Advertised BMS Capacity 120A, Inverter Needs More Amps
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lmwilco1
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You could use a contactor/relay, connect the negative inverter lead to one side of the relay and the other side goes to the battery negate terminal. Use the bms output to drive the relay. It will throw off all of the power usage data from the bms though as the current won't pass through it.
lmwilco1
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I have a 2000w inverter connected to my Overkill BMS and it is working fine. I have to limit the current I pull out of the inverter to keep it safe. The highest load is my microwave which when running pulls 110A out of the battery so I have 10A of headroom.
John Frum
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I already touched on the workaround in your other thread.
Appreciated very much.
However framed the question from a different perspective and got different views.
I'm not as quick on the uptake and obviously there is some real talent here, like you, so sometimes I need to ask again. Did you notice the different answers? But I do appreciate your perspective and understanding. Or, I ain't the sharpest tool in the shed, but I keep a diggin'.
John Frum
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Quality contactors are expensive and they draw power, but it can work.
They also wear out with use.
Please post the make and model of your inverter with a link so that we can see if a remote remote switching solution is practical.
They also wear out with use.
Please post the make and model of your inverter with a link so that we can see if a remote remote switching solution is practical.
In my world simple is better, not likely to go with a relay unless it is clearly the only option. Limited use of the inverter is a possibility.
A fused separate circuit likely. But if there is a BMS, why not size it to the job?
Also have a second solar setup planned in my tag-a-long trailer with AGM batteries, could be the inverter's home. My on the road workshop/toilet room
www.aimscorp.net
A fused separate circuit likely. But if there is a BMS, why not size it to the job?
Also have a second solar setup planned in my tag-a-long trailer with AGM batteries, could be the inverter's home. My on the road workshop/toilet room
1500 Watt Pure Sine Inverter 12Vdc ETL Listed to UL 458
1500 watt pure sine inverter
www.aimscorp.net
John Frum
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Yes that is what you should have done.
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John Frum
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amaclach
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I've been thinking about just this - is anyone using an AGM or FLA for relatively short heavy currents (2kW) that the LiFePO & BMS won't like?
I'm thinking Microwave, Espresso machine, even an induction hob?
In the offroad world, you should run your heavy loads like winch & compressor off your starter batteries (engine running of course, otherwise you might have issues restarting)...
Of course, this would require a second dc-dc charger as well, but may be better & cheaper than scaling up a lithium system for short peak loads. Just a thought.
I'm thinking Microwave, Espresso machine, even an induction hob?
In the offroad world, you should run your heavy loads like winch & compressor off your starter batteries (engine running of course, otherwise you might have issues restarting)...
Of course, this would require a second dc-dc charger as well, but may be better & cheaper than scaling up a lithium system for short peak loads. Just a thought.
Asking again and again looking for a different answer. You have a 120 amp BMS. That will cover all the amps you need except the inverter. Use a relay to cut off discharge positives before the BMS switches off. Bypass BMS negative inverter direct to battery negative. The BMS is not a control device it is a safety device. Your positive should disconnect before the BMS finds something wrong with one cell.
You can run about 1200 watts of equipment off of the battery and stay within the 120a limit. Do you have more than 1200 watts of equipment you plan on plugging in? If you do, the BMS will shut down when it detects the overcurrent condition.
Note that there's a timeline for overcurrent. The BMS can handle brief overcurrent conditions. See this thread:
Note that there's a timeline for overcurrent. The BMS can handle brief overcurrent conditions. See this thread:
You have run afoul of one of the main inverter connection issues. Inverter companies love 12 Volts because cars have 12v. People love large inverters to run that hair dryer and such. But, the two do not really go together. Something like a hairdryer will draw about 12.5 amps at 120 volts AC, but for the same 1500w of power will need to draw 125+ amps at 12v DC. Ohm's law says that if you use a 12 volt battery, you will need roughly 10x the current as is needed at 120v.
A BMS limits the current to prevent damage to the lithium batteries, so any attempt to bypass it is not advised. 100ah CALB LiFePo4 batteries are good for 100 amp draw long term, but can output more than that for short durations (seconds up to a few minutes, infrequently), but will be mildly damaged by going higher for very long (shortened lifespan - sometimes 50% fewer lifetime cycles). Other lithium chemistries generally have higher output specs, but can literally catch fire if you violate their specifications. For sure don't do it with those.
