Understanding your HEV NiMH battery pack

Discussion in 'Articles' started by msantos, Sep 9, 2009.

  1. msantos

    msantos Eco Accelerometrist

    [​IMG] Down and dirty inside your pack.

    [xfloat=left]http://www.cleanmpg.com/photos/data/500/NiMH_pack_cutout.jpg[/xfloat]Manuel Santos - CleanMPG - Sep 6, 2009

    As many of us know, our hybrid electric vehicles are equipped with battery packs consisting of chains of battery cells. Depending on the type of manufacturing technology and packaging, these battery cells can either by prismatic or cylindrical and there's much to be said about their management and longevity.

    In any case, as your vehicle ages some new found behavior's may become more pronounced and an explanation may be needed in order to avoid undue concern or panic.
    Explaining the basics (101) of the NiMH chemistry

    In its most basic interpretation, the NiMH cell chemistry is intimately related to the traditional and much vilified Nickel Cadmium battery technology in that very few differences actually separate the two. Perhaps the most significant, is that the NiMH chemistry replaced the highly toxic cadmium for hydrogen which is absorbed by the cells negative electrode. This electrode is manufactured from a hydride metal alloy that contains Lanthanum and other rare earth chains that basically create the protons as a result of the oxidization of hydrogen.
    In essence, the metal hydride cell chemistry depends on the ability of some metals to absorb large quantities of hydrogen.

    These alloy metal structures can then act as storage pools for hydrogen which can later reverse the absorption process to release the protons – which leads to the electric imbalance we seek. Remember it is this electrochemical imbalance that creates a difference of electric potential between the two electrodes and the measurement of voltage we need.

    Separating this particular electrode from its positive counterpart is an electrolyte consisting of a thick solution of potassium hydroxide. Although this solution is not involved in the chemical reaction it does act like a conduit for the hydrogen to travel between the negative and the positive electrodes.

    This chemical combination of materials usually yields 1.2 volts and is typical of the majority of consumer and industrial cells. Not surprisingly some hybrid electric vehicles leverage this very same attribute albeit with some rather interesting enhancements which we will also discuss later.

    In reality, the first commercially available NiMH cells were introduced into consumer level goods in 1989 and this makes this battery chemistry a relatively young player when compared to its competitors. Despite this, the technology has proven itself and found to be quite capable in a variety of applications including automotive use. As we speak, the existing NiMH chemistry is continuously being improved and until a better technology is identified and proven to meet and exceed its declining costs and longevity, the NiMH tech may remain with us a little longer than expected.

    Some of the main advantages over its older competing technologies, is that NiMH cells hold approximately 40% more energy than the NiCad and 100% more than a typical lead acid cell. One other advantage is that NiMH can not only safely handle higher charge and discharge rates but also very small charge and discharge rates thus making them ideal for HEV applications.

    However, like its despised structural sibling the NiMH is also susceptible to the memory effect and despite the lower magnitude than what is usually observed in the NiCad chemistry, manufacturers of the charge control circuitry in our hybrids must still include battery pack management routines that mitigate this chemical affinity.

    Charge and Discharge rates and depth of Discharge

    The typical life cycle of the NiMH chemistry is often pegged at 500 cycles. However, higher yield designs have been demonstrated to sustain over 3000 cycles while incurring a complete discharge.

    Of course, even without an improved design, the number of charge and discharge cycles can be dramatically increased to well over 300,000 cycles if a DOD (depth of Discharge) of just 10% is used. Other factors related to thermal management also have an influence on this life cycle which cannot be ignored.

    One other unsung advantage that this tech has over others (especially LiION) is that it can occasionally tolerate overcharge and deep discharges and depending on the application it can simplify the battery management process. Somewhat implicit to this characteristic is that the NiMH has a very wide operating temperature range, again a strong asset for space and weight constrained applications such as automotive use. This does not mean that a design including these batteries can forego proper thermal management. Instead, the requirement will be less stringent often abdicating the use of extreme cooling accommodations.

    Also, on this topic of temperatures; because there isn’t a reliable and detectable voltage change that indicates a cell is overcharging (as it is common in other battery chemistries), the charge cut-off is triggered instead by measuring the effective temperature of the cell while on the cell module.

    This approach is somewhat more expensive and less accurate but it works reaonably well especially when the charging circuit detects the higher temperature and then switches to a reduced charge rate (managed trickle) as one way to avoid a severe overcharge… and the primary means to ensuring longer battery life. That is also why this reduced rate is also timed and after a pre-programmed time span the charging will cease completely.

