Scion xB EV – Lithium Batteries and BMS

This page discusses the 32kwh GBS 100ah, 320V lithium battery and Elithion pro BMS which I am using in my Scion xB.

LiFePO4 Batteries

Lithium  batteries are the state of the art for electric vehicles.   All modern production EVs from Tesla to the Leaf and the Volt are using a Lithium ion battery chemistry.   Some modern Hybrids, such as many Prius models, are still using NiMH (nickel metal hydride) batteries but these are being phased out in favor of lithium as an equivalent pack is smaller and lighter.

Today, virtually all EV conversions are also using lithium.   While commercial manufacturers are able to construct very complex battery packs and use exotic lithium chemistries,  most conversions use LiFePO4, or lithium iron phosphate chemistry.   LiFePO4 cells are safe, reliable, easy to work with, and ultimately cheaper than lead acid, as long as well understood design rules are followed.

Chemistry

LiFePO4 cells are the most commonly used lithium battery chemistry in EV conversions.   They have a nominal voltage of about 3.2V per cell, and their operational range is typically about 2.5V to 4V.  This type of cell usually has a 2C to 3C power density and can handle intermittent bursts somewhat higher than that without damage.  These cells mostly come in easy to handle prismatic housings with two terminals on top.   They come in sizes ranging from 40ah up to 200ah for EV applications, and cells much larger than that are available from some manufacturers, in case you ever decide to convert a greyhound bus.

Each of the several manufacturers have their own specific recommendations as to charging and discharging voltage limits and maximum charge and discharge rates and operating temperature.   These recommendations should be followed, ideally with considerable safety margin, to ensure the longest possible service life.

Advertised cycle life for LiFePO4 is usually greater than 2000 cycles at 80% DOD, or depth of discharge.   To put that in perspective, that means if you build a car that initially will run 100 miles at 80% DOD, then at the end of the battery’s life the car will have an 80 mile range to the same DOD.   So assuming linear degradation,  the car will have averaged 2000 trips of 90 miles, or 180,000 miles.   Most ICE cars are getting tired by that point as well.    As any battery ages, its capacity slowly degrades and usually its internal resistance increases.   In most cases a battery is considered worn out when its usable capacity is at 80% of its original capacity.    This said,  LiFePO4 has not been around long enough to really prove its advertised cycle life in real world driving, and a lot of factors, including temperature, charge/discharge profile, vibration, corrosion, and other factors may affect lifespan.   But,  It seems that most well designed LiFePO4 packs are not seeing excessive failure rates, and I know several people who are using LiFePO4 cells that are six to eight years old in EV applications with success.   That would be virtually impossible with lead acid.

Manufacturers

There are several well known manufacturers of this type of cell.  All are Chinese.   There has been consolidation and several of these names are now owned by common parent companies.   They include:

  • CALB   (China Aviation Lithium Battery)   (Blue and Gray cells)  :  These cells probably have the best track record and one of the bigger installed base among EV conversions, and most people with well designed EVs have had good luck with them.
  • Winston Battery  (formerly Thunder Sky)  (Black and Yellow Cells).   The original Yellow Thunder Sky’s were the first Chinese LiFePO4 cells that were of respectable quality and affordable for EV conversions.   Today CALB cells are generally considered better relative to the original thunder sky cells insofar as I can tell, but these are still decent.   You can still buy the original Yellow Cells (with a LiFeYPO4 chemistry, “Y” for Yttrium, supposedly better but apparently mainly changed to avoid patent infringement).  Newer cells have black casings and different geometry and slightly different ratings and characteristics, but still the same basic thing.
  • GBS  (Turquoise cells).   These are the ones I am using.   While I would have preferred to use CALB or Winston, the form factor of these cells worked much better for my xB given the size of the battery I wanted, and these cells still seem to have a good reputation.   These cells have a slightly different chemistry than most of the other LiFePO4 cells.   This difference allows a somewhat higher energy density, but the down side is these cells do have higher internal resistance and a lower C rate than many of the other LiFePO4 cells.   They also have a different geometry relative to other cells of the same amp hour capacity and a different terminal design (4 small screws instead of a single bolt) and the cells have snap-on terminal covers.
  • China Hi-Power   (White cells).    These were sold for a while but did not develop a good reputation due to low C rate and high failure rate, and not many were used in EV conversions.    I do not believe anybody is currently selling new HiPower cells.

With the exception of China HiPower,  all of these manufacturers seem to produce a decent product.   Early on (mid 2000s) with Thunder Sky there were some problems, but they and the other manufacturers figured out pretty quickly that the EV conversion market in the USA is small and everybody talks to each other, and everybody heard about it when there were problems.    Still,  while it is possible to order direct from the manufacturer and maybe save a few bucks, especially if putting in a large group order as is sometimes done,  I personally feel it is prudent to buy from a reseller who does a lot of repeat business with the manufacturers, as they will have incentive to continue providing quality products to keep the relationship with volume purchasers.

