Voltage drop in cable tripping low voltage alarm - batteries OK

[sailingunity]

I have a Victron multi-plus 3000, 900 AH lithium bank. The bank is in the workbench which is about a 12 foot run to the inverter / charger (stops at the battery switch first). When running multiple devices in the galley (say the induction cooker and the toaster oven), the low battery voltage alarm will go off and cut the 120V power.

I've measured voltage at the battery terminal during this event and the batteries are fine. I don't want to lower the alarm voltage, as there could be a legit case when the battery voltage is too low to run the inverter, but how do I stop the alarm going off just because of sag in the cables?

Or do I replace the 4/0 with 2 - 2/0 cables?

[smac999]

the multiplus low votage alarm is measureing the voltage at the posts inside the multiplus. it doesn't care about the voltage at the battery itself. so yes it sees the voltage cable drop.

you need to measure the voltage at the terminals under the large load. and adjust the low voltage alarm set point or the cable size to stop it.

for the alarm setting you need a MK3 usb cable and computer. (which would have been used for the origal setup)

at 12' away single 4/0 might be too small for 3000w. normally that's for under 5'

stops at the battery switch first

also the multiplus should have it's own battery switch direct to the battery. and should not be off the boats switch. you can't have chargers and loads sharing the same swtich. otherwise when you turn it off the charger directly powers the loads

[GordMay]

Originally Posted by smac999

...
at 12' away single 4/0 might be too small for 3000w. normally that's for under 5' ...

Indeed.

3,000 Watts at 12VDC Nominal is about 235 Amps.
235 Amps at 12 Feet distance is about* ±5,640 Amp-Feet. [235A x (2 x 12 Ft*)]
* Since the Amp-Feet is a measure of total round-trip wire length, the figure may be higher, due to ups & downs, and other diversions in the wire routing, resulting in length greater than 2 x 12 Ft.

#2/0 AWG Cu. Is only good for ±4,350 Amp-Feet.
#3/0 AWG Cu, is good for ±5,600 Amp-Feet.
#4/0 AWG Cu. Is good for ±6,900 Amp-Feet.

See:
“Ohm's Law & Boats" ➥ https://www.cruisersforum.com/forums....html#post1256

“Wire Size Chart 1" ➥ https://www.cruisersforum.com/galler...r&imageuser=79

“Wire Size Chart 2" ➥ https://www.cruisersforum.com/galler...r&imageuser=79


“VOLTAGE DROP” ➥ https://www.cruisersforum.com/galler...r&imageuser=79

[SailingHarmonie]

Originally Posted by sailingunity

I have a Victron multi-plus 3000, 900 AH lithium bank. The bank is in the workbench which is about a 12 foot run to the inverter / charger (stops at the battery switch first). When running multiple devices in the galley (say the induction cooker and the toaster oven), the low battery voltage alarm will go off and cut the 120V power.

I've measured voltage at the battery terminal during this event and the batteries are fine. I don't want to lower the alarm voltage, as there could be a legit case when the battery voltage is too low to run the inverter, but how do I stop the alarm going off just because of sag in the cables?

Or do I replace the 4/0 with 2 - 2/0 cables?

Once again we have a bunch of answers being given with out enough data.

We need to understand what is going on here, because my quick calculation of voltage drop suggests that 4/0 wire is sufficient to keep voltage drop for 250 Amps over a 24 foot round trip run to less than 3%, and Victron’s manual agrees.

I strongly suspect something else is going on here, something potentially dangerous.

• First, we re assuming this is a 12V system, is that correct?
• what is the setting for the low voltage alarm?
  
• Has the voltage been measured at the terminals of the inverter at high load?
• What voltage at the batteries during a high load is considered “fine?”

If we truly are looking at voltage drop along the wire run, and that voltage drop is large, it is not 24 feet of 4/0 wire that is doing it. It is a bad connection. A bad connection at these currents will get dangerously hot and needs to be found and fixed. I am not yet sure that’s the case, but voltage drop from wire resistivity is not the answer here.

Running two wires is frequently done incorrectly and has significant safety issues.

[GreenWave]

OP stated he is using 2/0 cable. A responder implied 4/0 was good enough. In short Gord nailed it. Change to 4/0 cable then see about alarm adjustment.

[sailingunity]

Originally Posted by GreenWave

OP stated he is using 2/0 cable. A responder implied 4/0 was good enough. In short Gord nailed it. Change to 4/0 cable then see about alarm adjustment.

Actually I'm running single 4/0 cable today. My question was if I needed to run 2 - 2/0 cables instead. I'll check connections again.

And I misspoke originally, the bank is connected to the inverter charger through a fuse and then a on/off switch, that's it.

[CaptainRivet]

Originally Posted by SailingHarmonie

Once again we have a bunch of answers being given with out enough data.

We need to understand what is going on here, because my quick calculation of voltage drop suggests that 4/0 wire is sufficient to keep voltage drop for 250 Amps over a 24 foot round trip run to less than 3%, and Victron’s manual agrees.

I strongly suspect something else is going on here, something potentially dangerous.

• First, we re assuming this is a 12V system, is that correct?
• what is the setting for the low voltage alarm?
  
• Has the voltage been measured at the terminals of the inverter at high load?
• What voltage at the batteries during a high load is considered “fine?”

If we truly are looking at voltage drop along the wire run, and that voltage drop is large, it is not 24 feet of 4/0 wire that is doing it. It is a bad connection. A bad connection at these currents will get dangerously hot and needs to be found and fixed. I am not yet sure that’s the case, but voltage drop from wire resistivity is not the answer here.

Running two wires is frequently done incorrectly and has significant safety issues.

This is spot on, bad connection and/or bad fuse is most likely the cause. Had this also when fused with a 300A Megafuse from „bussmann“ that turned out to be a fake from china.
Put the high loads on and touch every connection and also the cable.
What 4/0 cable was used, welding cable or marine grade with tinned fine strands?