When designing a system, you will always keep in mind that voltage drop varies for different battery voltages.
If we look at the same 2400 W load, but now in a 24V system: • The 2400 W load @ 24V will create a current of 2400/24 = 100A • The total voltage drop will be 2 x 100 x 0.0016 = 0.32 V (= 1.3%)
Battery voltage
Percenta ge drop
Voltage drop
12 V
2.5 %
0.3 V
And at 48V the current is 50A. The voltage drop is 0.16 V (= 0.3%)
24 V
2.5 %
0.6 V
This leads to the next question; how much voltage drop is allowed? The opinions vary somewhat, but we advise to aim for voltage drop no bigger than 2.5 %. For the different voltages this is indicated in the table on the right. It is important to realize that resistance does not only occur in the cable itself, but additional resistance is created by any items in the path of the current. This is a list of possible items that can add to the total resistance: • Fuses • Shunts • Switches • Cable lug crimps • Connections
48 V
2.5 %
1.2 V
And especially watch out for: • Loose connections
• Dirty or corroded contacts • Bad cable lug crimps
Each time a connection is made, or something is placed in the path between battery and inverter resistance is added.
To give you some idea how much these resistances can be:
• Each cable connection: 0.06 mΩ • 500A shunt: 0.10 mΩ • 150A fuse: 0.35 mΩ • 2m 35mm 2 cable: 1.08 mΩ
2.5 Negative effects of cable voltage drop
We now we know that we need to do to keep resistance down to prevent a voltage drop. But what are the negative effects of a high voltage drop in a system? This is a list of the negative effects of a high voltage drop: • Energy is lost and therefore the system efficiency is reduced • All devices connected to the system have a shorter lifetime because of DC ripple. • The system current will increase. This can lead to DC fuses blowing prematurely. • High system currents can lead to premature inverter overloads. • Voltage drop during charging will cause batteries to get undercharged. • The inverter receives a lower battery voltage. This this can potentially trigger low voltage alarms • The battery cables can heat up.
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