Li-Ion BMS

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White Paper - Li-Ion BMS chips

Comparison of Integrated Circuits for Battery Management Systems for Li-Ion batteries

This paper discusses the chips available today for a BMS design. Please also see a detailed spec comparison

Designers of a BMS for Lithium Ion batteries may use one of the following in their product:

  • Generic ICs that can be used to design the Front End of a BMS from scratch:
    • Analog ICs, such as power supply supervisors, multiplexers, for "dumb" Front Ends with analog communications
    • Microprocessors, to take analog readings, for "smart" Front Ends with didital communications
  • Specifically designed ICs for Li-Ion BMSs
    • ASICs - (Application Specific Integrated Circuits)
    • Microprocessors or FPGAs that are pre-programmed / pre-configured for BMS operation

Today, only a few ICs that are specifically designed for Li-Ion BMSs are produced.
Thay can be grouped as follows:

  • ICs for small batteries
  • ICs for large battery packs...
    • that are only available to a select group of large manufacturers
    • that are available to the general public...
      • Front End only (dumb)
      • including smart BMS functions

The following ICs are generally unavailable: they are offered only to very large manufacturers, and by appointment only. They are listed here for curiosity only.

  • Maxim's MAX11068 - Front End only, 4 to 12 cells each, stackable to 372 cells (1.4 kV)
  • O2Micro OZ890 - Front End only, 5 to 13 cells each, stackable to 208 cells (0.7 kV)
  • A40MX02-PL537 (an ACTEL FPGA customized for BMS operation)

The following ICs are designed for small Li-Ion batteries for consumer products, such as laptops.
They are either just the analog Front End, or a complete BMS.
They are available to the general public.
Some may try to shoehorn them into a large battery pack design, with mixed results

  • Texas Instruments vast selection of smart ICs for up to 4 cells (complete BMS)
  • ATMEL's ATmega406 for 4-cells (complete BMS)
  • Intersil's ISL9208 for 5 to 7 cells (analog Front End only)

These ICs are designed for large Li-Ion battery packs. They are available to the general public.

  • Linear Technology's LTC6802 ICs for HEV traction packs
  • Analog Devices's AD7280 Lithium Ion Monitoring ICs
  • Elithion's chips for large traction packs
back to topICs for small batteries

ICs for small batteries are readily available and relatively inexpensive. Unfortunately, there are limitations to these chips, that limit their usefulness in large Li-Ion packs:

  • They are designed for low battery currents (a few Amps at most) sensed by a resistor (not for the high currents typically present in large packs, sensed with a magnetic sensor)
    • Without knowledge of the battery current, any powerful intelligence in the chips goes to waste
  • They handle at most a few cells in series. To do larger packs:
    • A centralized controller is required ($ ~200), with a multiplexer and with individual optical isolation to talk to each chip
    • As each chip has a mind of its own, it's hard to coordinate them, and to make use of the values they each calculate
    • Each chip expects its own current sensor, but there is probably only one current sensor for the entire pack, and no way to send its reading back to the individual chips
    • There may be no way for the centralized controller to control the loads on each chip; so, additional cell balancers are required
Texas Instruments anchor

Texas Instruments is the de-facto leader in ICs used in small Li-Ion batteries, such as cell phones and laptops.

Highlights include:

  • Various chips cover a whole gamut of options
  • One chip handles up to 4 Li-Ion cells in series
  • Include the smartest algorithms available to manage Li-Ion batteries
  • Additional chips provide redundant protection, for bullet-proof safety of the pack and user
  • Ideally suited for mounting directly on the cells, for cleanest design, minimal risks of short circuits
  • Very few additional parts required
  • Cell balancing, using on board components (external loads can be used as well)
  • Fuel gauge (SOC and DOD calculation) functions
  • Nondissipative balancing available
  • Inexpensive; in high quantities:
    • the chip price is about $ 1.5 ~ 2 / cell in series
    • the complete cost is about $ 4 / cell in series
  • SMB (I2C) serial interface
Atmel anchor

AtMel's ATmega406 is comparable to the best TI chips, but it also programmable

Highlights include:

  • One chip handles 2 to 4 Li-Ion cells in series
  • Include the smart algorithms available to manage Li-Ion batteries, field programmable
  • Ideally suited for mounting directly on the cells, for cleanest design, minimal risks of short circuits
  • Requires a few additional parts for protection
  • Cell balancing, using on board components
  • Fuel gauge (SOC and DOD calculation) functions
  • SMB (I2C) serial interface
Intersil anchor

Intersil's ISL9208 doesn't include smart algorithms, but handles more cells in series

Highlights include:

  • One chip handles 4 to 7 Li-Ion cells in series
  • Analog Front End, protection circuits
  • Ideally suited for mounting directly on the cells, for cleanest design, minimal risks of short circuits
  • Requires a few additional parts for protection
  • Cell balancing, using external resistors (internal MOSFETs)
  • SMB (I2C) serial interface
back to topICs for large battery packs
Linear Technology anchor

Linear Technology introduced the LTC6802, designed specifically for HEV traction packs.

