LiFePo4 Batteries: Care and Feeding For Beginners

[PeteCress]

2010 02-01:
I am starting this thread with the aim of getting consensus on a set of rules on taking care of LiFePo4 batteries for noobs like myself.
As Those Who Really Know chime in with additions and corrections, I will update this first post in the thread in order to get it all together in one place.

2010 02-02:
Added Rule #8 per Rifle.

2010 02-03:
Made some changes to Rule #1 per DogMan's observation that "Break-In" is really just balancing cells.
Qualified Rule #5 to specify smart charger and added Rule #00 - per NorthernMike, Dogman, and lester12483's comments.
Added some qualifications/questions to Rules 2 & 4 around the percent discharge/BMS limit issue

Fellow Noobs: Don't take this too seriously until the experts have started to chime in and it's had some time to mature.


___________________________________________________________________________________________________________
00) Use A "Smart" Charger And A BMS:
Many of the following rules presuppose both:

• A charger that auto-senses battery voltage and cuts power accordingly. Not all chargers do this. Some just keep stuffing electrons into the battery forever - regardless of the battery's charge state. These "Dumb" chargers will cook the battery if left on too long beyond full charge. A "Smart" charger will not.
  [pre][/pre]
• A Battery Management System circuit board between the battery and the charger and motor.
  Said board is responsible for cutting off the battery before it is totally drained and for helping to ensure that each cell in the battery is charged to the same voltage. Some batteries come in a hard case with the BMS board inside the case. Other batteries, especially shrink-wrapped or duct-taped come without a BMS and have to have one added by the user.

01) Break It In:
A new battery should not be drained beyond 30% of it's rated capacity for N cycles. Opinions on N vary from three to twenty.

"Break In" is technically incorrect, but it gets the message across. The real reason for doing this is to thoroughly balance the cells so that they are all at the same voltage. This process takes a number of charges. One of our resident experts cites 5 as the number that got one of his batteries into acceptable balance.

The liabilities of running the pack beyond 30% on those first few cycles are:

• Possible technical violation of the battery's warranty contract. Read yours tb sure.
  [pre][/pre]
• The possibility that some cells are so out of balance that they could be drained totally before the other cells reach BMS cutoff on a BMS that does not monitor cell-by-cell. This would introduce the possibility of damaging the mis-balanced cells.

Bottom Line: If you have a 10AH battery, don't feel the least bit bad about only running it for 1 or 2 or 3 AH each ride in the beginning.


02) Never Drain It:
Batteries should not be run to complete exhaustion. 80% seems tb the commonly-accepted limit.

(We have some uncertainty here: do we mean 80% of what trips the BMS' cutoff? Or, in the case of a BMS that is programmed to cut off at 80% already can we just run to cutoff?)


03) Do Not Charge A Cold (or hot) Battery:
This one is from DeWalt's web site: http://www.dewalt.com/us/articles/article_cordless.asp?Site=cordless&ID=702
"Attempting to charge batteries outside the 40-105 degrees F range can result in a permanent loss of runtime. When batteries are being charged and discharged a chemical reaction is taking place, and if it is too hot or cold the chemical reaction is disturbed causing a loss of runtime."

Bottom line: when you get home from that 3-hour ride in thirty-ish temps make sure the battery has time to get up to room temperature before putting it on the charger. Likewise, don't leave it on the bike charging overnight in a garage where the temp gets below 40 degrees.


04) Consider The Weather:
Batteries have different capacities depending on temperature.

Charts showing voltage against temperature abound, but I have yet to find one for AH against temperature for LiFePo4 batteries.

The spec at http://www.batteryspace.com/prod-specs/LFP18650.pdf seems to imply a 20% loss at freezing temp (0 degrees C, 32 F).

In the thread "LiFePO's range in different temperatures" (http://www.endless-sphere.com/forums/viewtopic.php?f=3&t=13917), Rifle and DocNjOj report about a 20% drop at 50F vs 80F.

