then let them sit.. if one cell in a parellel group is self discharging slowly, it's best to leave it with as much energy as possible to prevent them going below 2.0 over time.

The thumb rule is keep them between 40 and 50% SOC ( state of charge).. so ue the Ah not the voltage since the LiFePo4 cells have a very flat charge or discharge curve and 50% son is still in the 3.2V area while the 70% and 20% are in the 3.15 to 3.3V..

Most of the commercial lithium cells/battery that are stored for general purpose or included in a product ex: iphone etc.. are stored at 50% SOC

Doc

Thank you guys. My pack capacity is 334 Whs. So I will remove around 150 Whs before storing.

You can also store it in the fridge (not the freezer) to slow down the degradation process.

That's what the lipo guys do, should also work for Lifepo4.

The lower the SOC the cells are at during storage, the better. But if you go too low, this risks the cells self-discharging too far. As Doc mentioned, 50% SOC has become the compromise. This is a fairly low SOC to help slow the degradation of the cells but is still high enough to help prevent self-discharging too far before you can charge back up to 50%.

Don't use voltage as the measurement for long-term LiFePO4 cell storage. Fully charge and balance the cells and then discharge to 50% of their rated capacity or less. If you're around to check the cells every month, you can go down a lot lower than 50% SOC. But, I don't think that's going to give you enough of a benefit (i.e., longer cell life) to make the extra checks worth your time.

CamLight said:

The lower the SOC the cells are at during storage, the better. But if you go too low, this risks the cells self-discharging too far. As Doc mentioned, 50% SOC has become the compromise. This is a fairly low SOC to help slow the degradation of the cells but is still high enough to help prevent self-discharging too far before you can charge back up to 50%.

Don't use voltage as the measurement for long-term LiFePO4 cell storage. Fully charge and balance the cells and then discharge to 50% of their rated capacity or less. If you're around to check the cells every month, you can go down a lot lower than 50% SOC. But, I don't think that's going to give you enough of a benefit (i.e., longer cell life) to make the extra checks worth your time.

Click to expand...

So for really long term storage, would it be beneficial to have a charger trickle the battery? Like a purpose-built charger that keeps the battery at a low SOC, but above the damage line?

cal3thousand said:

CamLight said:

The lower the SOC the cells are at during storage, the better. But if you go too low, this risks the cells self-discharging too far. As Doc mentioned, 50% SOC has become the compromise. This is a fairly low SOC to help slow the degradation of the cells but is still high enough to help prevent self-discharging too far before you can charge back up to 50%.

Don't use voltage as the measurement for long-term LiFePO4 cell storage. Fully charge and balance the cells and then discharge to 50% of their rated capacity or less. If you're around to check the cells every month, you can go down a lot lower than 50% SOC. But, I don't think that's going to give you enough of a benefit (i.e., longer cell life) to make the extra checks worth your time.

Click to expand...

So for really long term storage, would it be beneficial to have a charger trickle the battery? Like a purpose-built charger that keeps the battery at a low SOC, but above the damage line?

Click to expand...

Unfortunately, that wouldn't work. Chargers are based on voltage, not state-of-charge. The charger could never maintain the pack at a low SOC, it would always charge to the selected voltage. You could select a very low voltage, obviously out of the packs middle-SOC area where the voltage is so constant, but IMHO it's not worth it. LiFePO4 self-discharges so slowly it's typically always better to discharge the pack to about 50% and just check it every couple of months. If you're approaching a low voltage, just charge the pack up a bit. No need to fully charge and discharge to 50% every time.

After a few months of doing this you'll have a good idea of how often you have to add a bit of charge to the pack. But, if you're storing the pack for that long, it might actually be better to sell the pack and buy a new one when you need it. The pack ages even if not being used and storing a pack for longer than a season or so might not be the best strategy if that happens a lot.

Ok, here's a query that applies to me and that I was thinking, with winter approaching, of making on this forum. I have a Ping LiFePO battery and, with the marginal weather we've been having these last few weeks, we're doing less and less outdoor riding. Soon we'll stop riding outdoors altogether and will switch to riding indors on stationary bikes. The battery will be sitting idle for 4-5 months, and I'm trying to figure out the best way to store it.

So, is draining the battery to about 50% the right way to store it for that period of time? There seem to be varying opinions about this. And, by the way, how do I know what 50% (of this 36 volt, 20 Ah battery) is? I've got a CA v 3 that tells me how many amp hours I've used. Do put a full charge in the battery, then run it down about 10 Ah?

On a somewhat related note, there is talk in this thread--and I've seen similar statements in other forum threads--about "balancing cells." Not being terribly well informed about electricity and electronics, I presume this means bringing all cells that make up the battery to roughly the same voltage level: is that correct? If so, how does one go about balancing a battery like the Ping LiFePO? After all, it's got a circuit board (the BMS, I assume) taped to the top and the whole thing is shrink-wrapped. Would the balancing being spoken of require gaining access to individual cells? Or is there some other way of doing it?

