Here are the definitions of the words currently used when it comes to batteries...

CellOne elementary part of a battery. (Lithium Ion 18650 elements in my case)
Lihtium Ion 18650 cellA cylindrical cell having a diameter of 18 mm and a length of 65 mm. Weighs approximately 45 gramms. These cells a very common. LG, Samsung, SOny, Panasonic and others produce several billions of these cells each year. A Tesla Model S car contains 7 104 of these cells.

ModuleA group of cells wired in parallel to increase the current capacity and decrease the internal resistance.
Internal resistanceA parasitic characteristic of a battery that creates losses, heating and degradation of the battery. The internal resistance cannot be null, but should be kept as low as possible.
A group of Modules wired in series to increase the voltage and therefore decrease the joule losses at high currents. In a battery design, one always tries to increase the voltage as much as possible. 10 % of increase of the voltage will decrease the joule losses of 20 % at the same current. In a Tesla Model S, the strings are made of about 100 modules in series, giving a total of 400 Volts.
Occur each time a current flows through a cable or a battery. These losses increase with the square of the current. If you can achieve to divide the current in a system by two, the joule losses will be divided by four. In general to decrease the current, you must increase the voltage accordingly to keep the same power rating.
StackThe whole battery made of one of several strings wired in parallel.
PowerThe current flowing through the stack times the voltage of the stack (in watts)
EnergyThe power going in or out of the stack times the time (in Joules or Watt-hours).
1 JouleEnergy of 1 Watt of power produced during 1 second
1000 JouleEnergy produced by a sports rower at each stroke
1 kWhEnergy of a 1 kW (= 1000 Watts) of power produced during 1 hour. One kWh is equivalent to 3 600 000 Joules
10 kWhAverage Energy consumed by my house during one day ( = 3 600 kWh per year)
100 WhYou need approximately 100 Wh to make one liter of water boil
200 WhYou need approximately 200 Wh to drive one kilometer with an electric car
Phase AFirst step of my Powerbucket project where the total capacity will be 10 kWh
Phase BSecond step of my Powerbucket project where the total capacity will be 110 kWh
BMSBattery Management System. Takes care of the battery by measuring the individual module voltage and temperature. Furthermore, the BMS insures also proper balancing of the cells, avoiding dangerous overcharging. The BMS also maintains counters of the energy going in and out of the battery stack
Specific energyThe amount of energy stored in one Kg of battery. The specific energy of a good lead acid battery is 40 Wh/Kg. The specific energy of a good 18650 Lithium Ion element is 250 Wh/Kg, 6 times more.

Powerbucket Specifications

This table contains the main characteristics of the Powerbucket

ItemValueWhy ?
Number of cells in parallel in one module5 * 14 = 70Fits well in my wooden bucket
Number of modules in a string15Fits well in my bucket AND is OK for my inverter
Nominal Powerbucket stack voltage15 * 3.6 = 54 VoltsWell In the input voltage range of my inverter (38 to 66 V DC)
Minimum Powerbucket stack voltage
15 * 3.0 = 45 VoltsWell above the minimum input voltage of my inverter (38 V DC)
Maximum Powerbucket stack voltage
15 * 4.2 = 63 VoltsWell below the maximum input voltage of my inverter (66 V DC)
Number of cells in phase A15 * 70 = 1050 cellsNumber of modules in series times the number of cells per module
Nominal voltage of one cell3.6 VThis is the average voltage of one cell during discharge.
Capacity of one cell2900 mAhI will use PANASONIC NCR18650PF cells having 2900 mAh capacity
Energy in one cell3.6 * 2.9 = 10.44 WhNominal voltage in volts X nominal capacity in Ampere hour = nominal energy stored in Watt hour
Total energy in the Powerbucket stack in phase A
10.44 * 1050 = 10962 Wh
Number of cells in phase B11 * 1050 = 11550Full bucket capacity
Total energy in the Powerbucket stack
in phase B
10.44 * 11550 = 120 582 WhMore than 120 KWh
Weight of one Module4.5 KgsTotal weight of the 70 cells, nickel strips, copper bars, plastic holders and solder
Specific energy of one module(10.44 * 70) / 4.5 = 162 Wh/Kg

About safety

Lithium batteries become dangerous when overcharged or short circuited. In order to mitigate the consequences of a single cell failure, I have decided to equip each indivual cell of a "Tesla Style Fuse". This fuse is made of a short piece of 0.5 mm diameter tinned copper wire welded on the negative pole of the cell. If the current flowing through this fuse exeeds 15 A, il melts, isolating the faulty cell.

A side project: Microwave Oven Transformer Spot Welder

The best way to solder the cells is to use a spot welder. I have designed and built my own because I wanted a maintainable, long lasting and improvable tool. Furthermore it is dirt cheap to build.

The Battery Management System

I have decided to design my own BMS based on a modular approach. It will consist of 15 Module boards communicating with a main board through Bluetooth Low Energy. Thanks to the BT communication I will get rid of all the insulation problems.

Modular solution

The Cell module is organized as follows:

I have populated and tested the first board - Everything works fine. So far so good...

Bare PCBs of the module boards:

The Main module implements an NXP LPC1549 Cortex M3 processor running at 72 MHz. This chip is really the perfect choice for this application.

I have chosen the LQFP 48 package. Nearly all the pins are used.

More about this awsome device here.

The board features:

Schematic of the BMS main module

First page - Microcontroller, Power Supply, I2C and CAN bus

Second page - I/Os, RaspBerry Pi interface and wireless communication