Victron energy Phoenix Smart 12/30 (1+1), Manual

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4.13. Automatic voltage compensation
The charger compensates for the voltage drop over the DC cables by gradually
increasing the output voltage if the charging current rises.
The fixed voltage offset is 100mV. The voltage offset is scaled with the charge
current and added to the output voltage. The voltage offset is based on 2x 1-
meter cable, contact resistance and fuse resistance.
Example calculation for the 12/50 (1+1):
The cable resistance R can be calculated with the following formula:
𝑅𝑅 =
𝜌𝜌 × 𝑙𝑙
𝐴𝐴
Here R is the resistance in ohms (Ω), ρ is the resistiv ity of copper (1.786x10^-8
Ωm at 25°C), l is the wire length (in m) and A is the surface area of the wire (in
m²).
A widely used distance from charger to battery is 1 metre. In this case the wire
length is 2 metres (plus and minus). When using a 6AWG cable (16mm²) the
wire resistance is:
𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 =
1,786 × 10
−8
× 2
16 × 10 −6
= 2.24 𝑚𝑚Ω
Installing a fuse close to the battery is highly recommended. The resistance of a
standard 80A fuse is:
𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 = 0.720 𝑚𝑚Ω
The overall resistance of the circuit can then be calculated with the following
formula:
𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑙𝑙 = 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 + 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅
Therefore:
𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑙𝑙 = 2.24 𝑚𝑚Ω + 0.720 𝑚𝑚Ω = 2.96 𝑚𝑚Ω
The required voltage drop compensation over the cable can be calculated with
the following formula:
𝑈𝑈 = 𝐼𝐼 × 𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑙𝑙
In which U is the voltage drop in volts (V) and I is the current through the wire in
amperes (A).
The voltage drop will then be:
𝑈𝑈 = 50 × 2.96 𝑚𝑚Ω = 148 𝑚𝑚𝑚𝑚
for the full 50A charging current.