As those Amp draws go up, you will need wire to match. 125 Amps needs 2 gauge wire. if the inverter will surge, you have to size for that. 250 amps need at least 000 gauge, probably 0000 gauge. Keep in mind there are 2 reasons for this. The resistance of the wire plays into both of them - Ohm's law again. So, the first is that the resistance causes a drop in voltage across the wire. The larger the wire, the less voltage drop for a given current. Most wire size charts are based on keeping voltage drop to within 6-12 volts from that 120v AC line, but at 12v DC, a 2v drop will be enough to trip the low voltage cut-off for most inverters - voltage drop is a function of the current and the resistance, not the starting voltage. So, you need really large, short wire for 12v inverter setups. The second, related reason for big wires is heat. A 2 volt drop across the wire at 250 Amps will generate 500 Watts of heat in the wire - it has to have a way to dissipate or things start to melt and burn. The wire size regulations are entirely focused on that heat - they couldn't care less if your system works.
Big inverters really want larger voltage. At 48v, the current in the wire is 1/4 that at 12v. For instance ~65 amps instead of 250. 65 amps is easily carried by a 4 gauge wire - and a 2 volt drop will not be noticed that much.
People run large 12 inverters all the time and you'll hear "it works for me", but its because they either planned for all the high current and this voltage drop stuff, or they got lucky, or they don't actually use the inverters capacity (an efficiency waste).
A BMS limits the current to prevent damage to the lithium batteries, so any attempt to bypass it is not advised. 100ah CALB LiFePo4 batteries are good for 100 amp draw long term, but can output more than that for short durations (seconds up to a few minutes, infrequently), but will be mildly damaged by going higher for very long (shortened lifespan - sometimes 50% fewer lifetime cycles). Other lithium chemistries generally have higher output specs, but can literally catch fire if you violate their specifications. For sure don't do it with those.
As those Amp draws go up, you will need wire to match. 125 Amps needs 2 gauge wire. if the inverter will surge, you have to size for that. 250 amps need at least 000 gauge, probably 0000 gauge. Keep in mind there are 2 reasons for this. The resistance of the wire plays into both of them - Ohm's law again. So, the first is that the resistance causes a drop in voltage across the wire. The larger the wire, the less voltage drop for a given current. Most wire size charts are based on keeping voltage drop to within 6-12 volts from that 120v AC line, but at 12v DC, a 2v drop will be enough to trip the low voltage cut-off for most inverters - voltage drop is a function of the current and the resistance, not the starting voltage. So, you need really large, short wire for 12v inverter setups. The second, related reason for big wires is heat. A 2 volt drop across the wire at 250 Amps will generate 500 Watts of heat in the wire - it has to have a way to dissipate or things start to melt and burn. The wire size regulations are entirely focused on that heat - they couldn't care less if your system works.
Big inverters really want larger voltage. At 48v, the current in the wire is 1/4 that at 12v. For instance ~65 amps instead of 250. 65 amps is easily carried by a 4 gauge wire - and a 2 volt drop will not be noticed that much.
People run large 12 inverters all the time and you'll hear "it works for me", but its because they either planned for all the high current and this voltage drop stuff, or they got lucky, or they don't actually use the inverters capacity (an efficiency waste).
@amaclach, Lead acid batteries, particularly starting batteries, are good for high current, but do not like to be left discharged, even partially. They thus make for poor energy storage devices compared to lithium. But, if loads were connected to the lead batteries and the lithium was used to continuously recharge them, it could work - it would just be more expensive and complicated then a single battery chemistry. Lead acid and lithium have different voltage curves, so you'd need to connect them through a DC to DC converter/charger - again, adding to the complexity. Probably simpler to upgrade the battery bank / BMS (and perhaps the system voltage), then to mix chemistries, but I'm sure it has its place.
I have no problem with my inverter negative 150A by-pass the BMS. If low cell shuts down the BMS the control circuits are un-powered and the inverter also shuts down. The inverter also has my positive side low voltage disconnect. My BMS is only there as a safety device if individual cells go funny. It doesn't control anything on it's own. 2000W is the largest inverter I would operate at 12 volts. Larger I would use at least 24 volts. Here is a good place to check wire sizes. http://circuitwizard.bluesea.com/
amaclach
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That's exactly what I was thinking. A dc-dc charger direct from the lithium batteries. Think of the lead-acid as a way to boost current short term without screwing up your expensive LFP batteries or roasting the BMS.
I have no need for such a setup (purposefully no inverter).
