Fuse and cable selection for batteries used in backup systems.

**Tradie** · 18-Mar-25, 08:29 AM

This has turned into an interesting topic, after receiving a notification from the battery supplier tech support, to notify me that the volt drop on a site is higher (2.0VDC) than it should be (V batt and V BMS) which could result in issues with BMS
and charging and discharging the batteries. It could also compromise the battery warranty if the volt drop is higher than 1.5 VDC, we dont want that to happen.

Lets look at what would normally be a straight forward selection, using a5 kw system as an example, then I will do the most common setup in 8 kw.

5 kw inverter

5 kwh lithium battery. Rated at C0.5 (charge discharge rate 50 amps)

25 mm sq multi strand cable (Alvern cables permopower SABS 1507 cable 1000V) rated at 115 amps.)

100 amp DC fuses

Distance from the inverter to the DC fuse disconnect 1 metres and from the DC fuse disconnect to the battery 1 m.

We need to look at the volt drop per metre and at what point you need to consider using a bigger cable.

Then it might also be a good idea to look at the fault level.

**Tradie** · 18-Mar-25, 08:37 AM

The part that is confusing me about the notification, when the batteries are charging or discharging V batt and BMS voltage are the same, there is no difference. The voltage difference is when the battery is at 100 % the gap open by a volt or 2. Once a load is applied the gap closes. I need to figure out how to screen shot and paste copy and paste.

**Tradie** · 18-Mar-25, 08:51 AM

Try call Alvern cables to get technical info about their cables, I wish you luck.

**Tradie** · 18-Mar-25, 04:28 PM

After a site visit and a couple hours of reviewing other site data. There will be about a 2 VDC split (V batt and BMS voltage) just after the battery reaches 100% SOC when viewed in the custom setting in the plant info.

At any time the battery is charging or discharging, the gap between the V batt and BMS voltage will close to within 1 VDC.

Carried out numerous Volt drop test at the inverter, DC fuse disconnect, master and slave batteries, while discharging the battery, and the volt drop at any point was less than 0.2 volts, so I would say the cable length and thickness is well within spec.

**felyn443** · 20-Mar-25, 10:47 PM

It looks like your 25 mm² cable is giving a voltage drop a bit above what you'd want, so it might be a good idea to either consider a larger cable or check if there are any connection issues that could be causing extra resistance. Also, factoring in the fault current level is key to ensure that your fuses can handle any potential surges.