LiPo battery care and basic information

[recumpence]

I am starting this thread in response to many emails I have recieved, asking about LiPo batteries and their applicability to E-bike use.

To that end, I am starting this thread for information related to the use of LiPo (Lithium Polymer) battery packs, (not to be mistaken as LifePo4, Lithium Ion, and other Lithium based chemistries).

Now, let me begin by staying I am not an electronic engineer (or any sort of engineer, for that matter). However, I have been involved in RC for many years, both as a user and a manufacturer. As such, I have a large amount of accumulated knowledge and experience using LiPo packs in RC cars, helis, planes, motorcycles, etc, as well as E-bikes. So, I will share my experience here.

There are many pros and cons to each Lithium chemistry on the market. LiPo is no different in that respect.

I will start with the cons;

The primary [negative] issue with LiPo is safety. There are far more reported problems related to safety in the use of LiPo than any other chemistry (with the possible exception of very early Li-Ion cells). Their safety problems are mainly related to over charging and sometimes related to overdischarging. ----------- If LiPo cells are charged to more than their rated capacity (normally 4.2 volts or so), they can, and often do, explode in quite a spectacular fireball! Now, being that is such an extreme outcome of misuse, most people stay away from LiPo. I must say, I completely understand! I have had one small LiPo pack explode in my house. That happened when a low quality charger locked on the wrong cell count and overcharged the pack. They are dangerous! However, proper care is quite simple, and that proper care is all that is needed for a long, trouble free life of your LiPo pack (more on that later).

The other negative is related to the first and that is charging. For safety reasons, a very high quality charger is needed to properly charge a LiPo pack. This is frustrating because it adds cost to an already expensive chemistry. I will explain more on charging later.


The positives;

#1 Extremely high power density. I have one LiPo pack in my recumbent that is 48 volt, 10ah pack. That pack weighs in at 5 pounds with all wiring and connectors! That is crazy compact and light weight!

#2 Light weight and small size. That pack I just mentioned measures 7 inches long, 6 inches wide and 2 and 1/2 inches thick. With that tiny pack I can go 25 miles at 18 to 20mph or 12 miles at 40mph!

#3 Very high discharge rate. Even relatively basic LiPo cells can run 10C continuous. Most higher quality packs can sustain 25 to 30C. There even extremely high end packs that are 60C! These are true discharge capabilities, not overestimates. I have run packs at well over 50C before and, yes, they handle it!

#4 Price. Now, LiPo cells are not the cheapest cells, but, for the technology you get, they are quite resonably priced. I paid $435 for that 48 volt, 10ah pack (of course, I also need a $250 charger to charge that pack properly). So, price is also relative.

#5 Cell ballance. I have had hundreds of LiPo packs. I have abused them and treated them nicely. I have over charged them, over discharged them, and just plain worn out a few and I have never, EVER had one go out of ballance! I have a cell ballancer. But, every pack I have ever hooked it to, has always been ballanced within spec (I think 2%) of each other. So, active cell ballancing is completely uneccessary. It is best to just check them with a ballancer occasionally to assure proper ballance.



Now, considering the charging issue (overcharging resulting in possible catastrophic failure), I will walk you through specifics of cell care, both charging and discharging;

Discharging;

When running a LiPo pack, the first rule is NEVER discharge the pack beyond 80% of its capacity (20% remaining). So, on my 10ah pack, I always keep at least 2ah of power left in the pack. This is extremely important. If you do not keep at least 20% capacity in the pack, the cells will have a VERY short life!

Second, never pull more C out of the pack than it is rated for. If you have a 15C pack, do NOT exceed 15C! That rule, combined with the 20% rule stated above, are critical to a long life for your pack. Also, those rules are interrelated and relative. What I mean by that is, the less C you pull from the pack and the more capacity you leave in the pack before each recharge, the longer the pack will last. I have one pack in an RC transmiter that is always recharged with 40% of the capacity left and I never pull more than 1C out of that 10C pack. That particular pack is over 5 years old and runs like new! However, if I pull the rated C out of a pack and discharge it down to 20% capacity, it will need replacement in roughly 500 to 1000 recharge cycles depending on the pack and how careful I am with it.



Charging;

The primary concern with LiPo charging is to keep the voltage constant while varying the current. So, a "Constant voltage, current tapering" charger is required for safe recharging. I personally like Hyperion chargers. However, there are many good chargers out there. Again, as long as the pack is not over-volted, it will normally charge perfectly without any issues at all.




In closing, I want to stress, in general, unless you are already experienced with LiPo, I would say to run A123 cells instead. They are safer, still very powerful, and not so fussy.

