Design of the Lithium Battery Pack for the Golf Cart

Cells and Batteries

Let’s start with some basic battery fundamentals, go a bit deeper into details about Lithium cells, then design a pack for my golf cart. 

A "battery" may be composed of one or more cells.  Cells are connected in series to create the desired voltage for the specific application.  For any given cell chemistry, the cell size increases with increasing capacity.  For example, a 12-volt lead-acid battery is composed of 6 individual cells packaged into a battery with each cell typically measuring a touch over 2 volts (higher when freshly charged; lower when discharged).  Here is the link to “battery” in Wikipedia.  http://en.wikipedia.org/wiki/Battery_(electricity)
 

Battery Pack Voltage and Charger  (or when "48 volt" does not equal exactly 48 volts)

Many electric golf carts are designed to run on a nominal 36 or 48 volts requiring three or four 12-volt batteries respectively.  With the exception of the 12-volt “drop-in” lithium replacement batteries, lithium ion batteries are commonly sold into the EV (electric vehicle) market as large individual cells and installed in series to make the battery pack.

Lithium iron phosphate cells are rated for a maximum charge voltage of 3.65 volts/cell, but under load will settle to 3.2 volts and stay relatively flat over the discharge, only dropping about 0.3 volts over most of the useful range of charge.  This characteristic is desirable in an EV because performance of the vehicle is consistent.  The disadvantage is that measurement of the total pack voltage drop is not as useful a tool to know the amount of remaining battery charge.

In the Lithium Boost design mentioned in the last post 15 cells are used providing 48 volts at nominal 3.2 volts per cell, (about 52 volts with a fresh charge and 45 volts just prior to the end of charge).  Because a 48-volt golf cart controller is typically rated to 60 volts or more, adding an extra cell is possible and increases total driving distance.  For example, 16 cells come pre-packaged in the 48-volt GBS kit.  To fairly compare the cost of a 15-cell design to a 16-cell design, be sure to consider the difference in the cost of the extra cell.

Caution: Overcharging or over discharging a lithium cell will damage it causing a loss of capacity or possibly destroying it.  It is important to ensure the charger cutoff voltage does not exceed 3.65 volts / cell and that in use the cells are not drawn down below the recommended minimum of 2.5 volts / cell.

I chose to use a non-adjustable off-the-shelf lithium charger for a 48 volt pack that has a cut-off voltage of 58.4 volts.  This dictated that I install 16 cells.  OK, now what size cells?

Selecting Cell Capacity

Capacity of cells / batteries is often expressed as amp-hours, meaning the sum of the area under a plot of amps over time.  For example a steady 40 amps of current draw over 1 hour would be 40 amp-hours, or 30 amp draw for 2 hours would be 60 amp-hours.  For any given voltage, the larger the rated amp hours the more stored energy the battery will hold and the longer the driving range.

Lithium cells have a useful capacity, or recommended depth of discharge of about 80%.  The common size lithium cells to consider for most golf carts are the 60 or 100 amp-hour capacity units. 

Given these parameters we can now design a pack for the EZGO cart.

Using a very general rule of thumb, based on experience from the conversion crowd, you might expect to move 10 to 13 pounds of gross vehicle weight over 1 mile for every watt-hour consumed.

• Weight of the EZGO RXV = 635 without batteries
• Weight of the battery pack = 7.1 lb / cell x 16 cells
• One to 2 passengers with a weight range = 150 to 300 lb

I calculated that a pack using the 60 amp-hour cells should provide a range on my EZGO golf cart between 25 and 30 miles over the recommended 80% useful discharge capacity. 

This is probably adequate, but I chose the 100 amp hour cells, so that my useful driving range can be extended to between 40 and 50 miles.  While highly unlikely that this range will be needed on any given day, this overdesign will help prevent over-discharging the pack on a very long run and-or allow fewer charge recycles each week.  Fewer recycles also increases the pack life.  It is also comforting to know that there will not be anywhere inside The Villages that is outside my golf cart range. 

Lithium Cell Brand

There are a number of recognized lithium iron phosphate prismatic cell brands including: CALB, GBS, Winston, Sinopoly and HiPower.  The new generation of CALB cells are reported to have some of the lowest manufacturing variation as determined by cell capacity and internal resistance.  This is important when coupled together in a pack, but there are many happy users and proponents of the other brands. 

My selected on-line supplier, Electric Car Parts Company, suggested the HiPower brand due to a 10% lower price and their personal experience with this manufacturer standing behind their warranty.  

To summarize, for my pack design will be:

• 16 lithium iron phosphate prismatic cells
• 100 amp-hour rated capacity
• HiPower brand

Since the useful life of the pack should be far beyond the 2-year cell warranty period, I ordered a spare cell.  Batteries are somewhat analogous to light bulbs in that any given cell may fail long before the typical rated life expectancy.  The likely hood of finding the exact brand, in the size and chemistry 5 years down the road is questionable.   With some experimentation, I may try adding the 17^th cell to the pack if the charger cut off voltage does not seriously limit the overall pack charge capacity.  Most likely it will remain in storage as a critical spare and occasionally swapped out with a used cell in the pack to keep all of the cells somewhat matched in capacity.

• Battery pack cost including bus bars:  $116/cell x 17 cells = $1,938
• Shipping for the cells:  $210

Battery Charger

Warning:  Do not try to use your old lead-acid battery charger on lithium cells.  They have a different charge curve and different cut-off voltage.  Yes, this means purchasing a new charger with cost increasing with the chargers wattage. 

I did not spend much effort comparing chargers and chose a small 10-amp Intelligent brand charger at a cost of $374. 

At this amperage, I expect 8 to 10 hours for a charge.  Not a problem since I typically plug the cart in at dark and it should be  ready to go first thing the next morning.  I also had to purchase a $30 EZGO style 48-volt plug for the business end of the charger cord.

As of today, the cells with bus bars and bolts, charger and other parts have arrived at my home via UPS in good condition. 

Next post will cover the two most hotly debated issues in the use of Lithium cells for EVs:

1. How to balance cells in a battery pack
2. The need for a Battery Management Systems (BMS). 

The decisions on cell balancing and BMS will dictate additional equipment options and operating practices for the golf cart.