The question of cell balancing really comes to the surface when we are considering multi-cellular battery packs and when we are considering chemistries based on Lithium. The reason for the interest in Lithium is its high specific energy density i.e. for a given weight it stores the highest Joule level or watt-hrs/kilogram. The drawback with Lithium is its sensitivity to over-charging or over-discharging. Either condition causes permanent loss of capability for the cell. The writers experience is with the hobby industry and interest in electric powered vehicles.
SOME BACKGROUND: For the electric vehicle industry a few different chemistries have been used. Specifically lightweight lead-acid, nickel-iron, nickel cadmium and nickel-metal hydride. All these chemistries can withstand moderate over-charging and even over-discharging to the point of cell voltage reversal. Experience by Toyota (and others) has shown by keeping the SOC, State Of Charge, within a 20% to 80% range the batteries will yield long life and support peak loads such as by acceleration and fast recharges by dynamic braking. The negative point about these chemistries is weight-----compared to Lithium they are nearly 2 to 1 heavier.
MORE BACKGROUND: Charge is measured in Coulombs and given the term Q for our purposes. Here the equation Q=I x T is useful where T is measured in seconds and I is measured in amperes. For a battery cell this "Q" is measured in ampere-hours, Ahr and you find it marked somewhere on the container. For a series connected pack which is charged and discharged serial connection, by inspection each cell receives the same I x T product or Q. IF the cells are matched AND balanced, the cell voltage (showing its SOC) will be the same for all cells over the SOC range. But cells, even those made by the same
manufacturer, after many charge-discharge cycles will show differences in SOC and eventually lead to overcharge of an individual cell in the pack. Now for Nicad, NiFe, and NiMh, which can tolerate over-charging, this is not a problem.
A word about "match" and "balance". In the hobby industry pack builders stressed "matching" which meant the cells in a pack all had the same Ahr measured rating and would reach end of discharge simultaneously. Gradually this has been dropped in favor of "balancing" meaning that cells start at full charge condition and end of charge determined when a the first cell hits the minimum voltage. This is nearly universal for NiMh packs made for the hobby industry.
In the current hobby industry because of the Lithium's superior Wh/Kg performance, virtually all packages are Li-ion, Li-Fe, Li-poly or LiPo and all have a "balance" connector option in addition to the two power connectors.
OTHER ITEMS PECULIAR TO Li BATTERIES:
NOTE 1: C rating, a current rate of 1C will discharge the cell in 1 hour. A current rate of 3C will discharge the cell in 1/3 hour or 20 minutes. So when a manufacturer rates his cell at 5C he is asserting the cell will support a 5C rate for 1/5 hour or 12 minutes. In the Hobby industry users are looking for power between recharge of nominal 10 to 20 minutes.
NOTE 2: Parallel connected cells are inherently "balanced" within that group.
NOTE 3: Single cell systems such as cell phones are inherently "balanced"
The charging systems vary. The balancing connection is in fact a tap connection between 2 adjacent cells. Most pack builders provide a redundant connection to each end of the packs so that charging can be done via the "balance" connector, which is polarized and has a "lock" feature. Now some charging schemes.
SCHEME ONE: A Constant Voltage, Constant Current (CVCC) source is connected to the battery packs power terminals. The constant voltage, CV, is set for the number and specific type cell and the Constant Current, CC, set for Ahr rating of the cells, typically 1C is the value. As the cells charge and the cell voltage approaches the set CV, the current is smoothly reduced to allow the pack to reach the CV set without over-shoot. Charging is stopped when the current reaches a set low safe value, (.03 x C). During this process the "balance" connector is connected to a multi-line voltage display that displays
each cell voltage in the pack simultaneously showing the degree of "balance". Cells are considered balanced if within +-.05 volt.
SCHEME TWO: In this arrangement, the cells are sequentially charged through the "balance" connector. The source of CV (adjustable for specific sell type) is connected to cell#1 and charged until the turn-off current level is reached. It then switches to cell#2 and repeats the process. This process is repeated until the pack is charged. This scheme is semi-automatic but takes longer time. It is quite safe.
SCHEME THREE: This is catching on. Each cell group has a pair of polarized connectors and the cell groups are separated for charging. Within each group the individual cells are parallel connected so that group is balanced. A charging rail driven by a husky CV source with sufficient Amp capacity allows parallel charging. After the current has dropped to the end value, the cell groups are separated and assembled in serial fashion to make the pack. As mentioned, this is becoming increasingly popular; its fast, its flexible, its safe and makes repair easier when you need to replace a part of a pack.
ECARS&HYBRIDS: Battery chemistries for Ecars, hybrids and PHEVs have used lightweight lead acid, nickel cadmium and NiMh. These do not require cell balancing and pack balancing is via over-charge if needed. But the recent interest in the Lithium chemistry because of its superior Wh/Kg is bringing the cell-balancing requirement to the fore. Compared to the hobby use the pack sizes (and voltage) are much larger. For Ecars & Hybrids the desire is to preserve "balance" while the overall pack is being charged or discharged in normal usage, so a bit different approach is required. Pack voltages have
ranged from as low as 100 volts to as high 400-500 volts. Example: The new Buick LaCrosse has a Li-ion pack delivering 115 volts and estimated at 11Kwh capacity and is considered a small pack.