(More uncertainty here - as in Rule #2)


05) Charge More Often Rather Than Less Often:
These batteries don't mind being charged more often. The rated cycle life is for full cycles. Charging a battery that is only depleted by 1AH does not impact it's charge cycle life as much as charging it when it depleted by 8AH. This noob would guess that the impact is only 1/8th as much.

In the same vein, leaving a battery on the charger after it has reached full charge will not hurt it as long as the charger is a "Smart" charger - although, from the context of various sources, it seems like this assertion is for hours, not weeks or months - but we have no idea why. NB: If the charger is not a "Smart" charger and you leave the battery on it beyond a full charge it will damage the battery.


06) For Long-Term Storage, Cooler Is Better:
If you need to store a battery over the winter, for instance, it is best stored in a refrigerator.

(maybe Somebody Who Really Knows can confirm or refute this.... and add something about self-discharge and how often it should be topped off. Somewhere, I read that storing a battery at partial charge was less wearing on it that at full charge... but I think that was for another chemistry)


07) Storing A Dead Battery Will Kill It:
Maybe a corollary to Rules 2 and 5: if it's down, charge it.... The long it sits in a "dead" state, the greater the chance/extent of damage.


08) Disconnect The Controller:
The controller draws current even when everything is turned off and the parasitic load from the controller can drain the battery.
Drain Battery=Bad Thing per Rules 2 & 7. Leaving it on overnight isn't going to be any big deal.. maybe a quarter mile less range the next day, but day-in-and-day-out can be another story. Suggested rule of thumb: unplug the battery when the bike is not being used.
___________________________________________________________________________________________________________

 

[SilverSurfer]

Great thread Pete. I dont even have all my LiFePo4's yet but will be watching this thread closely. If I may be so bold as to add a question within the topic, I already bought one 24v20ah LiFePo4, and now heard that some dealers are selling L-ions as fake LiFePo4"s! Whats a newbie to do? How do you know if you got - RIPPED OFF?!

 

[Rifle]

Under storage, you should add that the BMS needs to be disconnected so it doesn't kill the cell group it uses to power itself.

 

[dogman dan]

I have to agree with all you have said Pete. It's a great summary of the best approach. But some of the rules can be bent and other not at all.

Some totally disagree with the need to break in a battery. I just say why not start out easy on the thing, that way any problem that may ocurr can not be blamed on you by the vendor.


I've been getting away with bending a few of the rules with my first pingbattery.

I do try to limit discharge depth to about 75% in routine use. But come winter, it cools off, and the wind blows in my face riding home. So all winter dod may be more like 90% to get home, and often enough, I'll pop the bms lvc as I ride up the driveway. I never hesitate to discharge 100% when I need it. But I do see some occasional loss of capacity the next day after a 100% discharge, so it seems more likely to leave the battery unbalanced on the next charge if I ride till the bms cuts it off. If I expect to discharge 100%, I like to go easy on the battery towards the end, and ride slower the last few miles.

All summer, my bike charges outside at work, where it may be 90F, or in the garage at home where it may be 120F. Two years and 5500 miles later the battery still has great capacity. I personally belive that if you body can stand the temperature, your battery can too. The operating range of lifepo4 is beyond what humans can stand. Don't misunderstand, 50-80F for everything may be better, and measurable with science. I just don't think you will see a difference from storage or charging temps that is as noticeable as the difference you will see from how you discharge the battery.










For sure disconnect the bms for long period storage. If you have a long winter, you probobally have a room in the house that is 60F or lower. I don't think you need to put it in the fridge in summer either. Just don't store it in the attic. Mabye in an ice chest if it must be put in a garage, to lessen the peak temps. You could ride the bike around the block to keep the battery from being stored with too much charge. Taking it down to 50% charged sounds risky to me if there is some dang trickle short somewhere.

For shorter storage times, like under 30 days, just top up the charge. Again, it won't make as big a difference as other things, like running too high amp a controller, or living where it's hilly.

 

[nicobie]

I think Rifle meant to suggest unplugging the battery management system (BMS) when storing the battery. Isolating the controller is a good idea too.