Resellers

There are at least a dozen resellers of Chinese LiFePO4 batteries in the USA.    When it comes to batteries, most of these resellers all just drop ship from the same warehouses operated by the manufacturers and thus the batteries never actually pass through their hands.   Most of them will tell you this if you ask whether they actually stock the cells or not.   It is not bad necessarily that they are doing this, but it does mean that there is no difference whatsoever in the product between parties who do business this way,  so the only thing to consider is what price they are offering and any shipping and knowledge/support they offer.

A few resellers do stock the more popular cell sizes.  Obviously,  they still get them from the same manufacturers.  However these guys do have the chance to look over some of the cells and potentially weed out defective ones before shipping to you, and they may do some value-add work like testing, packaging, or balancing of the cells prior to shipping them.

A reseller may end up with a batch of surplus cells for many reasons.  If you can find one who has cells on hand that they want to move, you may be able to get a somewhat better deal than normal.   However in such case some research into the cells to verify their age or condition may be warranted.   It depends on the price, reputability of the seller and a thousand other factors.    My GBS cells were in this category.  The cells had been purchased new by a company’s R&D department apparently.   The company folded before the cells were used.    The reseller I purchased them from acquired them, presumably at auction, and had them on the shelf.   The serial numbers indicate the cells were manufactured in late 2010, but they were still in their original shipping boxes.  I got the reseller to agree to a written 1 year replacement guarantee (same as new for this type of cell) and that was enough that I was willing to buy for about a 20% discount relative to the going price for new cells of the same type.

Pack Design

Voltage Selection

Generally speaking it is a good idea to build a battery for your EV conversion that has the highest nominal voltage that the inverter/controller is designed to handle.  This will ensure maximum power, maximum range (for a given battery AH capacity) and minimum electrical current for a given amount of power.

Inverters/Controllers, DC/DC and other high voltage components do however work over a fairly wide range of voltages,  so it is fine for example if budget or space do not allow to run a 156V nominal controller on 120V, 132V, 144V, or whatever works.  If you go too far below the rated voltage of your major system components with your battery voltage you might start having brownout problems under certain circumstances, depending on the components you use.

A freeway capable compact to midsize EV should have a nominal battery voltage around 120V or more. Conversions that will not be driven on freeways can get away with 72V.   A performance minded EV should have 250V or more, to keep amps down.   These are rules of thumb, and performance will also be affected by the motor/controller selection, weight of the vehicle, and whether or not it has a transmission.   Generally speaking however, higher voltage equals better performance.    Another advantage of higher voltage is that the performance of the car does not suffer as much when the battery starts getting low.   The only major disadvantage, seen mainly once you go past 156V, is that the price of proper battery pack components (fuses, breakers, high voltage rated wire) starts to go up.  At or below 72V, many components get much cheaper and more common as this is in marine, golf cart and utility vehicle territory.

Battery Sizing

I go into this in detail on my Performance Analysis page.

Battery Box

mid and lower battery boxes installed.   Back seat and upper battery box go above.

mid and lower battery boxes installed. Back seat and upper battery box go above.

I’ve got a whole page on just this topic.  See Battery Boxes and High Voltage Conduit.   Basically,  the battery boxes need to safely contain the batteries, insulate them thermally and electrically, and protect them from water spray and road dirt.  How much protection is required increases with voltage and the power density of the battery, basically.

Banding

GBS cells re-banded in banks of 12 to save space and weight

GBS cells re-banded in banks of 12 to save space and weight

LiFePO4 Lithium batteries need to be clamped to prevent swelling.  Their plastic shells are not strong enough to prevent the internal layers of the cells from spreading apart during multiple charge and discharge cycles.  If the cells are allowed to swell they will first start losing capacity and power density, and will eventually fail.

Some LiFePO4 cells like my GBS ones come prepackaged in banks of 4, but owing to space constraints I had to repackage mine in banks of 12 and 14 as the extra couple inches needed by the extra end plates took too much room for me.

Cells that do not come pre-packaged should either be supported by the battery box if it is strong enough, or clamped together firmly but not extremely tightly using some other mechanism.

Bus Bar

rear box cells exposed showing bus bar and cell boards.

rear box cells exposed showing bus bar and cell boards.

