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What happens when loads ask for too much current?

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chadmichael

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Feb 26, 2022
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Two 170 Ah 12V LiFePo batteries in parallel.

175 Amp Max continuous discharge
350 Amp Max Peak Discharge Current
300 Amp internal fuse
BMS has Over Current Protection

I have NO fuses or anything between the two batteries and my busbar. I have three circuits exiting the busbar. Each circuit has a bolt on terminal fuse (Blue Sea MRBF Terminal Fuse Block): a 300 Amp fuse between my busbar and 2200 Watt inverter, a 50 Amp fuse between my charge controller and the busbar, and a 150 Amp fuse between my 12 V fuseblock and my bus bar.

Obviously, if all circuits were maxed out, I could have a situation where the cumulative ask is higher than the combined 350 Amps of continuous discharge. Based on my intended usage, this will never happen (famous last words I'm sure). My 12 circuits off the fuse block will likely never exceed 50Amps ( I put the large fuse on because I'm using large copper than necessary. ), the charge controller isn't "supposed" to be a load (but maybe it is in some fault cases?), and the inverter will only draw anything near the full 300 Amps when I turn on a very high consumption cooking appliance that I intend to only have in use for 20 to 30 minutes a day maximally.
1) Is this bad design? Should I do something different? Put smaller fuse on the DC fuse block that puts a hard limit on my intended low loads from that branch?


2) What exactly would happen if I managed to ask my batteries for the full load of 400 or 500 Amps? BMS shuts down the batteries, correct?

Open to all suggestions and advice :)
 
The BMS will shut down, as long as it is not destroyed by the high load. A fuse is recommended for each battery to protect the wires and the BMS.
 
chadmichael said:
2) What exactly would happen if I managed to ask my batteries for the full load of 400 or 500 Amps? BMS shuts down the batteries, correct?
Click to expand...
in my system I have a 400a ABB SACE shunt-trip. It's both a circuit breaker (will trip if overall current goes over 400a) but can also be triggered by the BMS if a cell voltage is too hi/low or temp is too hi/low.

As @timselectric said - you should have a BMS and some way for the BMS to disconnect the battery from the bus-bar (e.g. isolate the battery from all loads) either directly or to fire a shunt-trip breaker to do it. If its BMS only, I'd recommend adding a manual breaker (or switch) so you can affirmatively disconnect the battery from all loads to work on things :)
 
You are not ever going to have more than 350A of loads.

If you maxed out the 2200W inverter it would be roughly 2200W / 12V / 85% = 216A. You may have used a 300A fuse but the loads will never get that high.
So that leaves up to about 135A of DC loads from your fuse block. All of your DC loads running at once are unlikely even that high.

So from the point of view of possibly overloading your battery's max continuous discharge current you have nothing to worry about even if you max out the inverter and all of your DC loads.
 
timselectric said:
The BMS will shut down, as long as it is not destroyed by the high load. A fuse is recommended for each battery to protect the wires and the BMS.
Click to expand...
For this purpose, couldn't I just add a smaller terminal bolt on fuse at each battery to positive busbar connection? Rated a bit above the max continuous current? So maybe 200A?
 
OffGridInTheCity said:
in my system I have a 400a ABB SACE shunt-trip. It's both a circuit breaker (will trip if overall current goes over 400a) but can also be triggered by the BMS if a cell voltage is too hi/low or temp is too hi/low.

As @timselectric said - you should have a BMS and some way for the BMS to disconnect the battery from the bus-bar (e.g. isolate the battery from all loads) either directly or to fire a shunt-trip breaker to do it. If its BMS only, I'd recommend adding a manual breaker (or switch) so you can affirmatively disconnect the battery from all loads to work on things :)
Click to expand...
Well, this raises a question I've been wondering about. These batteries have a button to push on the external box. This button turns off current via the BMS as I understand it. Can these be relied upon for turning off the batteries before working on the system? The also feature anderson connections that I can easily disconnect in addition to the button that turns off the BMS.
 
chadmichael said:
For this purpose, couldn't I just add a smaller terminal bolt on fuse at each battery to positive busbar connection? Rated a bit above the max continuous current? So maybe 200A?
Click to expand...
It should be placed on the battery terminal, to protect the wires.
 
timselectric said:
It should be placed on the battery terminal, to protect the wires.
Click to expand...
these batteries do not have exposed terminals. They take anderson SB175 connectors. So, I don't see how I can avoid the short runs ( 12 inches) from battery to bus bar, and it seems like my first opportunity to fuse is at the battery to bus bar connection, unless I splice these wires and insert a fuse.
 
Sorry, I didn't know what type of battery it was.
Is this a "BigBattery" brand battery?
If so, it already has a fuse inside.
 
chadmichael said:
Well, this raises a question I've been wondering about. These batteries have a button to push on the external box. This button turns off current via the BMS as I understand it. Can these be relied upon for turning off the batteries before working on the system? The also feature anderson connections that I can easily disconnect in addition to the button that turns off the BMS.
Click to expand...
Sure - and it's easy to check (that the switch works) with a voltmeter :)
 
chadmichael said:
Two 170 Ah 12V LiFePo batteries in parallel.

175 Amp Max continuous discharge
350 Amp Max Peak Discharge Current
300 Amp internal fuse
BMS has Over Current Protection

I have NO fuses or anything between the two batteries and my busbar. I have three circuits exiting the busbar. Each circuit has a bolt on terminal fuse (Blue Sea MRBF Terminal Fuse Block): a 300 Amp fuse between my busbar and 2200 Watt inverter, a 50 Amp fuse between my charge controller and the busbar, and a 150 Amp fuse between my 12 V fuseblock and my bus bar.

Obviously, if all circuits were maxed out, I could have a situation where the cumulative ask is higher than the combined 350 Amps of continuous discharge. Based on my intended usage, this will never happen (famous last words I'm sure). My 12 circuits off the fuse block will likely never exceed 50Amps ( I put the large fuse on because I'm using large copper than necessary. ), the charge controller isn't "supposed" to be a load (but maybe it is in some fault cases?), and the inverter will only draw anything near the full 300 Amps when I turn on a very high consumption cooking appliance that I intend to only have in use for 20 to 30 minutes a day maximally.
1) Is this bad design? Should I do something different? Put smaller fuse on the DC fuse block that puts a hard limit on my intended low loads from that branch?


2) What exactly would happen if I managed to ask my batteries for the full load of 400 or 500 Amps? BMS shuts down the batteries, correct?

Open to all suggestions and advice :)
Click to expand...
hey op, i've got the same or a very similar battery setup to you. Mine is the batteryevo 12v 120ah with same BMS, fuse, etc. Do you have a schematic or can you describe your system? I am trying to figure out what connectors/wire sizes to use coming out of the sb175 to my busbar and on to 2500w inverter. Appreciate any help!
 
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