    Also note that a battery pack’s operating temperature is determined by adding the ambient temperature to the heat generated by the battery pack assembly, its circuitry and even adjacent systems. If a battery is subjected to excessive charge or discharge currents then thermal generation may exceed its heat dissipation abilities and then we have what is commonly referred to as a potential thermal management mode. This mode is often incurred to reduce the chances of a potentially bad thermal run-away.

    Measuring the SoC (State of Charge)

    Unfortunately, during the charging process the measured voltage is actually lower than the effective voltage of the battery – especially on higher current setups such as the ones found on Hybrid electric vehicles. This often forces manufacturers to measure the voltage levels of the battery pack at the cell or module level instead of measuring it at the battery pack's terminals.

    While the cell voltage is dependent on the design and chemistry and cannot be easily managed after production, the cell’s energy capacity also depends on the surface area of the electrodes and the effective quantity of electrolyte present in the cell. Also, with significant bearing is the conductivity that exists between the cells when assembled into a pack. Obviously, the design of the cells will also impact the design of the pack as we shall see in a few moments:

    The cylindrical cell

    Perhaps the most prolific design, it is found in the vast majority of consumer level application and some of the Hybrid vehicle architectures (Honda, Ford, etc). This cell format is also the easiest to cool on an individual basis, but when assembled in to a pack the inner cells may be subject to less optimal temperatures which often requires manufacturers to study the air flow patterns and rates to ensure that the thermal management is as even as possible.

    Example of a cylindrical cell design

    The Prismatic cell

    The shape of this cell is usually determined by the cell packaging which is often flat and effectively packaged into a rectangular packet-like shape. This not only leads to a more efficient use of space which is a critical requirement of HEVs but also gives the manufacturers (i.e: Toyota and others) greater freedom in the final positioning of the battery pack. Also as a result of this arrangement, the connections between the cells are shorter and less subject to losses. This arrangement is also more difficult to cool, often requiring a more elaborate and robust thermal management strategy not to mention accommodations for two dimensional physical expansions due to the increase in cell volume during the charging process.

    An example of a prismatic cell design

    The electrical connections between the individual cells (Power BUS)

    In both cases, the connection between the cells and/or modules is established through careful soldering or attachment to a copper bar (often dubbed a power bus). In many cases and depending on the design requirements, a combination of the two is used. Once these basic requirements are met the assembly will be fitted in the HEV pack which may also include support circuitry and thermal management accommodations.

    An example of a battery pack with prismatic cell assemblies


    A common example of cylindrical cell assemblies used in an HEV battery pack.

    The battery packs management circuitry

    Nowadays, this is usually a combination of hardware and software designed to not only manage the multi-cell assemblies, but also offer safety and health monitoring of the battery pack and downstream systems. In terms of cell management, the circuitry will accommodate a variety of basic functions ranging from collective cell balancing, thermal management, and charge and discharge management and monitoring.

    More about that dirty word: "balancing"

    The biggest issue with multi-cell battery assemblies is the underlying high number of cells that are needed in a high power application such as the ones we have in our hybrid electric vehicles.

    Because each cell represents a single and unique (often un-managed) point of failure, an entire assembly made up of dozens and even hundreds of these cells will dramatically increase the opportunity for failure at a larger scale. On this note, the higher the target voltage, the larger the battery pack and the higher the chances of failure… unless... something is done to manage this probability of failure.

