First Test Drives of Partially Completed Cart and Pack Charge

When the battery disconnect switch and the key were switched on, it ran! 
No sparks or smoke and no electrical components failed at the maximum pack voltage (charger cut-off) of 58 volts. 

I had found that the Sure Power DC-DC converter was rated to take 100 Volts, but I was not able to find the upper voltage limit of the controller.  The local EZGO maintenance techs didn't know either.  I did find out from the service techs that the regenerative braking system "shunts" some of the current through a big resistor to protect the battery pack.  There is no public information on the settings of how this works.  Does it set a limit on the peak current or battery voltage?  Does it vary with the pack voltage?

Not having all of the parts to install the JLD-404 amp-hour meter or the odometer, I took my Fluke 115 multimeter along on the test drives.

Using the open ended pair of lead wires from the 200 amp - 75mV shunt, the cart was pulling from 15 to 40 amps with instantaneous peaks up to 160 amps during rapid acceleration.  The pack voltage and the individual cell voltages were checked every mile or so. 

To be sure we stayed away from the battery bottom, I put the cart on first charge with the pack at 51 volts. 

This is the EZGO charge port on the front of the seat bench.  The plug-in-adapter was ordered from EVDrives.com for the cord from the new Lithium charger.

Since the pack was at a healthy SOC, the Intelligent charger went directly into constant current at full 10 amps instead of half current and then ramping up to 10 amps.  10 amps is only 0.1 C (well within the battery limits).

 

Here is the charger at 54.5 volts.  At 10 amps the voltage reading is about 1.3 volts higher than the actual pack voltage due to the voltage drop across the cord.

 The back of the charger with the 110V AC power plug and the On-Off switch.

The charger did shut down the constant current mode when it reached 58.1 volts and went into float applying only enough current to hold the voltage.  Average cell voltage was 3.63, just shy of the recommended 3.65 cut-off voltage for Lithium Iron Phosphate cells.

Next posts, installation of the JLD-404 meter and the EX-RAY speedometer/odometer.

Lithium Batteries - Top Balance and Install

All 17 of the HiPower 100 amp-hour 'short' lithium iron phosphate cells arrived by UPS in good condition.  They were packaged in pairs in cardboard boxes padded with foam inside and secured by straps outside.  Each battery has a sticker with the serial number. 

A separate capacity and internal resistance certificate was provided (paper left of the cell above).   For best overall battery pack performance the values should be tightly clustered (+/- 5%).  A scatter plot of the capacity versus resistance is shown below.  All cells are shown to be greater than the 100 amp-hour rating.  The lowest capacity was 101.5 amp-hours and the highest 107.7 amp-hours.  The internal resistance varied from a low of 0.20 to a  high of 0.32 u-ohm.

Here are the batteries hooked in parallel so they can balance with each other and get their initial charge from a Cell Pro Powerlab 8. 

While the batteries were being charged, I removed the four dead Trojan T-1275 deep cycle lead acid batteries from the golf cart.  Here is a picture of the thick wall plastic battery box.  Note that the bottom has ridges and curves which does not conform to the smaller footprint of the lithium pack.

I created a template of the cart battery box and custom cut a plywood deck. 

Sixteen of the cells fit nicely in two rows.  They are secured by custom fitting some wood around the base.  Three-quarter inch diameter vinyl tubing was cut and split to fit over the bus bars and terminals to help prevent accidental contact.

A 48-volt 600 amp Blue Sea Systems battery disconnect switch was installed under the seat.  The switch provides a way to ensure no parasitic current drain and provide a safety cutoff during maintenance. 

An inexpensive +/-1% Amico 200 amp-75 mV shunt was installed in the negative lead to provide a way to measure current flow.  A more accurate (+/- 0.25%) shunt will be ordered after testing the actual amp draw in operation.

Next steps are to install the JLD-404 meter.  This will require un-switched (continuous) power out of the Sure Power 48V to 12V DC-DC converter (model 41020C00).  Unfortunately the un-switched pin #4 was not being used and did not contain a lead wire.  Had to order a new Deutsch DT style 6-pin connector.

I had also ordered an EX-RAY speedometer / odometer for an E-Z-GO Golf Cart, but did not specify the RXV.  When it arrived the mounting plate did not fit the steering column.  Called EV-Drives.com and ordered a replacement.  Got a return authorization and will send back the one I have for a refund.

Next first test drive and battery pack re-charge.

Lithium Iron Phosphate Cell Behavior in a Series Battery Pack

Cell Capacity

Not every cell has exactly the same capacity as illustrated in the figure below.  In this case cell “B” has only 95% of the rated name plate amp-hour capacity while cell "C" has 105%. 

Also, remember from the last post, lithium cells do not tolerate over charging or over discharging. 

The combination of these two factors cause the useful amount of amp-hours in the entire pack to be limited by the lowest cell.  In this example, cell "B" will be the first to discharge / charge.

Let's see why the lowest cell is the limitation and some alternatives to deal with it.








