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.
#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;
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.
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.
Last edited by recumpence
on Mar 16 2009 8:45pm, edited 1 time in total.
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