BATTERY TECH 101
LITHIUM BATTERIES 101
We have been importing
Lithium battery cells for over 10 years now, dealing with various types of cell
from LCO (Lithium Polymer), LifePO4, NiCoMn (Lithium nickel cobalt manganese),
and Mn2O4 (Lithium manganese dioxide). While there are many types of Lithium
chemistry available, certain chemistries are better suited to electric vehicles
and the quality of the cell production has a huge factor on lifespan and
maintenance. There are hundreds of factories in China producing Lithium cells,
but only a handful that produce High grade cells that are suitable for vehicle
use and the high power cells suitable for 2000w up bikes are VERY expensive,
some beyond reach of us mere bike riders!
There are too many battery packs being sold on Ebay and out of
LiMn2O4 Cells have similar safety features to LifePO4, low risk of rupturing or catching fire, but also a lower energy density of only 110-120WH/KG, and a much lower cycle life. They do maintain their voltage and internal resistance better but lose capacity over time/use just like all Lithium cells. They are cheap but not really a good choice for long term use or for bikes more than 200w so we don’t import them.
NMC LiNiMnCo Cells are rated medium on the safety chart, they can be damaged by overcharging and it is possible to rupture the cell with the result a battery that burns itself out and smokes the area out, (this is of course possible with any Lithium battery which is why we have a set of instructions on safe handling and storage of Battery packs, and recommend only hi quality computerised chargers). LiNiCoMn battery packs have an energy density of 180-200WH/KG and a higher cell voltage of 3.7v, this means 6 cells has the equivalent voltage of a 7 cell LifePO4 battery, but at a lighter weight, higher energy density and lower cost. The issue with these cells is if over discharged they die instantly, they have a lower power output ability and must be run with a PCM protection board which limits the current output making them a poor choice for bikes over 250w.
LifePO4 batteries have been used with E-Vehicles for a while now, they are very safe, low risk of rupturing or catching fire, but they are a lower power cell with an energy density of 140-150 Watt-hours/KG and cell voltage of 3.2v so you need more cells to make up the same voltage as a LiPO pack. Their cycle life is very dependant on cell quality, there are a lot of LifePO4 batteries being sold on Ebay and out of China that are low grade 2 cells and many only survive a few hundred charges if you are lucky.
We have run cycle life tests on Grade 1 LifePO4 cells and after 1000 cycles their internal resistance rises which reduces their capacity and retained voltage under load around 20%. LifePO4 chemistry is also expensive to produce in high quality.
Lithium Ion 18650 cells are a well known cylindrical cell with 3.7v nominal and 4.2v fully charged. Found in laptop batteries they are usually a low output cell and not really suitable to hi power Ebikes, good for 200-250w bikes if the pack capacity is high enough.
There are new cells available with hi discharge ratings from Samsung, Sanyo and LG but they are pretty expensive. New Chinese cells are proving to be quite reliable and have a decent cycle life even under heavy load and in the years to come should be a good option for 1500-3000w bikes. Another issue with 18650 cells is the need to put many of them in parallel in order to make up enough capacity. Typically there are 6 or more cells connected in parallel, then (in the case of a 48v battery) 13 sets of 6 are connected in series to get the required voltage. This is 78 cells in one pack, if one goes bad it can be quite time consuming and costly to find and replace the bad cell. A better alternative is the larger cells available, 21mm, 25mm and even 32mm diameter which have much higher capacity per cell. The 32700 we are using has an 8Ah capacity, we only need 2 of these in parallel to make a 16Ah battery pack, 26 cells in total, much easier to track down a bad cell!
LiPo batteries (Lithium Polymer) are extremely high power cells, they feature the highest power density (210WH/KG) and highest output ability, but are also the most dangerous of all lithium cells, overcharging can cause them to rupture and burn, as can physical damage, if the cell is shorted internally it will overheat and rupture within minutes producing a lot of smoke and in severe cases some flame from the end of the pack. Of course this is a worst case scenario and it is very difficult to damage Lipo cells in this way. The power of the cell is unbeatable for the price and are popular with the DIY guys and those using high power bikes (2000w upwards) especially as weekend recreation. Storing them safely in a steel tool box are necessary habits to get in to with Lithium polymer batteries and something EXPECTED, it’s not a suggestion!
CHARGING & STORING LITHIUM BATTERIES
NEVER Charge batteries inside your house! We say this to every customer, its in our battery instruction notes, and it NEEDS TO BE FOLLOWED. We will not be held responsible for you damaging your property by charging or storing any lithium batteries inside the house! Charge outside under cover, or in the garage on a non flammable surface away from everything!
ALL Lithium cells start to degrade as soon as they are manufactured, they have a finite shelf life, storing them correctly at 50-60% capacity increases their lifespan a lot. It is this reason that claims from sellers of thousands of cycles is completely pointless, as most of these batteries will fail due to age and not cycles!!
All our cells are quality products and the cells are matched for internal resistance and capacity in the factory before being built into battery packs. There is no need to use 30c, 45c, or 60c discharge LiPO batteries in a bike! A 10-12c rated battery in an electric bike is more than enough headroom. Its almost 4 times what is required, so by using lower C rated packs you can keep the cost down around 50% over the hi output specialist RC packs.
10-12c Lithium Polymer battery packs typically get 2-300 cycles (sometimes more, often less, depends very much on the user) if correctly maintained and not over discharged or overcharged. I know some users who are reaching 300-400 cycles when doing daily commutes, they are taking good care of their batteries though, leaving 20% in them and storage charging them when not in use. Checking and maintaining balance is also very important with LiPO batteries.
Following strict guidelines will get the most life from your batteries, being careless with how you use and store them will result in short lifespans, its possible to kill a Lithium Polymer battery in one go by over-discharging it or leaving it connected to your bike for a long period of time. Whether you have a switch or not makes no difference, if you leave a battery plugged in it will discharge, leave it long enough it will discharge to zero, the cells will drop below 2.9v and the battery will die. They usually cannot be recovered. All packs have warning labels on them and you MUST get into the habit of double checking everything you do with them.
Running your packs down too low will kill cells, most controllers have cutoffs set into them but you can still continue to run the bike at lower throttle settings after hitting the cutoff. I know users who have continued to squeeze as much as they can out of their batteries this way, too far from home, don’t want to ride the bike as a bike, whatever the reason, the end result is the battery cells go way out of balance, sometimes cells die, you cannot get them back, 3.7 at resting is as low as you should go which is 44.4v on your throttle display. Under load the voltage drops, this is why cutoffs are set around 44v, once the load is removed, IE cutoff hits, the voltage will climb back up to around 48v at rest. This is where you stop!! If you try to squeeze more out of them you WILL damage them!