ElectricSubaru


Last update: Aug 17 2018

Motive Battery Pack

Batteries are stowed in two groups, both in the back of the car. The main battery box is below the trunk floor, where the spare tire normally hides. A smaller secondary rack is located above the trunk floor. If not for the rear sway bar, the main box could have been bigger and held all the cells.

Cells

Propulsion power comes from 50 CALB CA100 cells. The nominal pack voltage is 160V, capacity is 16kWh. The cells can supply 300A continuous or 1000A for 30 secondsPDF. Total weight of all the cells is 370lb (170kg).

CALB CA100 Cell

I carefully sanded the terminals flat. Some were misshapen and few were corroded.

Main Battery Box

The main box is a trapezoidal shape containing 36 cells. The trapezoidal shape fits between the frame rails, and leaves space at the rear corners for passing cable through the side. The box is made from aluminum plate (0.125in thick) welded at the sides and bolted to a flange of structural steel angle (1.5in x 1.5in x 0.125in). The box was fabricated by Johansen Mechanical for $300.

Lower box fabricated Lower box fabricated Lower box flange Lower box flange Lower box painted

The box floor is strengthened by aluminum T-bar (2in x 1.25in x 0.125in) from Orange Aluminum. The box sides are filled with plywood wedges. The wedges are just over half the height of the cells so that they cannot tip.

Lower box internal structure

The box flange rests on two thin strips of MDF, contoured to fit the profile of the top of the frame rails. Six bolts secure the box to the frame rails, three on each side.

Cross section

The box lid is polycarbonate with five straps screwed to the flange. The straps hold the cells in the box in case the vehicle rolls over. The straps are made of wood on the underside of the polycarbonate and a strip of aluminum (1in x 0.125in) on top.

Lower box installed Lower box from outside

I used the Harbor Freight Electric Body Saw to cut the trunk floor. It worked best with the blade teeth pointing up.

Body Saw Body Saw

Trunk Rack

A rack in the trunk behind the rear seat holds 14 cells, the charger, and the battery disconnect switch. It is constructed of aluminum angle (1.5in x 1/4in) and is bolted at the four corners into rivet nuts in the frame rails. Some wood blocks hold the rack at the height and angle needed to clear a hump in the middle front of the trunk floor. The rack is on top of a section of the original trunk carpet.

Trunk rack Trunk rack Trunk rack Trunk rack Trunk rack Trunk rack Trunk rack

The sides of the cells are covered with foam board for insulation from heat given off by the charger.

Trunk rack

The lid is made of aluminum angle (1.5in X 0.125in) with a polycarbonate top. The polycarbonate is between the terminals and the aluminum. A ring of wood around the edge spaces the lid so that the polycarbonate clears the terminal bolts. The lid is held down by two Keeper 85512 ratchet tie down straps, rated at 1200lb breaking strength (each). Two cutouts for power connections are insulated with rubber grommets cut into a 'C' shape.

Trunk rack

Connectors

The connector straps are from EVTV. They are very thick and wide, with approximately the same cross section as 4/0 AWG cable and are available in lengths for both side-to-side and edge-to-edge configurations. They have an oval hole at both ends that has a smaller terminal contact area than would a round hole. Adding a copper washer between the battery terminal and the connector strap improves the amount of contact area. The washers are Dorman 725-002. The straps come with M8x16 stainless bolts. With the extra washer, M8x20 works better.

EVTV Battery Connector EVTV Battery Connector with Copper Washer

The diagram below shows how much the contact area increases by using the washer. The contact area without using any washer is shown yellow. Adding the washer, this contact area increases to the full size of the terminal, shown green. The contact area above the washer is violet, which is much larger than the terminal.

Surface contact

The copper washers that make contact with the aluminum battery terminals are tinned with Tinnit Tin Plate to reduce galvanic corrosion.

Tinnit Tin Plate

One Bad Guy

One of my cells had a soft short. This kind of defect cells can easily be detected by checking the voltage after fully discharging. If the voltage drops over night, there is a soft short. The bad cell was replaced.

Bad Cell

What! No BMS?

Instead of using a battery management system, I manually balance the cells every hundred charge/discharge cycles or so. I think using a BMS is a good idea for peace of mind, but I decided to see if the cells would stay balanced as indicated by the extensive experimentation and testing performed by Jack Rickard and John Hardy.

For the first four years, I kept the pack bottom balanced. To do this, the pack is fully discharged, rested for 12 hours and then each cell voltage is measured using a good multimeter. Each cell is adjusted downward with a power resistor or brought up with a battery charger.

Junsi iCharger 306B Power Resistor Multimeter

Bottom balancing worked well for the first four years. I saw no significant unbalancing. But after four years, I found my weakest cell significantly overcharging. This may be due to the pack becoming unbalanced, or it may be due to that weakest cell wearing more quickly than the others. I also noticed that the voltage limit on my charger was a bit too high. I adjusted the charger down a bit and that alone might have solved the problem, but I also made the decision to top balance the pack from then on.

Bottom balancing and top balancing both have advantages and disadvantages.

The key advantage of bottom balancing is that it minimizes over discharging the weakest cells when the pack is fully discharged since all the cells reach their discharged state at the same time. However, this strategy increases the chances of overcharging the weakest cells, because they reach full charge first, and then continue charging while the rest of the cells catch up. The overall pack voltage is not greatly affected by one or two cells. The charger keeps going until the total voltage reaches its limit, regardless of any particular cell's voltage.

The main advantage of top balancing is that it avoids overcharging cells. All the cells reach their full state at the same time. This is significant because full charging is routine whereas full discharging is rare.

Another key factor is convenience. Bottom balancing requires fully discharging the pack, which is time consuming. The fastest way to discharge the pack is by driving, but that puts you at risk of being stranded. Any other method of discharging is tedious. Top balancing is done at the end of charging. This happens on a near daily basis, which allows checking the pack almost any time without planning ahead. Top balancing is easier to perform on a more frequent schedule.

One issue affecting this decision is whether the car draws from the main pack when parked. If so, leaving the car parked and unplugged for a period of several months may fully discharge the pack and damage the weakest cells. This car does not draw power from the main battery pack when parked.