 

[northernmike]

Am I incorrect to assume that Lithium batteries of all chemistries are happier resting at 40-50% SOC?

I understood from my research that storing Li at 100% SOC decreases lifespan.

Also, these % values for discharge should be properly quantified. Ie. how is SOC measured? Wh consumed? Ah consumed? Resting voltage?

 

[northernmike]

Also, regarding point #5, "leaving a battery on the charger after it has reached full charge" WILL destroy the battery.

 

[PeteCress]

For sure disconnect the bms for long period storage.

BMS or controller?

I've been fixated on the controller.

Never even thought about the BMS - which is integrated into my battery behind "Removal Voids Warranty" stickers.

 

[DerekG]

I was under the impression leaving it on the charger should be fine as long as you have a BMS capable of bleeding away any sort of trickle charge being applied. Most chargers meant for LifePO4 will cut current completely I believe, however, other chargers people have built should not be left connected if they trickle charge.

 

[USAEBIKECO]

I have a Vpower 36v lifepo4 battery and I disagree with you on 1 and 2.

You do not need to break in a lifepo.

When you first get the battery plug it in for a few hours and the BMS balances the cells. This "break in" theory is unfounded.

I have drained my battery many times and have seen no difference in performance. The BMS cuts it off slightly before its totally drained.

 

[dogman dan]

It's an argument based on a misuse of a phrase.

The battery does not "break in" but this is a phrase even the stupidest noob understands. You are absolutely correct that the batter needs only to get balanced.
You are abosolutely incorrect that simply leaving it on a charger is going to garantee a perfectly balanced the battery before cycle 1. Both of my pings balanced OK on the first charge, but balanced a lot better on charge 2, and better still by about cycle 5. After that they both seem to stay balanced pretty nice. It's a small sample, two batteries from one vendor, but this is what I have measured and observed my batteries doing. Some jackass battery vendors have used the way the customer did the first few cycles as an excuse to refuse to honor a warantee. So go ahead, give them another reason to screw ya if it makes you happy. It's not so much the battery needs it as that covering your own ass is a good idea. A light discharge for a few cycles is mostly a way to get the first few cycles done before it takes a week to put that many miles on the battery. If the bms happens to be defective, then you are screwed anyway, and may ride till cells die. At some point, you may want to very carefully confirm that the bms lvc does function on the bms.

Re no 2..

It's very well documented that ALL chemistries give a longer cycle lifespan if cycles are shallower, and discharge rates are less than the max continuous rate. With some chemistries the difference is dramatic, such as lead acid. With others, such as lifepo4, the practical real world difference may not matter.

1000 cycles vs 1500 cycles may not mean a rip unless you ride the kind of miles I do. Some buy a lifepo4 pack and then calculate it will take them 20 years to use up the 1000 100% dod cycles. So yeah, I do agree that for most people it's pointless to try to extend the batteries lifespan by not riding it to cutoff. My typical use has been a 60% discharge in the morning, and in the afternoon a 75-90% discharge. As the battery aged, I began to see a less balanced recharge on the first cycle after a 100% discharge. But if you have a functioning bms, discharging 100% is pretty much harmless. Many people will never ever use up a good lifepo4 battery since the may never ride 5000 miles in a lifetime. I will hit 1000 75% cycles in 3-4 years. If my battery dies early at only 800 cycles, whoop de doo.

 

[dogman dan]

Re northernmike.

My ping v1 has been left on a charger almost 22 hours a day for 2 years. If I get up in the morning and will not use the bike, I un plug. But when using it, I ride to work, and plug in, ride home and plug in. The pack is not on a charger only when my butt is in the saddle. The pack is fine, after 4500 miles and an estimated 400-500 cycles any capacity loss is still undetectable. Now that is what I call a ruined battery. Don't try this without a bms though. Don't try this without a "smart charger" either guys. You betcha leaving a lifepo4 on a dumb charger too long will wreck it. Dumb chargers can wreck any battery.