    Please note that most battery packs on our HEV’s tend to target higher voltages and that means that cells are often chained in series. Parallel assemblies tend to exhibit less problems and are frequently defined as permanently balanced.
    1. Quite often manufactures tend to cherry pick production batches by identifying good production runs and grouping cells of the same production batches that are likely to have the exact same characteristics. This BIN sorting is very common in a multitude of industries where some deviation of the manufacturing precision and quality varies with time. The goal is to ensure that a battery pack is made of cells that are as similar as they can be and by using cells from the same production batch, these similarities are more likely.
    2. Because the differences between the cells become more pronounced when the assemblies are subjected to rapid and frequent charge and discharges the design accommodations of a battery pack can be of detrimental importance, so much so that a failure can occur in just a couple of years of normal use.
    3. If a cell of diminished capacity exists within a pack, then there’s a good chance that that cell may frequently experience an overcharge before the surrounding cells reach their fullest charge level. This will lead to an early failure of that cell because of the excessive temperature levels. These higher temperature levels will then spread to the surrounding cells which only compromises their charging profile as well as their longevity.
    4. A cell of diminished capacity will likely lose its entire charge BEFORE the surrounding cells. This will likely raise the chances of a reversal in polarity. If left in this state long enough then a premature failure of the cell is almost guaranteed.
    5. Once a bad cell is identified then the entire battery pack must be replaced. The alternative would consist of locating a replacement cell of the same age, capacity, and electrical and thermal characteristics… which not only requires great expertise and understanding on the part of the shop performing the operation, that although cheaper, also cannot guarantee good enough reliability to make it worthwhile for the typical consumer.
    6. Pack design is a veritable science requiring not only a good understanding of the underlying cell chemistry and its characteristics under a broad operational range, but also how to design and integrate a pack assembly in a vehicle. This integration must take into account the temperature distributions and also the consequential effects imposed on its operating regimen by the interacting upstream and downstream systems. For example, did you know that while the operation of the Air conditioning in the passenger cabin benefits the battery pack, it can also have an adverse effect on the overall temperature signature if set too aggressively. Remember, in many of today's hybrid vehicles, the AC compressor is electrically driven which also places some stress on the battery pack. And for that same reason, the higher and more aggressive the cooling demands are, the greater the thermal stress on the battery pack will be.
    7. Manufacturers often select one of many schemes for managing the inequality between the cells in a battery pack. The choice of a particular method over another depends on many factors but they all aim to avoid the same problems while ensuring the long life of the battery pack while focused on each individual cell. Frequent re-calibrations on some of today’s hybrids are meant to ensure that this equality exists in a somewhat aggressive schedule often driven by: Age, frequency and amplitude of discharges, peak and temperature hysteresis and voltage deviation patterns. Please do not panic when these re-calibrations occur. Take note of them and contact the manufacturer/dealer if your fuel economy is severally impacted.
    Let’s have a glance at the positive attributes of NiMH first:
    • Accelerated charging is possible
    • Wider operational temperature ranges (-20C to +60C)
    • Physically robust and often able to withstand physical fracture without harmful spill effects.
    • Non-toxic when compared to other chemistries
    • Raw building materials that are relatively abundant and can be recycled
    • Low internal resistance
    • Deep cycling is possible under some scenarios and conditions
    • Relatively high energy density when compared to other chemistries
    • Relatively flat discharge curve (which often demands greater accuracy in measuring the SoC since the changes are very minute).
    • Steady lowering of costs as production volumes increase and manufacturing quality ramps up.
    • Relatively mature and predictable manufacturing processes which help minimize lower yields and quality rejections.
    Negative attributes:
    • While refurbishing a battery pack is possible, the results may vary often making such attempts less cost effective and even discouraged.
    • Small charge rates (trickle rates) is not usually supported since slow overcharging can occur and lead to premature failure of the battery (typically through loss of capacity). That is why timer circuits are usually built into the charger circuitry to ensure limited exposure to overcharge.
    • Self discharge rate is very high often dwarfing all other competing chemistries. That is why hybrid cars should be powered up at least once every 30-60 days to prevent a full and crippling charge depletion. The SoC indicators ARE NOT a reliable indicator of health when a vehicle is in storage.
    • Can be conditionally stored in a fully charged or fully discharged state.
    • Like NiCad, it too has a memory effect requiring battery management routines to contain it.
    • Battery venting for the release of gases must be designed into the package.
    • Requires cell reconditioning if SoC is kept at a steady level for longer periods of time.
    • With the allowed packaging constraints, battery packs must be made from many individual cells – each at 1.2V – in order to produce the required voltage levels. This compounds the probability of failure since each cell is now a potential point of failure in the entire chain of cells.
    What can be done to ensure a longer battery pack life?
    • Cause a deeper discharge periodically to nullify the inherent memory effect. This is automatically done by the governance and battery management systems.
    • Ensure that the amplitude of your regenerations and electric assists are as small as possible.
    • Use some electric assist once in a while. Please remember a static SoC is just as bad as excessive use.
    • Keep your battery pack cool. While there are many things we can do some do not involve hardware modifications at all.
      Again, keeping the amplitude of your regenerations low goes a long way towards keeping your pack happier. AC use? Yes, a properly governed use of AC is instrumental and while you may be able to stomach the convection baking for long periods of time in a hot car, your pack may not be able to do the same for long.
      When required, please use your AC sensibly and avoid invoking arctic temperature settings as soon you power the vehicle up. Instead, start with the highest temp setting first and gradually lower it until comfort is achieved.

      Trust us, your pack will thank you.
    As usual, if you have any comments or additional information to provide please don’t hesitate posting your views and even contacting me with the details. ;)
    Last edited: Sep 9, 2009
  2. lowandslow

    lowandslow Member

    Excellent article! Thanks so much for this (and all the other) great info you've shared with us. I liked the practical recommendations on how to ensure a longer battery life. And for me at least, understanding more about how these hybrids work from a technical standpoint, just makes me appreciate them all the more.
    Best Regards,
  3. Harold

    Harold Well-Known Member

    Thanks for this. Hal

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