Completed Lithium Battery Overview

  • 25kW of total capacity. Charge and discharge limits fence this to 19kW of usable capacity with an expected lifetime of 14-18 years before replacement of cells is necessary.
  • 3kW @240 VAC capacity, or put another way, 6kW @120VAC.
  • Recharges via 50 AMP RV connection or 2.5kW of roof solar.
  • Remote monitoring via cellular data with packet radio fallback.
  • Weighs about 500 lbs including frame
  • Equivalent to approximately a metric ton* of Trojan T-106 batteries.
    • I’m not really feeling like doing the math right now. Maybe later, or calculate it for me! Be sure to include factors like peukert’s law, max discharge capacity, recharge rates, and all that other stuff!
  • Thank you Juan from http://www.beginningfromthismorning.com/
    for encouraging me to try this project out, and get me out of a few binds with the BMS!

This is an overview of the lithium battery pack I built for the broccolibus. As you’ve seen in other posts, it’s based on a Nissan Leaf with the second generation batteries.

Depiction of the guts inside a Nissan leaf battery (generation 1 and 2). The silver rounded things are the foil packs that contain the Lithium Cobalt Oxide cells.

You can identify those batteries because they’re a 2p2s group with a split shell. The first generation cell groups are also 2p2s but have a solid shell. The battery is configured as a group of 14 “cells”, two of which are split by the case itself. This gives the cells within an individual case an opportunity to stay closely related to their charging and discharging activities.

The REC-BMS SI-16 channel controller

Management of all 14 cell groups is with the REC-BMS, which has 16 available cell channels for passive balance charging. Basically, each “cell group” positive voltage end is wired back to the BMS. The BMS will passively bleed an amp or so of current from the highest cell, and at the same time communicates via CANBUS the allowed maximum charge current to put into the battery.

14 unique cell groups for the BMS, attached to bus bars.

The battery is held together with a large steel frame which is bolted via frame hangers off the bus body. You can see the black vertical bar things, which are the threaded rods suspending the battery pack. It is also bolted to the floor of the cargo box on the bus.

Structurally, the cargo box has hangers off the truck frame rail, and it’s “face” is attached to the skirt on the outside of the bus. This attachment method seems secure and protects the batteries from severe trauma in a moderate collision, since the cargo box itself provides an envelope of security.

The battery has several required control features beyond the cell voltage balancing of the BMS. The primary contactor is under primary authority of the BMS which monitors voltage, amperage, and temperature envelopes and “pulls the plug” when the envelope is exceeded.

There is an E-Stop button on the opposite side of the vehicle that also disconnects power to the primary contactor, as well as shuts down all AC power and solar inputs. Activating the E-stop under load may damage equipment, but it’s good to have a second way to remotely disable systems.

There are two independent DC busses that are for the inverter/charger system (200 Amp) and the solar charging system (100 Amp). The other end has mega fuses, due to the way power flows on the bus from both directions. The primary disconnect plug comes before both breakers has a 350 Amp fuse as well.

The thermal control system is independent from the BMS, and is capable of ventilating the battery compartment or heating it. It’s primary purpose is to keep the battery pack in the center of it’s thermal performance envelope.

Each of these safety layers are intended to provide multiple opportunities to shut down the system in the case of a catastrophic fault.

The 48 volt DC power bus energizes several systems. Primarily, it operates the inverter pair, configured for North American 240 volt split phase power.

This power allows us to operate a couple “big” appliances such as a washer, dryer, and an air conditioner. In my research I’ve found that the 240 volt appliances provide marginally better energy efficiencies than their 120 volt cousins. In the case of the clothes dryer, I don’t think any 120 volt heat pump condensing dryers exist.

Additionally, there is an array of buck-boost voltage converters to provide 12 volt “rv” style power. These are fairly efficient and power things like the furnace, water pumps, hot water heater, fans, and lights. The DC distribution box has additional logic for switching back to coach power in an emergency in case the primary battery is not available.

A standard 50 Amp RV shore power connector can power the system. All power is available to run all devices regardless of voltage input, due to the Victron Quattro inverter’s power assist feature. Also, since our vehicle used to be a school bus, it also has an enormous high capacity alternator. When driving for long periods of time, there is an auxilary inverter to pull some power from that alternator (up to 8 amps @ 120VAC) and feeds the charging circuit to keep the lithium batteries topped up.

The solar system (an entirely different post, I think!) has 2550 rated watts of capacity on the panels. I have seen up to 3kw of power come out of the panels on bright cold days in October 2018. I’m looking forward to see what 2019 brings for power harvesting.

Some things I’ve discovered about operating this system for a while now:

  • Eliminating the requirement to fully charge batteries (like lead acid) is a lifesaver. If we don’t have full charge, it’s no big deal.
  • The system has been relatively trouble free. I paid a lot of attention to fasteners, connectors, and overall “quality” of the build. It took time but has paid off with relatively few problems.
  • Nobody in our family really worries if there’s enough power. We operate all the things we want to. When off grid, the most we have done is time laundry so it’s done during the day time when solar harvest is the most.
  • When we are completely off-grid in the winter time, we need supplemental power to maintain the carefree power sucking lifestyle. We are fortunate enough to pull power from friends, the vehicle’s alternator-generator system, or EV parking spots (with a J1772 to NEMA 14-50 adapter interface)
  • If we are careful about power consumption in the winter time, the solar panels (even on weeks of consecutive rainy weather) afford enough power to run lights, fridge, televisions, pumps, furnace, and charge laptops and cell phones.

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