electrical cable sizes for DC use and 12V batteries

Introduction

red is positive
black is negative / earth (connect black wires before red wires to a charged system)
always connect the red first to a live power source
cables are rated to a certain maximum current in amps - this current should not be exceeded
if you need a long cable > 5m then you should probably get the next larger size up to ensure there is less voltage drop
DC voltage drop along long cables is a major issue and is the main reason why we use 240V AC currents for longer distances and very high DC voltages with lower currents for very long distances

Buying a standard Anderson plug connection to your 12V lithium battery terminals
- battery terminals are usually M8 - the cable connection to these M8 connectors should have insulating heat shrink wrap protection at the cable connection
- you will need an inline fuse rated at 50-80A range (most Anderson plugs are rated to 50A)
- each copper cable should be short (<0.5m) and should be 8 BSB / AWG
- Don't buy a cheap 10A cable from Aliexpress - they will be totally inadequate for your needs unless you will only connect a cigarette lighter plug! - most AC-DC chargers are 15-25A

Installing a DC-DC charger into your vehicle
- as this requires several metres of cable at 40-50A, you should be looking at 6-8AWG cable (you may be able to use 8AWG if you are sure you will only use a 25A charger or distances are only a few metres)

1st calculate the resistance of the cable:
- if you know the resistance per meter then:
  - resistance in Ohms = resistance per meter x length of cable
- otherwise:
  - resistance in Ohms = resistivity in ohm-meters x (length in meters / cross-sectional area of the wire in square meters)
    - resistivity of copper at 20degC is about 1.68 x 10^-8 ohm-meters
    - resistivity usually increases with temperature
Next, calculate the current if it is not already known in amps
- current in amps = power in watts / voltage
Now, calculate the voltage drop per cable:
- voltage drop = current in amps x resistance in ohms
Finally, if you need a round circuit (ie. negative and positive cables), the voltage drop will be twice this if of the same length - as two cables are used

as a side note, this drop in voltage results in a drop in power (watts) with the lost power being converted to heat
- loss in power in watts = voltage drop x current
with very high voltage systems, voltage drop is not such an issue as current is lower for the same power which is one of the reasons why electrical cables running many miles are operating at hundreds of KV, this also allows smaller cables.

6 BSB / AWG is rated for over 50A and has resistance of 1.3mOhm/meter
- this is used when wire lengths need to be longer than 5m with high currents
8 BSB / AWG is rated for around 40-50A and has resistance of 2.1mOhm/meter
- this is the most common wiring for main car battery to DC-DC converter
- 5m with cross section 8.37mm² at 15A 12-14V will result in a voltage drop of 0.3V along the wire (0.5V if 25A)
10 BSB / AWG is rated for around 30A and has resistance of 3.3mOhm/meter
- this is a common wire gauge for solar panels
12 BSB / AWG is rated for around 20A and has resistance of 5.2mOhm/meter
14 BSB / AWG is rated for around 15A and has resistance of 8.3mOhm/meter
18 BSB / AWG is rated for around 10A and has resistance of 21 mOhm/meter
22 BSB / AWG is rated for around 3A and has resistance of 53mOhm/meter