Re storage state of charge and temps. My bike lives in the real world, as in, I want it ready to go anytime I choose. So I leave it charging endlessly, storing it for the last two years perpetually 100% charged. The temperature is whatever it is in the garage. Usually not below 32F in my climate, but for months it may be 100-120F in there. Again, after two years, I can't find a detectable loss of capacity. I can ride the bike the same distance, charge time is just as long, and just as many kwh are used by the charger. So what can I conclude from this? My ping cells kick ass? It doesn't matter so much unless the time period is longer than two years? It doesn't matter till temps are above 120? Possibly all of the above.

All I know is after two freaking years of "abuse" my first pingbattery is alive and kicking ass. Whatever you bought may act differently, especially if it is a different chemistry than lifepo4.

 

[dogman dan]

Re the controller or bms?

Disconnect the controller when you aren't riding the bike. It doesn't matter how, un plug, use a switch on the battery, whatever. Even controllers that have a switch on them will draw a trickle when still connected to the battery. Leave it for weeks, and it will drain the battery till the bms cuts off. Leave it for overnight, and you may just have a 1/4 mile less range. Times that I have forgotten, and left mine connected while charging have done no harm. At work, I never bother to unplug while charging, and often have forgotten to turn off a switch, or even don't have a switch. Any trickle the controller uses is replaced by the charger.

For long term storage of the battery, disconnect the bms if possible. For a pack built into a box, this may void the warantee. I can't say about any particuar bms, but some designs draw power from one 3.5v cell in the pack. Some have reported storing a pack for 1-6 months and finding it had one dead cell when they got it out in the spring. So now long term storage is advised to be done with the bms disconnected if possible.

Long term storage at 100% charged is documented to be less harmfull at low temps, so the best advice I can give to those who can't disconnect a bms is to store fullly charged in a cool place if possible, and check the state of charge at least monthly if possible. If I was getting deployed to Iraq what I'd do is void the warantee and disconnect the bms. Beats coming home to a dead pack.

 

[PeteCress]

A light discharge for a few cycles is mostly a way to get the first few cycles done before it takes a week to put that many miles on the battery.

In the interest of clarifying Rule #1, does that mean that there is no downside to running the battery all the way down to 80% right off the bat except for it's taking longer to accomplish the requisite number of charges to get a good balance?

 

[DerekG]

Ehhh as long as you have a good BMS which watches each parallel group you should be fine. Cells are formed in the factory and are ready to go when you get them, however, taking it easy for the first few charges is my rule of thumb too... If you need at or above 80% right off the bat chances are you should have gone with a larger battery

 

[amberwolf]

In the interest of clarifying Rule #1, does that mean that there is no downside to running the battery all the way down to 80% right off the bat except for it's taking longer to accomplish the requisite number of charges to get a good balance?

The only reason to even worry about this is that until the pack is balanced, then some cells are going to reach their LVC before others. As long as your BMS is a cell-level LVC, and actually *works*, either by warning you in a very clear way that you should stop *now*, or by actually cutting off pack discharge, even that doesn't matter.

 

[PeteCress]

I have drained my battery many times and have seen no difference in performance. The BMS cuts it off slightly before its totally drained.

This raises a question that's been in the back of my mind since the moment I pulled the trigger on Rule #2: Should the 80% apply to the amount of AH to BMS cutoff? Or should the 80% apply to the total cell life - in which case one would expect that many BMS' are already taking the 80% number into account?

Put another way: if I know that my BMS is programmed to pull the plug at 80% battery use, can I safely run the battery to cutoff every time?

If this is true, I need to do some editing on Rule #2.

 

[northernmike]

Right, regarding "leaving the battery on charge"...

There are those "beginners" who would try to BE the BMS.

They should not be fooled into assuming that leaving a Lithium battery "on charge" is the same as leaving a lead-acid or Nickel Cadmium or other chemistry "on charge", even if it is a so-called "trickle charge".

The reason for my post was to draw attention to this fact - that Lithium batteries have no chemical method of absorbing or displacing extra electrons.

Over-charging Lithium, AFAIK, will destroy it.

