Charging Lithium Ion Batteries
Lithium-based batteries are not new technology, but
recent advances are making them more practical for
consumer use in renewable energy storage systems.
Although their cost is still prohibitively high for most residential systems, lithium-ion (li-ion)
technologies are at the forefront of modern battery engineering and offer a number of
advantages over the more traditional chemistries, such as lead-acid. Li-ion batteries should not
be confused with batteries that use solid lithium, as they are two different technologies. The
latter possesses a very high energy density, but can also be inherently unstable over time in its
present state of development, so it is not a contender for larger battery-based storage systems.
The high energy density of lithium batteries is perhaps one of their most attractive features. Liion provides two to four times the energy density of lead-acid based on weight; about six times
based on volume. Li-ion batteries also exhibit very high charging efficiency (97-99%+), they can
operate at somewhat higher ambient temperatures than lead-acid, and can be charged at a
much higher rate. Their cycle life can easily be twice that of lead-acid - often much more than
that depending on the application – and much more of their capacity can be safely utilized
before a recharge is required. Compared to traditional flooded lead-acid, they are
maintenance-free and are considered benign from an environmental standpoint.
At the present time, their up-front cost is the most formidable barrier to the widespread use of
li-ion batteries, although the cost has been declining and continues to do so. Li-ion batteries are
currently (2014) about three to four times the cost of an equal-capacity AGM lead-acid battery,
and can be nearly ten times the cost of an equivalent flooded lead-acid (FLA) battery. The cost
over lifetime of li-ion can approach that of lead-acid under some conditions, based on li-ion’s
much longer cycle life and other factors.
There are around half a dozen different li-ion chemistries available. The most popular for
residential use, and as a replacement for lead-acid types, is currently the Lithium Iron
Phosphate type (LiFePO4). LiFePO4 batteries are among the safest of the lithium chemistries.
While some lithium technologies react violently with abuse and can be thermally unstable,
LiFePO4 is a relatively safe and abuse-tolerant variety. The terminal voltage of 3.30 volts per cell
is closely compatible with standard 12-volt systems when used in banks of four cells in series.
Li-ion and lead-acid batteries follow a similar charging profile when used with a 2-stage charger.
This makes it easier to charge li-ion with a charge controller that is meant for lead-acid.
Li-ion batteries must incorporate a battery management system (BMS). This allows a bank of liion cells to be charged as one battery without the risk of imbalancing or overcharging the
individual cells. Never attempt to charge a li-ion battery without using a BMS.
Typically LiFePO4 batteries undergo an initial bulk, constant-current charge similar to lead-acid
batteries. A common charging rate is 50% of capacity, compared to the typical 10-20% rate of
lead-acid. When the voltage of the cell rises to 3.60v (14.4v for a series bank of four cells), the
charger transitions to a constant-voltage “saturation” charge and continues at this voltage until
the charging current drops to about 3% of capacity. This is analogous to the “absorb” charge of
a lead-acid battery. Charging of the li-ion battery ceases at this point, whereas the lead-acid
battery would typically undergo an indefinite float charge.
It is important not to subject lithium batteries to overvoltage, as this will shorten their life and
can raise safety concerns with some types. With LiFePO4, the maximum applied voltage should
typically not exceed 3.60v per cell. Unlike lead-acid, li-ion batteries do not require a full charge
in order to sustain the service life of the battery. Therefore, it is usually better to undercharge
the battery slightly so that the potential of overvoltage can be minimized. This can be
accomplished by charging at a slightly lower (less than 3.60v per cell) voltage, or by eliminating
the saturation charge entirely.
Rogue’s MPPT charge controllers can be used in this manner to charge LiFePO4 batteries that
use an integrated BMS. The MPT-3048 with firmware version 1.0.4 or later, and the MPT-2024
with firmware version 2 or later, may be used. The following guidelines may be used to set up
either controller. The MPT-2024 will require an external device, such as a PC interface running
PowerNet, or a COM-4850, to enable setpoint adjustment. The COM-4850 will require firmware
version 1.1.4 or later. PowerNet must be version 1.2.0 or later.
The following guidelines are meant for illustrative purposes only. Always consult your
battery’s manufacturer to determine the proper charging profile and setpoint values for your
particular battery and application.
1) From the SETUP menu, first screen (or from the PowerNet Setpoint Adjust window),
adjust the ABSRB setpoint to coincide with Stage 2 saturation voltage for your battery
(typically 14.4v for a bank of four series cells). Save the setting.
2) Also from the same setup screen, adjust the FLOAT setpoint to match the Stage 1 (bulk)
re-entry voltage. This is the voltage at which the controller will begin charging the
battery again once its voltage drops after a full charge. This value should be obtained
from the battery manufacturer. Now select “DISABL” (leftmost button on the MPT-3048
or COM-4850), or check the FLOAT “Disable” checkbox in PowerNet, and save the
setting.
3) From the SETUP menu, second screen (or from the PowerNet Setpoint Adjust window),
adjust the FLOTR setpoint to approximately match 3% of the battery’s capacity in amps.
Save the setting and exit setup.
4) Do not use battery temperature compensation. The easiest way to accomplish this is to
leave the battery temperature sensor disconnected. However, if you’d like to monitor
battery temperature: affix the sensor to the battery, connect the cable to the controller,
and adjust the TCOMP setpoint to zero (last setpoint on the fourth setup screen). This
will disable temperature compensation, but allow the controller to monitor
temperature.
5) Li-ion batteries should be charged without any loading, so that the end of the charge
can be more accurately determined. If this is not possible, consult your battery
manufacturer for advice. Rogue’s charge controllers can also set a limit on the duration
of the saturation charge (as determined by the “FLOTM” setpoint), if the battery
manufacturer can provide assistance in choosing the proper duration.
6) To ensure best charging accuracy, first calibrate the controller as described in the user’s
manual for your model.
As mentioned previously, the saturation charge may also be eliminated to minimize battery
stress. In this case, battery charging ceases as soon as the saturation voltage is met. To
configure a Rogue charge controller to bypass the saturation charge, follow the above outlined
steps, and also the following:
7) From the SETUP menu, first screen, select the ABSRB setpoint (press “MOD” as though
you are going to adjust the setpoint), ensure that it remains at the proper saturation
voltage for your battery, and then select “DISABL” (or check the ABSORB “Disable”
checkbox in PowerNet). Save the setting and exit setup.
When the saturation charge is bypassed, the controller will charge the battery up to the
saturation voltage and will then terminate charging until the battery voltage falls below the
value set in step two, above. At this point, the controller will begin bulk charging the battery
again until the saturation voltage is met once more, and the cycle will repeat.
References:
T. Reddy, Linden’s Handbook of Batteries, 4th ed., McGraw-Hill, 2011.
I. Buchmann, Batteries in a Portable World, Cadex, 2011.
Rogue Power Technologies, Ashland, OR 97520, http://www.roguepowertech.com