 


Top Balancing

Let’s say that you carefully charged each cell so that they were all completely full as determined by the cell voltage.  A typical “cut off” charge voltage for LiFePO4 cells is 3.65 volts.  With each cell at this voltage you have effectively balanced the pack at the top of the charge.  See figure below (left).

Now you drive the cart until the lowest capacity cell reaches the discharge “cut off” voltage of 2.5 volts (figure – right).  If you continue to drive the cart the other cells will deliver their remaining amps, but because the pack is connected in series, those amps are still flowing through cell “B”.  That current will remove base material from the cell and do irreparable damage.  At $125 to $150 for each 100 amp-hour cell, you don’t want to go there.

The challenge is that it is difficult to know which cell is empty.  The voltage of entire pack may look fine, but one cell can still be below the critical value.  The only way to be sure is to have a system that measures the voltage of each cell, or at least small groups of cells.

Bottom Balancing

There is another approach.  That is called bottom balancing which is performed by individually draining each of the cells to the same lower voltage level (figure below – left).  Note that this illustration shows 2.50 volts, but bottom balancing is often done at a more conservative 2.70 volts.  Now when you drive the cart you can tell from just looking at the total pack voltage when to pull over and call the tow truck as all of the cells reach empty at the same time.

There is no free lunch.  During charge you have to know which cell reaches the charge cut-off voltage first and shut down the charge cycle as damage also occurs at the top end (figure – right). 

How do you do that without a cell-level monitoring system? 

Options include having individual chargers for each cell, under charging the pack and hope that none of the lower-capacity cells were damaged, or manually checking the voltage of each cell toward the end of the charge to find an optimal cut-off point.

Battery Management System

A third engineering alternative is to measure voltages on each and every cell during charge and discharge.   Alarms can provide warnings and, in some designs, the controller can turn off the charger on the top end and-or shut down the system when the lowest capacity cell is empty.  These systems are referred to as a BMS or Battery Management System.  In addition to adding cost to a golf cart application, some of my personal concerns regarding a BMS are the complexity (reliability), uncertainty (what the BMS is doing to my cells), and amount of exposed circuitry (failure rate).

Many designs also attempt to move small amounts of current from higher capacity cells to lower capacity cells, especially during charging.  The use of a BMS is frequently combined with top balancing.

Which is correct?  You will find very strong opinions with heated arguments for each of these techniques.  My view is that all with work just fine if you understand the approach and follow the procedures for that alternative.

My Design and Operating Choice

Which way am I going?  This is a golf cart and I want a simple low cost system.  Regardless of which approach you take, I am convinced that all lithium packs should have an amp-hour counter (or watt meter) to provide an indication on the state-of-charge.  These devices actually take mV readings across an externally mounted shunt (calibrated resistor).   I will be using the common JLD-404 meter, cost $75 and a 200 amp / 75mV shunt.

I would have gone with top-balancing combined with voltage measurement and alarm for four groups of 4 cells each.  The rational was that any cell that dropped into the danger zone would be easily detected.   This would only require 5 fused wires.  While searching for parts I found some interesting options designed for the radio-controlled toy airplane / helicopter hobbiest.

Now the plan is to go with top-balancing combined with full cell-level monitoring and low voltage alarms.  This will be provided by a pair of tiny Cell-Log 8M devices attached to the dash. 
http://www.progressiverc.com/celllog-8m.html

The drawback is that design will require 17 fused wires and then unplugging of the Cell-Logs when not in use as they have a small parasitic current draw that is un-even from each cell.  Thanks to Ken's MR2EV conversion blog for the tip and details.  http://blog.mr2ev.com/should-you-install-a-battery-management-system-to-protect-those-expensive-lithium-batteries/

Since only 2 knowledgeable and responsible adults will be using the cart, a mistake-proof auto-shut down system was not thought to be needed.  I can also forgo the cost and complexity of a full cell-level BMS. 

My pack will be protected at the top end where charging occurs frequently, I should have an indication of the state-of-charge from the amp-hour meter to let me know when to start home.  Should there be a surprise, an alarm will signal any problem toward the bottom.  The appropriate decision on how to handle can be made at that point.

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.

Available Lithum Battery Solutions for Golf Carts

Where can I buy a ready made Lithium Battery solution?
If you are just starting to investigate lithium for your electric golf cart, like I did this year, you want to know who supplies and installs lithium batteries, the options, performance and cost.  The following post provides an overview and links to some of the packaged solutions that I am currently aware of.    

“Drop-in” Batteries
The concept of the 12-volt “drop-in” lithium battery sounds like an ideal solution.  The term means that four cells have been placed inside a plastic housing along with some circuitry.  They come packaged in a traditional lead-acid size box with traditional battery terminals and literally just are a swap out.  The difference being is that they are lighter in weight and store more energy which provides longer range. 

Examples of drop-in replacements are made/sold by Lithionics Battery  http://www.lithionicsbattery.com/golf.html

Another source is Smart Battery.  Here is a re-posted picture of their 48V -100 amp hour battery.