That being said, for those looking for the lightest pack and are willing to pay for a good charger and learn what they need to learn for safety, LiPo is a good viable option.

Matt

[GGoodrum]

Great thread, Matt. I've thought about doing the same thing, as I too get lots of PMs and emails about using RC LiPo packs in ebike applications. I think we need to get the powers that be here make this a "sticky".

I just posted this in another thread, but I think it is worth repeating here:

What concerns me the most with using RC LiPo packs in an ebike or other EV application, is not discharging, although I agree that it is quite possible to have cells go into thermal runaway, and not stop burning until there's nothing left that is even remotely flammable, but shortcuts made in charging. About 75% of the LiPo fires I'm aware of, including those of three friends of mine who had their garages burn down, were caused by malfunctioning chargers. Almost all of the newer LiPo/RC chargers in use today, however, have safety features that monitor each cell, during charging, to make sure not one goes over 4.25V, period. Above about 4.35V, or so, the fireworks begin. One issue with all these very safe RC chargers is they are designed to be used at the flying field and so they use 12VDC on the inputs. To use them at home means you need an "extra" AC/DC 12V supply. Most of these also are limited to 6, 10 or 12 cells, so if you have a larger pack configuration, it becomes complicated.

What some have done instead, which worries the heck out of me, is to simply use big bulk chargers, and charge the whole pack. This is absolutely a recipe for disaster, in my opinion, becuase there is not only no balancing going on, more importantly, there's no cell level monitoring that will absolutely shut off the charger if any one cell goes over 4.25V. Even using a modified version of our BMS board is not really the answer, because if the charger control section is not "throttling" properly, the shunt circuits can get swamped, and th voltage will rise. If people are going to heed Luke's advice, and decide to go down the path of using cheap LiPo packs, my hope is that they will also figure out a way to also use the dedicated LiPo chargers, with the built-in safety interlocks. These chargers are the reason why you don't hear much these days about LiPo fires. It has nothing to do with advances in the cells. They are as volitile now as they've ever been, maybe more so. It is the advancements in how they get charged that has caused the reduction in fires, etc.

-- Gary

[recumpence]

I agree.

I have been puting off writing this thread because of possible battery battle arguements. Everyone has their preferred chemistry. But, I finally realized that LiPo is the most dangerous chemistry and is the one chemistry not really addressed here (in a dedicated thread, anyway).

I know there is much more that can be said about LiPo and I am sure we will add to this thread as time goes on. But, for now, this is a good start.

Anyway, I love my LiPos! But, I am an expert in their use (or at least extremely experienced). So, I am confortable with them. That being said, for those who want the ultimate, this is the way to go. Just, be warned, you need to become thoroughly familiar with LiPo before you get them and make sure you threat them REALLY nice!

Matt

[liveforphysics]

You don't need to treat them nice during discharge or charge, as long as you follow the golden rule. You can abuse them harder than anything else out there, short of perhaps A123s.

***Golden Rule of LiPo*** If you break this rule, you are pulling the pin on the grenade.

Absolutely NEVER Exceed 4.3v/cell, never Discharge below 2.7v/cell, never puncture the cells.

With other cells, they often give warnings similar to that, and you can break the rules as much as you want, and suffer nothing, or maybe a little drop in cell capacity. LiPo is NOT like other batteries in this regard, when you break the golden rule, often even just one time, you are effectively telling the battery to fail into a ball of fire.


If you wish to use LiPo, you MUST follow that rule. No exceptions here. No room for error. No Oops, my charger reset, no excuses.

The most simple way to follow that rule is to use the wide range of awesome microprocessor controlled chargers made for LiPo batteries. They will never let the first part of the rule be broken.

For the second part of the rule, you need to watch your pack voltage while riding, or setup a LVC if you are the forgetful type. If you do ever deep discharge a cell, you are in no immediate danger. As long as the cell doesn't reverse polarity it wont explode just from excessive discharge. However, when you go to re-charge that cell, this is when you will see the fireworks. If you want to attempt to re-charge a deep discharged cell, do it in an area that you wouldn't mind making a bonfire. Maybe invite buddies over to watch. You can always stab it to make it go off if it doesn't go by itself. A deep discharged cell should never be trusted again in a string with good cells.

The cells need to be mounted in a place where they won't be poked by anything. If they get stabbed, they often just burst into flames. If you don't protect them, you are risking a fireball when you wreck. Other unprotected batteries generally just get damaged by a wreck, LiPo may burn your bike to the ground in a wreck.





If you follow these rules, you will enjoy LiPo as the highest energy density and highest power density battery you can buy. It's also been having prices rapidly fall, and now you can find quality LiPo for around the$ 0.50-0.55/wh mark.

