how to measure the state of charge and voltage of your 12V batteries - shunts, etc

Introduction

measuring the voltage of your battery is very easy:
- use the battery inside a battery box or power station which has a volt meter display, or
- use a voltmeter and attach to battery terminals, or,
- use an inline ammeter which will also display the current flowing
measuring the state of charge (SoC) is NOT so easy if you don't have a power station with this function or a battery with Bluetooth
- you must insert a shunt into the wiring and this will then allow you to see a SoC once you have set it up for your battery
- if you don't have a SoC meter, you can get a very approximate estimate of SoC of a 12V LiFePO4 battery by its voltage when there is no load:
  - >13.6V = fully charged (it will usually be 14.6V on a charger when fully charged in boost mode - chargers will usually drop this to 13.6V in storage mode)
  - 13.4V = ~90% SoC
  - 13.3V = ~80% SoC
  - 13.2V = ~ 70% SoC
  - 13.1V = ~60% SoC
  - 13.0V = 40-50% SoC
  - 12.8-12.9V = 20-30% SoC
  - 12.0V = 10% SoC
  - most have a low voltage cutoff of around 10V (ie. “zero charge”)
measuring the cranking capacity of a 12V cranking battery (a battery designed to start your car):
- you will need a cranking battery tester and follow the instructions

these are needed for measuring State of Charge (ie. percent charged) of 12V batteries if they don't have Bluetooth SoC functionality
if you have a caravan or 4WD with permanent power system installed then having one of these wired in is very handy
some shunts have a built-in LCD display
buy one with Bluetooth if you wish to be able to set values and see SoC on your phone eg. Victron SmartShunt
one terminal of the shunt is wired to the negative of your lithium battery (and this has to be the ONLY connection to the battery negative terminal for this to work), the other terminal is wired to the negative of your system - this could be a bus bar which allows multiple negative end connections from your system
there is a much thinner cable (usually red and with a small fuse) which connects to a “V Batt” port on the shunt and then to the positive terminal of your lithium battery
if using a Victron shunt it will have a VE port to optionally connect to your Victron server and the Aux input allows a thermometer probe or other options
see https://www.youtube.com/watch?v=oY4Dcuny7Ao

you need to input the battery capacity in Ah
charged voltage is the voltage which the shunt will assume equates to 100% charge status - this is usually 14.0V (0.2V lower than the charge absorption voltage)
tail current - is the current which the charging falls to when it approaches 100% charge status - this is usually set to 2%
discharge floor is the lowest % of capacity which you want the battery to discharge to - this is usually 0-10% (if using lead acid batteries, this should be 50-60%) - this is used for the shunt to calculate how much time you have left at current power usage before it reaches this level of SoC
Peukert exponent is a parameter to indicate charge efficiency of the battery - this is usually 1.05 (1.25 for lead acid batteries)
charge efficiency is usually 98% (lower for lead acid batteries)
current threshold is the min. current you ant the shunt to recognise as current - usually 0.10A
see https://www.youtube.com/watch?v=NdiHtaCMeck