Highlights include:

  • One chip handles up to 12 Li-Ion cells in series
  • Multiple chips can be used for packs of just about any size
  • Communication uses a 3-wire daisy chain between adjacent cells, with no need for optical isolation
  • Ideally suited for centralized ("spaghetti") or modular topologies
  • No practical limitation on battery capacity and battery current
  • Reports cell voltages and temperatures
  • Very few additional parts required
  • Cell balancing, using on board components (external loads can be used as well)
  • Inexpensive; in high quantities:
    • the chip price is about $ 1 / cell in series
    • the complete cost is about $ 1.50 / cell in series, plus the cost of the controller ($ ~200)

The Linear Technology chip does have some disadvantages, though:

  • If the 1st or the 13th wire (the ones that provide power to the chip) open-up, the chip is destroyed
  • While the BMS works well on the bench, in real life applications it is unable to communicate n the presence of the EMI interference typically present in real life applications (from devices such as chargers and motor controllers)
  • Cell voltage sense wires and thermistors are routed through the pack, adding a risk of short circuits and paths for plasma events
  • Has no intelligence of its own: requires a BMS controller, designed by the end user
Analog Devices anchor

Analog Devices's AD7280 Lithium Ion Monitoring IC is similar to the Linear Technology chip.

  • One chip handles up to 6 Li-Ion cells in series
  • Multiple chips can be used for packs of up to 300 cells in series (~1.1 kV)
  • Communication uses a 7-wire daisy chain between adjacent cells, with no need for optical isolation
  • Ideally suited for centralized ("spaghetti") or modular topologies
  • No practical limitation on battery capacity and battery current
  • Reports cell voltages and temperatures
  • A few additional parts required
  • Cell balancing, using external components

Without experience on these chips, not many disadvantages are yet known:

  • Its price is not publisehd by the manufacturer nor by distributors
  • The number of additional parts is noticeably more than for the previous 2 manufacturers
  • Cell voltage sense wires and thermistors are routed through the pack, adding a risk of short circuits and paths for plasma events
  • Has no intelligence of its own: requires a BMS controller, designed by the end user
  • A relatively high number of daisy-chain wires is required between sections
Elithion anchor

Elithion offers the chips used in its standard BMS for inclusion in other companies' BMSs

Highlights include:

  • 1 to 255 cells in series (~1 kV max)
  • No practical limitation on battery capacity and battery current
  • Cell-mounted cell-boards with only 1 daisy-chain wire result in clean layout, little space requirements and simpler installation
  • Two different chips:
    • A cell board chip (one per cell in series)
    • A BMS controller chip
  • Fully programmable BMS controller, to match the needs of the system
  • Reports cell voltages and temperatures
  • Fuel gauge (SOC and DOD calculation) and SOH functions
  • Cell balancing, using external components (nondissipative balancing possible)

The Elithion solution does have some disadvantages as well:

  • The algorithms are not as evolved as in the Texas Instrument chips
  • Not fully integrated: 14 additional parts required to complete a cell board
  • Relatively expensive; in high quantities:
    • the chip price is $ 3/ cell in series
    • the complete cost is about $ 5 / cell in series, plus the cost of the controller ($ ~200)
back to topConclusions

This is our take on the present situation on chips for Li-Ion BMSs:

  • ICs for small packs may seem attractive, but trying to shoehorn them in large packs is an exercise in futility
  • The Texas Instrument Chips in particular are wonderful, but their features are lost when shoehorned into a large pack. One day soon the TI designers will finally understand what it takes to translate their great understanding of Li-Ion management into something that will also work in large packs
  • Linear Technology's approach is great, but it was poorly implemented by people who had little understanding of the real world challenges of large batteries. They will soon gain that understanding from disappointed users who had the misfortune of being their Guinea pigs, and will design it right
  • With the little that is known about the Analog Devices chip, we assume that it has similar advantages and disadvantages as the LT chip
  • Maxim's and O2Micro's offerings might provide effective solutions, but what good is it to the designer if they are not available?
  • Elithion's chips may be seen as the best viable option for large Li-Ion packs available today; but unless they advance significantly and soon, they will lose advantage once TI and/or LT get their act together

 

Creative Commons License "Comparison of Integrated Circuits for Battery Management Systems for Li-Ion batteries" by Davide Andrea is licensed under a Creative Commons Attribution-Share Alike 3.0 Unported License. Permissions beyond the scope of this license may be available by contacting the author.

Davide Andrea, Elithion, 7/21/11

© 2008~2017 Davide Andrea. All rights reserved, except where noted by CC mark. Page published on Jul 21 2011. Graphic design by morninglori
 

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