Battery

Opening the Battery

Honestly, I was a bit reluctant to even open an expensive working battery.  Most of my battery knowledge is decades old and applies to lead-acid and nickel-cadmium cells, so there was much to learn.  What better way than tearing right into it?

I'm surprised how cavalierly some YouTubers approach high-energy battery systems (wearing jewelry, etc. while working on them).  Not only is there a risk of electrocution, there is also a risk of fire.

In order to remove one side of the EM's battery you must drill out 16 aluminum pop-rivets (which includes one for the carrying strap).  I considered doing this in my vertical mill but decided against it.  If something were to go catastrophically wrong, I did not want the battery clamped down inside my workshop.  It turns out the rivets were very easy to drill out by hand, but I was not going to do so without some precautions.  I would have preferred to do the drilling outside but the weather was pretty nasty.  I decided to perform the dissection inside my garage with the battery on a rolling cart and two large CO2 fire extinguishers at the ready.  First I discharged the battery until it was nearly depleted, figuring if something did go wrong, the less stored energy the better.

The rivet heads came off with a 5/16" bit.  I vacuumed shavings continuously.  Unfortunately, this did not make the side cover any more willing to be removed.  I then had to drill into each rivet pin using a 5/32" bit.  This caused some mandrel heads to fall inside the battery compartment.  This was not comforting, but I could not detect anything amiss and cautiously continued.  

With all the rivets removed, the side cover was still held in place with silicone sealer.  Fortunately, it was a simple matter to loosen it with a razor knife.  Like any good surgical nurse, I counted clamps (I mean mandrel heads) before saying the job was done.  Below is what I found.

Inside the Battery

In addition to the lithium-polymer cells themselves, there is also a battery management system (BMS), a large discharge relay, a small charge relay, an LED state of charge (SoC) indicator, the main on/off switch, a Hall-effect current sensor, and a couple of discrete components.  The enclosure itself even serves as a heat sink for the BMS.  The battery connector is a clone of the ubiquitous Anderson SB 120 and was manufactured by K.S. Terminals of Taiwan.

The battery is arguably the most important aspect of any electric vehicle – it certainly is in terms of safety.  Overall I'm extremely impressed with this battery.  It is very well constructed.

The green area contains the actual soft pack (pouch) lithium-polymer cells.  I learned from an EM 5.7 owner that the cells are made by Kokam in Korea.  One cell in his pack went bad and the replacement pack he ordered took almost a year to get from the factory in France.  He added that there was a lot of runaround with multiple people and the cost was $2,800.

All of Kokam's part numbers start with “SLPB” which stands for Superior Lithium Polymer Battery.  One useful characteristic of lithium-polymer chemistry is that it exhibits no “memory effect.”  This is great if you do a mix of medium and long rides, but always want the battery fully charged before setting out.

I'm wondering if the battery assembly was made in Korea by Kokam or a subcontractor there.  I can only see identification on one component, the small charging relay which says “Made in Korea.”  Indeed, everything inside appears to be well above the quality of typical Chinese manufacturing – other than the gobbing-on of silicone sealer.

Cindy's battery is stamped “EMBA2-13-277.”  Her frame number ends in “279.”  I have learned that the “13” indicates a 13-cell pack.  Presumably, the later 14-cell packs would be indicated in a similar manner.

Some brief comments regarding the individual components follow.

Lithium-Polymer Cells

The battery pack itself is rated at 1.17845 kWh.  Each of the 13 cells is rated 90.65 Wh.  At some point, Kokam may have had a US presence, but it appears to have been acquired by SolarEdge back in 2018. 

Large Discharge Relay

Physically, this is a very large relay - often called a contactor.  It likely also contains a precharge resistor for the motor controller.  A note in the Kelly User Guide says, “All contactors or circuit breakers in the B+ line must have precharge resistors across their contacts.  Lack of even one of these precharge resistors may severely damage the controller at switch-on.”  Kelly specifies the precharge resistor as being 1k ohm at 10 watts.

Small Charge Relay

This appears to be the only component in the enclosure with a country of origin label.  It says Made in Korea.

Hall-Effect Current Sensor

The current sensor is a blue toroid.  Both the charge and the discharge wires run through it.  Using a Hall effect sensor does not waste any power in a current shunt (although it does require a small source of power for the signal-conditioning electronics).

State-of-Charge Indicator

The SoC indicator appears to be nothing more than a special LED voltmeter (which would explain why it is so non-linear).  It seems quite similar to the ones used for lead-acid batteries sold on Amazon.

From the 5.7's Owners Manual, “Only when the battery is fully charged will the 10th LED (far right) be lighted.  As the battery’s state of charge decreases, successive LEDs light up, one at a time.  When the 2nd (from left) LED flashes, this indicates 'energy reserve'.  When both 1st and 2nd LED flash, this indicates 'empty' and a new battery is needed to replace the old one.”

On/Off Switch

This is a simple SPST switch.  It applies power to both the BMS and SoC indicator.  Honestly, it is a very poor quality switch (and it's installed upside down compared to a normal residential light switch in which the lever is flipped up to turn on the light).  I expect to have to replace it eventually.

Charging Port

The charging port is similar to the 3-pin XLR used in professional audio work, but it is keyed slightly differently.  These connectors are typically rated for a maximum of 16 amps.

Discharge Cable

The discharge cable conductor is 25 mm²  wire, which is approximately equivalent to a #3 AWG.  It has a resistance of 0.222 ohms per 1000 feet (222 micro-ohms per foot).  It is rated to continuously carry about 150 amps at 30 degrees C.

Assuming a 150 amp load, the voltage loss is about 33 millivolts per foot (by E = I * R).  The self-heating (power loss) is about 5 watts per foot (by P = I² * R).

The power connector is equivalent to an Anderson SB-120 which has an average mated contact resistance of under 0.136 milliohms (which includes 5.5 inches of #2 AWG wire).

Reassembly

Although drilling out the rivets is not a horrible job, I prefer a simpler method if I decide to change something inside the battery box later.  So, I replaced the rivets with M4 rivet nuts and screws. (Originally, I had planned to use M3 rivet nuts, but they proved to be just too small.)

I applied some natural-cure RTV silicone to the cover first before screwing it down.  It is imperative to use a natural-cure silicone.  Typical RTV silicones off-gas acetic acid which will corrode electronic systems.

14-Series Battery Pack

According to an old brochure I found, EM changed the 5.7's battery pack starting with 2017 models.  They increased the number of lithium-polymer cells to 14.  This makes the nominal pack voltage 51.8 volts (14 * 3.7).  The fully-charged voltage theoretically would increase to 58.8 volts (14 * 4.2).  Although the Kelly controller is rated 48 volts nominal, it is specified to operate up to 60 volts.  A 14-cell pack would be a possible upgrade path.