Home PV - Introduction to the Lithium-Ion battery conversion

  This project is the next phase on my road toward a sun powered quasi real time net-zero electric system for the whole house.

 What does that mean ? Simply that instead of aiming for net zeroing my electric consumption over a year, it'll be done quasi instantaneously: the power drawn from the grid will be zeroed out by the inverter if within its max power output spec, and if there's enough solar power or stored energy left in the batteries. If not, the system will keep track of the deficit over time and compensate for it next time there is solar power available. My goal is to lower the power drawn from the grid at any time.

 The first step was to replace the propane powered DWH and forced air furnace, by an electric DWH and small convection electric heaters distributed around the house. The 2nd step was to install a photovoltaic (PV) solar system, grid tied and backed by lead batteries. All that was completed last year.

 Now the goal I am working toward is to consume as little electricity from the grid at any time, thanks to our quasi year-round sunny Colorado weather and blue skies, and thanks to the batteries that can provide power even at night or on cloudy days.

 Getting there will be done in several phases:

1. Replace the photovoltaic (PV) solar system's current lead batteries with a ooomphy 24KWh Lithium battery pack salvaged from a Nissan Leaf car
2. Continuously control the inverter's grid/battery management parameters to push into the grid as much power as the house consumes at any time, or to compensate a previous consumption / production deficit (afaik no grid-tied battery-backed inverter on the market today implements a real time net-zero policy for the non backed up loads in a grid-tied house. And none keeps track of, and compensate for, deficit overtime)
3. Develop a distributed smart outlets control system to turn on / off / delay the large household's loads without impacting the occupants' lifestyle (Heating, DWH, A/C, EV charging, etc), nor requiring any in-wall wiring work

 Hopefully when all that is done, I'll be able to power the house for a couple of days without sun during the inter seasons, and maybe be net-zero on a daily basis during sunny winter days despite the heating & EV charging loads. More on the math behind this in later posts, if I don't procrastinate 😅

 So, on to the first step. At the top is a 30.000ft diagram of the PV system. Note that it does not have a secondary panel behind the inverter for backing up some loads. I decided not to install one, as it would have (1) required a lot of work and money due to that house's situation and (2) constrained the backed-up loads to the inverter's 8KW max output no matter my future load needs. One consequence of that choice is that if the grid goes down all loads will lose power (except for 2 backup outlets in the utility room, not show in the drawing), but I am ok with that.

  As for the lithium battery, I bought a 2015 Nissan Leaf 24KWh battery pack, from a salvaged car, for ~$4000. Those have become quite commonly available, are more affordable than a salvaged Tesla pack, have much larger capacity than a Volt's pack, and are arguably easier to reconfigure than a Tesla pack. Here is the 600+lbs beast after removing the lid:

Modules disassembled and undergoing equalization:

 Equalization is necessary as the pack will be reconfigured from 48 modules in series (ie ~400V) to 6 groups in series, each with 8 parallel modules (going down to ~48V to be compatible with the solar inverter). Before grouping the modules in parallel via bus bars, it is important to ensure that all the modules within a parallel group are at the same voltage. If not, high currents may occur when connecting them in parallel, which could possibly create sparks and/or deteriorate the most unbalanced modules. [update] actually, top balancing is the only practical way to ensure cells are initially all in an identical SOC.

 The largest voltage difference between my cells was less than 20mV, so I connected them all in parallel with 1ohm 1/4W resistors (Power dissipated = V^2 / R = 20mV x 20mV / 1ohm = 0.4mW, ie well below 1/4W). The max balance current will therefore be 20mA, which these Leaf cells can easily handle without overheating or deteriorating. In a couple of days the worst voltage difference had come down well below 2mV. Mission accomplished. [update] again, top balancing is the best method to use. Equal voltages between lithium cells does NOT equal identical SOCs...

 By the way, in this blog series the definition of the words module and cell will be very specific, to avoid confusion due to the Leaf modules' construction. A module is the smallest physical battery "brick" a Leaf battery pack is made of, i.e. each of the 48 shiny rectangular boxes with the 3 terminals. Inside a module there are 4 battery pouches in a 2S2P configuration (i.e. 2 groups in series, each with 2 pouches in parallel). 2 pouches in parallel form a cell, which is the equivalent in voltage of a single "common battery", like an 18650 or AA battery. So the voltage is anywhere between ~3V to ~4.2V per cell (whatever this Leaf's chemistry spec is), and a module is ~6V to ~8.4V with a 500Wh capacity. I'll share in a later post more detailed specs for that 2015 Leaf battery type.

 That's it for this introductory blog entry. Next time I'll cover reconfiguring a Leaf pack's modules from 96S2P to 12S16P. Boop !