Bus bar (or cable) is the usually-copper bar that connects adjacent cell terminals to each other.  Usually the battery manufacturers offer bus bar,  and in most cases it is sufficient though high performance EVs and unique battery layouts may require custom work.   If making your own,  ensure the bus bar has adequate cross section area to carry the maximum current you expect on a continuous basis, and that the alloy of copper you are using is appropriate for carrying electrical current.   (for example,  copper plumbing pipe can be flattened to make cheap bus bar,  but that particular alloy of copper is not a particularly good conductor compared to other types of copper, as I discovered in my MR2)

Most of the bus bar I used came with the cells.   However I still needed to make some extra bus bars for the terminal lugs and a few inter-cell connections.   Mine are the copper ones, the originals are the nickel plated ones.

Fusing

upper battery box with cells installed.  Form 101 600V, 400A fuse in lower left corner.

upper battery box with cells installed. Form 101 600V, 400A fuse in lower left corner. Each battery box has the same type of fuse, positioned midway.

There should be at least one fuse in each battery box.   This fuse should be placed as close to the middle of the cells in that battery box.  For example,  If a given battery box contains 20 cells and about 72V,   the fuse should go between the 10th and 11th cells.   This means removal or blowing of the fuse will divide the maximum voltage in the box by two.

Fuses for an EV traction pack should be Semiconductor fuses, rated for at least the maximum on-charge DC voltage of the system.  Amp rating should be about 20% to 30% greater than the maximum expected Amp draw of the system.  You do not want the Amp rating to be too much higher than the expected maximum, as if it is too high the batteries may not be able to deliver that much current even in a short circuit situation, and the fuse will provide no protection.   For my Solectria system,  It can pull at most 250 battery amps and the battery voltage on charge is about 360V .  This means 400 amps and at least 400VDC is a good size for the traction battery fuses.

Charger

There are several manufacturers of chargers for EV Conversions.   I am using a Manzanita Micro PFC-30 charger, which is the only part I kept from my 1985 Toyota MR2 EV conversion  (I put back its older, original Russco charger).    The Manzanita chargers seem to be among the more popular in the EV conversion world, owing mainly to their flexibility and high power capability.   They will run off almost anything, and charge almost anything.

There are other charger manufacturers, but many of these are not user programmable, which can be a pain in the neck for an EV conversion owner.

BMS

The BMS, or Battery Management System, is the system that monitors the traction battery.   A good BMS monitors for maximum and minimum voltage on a per cell basis,  Maximum charge and discharge current,  and maximum and minimum operating temperature.    A BMS will also perform active cell balancing, or actively discharging cells with relatively higher voltages, to ensure all the cells in the battery stay at the same relative state of charge.

Many BMS systems also record and/or transmit telemetry which may be recorded or displayed to allow a detailed view of the health and state of charge of the battery pack as the car is being driven or charged.   In my experience, this information is vastly more valuable than a simple volt meter or SOC meter.   It allows you to detect when there is a problem in the battery pack before damage is severe enough to be evident with a single voltmeter, or before you break down on the side of the road.

The BMS must communicate both with the charger (to manage charging) and with the inverter/controller (to manage discharge and regen) in an EV.

Depending on the complexity of the car and how it is used,  a simpler or more complex BMS may be used, but there should be something for reliability and to protect the rather expensive LiFePO4 cells.  All production EVs have a BMS system.

Options

Commercial

There are several manufacturers of BMS systems for Conversion EVs.   The big ones right now are Manzanita Micro,   Elithion (mine) and Orion.   Each system, while performing similar functions, has a different architecure and with that different advantages and disadvantages.

Elithion

I chose the Elithion “Pro” system (their more expensive one) owing to its detailed telemetry capability that their standard system does not provide.   The Elithion system is made in USA I believe.   Their architecture uses very compact individual cell boards which communicate on a serial bus back to the main BMS unit.   It can monitor and control many functions in the car and can be adapted to interface with many different Inverter/Controller/Charger combinations, though some interfaces are more sophisticated than others.   I did have to build a fair amount of custom hardware to interface the Elithion system with my Manzanita Micro PFC30 charger and my Solectria UMOC445TF inverter.