For Ecar , PHEV and Hybrid usage, "balancing" has to be a continuous action. So putting in a 100 pin cable is NOT the solution---balancing circuitry has to be built right in the battery pack itself. From much of the experiments of using Li-ion packs in the so-called California Prius two reactive balancing methods have emerged; capacitive and inductive. The objective is to keep ALL the cells in the pack within +-.05 volts throughout the battery voltage range of 4.0vdc down to 2.5vdc.This will assure that the pack safely can deliver high current surges for acceleration and absorb high current surges for regenerative braking.
ACTIVE BALANCING USING CAPACITORS: The idea is to use a set of capacitors referred to as "shuffling" caps to carry the balancing currents. Visualize the Li-ion pack as series connected string of cells stacked, for explanation, as a stack say 100 cells high for a 380 volt pack. Cell #100 is at the top; cell #1 is at the bottom. Associated with each of the 100 cells is PMOS/NMOS pair configured such that the gates are parallel and driven by a 3Khz square wave. The drains are connected in parallel so that alternately the node is connected to the pos(+) and neg(-) of the battery cell. A single shuffling cap i.e.
500 Mfd connected between PMOS/NMOS pairs for cell #99 and cell#100 will alternately connect to cell #100 and cell #99 3000 times a second. In our example there is a shuffling cap for each pair of adjacent Li cells. So for a 100-cell string there are 99 shuffle caps.
How is balance achieved? The shuffle cap has the voltage of the cell last connected to, hence has the SOC of that cell. When connected to the next battery cell, current will flow into or out of the shuffle cap depending on the SOC of the next cell. Current will flow from the battery cell with the higher SOC into the batteries with the lower SOC----hence toward balance. In our example a single shuffle cap assures the two adjacent cells it shares are in balance since charge transfer will continue until the SOC of each cell match. Now to extend the number of Li cells, we just add the cell, its PMOS/NMOS pair, and its
shuttle cap. The "balance" extends because the new parts (given time) align voltage (its SOC). Parameters that can be varied to adjust "balance" performance are (1) the square wave frequency, (2) the size of the shuffle cap. Active balance via caps is NOT the only way; an alternate design uses inductors and will now be described.
ACTIVE BALANCING USING INDUCTORS: This time each Li cell has ONE PMOS switch again driven by a 3 Khz square wave. The source of the PMOS is connected to pos (+) terminal of the Li cell. The drain is connected to the neg (-) terminal of the Li cell through the "balance" inductor. The drain end of the inductor is connected through a Schottky diode to the neg (-) terminal of the next Li cell just below.
EXPLANATION: The 3 Khz square wave alternately connects the node of the PMOS drain, inductor and diode anode, to the pos (+) of the upper Li cell and then drives negative until the diode clamps it at neg (-) terminal of Li cell just below. The current through the inductor builds linearly from 0 to a value of I=E/L x dt. Where dt=1/(6000), E=SOC of the Li cell, L=470 microhenry; if E=3.5 volt, then I=1.25 amp. In the second part of the cycle the current drives from 1.25 amp through the diode back to 0 into the
next Li cell below.
So the balancing action depends upon the cell below receiving an energy input based upon the SOC of the cell above. For this circulation to work we need to get an energy sample from the bottom cell, cell #1, back to the top, cell #100.
This end around maneuver is done again using a 470 microhenry inductor that has a tightly coupled unity ratio secondary. The PMOS switch drives the primary of the inductor as usual for the first half of the 3 Khz square wave and achieves a current value based on the SOC of cell #1. The reference end of the secondary of inductor #1 is connected to neg (-) of cell #100. The output end is connected through the Schottky diode to the pos (+) of #100 cell. The "balance" action can now proceed and all 100 SOC
values will seek the same value i.e. balance.
COMMENT: Both capacitive and inductive balance action is controlled by starting and stopping the 3Khz waveform. So how do we use this way of control? The Ecar/Hybrid market requires long life use; for example warranted service for 150,000 miles over 8 years. Throughout the 8-year life of a Li-ion pack the many cells that make up the pack, will receive 100's of charge/discharge cycles. The efficiency of the various cells is CLOSE but not identical. The balance circuits will correct this IF turned ON. Why not
leave the balance circuits ON full time? Both capacitive and inductive balance actions are reactive and the loses are ohmic, small, but real.
So a strategy is necessary to determine when the "balancing" can be turned off. When the car is turned off the balance can be turned off. When the pack is supplying steady current; most surely the balance circuits will be turned on.
What is needed is a display that shows the "out-of-balance" cells that need balancing. All Ecars and Hybrids have some form of SOC display. This basically tells the driver how much charge remains in the pack. When the writer test drove the GM IMPACT/EV1 the display showed an accurate (conservative) of the miles remaining before charge was required. The IMPACT/EV1 was an Ecar.
It's all going to get sorted out quickly so drivers can be comfortable driving the cars.
Interesting.
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