If a "beginner" has the means to set up his own BMS and relies on the wrong metrics, this is a highly probable outcome.

Given the difficulty of understanding and measuring SOC in Li cells, I would hope those considering themselves "beginners" do enough research to at least approach a point where they no longer feel they are total "beginners" before engineering a charging / discharging management strategy for their pack.

I'm leaning towards the G/F BMS as the ultimate solution, but don't have a full grasp on it's parameters as of yet. The decsion to buy TS cells or stay with flooded NiCads will determine my "homework" required... :wink:

 

[RPA]

This raises a question that's been in the back of my mind since the moment I pulled the trigger on Rule #2: Should the 80% apply to the amount of AH to BMS cutoff? Or should the 80% apply to the total cell life - in which case one would expect that many BMS' are already taking the 80% number into account?

Put another way: if I know that my BMS is programmed to pull the plug at 80% battery use, can I safely run the battery to cutoff every time?

If this is true, I need to do some editing on Rule #2.

The 80% rule of thumb is for AH used of the pack (eg. 8 AH of a 10AH pack). BMS's work off of individual cell voltages not the amount of AH used. When the individual cell voltages get to low is when they intervene or warn you that the pack is at the limit.

 

[PeteCress]

The 80% rule of thumb is for AH used of the pack (eg. 8 AH of a 10AH pack). BMS's work off of individual cell voltages not the amount of AH used. When the individual cell voltages get to low is when they intervene or warn you that the pack is at the limit.

What about the known effects of low temperature?

Let's say a nominal 10AH pack is only good for 7AH at a certain temperature.

Would the 80% rule then suggest only running it to 5.6AH used?

 

[amberwolf]

Yes, generally.

 

[ET4041]

To revive a stale thread, here's a question: if you can get 1000 cycles with a 100% discharge or 2000 cycles with a 50% discharge, isn't that the same? Yes, you do get longer cycle life with a lower DOD, but in the end you would have got the same amount of energy from your battery, and you only have to charge it half as often. So that would be the better option in my books.

 

[BIG BEAM]

I think I should mention something that we have been doing in the flashlight world for a while with Li ion batteries.I've been reading a lot on here that failures with BMS is sometimes a problem.If you charge any set of batteries in parallel they will balance themselves.Of course with a 36v pack you would need 12 wires coming out and a BIG charger.As the cells charge the resistance goes up and the cells that are lower will get more amps.I do this all the time with 18650 and 17670 batteries.If the charger shuts off at say 4.15 v all the batteries will be 4.15v no matter what state of charge you start with(within reason).Even if you ck the batteries the next day after resting they would be at 4.13 and all the same.

I am planing on being a sunday rider and the plan is to balance my pack this way once a month or so and the same way over winter.I'll let you guys know if I cook cells or ruin any.
DON

 

[madact]

Of course with a 36v pack you would need 12 wires coming out and a BIG charger

Not such a bad idea, actually - the pack would have no internal connections, the charger plug would have all the positives and negatives on a bus each, and the bike plug would make all the series connections in the plug body and still have just two wires coming out of it. I'm thinking of something that looks like an old-school SCART plug... does anyone know of a modern standard connector with lots of big high-current blade 'pins'? Maybe 'ATX' power connectors salvaged from dead PCs could do the trick, though I suspect the current handling may not be sufficient...

 

[amberwolf]

No, the ATX molex connector isn't up to that task; I used one on my DayGlo Avenger's original friction drives, and had to use many paralleled pins to handle the current.

That said, if you are already using Anderson Powerpoles and finding them sufficient to handle your current, you can simply lock 12 of them together in an array so they're keyed and unable to connect to the bike connector in more than one way. They even make panel mounts to do this kind of thing (but you might have to go thru Anderson to get them, or at least to find someone that sells them).

If the smaller Powerpoles aren't enough, they make bigger ones that still lock together the same way.


Another option might be Canon connectors, like you see in military and aircraft stuff, usually round with varying amounts of pins, which can be different sizes or all the same. Those would also be waterproof, and also come in panel mounts, but I think they are pretty expensive.