LiFePO4: is it better to discharge to 80% or 40%?
- Thread starter callagga
- Start date
cell_man
100 kW
It's generally accepted that LiFePO4 will last longer if the DOD is limited a little but you can't compare it to SLA. So yes I would say that 40% is better than 80%, but even discharging to 80% or more is not really a big problem and you should still expect a long cycle like. The real killer is trying to pull more current from a pack than it is rated for.
oldhaq
100 W
I've also wondered if when running-in a new battery, it is said the capacity increases slightly after each charge. Could this capacity be dependant on how often and/or how far the battery has been discharged then charged, or will it eventually reach the same capacity anyway?
dumbass
100 kW
oldhaq said:
To best my knowledge it's a matter of run time. So if you make short runs rather then long runs it just takes a little longer to reach the max capasity of the pack. That being said most people will tell you lifepo4 cells should be condicioned when new. Meaning you should not run them more them 60% to 70% for the first 10 to 20 charges. So if you run them only 40% they will likely be at full capasity before you get them condicioned (20 charges/discharges).
PS...2 40% discharges is better then a single 80% discharge. Both will not be a problem but the 2 40% discharges should add to the life cycles.
I've been conditioning my lifepo4. I"m going to do about 15 runs before really going for a long distance. The reason I think that conditioning is good because of the way lithium is currently built. They have these electrons and the battery works because the electrons travel from one side (the anode side) to the other side (the cathode side...or vice versa...might have those backwards). Ok, the electrons travel in tiny little pathways in the carbon. If you've ever seen a lithium battery taken apart, it has carbon sheets. Now, the reason why (I think) that conditioning is good is because it gets the electrons used to traveling to and from one side to the other without generating lots of heat. My theory is that it's the heat that destroys the pathways. Without the pathways, the electrons can't move from one side to the other. So if you take short runs that don't generate lots of heat, the electrons have a chance to really form those pathways and generate new ones. LIke if you took your battery out and really used a lot of it and generated lots of heat on it and burned it up, the electrons move from the anode to the cathode side but now the battery is so hot that the electrons can't move back to the anode side and are stuck on one side because the pathways were destroyed.
I only understand it partially so I might not be completely correct but it's something along those lines.
I only understand it partially so I might not be completely correct but it's something along those lines.
The reason why A123's are so much better is that scientist discovered that with bigger pathways for the electrons to move from one side to another, the batteries can be recharged faster without damage. The heat generated is from the electrons having hard times moving from one side to the other. I think that's generally what heats up a battery. That's why they say that a battery with a very low internal resistance doesn't heat up very much because the electrons have big pathways that they can move in from one side to the other.
John in CR
100 TW
40% definitely. A123 released a test report showing a 100,000 cycle life using a 50% DOD with a conservative max charge with one of their newer cells. Yes charge at work, and ideally trim that max charge down, or use a timer. If given the option I'd always want my bikes near full any time I get on them. The only exception was while I was running a really big pack and only needed to charge once or twice a week.
A bigger benefit of charging at work, is the extra freedom of transportation it gives you in case you want to stop somewhere else on the way home, go a different route, whatever. Be sure to tell or show your boss how little electricity you are using, so he doesn't think you're getting one over on him. Also, tell him about the tax credit related to biking facilities at work, so the company can come out ahead. Be sure to get yourself, or better, have the boss use some of that $20/mo to get a charger to keep at work. No use carrying around a charger unless you're going out for a long ride.
A bigger benefit of charging at work, is the extra freedom of transportation it gives you in case you want to stop somewhere else on the way home, go a different route, whatever. Be sure to tell or show your boss how little electricity you are using, so he doesn't think you're getting one over on him. Also, tell him about the tax credit related to biking facilities at work, so the company can come out ahead. Be sure to get yourself, or better, have the boss use some of that $20/mo to get a charger to keep at work. No use carrying around a charger unless you're going out for a long ride.
dogman dan
1 PW
I haven't seen any charts like that for lifepo4. But theoretically if you are expecting 1000 cycles to 100% dod at a low c rate, then 2000 cycles at 50% dod should be possible.
As for the 40% vs 80% depth of discharge,,, My experience is that my v1 ping will stay balanced pretty good at 80% dod, and doesn't increase its ability to stay balanced with lighter dod. But once I do a few 100% discharges the pack may get out of balance. I have no info on how various dod levels affect cycle life, but having a battery out of balance a lot can kill cells for sure. I find some shallower depth of discharge cycles help the pack balance again once it gets unbalanced a bit. This is why I recomend a break in period for new batteries. I don't know if the cells are breaking in or not, but I do know some shallow cycles help it balance. They rarely balance really good on the first charge or two. The idea is to get the battery used to taking a full charge. It may be totally a fanasy that it helps, but I can't see how it could hurt unless the charger is set to overcharge in the first place.
So my conclusion is that keeping depth of discharge extremely light is not as important as with lead batteries. Somewhere between 80% and 100% is the point where my battery starts to recharge less balanced. So if you can go to work and back on 80%, I'd say go ahead and charge at home. But if you start running out on the way home charge at work too.
As cellman says, keeping the discharge rate in low enough for the cells is probobally much more important for all chemistries. If a cell is made for 5 c for instance, it's bound to last longer discharged at 2.5 or 3 c rather than 5. That's why I recomend at least a 15 amp hour pingbattery for a 20 amp controller.
As for the 40% vs 80% depth of discharge,,, My experience is that my v1 ping will stay balanced pretty good at 80% dod, and doesn't increase its ability to stay balanced with lighter dod. But once I do a few 100% discharges the pack may get out of balance. I have no info on how various dod levels affect cycle life, but having a battery out of balance a lot can kill cells for sure. I find some shallower depth of discharge cycles help the pack balance again once it gets unbalanced a bit. This is why I recomend a break in period for new batteries. I don't know if the cells are breaking in or not, but I do know some shallow cycles help it balance. They rarely balance really good on the first charge or two. The idea is to get the battery used to taking a full charge. It may be totally a fanasy that it helps, but I can't see how it could hurt unless the charger is set to overcharge in the first place.
So my conclusion is that keeping depth of discharge extremely light is not as important as with lead batteries. Somewhere between 80% and 100% is the point where my battery starts to recharge less balanced. So if you can go to work and back on 80%, I'd say go ahead and charge at home. But if you start running out on the way home charge at work too.
As cellman says, keeping the discharge rate in low enough for the cells is probobally much more important for all chemistries. If a cell is made for 5 c for instance, it's bound to last longer discharged at 2.5 or 3 c rather than 5. That's why I recomend at least a 15 amp hour pingbattery for a 20 amp controller.
cell_man
100 kW
callagga said:
What John is saying is that the the battery will last longer if not fully charged. Fully charged is typically taken as 3.65V with minimal current being pulled from the charger (100% SOC state of Charge). It's actually not quite right to say 100% SOC occurs at 3.65V, it depends on temperature but 3.65V is generally accepted to be a safe level, although many BMS routinely allow cells to go quite a bit higher than this.
If you could take off 10-15% of the charge at the top and always leave 10-15% at the bottom, Lifepo4 will last a lot longer. It's not so easy to charge to exactly 90% soc as the BMS will be set to balance at 100% SOC and the standard chargers also have a fixed charge voltage. If you reduced your charge voltage to say 3.55 or 3.60V, there will still likely be cells that reach 3.7V or so. To accurately control your charging level you would need a balance charger or a BMS/charger combination for the lower voltage.