This direct link to the Smart Battery golf cart kits provides various capacity options.  Forget the lower cost 40 amp-hour batteries, as you probably want the range of the 60 amp-hour version, or for a place like The Villages, FL possibly the 100 amp-hour size.  http://www.lithiumion-batteries.com/EVGOLFCART.php

Simple and easy, but wow the price is jaw dropping.  I called and got a quote for a set of 100 amp-hour batteries and charger from a dealer in west Florida.  The installed cost was approaching $6,000.  Ouch, I did not pay that much for my used golf cart.

“Drop-in” Module
EVTV Motor Verks, a Utah-based conversion shop and on-line parts store, has recently developed a 16-cell 60 amp-hour capacity drop-in module which they mention in their EVTV Friday show - October 4, 2013.  Quoted cost is about $2,850 with a built in charger;  Their concept is to just plug your 110 Volt extension cord into the module.  Skip to 26 minutes of  this 2 hour show to watch:  http://www.youtube.com/watch?v=M8fE5cIZag4

The EVTV option is not yet on their products page and may or may not yet be available.  Note: While EVTV has a strong web presence and provides some good information and well researched but pricey products to the EV community, I was taken aback by the strongly biased opinions and name-calling rants in their posts and videos. 

Prismatic Cells
There is quite a bit of information available on conversion of gas powered cars to electric vehicles “EVs”.  The EV conversion crowd has done the hard work of figuring out the design alternatives and what works.  With the exception of some EV car manufacturers, the conversion crowd is almost exclusively using, Lithium Iron Phosphate (LiFePO4) chemistry.  Safer and more stable than the higher energy chemistries such as lithium cobalt cells;  Think Boeing Dreamliner and Tesla.  These LiFePO4 prismatic cells come in the form of a flat brick, with a rugged plastic case in typical sizes for EVs of 60, 100 and 180 amp-hours.  Under the hood of the drop-in batteries and module solutions mentioned above you will likely find LiFePO4 prismatic cells.

Here are a couple of re-posted pictures of popular brands of cells in the 100 amp-hour capacity size. This is the CALB design which uses the large threaded terminals (one bolt for the positive and one bolt for the negative) to interconnect the cells with bus bars.
 

 

 

And the GBS design which uses a brand-specific connection with 4 small screws on each terminal to interconnect the cells with straps.  They come banded together in groups of 4 with lifting cords on each end.  The removable purple plastic cap helps prevent short circuits form accidental contact.



Dealer Installed Systems
With a typical charged cell voltage of 3.2 volts, enough lithium iron phosphate cells are connected in series with bus bars to get the desired power.  For a 48-volt golf cart you could use as few as 15 cells since that would provide exactly 48 volts.  This is what Lithium Boost, based out of San Diego, does in their golf cart conversion packages.  http://lithiumboost.com/

The "Plus" package is the 60 amp-hour cells and the "Pro" package is the 100 amp-hour cells.  There is now a dealer in The Villages, FL and at least one local customer case history posted who installed the "Ultra" 180 amp-hour cells in a heavy custom 4-seat model.

While the posted design details are limited, the Lithium Boost option uses individual cell chargers, one for each of the 15 cells, mounted in a module on-board the cart.  Individual chargers ensure that each cell receives a full charge.  A BMS “Battery Management System” (more on this topic in later posts) also protects the cells by balancing the voltage of the cells when charging, and then to prevent over discharging, shuts off the cart if you run the batteries to the capacity limit.  A limp mode provides a very short interval of power to move the cart to a safe location.

On-line posted prices for 48-volt versions of this alternative which uses CALB brand cells are about $2,000 for the 60 amp-hour pack and $2,900 for the 100 amp-hour pack.  I still have not confirmed if the price includes installation, but with local dealer support it looks like a viable option for many.

Do-It-Yourself Kits
A turn-key DIY “kit” is available which includes 16 of the GBS brand 100 amp-hour batteries, a top-of-the-line BMS (they call it an Energy Management System) and external charger for about $3,250.  This package comes with a cockpit display screen that gives the driver more information that you probably care to know on various parameters regarding the state of the pack.  Nice elegant solution with an upgraded display.  The webs site mentions that you may need to add a separate DC to DC converter to keep power supplied to the on-board computer.

from Elite Power Solutions
http://elitepowersolutions.com/products/product_info.php?cPath=27_29&products_id=164

from Electric Motor Sport
http://www.electricmotorsport.com/store/ems_ev_parts_batteries_lpf_gbs_kit48.php

The total installed cost for any of these alternatives may be higher due to the cost of shipping the 100+ lb of cells and accessories.  There may also be add-on options which are required to make a complete package.

I will update this post with links and prices to more packaged solutions as they become available.  Please leave a comment know if you know of other viable options for golf carts.

With all of this information, which one of these solutions did I chose?  Actually, none of the above, as I went the a-la-carte design-it and build-it yourself route.  In the next post I will go deeper into the topic of LiFePO4 prismatic cells and how I sized a battery pack for the EZGO golf cart.