LATER EDIT: I see over at http://endless-sphere.com/forums/viewtopic.php?f=3&t=15784 that the BMS should be disconnected for long-term storage so that it doesn't drain down too far the cell it uses to power itself (note to self)

The dominate component of calendar decay is based around potential related decomposition and temperature, and the potential is buffered to be roughly the same from 20-80% SOC.


Keep LiFePO4 cells in a cool place between 20-80% SOC and they will be as well preserved from aging as is practical for a consumer to do.

Exactly. The main hazard with ping and many other bms equipped packs is that the bms runs on just a few of the cells, not the whole pack. So a bms can draw down just those few cells to 0v, without the lvc cutoff doing anything about it. The lvc will cut off the main discharge wires, but the bms keeps on running down the cells it powers up on.

So the number one thing is unplug that bms. Then store in a cooler place than right by the heater in the house. A garage than doesn't actually freeze would be ideal, or just a less heated room in the house is fine.

I'd say a good plan would be to draw about 5 ah out of the pack, then store it.

For those who cannot unplug a bms from the pack, storing it fully charged and recharging it periodically would be best. Still keep it in a cool room.

dogman said:

Exactly. The main hazard with ping and many other bms equipped packs is that the bms runs on just a few of the cells, not the whole pack. So a bms can draw down just those few cells to 0v, without the lvc cutoff doing anything about it. The lvc will cut off the main discharge wires, but the bms keeps on running down the cells it powers up on.

So the number one thing is unplug that bms. Then store in a cooler place than right by the heater in the house. A garage than doesn't actually freeze would be ideal, or just a less heated room in the house is fine.

I'd say a good plan would be to draw about 5 ah out of the pack, then store it.

For those who cannot unplug a bms from the pack, storing it fully charged and recharging it periodically would be best. Still keep it in a cool room.

Click to expand...

I'm gathering that BMSes are made to be cheap and that adding a DC circuit to handle the pack voltage would be costly.

Would there be any sense in adding DC circuits that run off the whole pack and supply the BMS with the power it needs? So that cells aren't drawn at uneven rates?

The only way to get my ping to balance itself was to leave it on charge for about 36hours letting it bounce innocent cells to 3.95 volts all night long.

I pulled off the bms and added a balance port instead so I could charge it with an rc hobby charger. Once balance charged I would set the charger to storage and it would take something like 3-4ah out of my 20ah pack (around 3.34v a cell). Three years later my lazy gf seems to have squeezed 22ah out of the pack draining it down to 3.18v a cell (maybe my charger isnt 100percent accurate but it cant be too far out).

each "cell" is actually 4 in parallel i think and if I leave it fully charged by accident for a month at 3.58v I come back to cell number 7 at 3.35v. All my other cells sit at 3.58v for weeks. I always thought I had mismatched cells from day one but a little attention and effort still has me getting a 50 miles range with my lazy gf out of a 24v 20ah pack and 500w conhis.

Im sure leaving 3 cells fully charged paralleled with a duff one over weeks is bad but as I stored it at the nominal voltage over 3 years/winters it never showed it up or decreased its range. So for anyone with no real want or need to measure storage capacity or to buy and install better chargers maybe just setting it to its nominal voltage at resting like I did is good enough.

I have left my ping battery unused and unchecked for over 6 months in the past. I just left it on the bike in the garage and did not disconnect anything. When I went to finally use that bike, I thought I should charge it first. The charger ran for just a few minutes and the green light came on. The bike rode fine and did not seem to lose any range or power.

I also have an old jimmywu/cammyc battery (remember those). I use it every Christmas to power a string of led Christmas lights that I set up for 36v. I don't disconnect anything and, same thing, I go to charge it after a year and the charger kicks off in just a few minutes.

Sorry for bumping an old thread, but does this info still apply today?

I have a 48V10AH LiFePO4 Aluminum Cased Battery (a little over a year old) and will be deploying for 6 months.

What I pulled out of the thread is:

You can also store it in the fridge (not the freezer) to slow down the degradation process.
Don't use voltage as the measurement for long-term LiFePO4 cell storage. Fully charge and balance the cells and then discharge to 50% of their rated capacity or less.
Keep LiFePO4 cells in a cool place between 20-80% SOC and they will be as well preserved from aging as is practical for a consumer to do.

Click to expand...

My only means of measurement is my usage to work (or using a voltmeter) My commute is about 25km round trip and uses between 5-6ah of my 10ah battery.

So putting the battery in the fridge after work on the last day before the deployment without charging, is what I am thinking of doing...
Is that the correct way of preserving the battery?

Thanks,
Chris

Only if the BMS is disconnected from the pack. The plugs to the small wires unplugged.

In any case, you would not want to store it if more than 50% empty. Charge it, ride around the block a few times, then disconnect the bms is what I'd want to do myself.

If you can't disconnect it, then charge fully, then put in a cool place. Ideally, have somebody recharge it for you about monthly.

I bought 2, 200ah 24v lifpo batteries that I will be using for an off grid house. I wont be hooking them up for several months. The BMS is internal and I can't disconnect it. I have them fully charged at the moment and wanted to know how to check if the batteries are discharging. I have a multi meter and it reads about 26v. How would I check if they are loosing charge and do I want them to loose charge?
If I don't want them fully charged, how do I take some of the charge off of them?