Electrically, these batteries are incredible and extremely robust. It's basically impossible to discharge the cells to hard, as long as they are quality cells with Ri <10mOhm. When the internal resistance is that low, the cells just can't seem to get any warmth in them even under extreme loading, like 50c+ for burst of acceleration. Cells with higher internal resistance need to have the C rating obeyed, as they do get warm, and the voltage drops on them under load as well.

They have a unique safety factor built into them. In properly designed packs, the cells are the failure points if the battery is shorted. I've done it by mistake a few times, even very serious shorting, like 8awg wire plopping onto another piece of 8awg wire. The cell tabs have always acted like a fuse and vaporized in an instant to protect, just like a fuse would. I've never had cell damaged from this, but it does require re-soldering the vaporized area of the tab, which requires special solder for aluminum.

For a high performance application, or for ultimate range, or for keeping a bike as light an stealth as possible, no other battery can compare with LiPo. However, if you aren't prepared or capable of following the golden rule of LiPo, you need to choose different battery chemistry.

[swbluto]

I think Liveforphysics basically won this thread. He actually outlined definite operating parameters and tolerances, which is one of the key things to know about LiPo.

[recumpence]

I think Liveforphysics basically won this thread. He actually outlined definite operating parameters and tolerances, which is one of the key things to know about LiPo.

Ahh, good point. My post had no definative information what-so-ever.

Matt

[swbluto]

I apologize if I implied superiority and subsequent inferiority and in the process created animosity.

Your post is very informative and "lays out the land of LiPo" in showing its applications, potential and so on. I'm just the type that has absolutely no experience with LiPo and am very scared of accidentally burning down an apartment complex and/or blowing myself up and having well-defined operating limits kind of assured my fears(I also like numbers - I'm a pseudo math major). So, I had gained quite a bit from liveforphysic's post although I suspect I won't be jumping onto LiPo any time too soon given how strict the boundaries are as I know how easy it is to make a silly mistake.

Many people who have had little experience and great interest in high performance batteries would also gain quite a bit of your post, but my applications are more pedestrian compared to the drag racers among us.

[recumpence]

I am kidding around.

As I mentioned, I am not an engineer. I was hoping others with more technical knowledge would jump in.

Matt

[steveo]

Hey everyone,

thanks for starting this tread with some valuable info on lipo batteries..

I'm presently doing battery testing on lipo's atm! these 4p packs consist of 4 individual cells rated at 2c & 2500mah with my guess.. I don't have absoulotly no battery specs on these... all i know is they came out of many china made bikes that were recall as per the bike frames where braking right where the lipo batteries where kept within the bike frame .. the cells are all new .. some sat with resonable voltages for over a year at 3.7v .. some lower around 3-2v




I just wanted to ask, I know it was metioned earlier .. but i've found that lipos that i got that were really low in voltage tend to have higer capasity then those that were stored at a higher charge, the mah readings and resistance readings i've had are perfectly sound. Do you still think the cells are bad to use?...

-steveo

[MitchJi]

Hi,

I think more information on the safety issues involved in building packs and safe enclosures (Lipo Sacks?) would be useful.

[liveforphysics]

The charger is such a crucial part of the saftey of using LiPo batteries. I made an elaborate post about charger options, and I'm going to include it in this thread.


I have 4 of these:

http://www.hobbycity.com/hobbycity/stor ... oduct=6609

I have a fluke 189 meter, and this charger drops them off at EXACTLY the voltage I set it to charge each cell. Works fantastic, never had a problem with any of them. You do need a 30amp supply for each charger to run it at full potential. If you don't have a 30amp supply, it runs fine off even a little 5amp supply, but you obviously have to lower the charging current settings. It does 1-10s cells, and as long as you run each one from it's own 12v source, you can easily run a pair of them for charging an 11-20s pack. They never get above luke warm during charging, and they are very quick, painless, user friendly, plug-it-in-and-walk-away. If they detect any errors or funny stuff, they will shut down the charging.

And one of these:

http://www.aero-nuts.com/product_info.p ... cts_id=570


This charger is elaborate in every way except the stupid little spining clicking wheel. You use that wheel for most everything, and it's tedious to setup things, but it does save 10 battery pre-sets, so you just select the one you want and plug in the battery once you have it all programmed. It will do fancy pants graphs of each indivdual cell voltage as it charges, much like a CBA, and it lets you discharge packs as well. If you have computer skills you can use it's data output port to connect it to a computer and make neat graphs of charging/discharging performance. It also has a temp sensor to shut down charging if a pack gets above the temp you set. It will do 12s cells, and if they detect anything funny during charging, they shut down the charging cycle. It has a disadvantage. It only balances at 80mA, so if you get a pack way out of balance, it may not be able to get things corrected in a timely mannor. However, the balancer does also have a mode to function as a standalone balancer, and it's very accurate, just slow for E-bike sized packs.

