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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
CHEMISTRY 101
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!
