Seen here is the majority of the 3 stage charger.  Each stage consists of a 100UF 370VAC Run type oil filled capacitor coupled to a common 50A bridge recitifier by a 25A solid state relay.  This makes a constant current source (though in practice it varies based on charge level), each stage contributing ~1.7A of charge current when switched on.  Charging begins with all 3 stages on, continues until 1 battery group reaches the gassing voltage (~14.1v for gels), cuts back to 2 stages until a second group reaches the threshold, then holds the final stage on until all batteries are “full”.  At this point a 1.5hr “accelerated finishing charge” is applied at 1.5amps as per the MK gel charging manual.

If this is completed without interuption then the charger shuts off and waits for a discharge cycle before starting the cycle again.

Some of the cables are left disconnected in these pics; these are the jumpers between groups.  These are the last to be installed, this minimizes the number of instances while working that lethal voltages are potentially present.  First jumpers are installed between every other 12v segment to create 24v groups, below the lethal voltage. These are then jumpered to create 48v packs, this is when it starts to get dangerous, and any action around the batteries is thought out carefully beyond this point as to eliminate the risk of a dropped wrench, accidental shock, or any other hazard associated with high voltages with a great deal of current potential.  All tools used around or in the engine bay are covered entirely with heavy duty heatshrink tubing. 1″ Works well for smaller wrenches. Electrical tape could also be used but I’d rather not get my tools covered in a sticky residue if possible, the heatshrink (being of the non-sealant variety) cuts off with relative ease at any time.

The configuration has changed in anticipation of a sale to a new owner in Boston.  Power is now supplied by a bank of 30 u-1 gel batteries, 33ah each.  Peak power is sacrificed for increased total capacity and better life under deep daily cycling.  They are arranged as 10 groups of 3, 6 groups (18 batteries) in the front and 4 groups (12 batteries) in the back under the floor of the hatch.  Total pack weight has decreased from 780lbs to 720lbs while total capacity increased to 12,000 watt-hours.  The use of lighter gauge steel and nylon straps to secure the batteries resulted in 40 lbs of reduction of battery hardware, plus at least 20 lbs saved by using 1awg cable instead of 4/0 awg.  Safety is being improved by moving all traction wiring outside of the cabin as well as the installation of a second contactor for full pack isolation while charging.  The 1000A controller will be turned down to ~700A to better suit the current limitation of the gels and to better suit the normal driving conditions expected.  Additional pictures will follow as the vehicle is readied for transport to its new owner.

So now I need to fab up a hold down for the 13th battery, jumper it in and re-locate the most-positive point. I am also in the process of re-wiring everything so that it will pass inspection in a few days. New wires must be run for the blinkers, brake lights and horn.

This chart demonstrates the reduction is usable energy from high amperage discharge. At the highest reported discharge current (570A) the cells deliver just 19Ah over the course of 2 minutes. I am more concerned with the 1hr and 30min specs, as these correlate with the amount of current usable for my commute with and without the ability to charge at my destination.

By plugging all the values appropriate for the 240z into this handy calculator I get a pretty good estimate for the current draw at different speeds for various incline and wind situations. It happens that the 30min draw of 96A works out to be about 70mph on flat ground, while a draw of 53A is 55mph. I also see that I can travel 30mph for 5 hours, or 90mph for just 12 minutes.

The Bussman 400A fuse just wasn’t up to the huge amp draws possible under hard acceleration so it had to be replaced. This is by far the biggest fuse I’ve ever seen.

Protective boots for the battery terminals will provide protection against a catastrophic event such as a piece of metal being dropped into the battery bay. 3,500 Amps of short circuit current is not something to play with.

A 13th battery was purchased today. This bring the voltage up to 156 volts from 144, and the total pack capacity up to 11,700 watt hours from 10,800. This is a 8.3% increase in available energy. I expect to see slightly more than an 8.3% increase in range however. This is because of something known as Peukert’s effect, which basically states that the more quickly you draw energy from an electrochemical cell the less total energy can be extracted ultimately. This holds true conversely; the more slowly you discharge your batteries the more total electricity they will provide. Say I’m traveling down the road at 50mph and consuming 100 amps at 100 motor volts.

Volts * Amps = Watts

100V * 100A = 10,000W

In this case the vehicle is consuming 10,000 Watts of energy. The batteries are not being discharged at 100A as you might expect, as the motor controller acts as a “power converter”. If we have a 144V pack voltage the amp draw from the batteries can be calculated thusly:

Motor Watts / Battery Volts = Battery Amps

10,000W / 144V = 69.4A

So 100A at 100V to the motor consumes about 69 Amps at 144V from the battery string. But now if we increase the battery voltage to 156V by adding an additional battery.

10,000W / 156V = 64.1A

The motor is drawing the same amount of power from the batteries, but because the pack voltage has increased the current required decreases.