I also have one of these that I use for my very small helicopters.

http://www.hobbycity.com/hobbycity/stor ... /_Balancer

It charges only directly through the balance taps, and treats each cell like it's a seperate battery. Because of this, you can't get a pack out of balance enough to bother this thing. It takes anything you throw at it. You could build a pack with a 1ah cell and a 50ah cell and a few full cells and a few drained cells all tied in series together, and this charger will top them all off at 4.25v/cell no matter what. No settings, no fooling around, no possible way to make an error with anything, you just plug in the balance taps and hit the power switch and wait for the cell LEDs to all turn green. It's not fast, it only does 5 cells at a time, but it has it's place.


For running these chargers, you need a 12-16v supply.

For running just a single 250w charger:
http://cgi.ebay.com/250W-13-8V-18A-Swit ... m153.l1262

For running a single big charger:
http://cgi.ebay.com/350W-12V-29A-Switch ... m153.l1262

If you are looking to power 700w of chargers, this little pair would run in series to give you that:
http://cgi.ebay.com/2-Pcs-350W-7V-Switc ... m153.l1262

If you are looking for a monster 1000w 12v power supply, they are available here.
http://cgi.ebay.com/1000W-12V-83A-Switc ... m153.l1262

[liveforphysics]

As far as charging saftey goes, guys claim that you can set a big pack in a LiPo sack, set it on your carpet and over-volt the pack until it blows, and it just leaves a smoke stain on the carpet.

I personally have no experience with LiPo sacks other than holding them. They appear to be a layer of nomex with a layer of fiberglass, and another layer of nomex sewn together. It would be pretty easy to make your own custom enclousure for a pack.


I have this big stainless steel baking pan that I have sitting on a couple bricks. I have a sheet of 1/4" scrap steel that I set over the top of it with a 1/2-1" gap on the edge for the charger wires to poke through. It's never been needed, because I follow the golden rule of LiPo, but I'm pretty confident if a pack were to burst, it would not cause any damage outside of smoke stains.

[dogman]

Nice post guys, I knew lipo had issues with charging, and now I know why. I'd be pretty worried if I had lipoly charging in my house every night but there are places to do it safely. How about just tucking it into the oven, off of course, so if it pops, it's in a metal box with an exhaust hood over it?

With my long ride home every day, I have had thoughts about adding a small lipoly, emergency pack to my bike. Something about 3-4 pounds, and just enough capacity to limp home at 10 mph. But more likely is I'll just buy a 48v ping and then have a bigger capacity to begin with.

[liveforphysics]

If you put a LiPo pack in the oven and over-voltaged it to blow it up, it would be fine. Oven would be a great place to charge, good idea.

You mentioned a Ping pack. For energy storage, it's fine. About twice the weight and bulk of LiPo for the same energy storage, but still fine. Now, lets talk about POWER differences. This is what LiPo is all about.

Let's choose Ping's biggest battery, the 48v 20Ah. This is a 2c battery, which tells us that this pack can discharge at 40amps. This battery weighs 20lbs, and can discharge at 1920w.

How much weight in LiPo do you need to match that?
Not picking anything fancy, just economical 30c zippy LiPo packs from hobby city, we can work out what it takes to equal the power of the biggest Ping pack in LiPo. What does it take? It just takes a single tiny 4cell 5Ah zippy pack, and it can all ready sustain 2200w VS the Ping battery 1920w, but the big difference is weight. The LiPo battery is just 1.2lbs vs the 20lbs of Ping battery! Yup, that's about 17 times the power density. It's also about the size of a regular non-flip-type cell phone, VS the size of a couple stacked shoeboxes. If you are looking for energy storage, lead-acid, LiFePO4, NiMH all offer energy storage at low prices if you don't mind a lot of weight and bulk. However, if you are looking for performance and power with a reasonable cost, nothing matches LiPo.

Just incase anyone wanted more specs on the battery I choose to compare power to the biggest Ping pack, here is a link. It's not anything special, just the upper model of zippy LiPo.



http://www.hobbycity.com/hobbycity/stor ... _4S1P_30C_

[recumpence]

Very good stuff!

I could not agree more.

Lipo is fantastic, but you need to understand what you are doing before you go off and jump head-first into them.

Luke, thanks for the detailed info.

Matt