The old saying that “A chain is only as strong as its
weakest link” is somewhat applicable to a series string of lithium batteries,
only with a string of lithium batteries this analogy is really much more
complicated. A lithium cell is much more sensitive to mistreatment than a
comparable lead acid battery.
CHARGING:
If lithium cells are overcharged, it will shorten their
lifespan or perhaps permanently damage them. So with a LiFePO4 (Lithium Iron
Phosphate) battery whose nominal voltage is 3.2 volts you should never charge
them above 3.65 volts and it is best to have something in the system that will
shut the charger off when they reach a maximum charge voltage of 3.65 volts.
Similarly, with a LiNCM or LiMn204 (Lithium Polymer) battery
whose nominal voltage is 3.7 volts you should never charge them above 4.2 volts
and it is best to have something in the system that will shut the charger off
when they reach a maximum charge voltage of 4.2 volts.
DISCHARGING:
Conversely, a LiFePO4 lithium battery cannot be discharged
below 2.4 volts without damage to the cells. So there must be some mechanism in
the system which will automatically throttle back and eventually shut the
battery pack down in the event of a discharge condition that goes below 2.4
volts. If that mechanism isn't in place you risk damaging some of the batteries
in your pack. This system is commonly referred to as a Battery Management
System or BMS or in the case of some smaller battery packs, like in a bicycle,
a PCM.
A PCM (Protective Circuit Module), which is often called a
PCB (Protective Circuit Board), is a passive system that will operate best at
voltages of less than 96 volts and peak currents that never go over 200 Amps.
BALANCING:
Having a
number of batteries all connected up together storing electric energy is
somewhat like having a barrel that is made from wooden staves which are of
varying length that is storing water. Some batteries will accept a charge
better than other batteries in the string. When they reach a predetermined
voltage like 3.65 for LiFePO4 batteries then there has to be a way to shunt
current from that highest charged battery and prevent it from becoming
overcharged while their weaker batteries in the same string catch up. To
simplify that I have used the storing water in a wooden barrel analogy and show
the longer staves as the good batteries while the shorter staves represent the
weaker batteries. This is the third function of a BMS. A good BMS will balance
all the batteries so that they reach full charge in about the same time. This
might be shown as a barrel that has all of the staves at the same length.
Passive
BMS systems contain large resister networks which merely redirect the charging
current through the resister network and burn up excess power while the balance
of weaker cells are getting to their full recharge voltage level.
Cheap BMS
systems do not do this they merely turn the charger off when the first battery
nears 3.65 volts. With those systems the battery pack could still be charged
with a greater amount of power but there isn't any way these cheap systems will
allow that to happen. Thus you may lose as much as 20% of the battery packs
full potential capacity for the sake of a few dollars saving when purchasing
the BMS system. In motor vehicles that can amount to a big loss of range.
The fourth BMS function concerns protecting and balancing the batteries from
over discharging. If the batteries are somehow not balanced so that all of the
staves are equal, which a properly engineered BMS system will do, than as the
battery is discharged the weakest battery, represented in my illustration by
the shortest stave in the barrel, will reach its full discharge shutoff point
ahead of the rest of the pack. A good BMS system will keep all the cells
balanced so that the weakest cell voltage with a LiFePO4 battery will not go
below 2.4 volts while the other batteries are allowed to drain their energy.
Similarly, a good BMS system will also keep all the cells balanced in a Lithium
Polymer battery so that the weakest cell voltage will not go below 2.8 volts
while the other batteries are allowed to drain their energy. A well designed
BMS will assure that all of the batteries are close to equal in storing and
discharging their energy uniformly. The last function of a good BMS system is
monitoring and controlling the temperatures within the cells or the total pack.
Many lithium chemistries like Lithium Cobalt can spontaneously burst into
flames if they are over charged or discharged. Metal fires are extremely hot
and water or even fire fighting foams alone will not extinguish the flames. Of
all the lithium chemistries Lithium Iron Phosphate (LiFePO4) is the least
volatile and theoretically will not combust. However, extreme heat can damage
and shorten the life in all lithium cells. A good BMS system will monitor the
temperatures within a pack and either shut the pack down or throttle back the
discharge or charging current so that the cells never reach a point of overheating.
There are people who claim that you don't need a BMS system! And, in fact they
are right. You only need one if you are concerned about the most available
power from the pack along with the health and longevity of the lithium cells
that you paid a lot of money for. You will add years to your pack life if you
use a good, well designed, BMS system.
In summation there is another saying that applies. “You get what you pay for!”
Lithium batteries are expensive and you deserve to get a long battery storage
life from every dollar that you spend on both the batteries and the BMS system.
A good BMS system will pay for itself several times over the lifetime of your
battery pack!