  • I liked that the individual cell boards on the elithion system are cheap, compact, and do not allow cell/battery voltage off of the board.   the cell boards almost fit under the GBS cell covers that came with my cells.  Later GBS cells have a taller cell cover than mine did which the boards would fit under with no issues.
  • In my opinion, the major downside to their boards are their choice to use very tiny, very hard to assemble connectors and wiring that require special crimping tools.  I wish they had used RJ modular connectors such as the Manzanita system uses;  it would have made assembly MUCH faster, neater, and easier.  I suppose their selections are partly due to shielding and bandwidth requirements, but still.
  • The elithion cell board balancing current is very low owing to their small size not allowing much heat dissipation.   This is not a big deal if the cells are already balanced and are healthy,  but it may begin to present a problem as the battery pack ages and the cells diverge from each other in capacity.  I’ll find out.   They do offer a bigger balancing board add-on but I think that is intended for very large cells.
  • The cell boards are delicate and very easy to damage during assembly.  I was lucky to only fry two cell boards during assembly.   I installed one backwards, which killed it instantly.   The other one I do not know why it died.   (Elithion recommends ordering 10% extra as spares and I had done that)  Even my (successful) homemade BMS experiments I had polarity protection on my cell boards.  Not sure why Elithion chose not to.
  • The Elithion system is also sufficiently complex owing to how many features and configuration settings it has that it takes a fair amount of time to digest everything and set it up right.   This is only a problem if you are impatient, like I am.
  • The Elithion system is configured via Serial port connection, so you need a laptop or other computer with a serial port, or a USB to serial adapter, and either hyperterminal (windows) or minicom (linux) configured for 57600 8N1.   This interface, once you get the hang of it, is easy enough to use.
  • I ordered my Elithion pro with a built in high voltage daughter board that has a built in charge current sensor and some full-pack-voltage related functions.   I was unable to get the charge current sensor to work, and ended up configuring the system to use the same Hall effect device used to sense discharge current for the charge current as well, though its resolution is not as good owing to its being sized to sample maximum discharge current which is an order of magnitude greater than maximum charge current.   I have not contacted Elithion or the reseller to diagnose it as it is working adequately.  Most likely I did something wrong I am sure.
  • Elithion provides no paper documentation.   It is all online.  While they strongly recommend you don’t do it,   It seems like a good idea to download a local copy,  in case they vanish on their own or the oil companies convince the government to ban them.

Manzanita Micro

Manzanita Micro has been making EV BMS systems since in the USA the lead acid days.   I do not have direct experience with any of their systems but I know many people who do.   Their design is distributed, without a central BMS unit like Elithion or Orion has.   Their system, unsurprisingly, plugs straight into the control port on Manzanita Micro chargers to provide charging control.    The signaling for their system is somewhat less sophisticated than some of the newer systems, but it does everything you need it to do,  and they now offer modules that generate some of the more detailed information that the other systems do if you need it.

Currently, they make 8 and 12 cell boards.   The boards mount near the bank of cells they manage, or directly on top of them with an adapter board if they are one of a couple common cell sizes.  The boards themselves have temperature sensing on each regulator and I believe there is one remote temperature sensor per board as well.   The individual boards communicate between the charger and other components with a single string of RJ modular cables.  This makes installation very quick and easy,  if there is room to fit the larger boards.  The manzanita regulator boards also can handle more balancing current that the other systems do,  though they have been moving to higher density cell boards, which cannot handle as much current, to offer a lower cost product.

Most of the racers and high performance EVs use Manzanita BMS systems.

I chose against them owing mainly to the tight space constraints in my setup.   While  quite compact for the power they handle, the 12 cell boards are still much larger than the individual cell boards Elithion has, necessitating more complex installation and more space needed above the cells.   Also, with my battery configuration (100 cells divided into 3 boxes), there was no way of using 12 cell boards (their compact ones) without wasting many channels since 100 cells does not divide evenly by 12.

I did feel somewhat bad about this decision since they are a local company and I know the owner through the local EV association.   If I had been using a more conventional cell choice (CALB 100ah) and had a bit more space in my battery boxes I would have gone with Manzanita Micro.

Orion

Orion is the newest kid on the block in BMS systems for EV conversions.   Like Elithion it has a central BMS unit.   Unlike Elithion though there are no individual cell boards.  Instead, each cell has two wires (one shared with the adjoining cell) running back to the BMS unit to allow the unit to perform sampling and balancing.   There are several temperature sensors, but not one per cell like Elithion.

The main advantage of the Orion system is possibly the cheapest system available now that can handle large, high voltage battery packs.

I did not like the Orion system owing mainly to the lack of cell boards.  This means every voltage point in the high voltage traction battery is hauled back to the main BMS unit.  There are MANY opportunities for short circuits and exposure to dangerous voltages.   Any damage to a given cell channel presumably will require repair, or replacement, of the whole unit.  Risk of this could be mitigated somewhat by adding a fuse on every wire.   Also with a 100 cell system,  I would need 101 wires going from all points of the battery pack back to the base unit.   For these reasons I did not consider the Orion system for very long.

GBS

GBS offers their own BMS system specifically designed for their cells.   I did not investigate it very closely as I wanted to use a BMS system explicitly designed for EV conversions and I wanted to use a BMS that I knew would be compatible with my Manzanita Micro charger.

Others

There were other BMS systems that have been made,  but are not currently in production.  Most of these were much simpler systems that did basic battery voltage management and provided very simple high and low voltage warning signals, but little else.   I do not know of any being sold right now.

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