as I said above, currently the max load would be running the landing gear up and down, which is a 20a max circuit. Down the road, eventually a 3000w inverter. Max charging current would be 20a (2 10a solar in parallel). So, does it make sense to plan for the down the road eventuality or get a bms that covers the max of the current system.
Help Requested: Recommended BMS Size
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John Frum
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3000 ac watts / .85 conversion efficiency / 12 vots low cutoff = ~294 dc amps.
~294 dc amps / .8 standard de-rate = ~367 bms amps.
That is well over 1c discharge from the battery which is going to cause some stress.
For 3 batteries each with 120 amp bms its pretty close.
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Ok that makes sense, but why does the inverter efficiency factor into it? Wouldn’t that only affect the output of the inverter?
John Frum
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The inverter almost is always the vast majority of the load.
converting dc to ac is ~%85 efficient which is significant to factor in the calculation.
The pure dc loads are usually not even worth calculating in comparison.
Go big on the ac and usually ignore the dc.
There are of course exceptions.
Do you have any significant pure dc loads?
The 15% efficiency loss is more than is necessary for the Victron. According to the datasheet below, the efficiency is 93%. So 3000 watts x 1.07 / 12 = 178 amps.
Inverter efficiencies are misleading.
That's the PEAK efficiency, which tends to occur around 25-50% rated. At low power, they can be VERY inefficient. My "background" load of 82W on my 48V Quattro 5K is delivered at a net efficiency of about 65%.
That's the PEAK efficiency, which tends to occur around 25-50% rated. At low power, they can be VERY inefficient. My "background" load of 82W on my 48V Quattro 5K is delivered at a net efficiency of about 65%.
ok, I guess I figured the efficiency would come out of the AC side, so a 3000w inverter may be pulling 250 amps on the DC side, but only putting out 2550 watts on the AC side. So you're saying it will actually put out 3000w but needs 290ish amps on DC to do it?
The only 2 decent sized loads I have on the DC side that I can think of are the landing jacks, which while I don't know their actual rating, have a 20 amp circuit breaker; and the 12v residential fridge, which has a 15a fuse but I've been told it shouldn't draw more than about 6 amps max. Other than that just led lights, stereo and exhaust fan in the bathroom all of which should be negligible.
Is there only one 20 amp circuit breaker for the landing jacks? I've heard of landing jacks and leveling systems with 100+ amp loads.
John Frum
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Yes.
I wouldn't worry too much about the jacks if you only use them for a couple of minutes when you setup camp.
The fridge should be considered as it runs all the time at some duty cycle.
Are you using a Victron inverter/charger?
If yes the efficiency is ~90% but the surge capability is high so the math will change.
As in the bms requirements will be even higher.
Generally when you see a rating, it's an output rating.
1000W microwave outputs 1000W of microwave power, yet it consumes closer to 1600W.
A 750W pump outputs 750W of pumping power, but it may consume substantially more than that. My 3/4 hp pump (about 560W) consumes about 1100W
1000W microwave outputs 1000W of microwave power, yet it consumes closer to 1600W.
A 750W pump outputs 750W of pumping power, but it may consume substantially more than that. My 3/4 hp pump (about 560W) consumes about 1100W
Good point, I'll have to check when I'm working on the trailer this weekend. Now that I think about it it might be 2 20amp breakers, one for each side of the jacks.
I don't have an inverter yet, but it will probably be a victron when I do get one. Unless someone changes my mind.Yes.
I wouldn't worry too much about the jacks if you only use them for a couple of minutes when you setup camp.
The fridge should be considered as it runs all the time at some duty cycle.
Are you using a Victron inverter/charger?
If yes the efficiency is ~90% but the surge capability is high so the math will change.
As in the bms requirements will be even higher.
Good to know. Thanks!
John Frum
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Victron inverter/chargers are top shelf kit.
As I may have said earlier in this thread they require an even beefier high current path including bms.
John Frum
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24 volt batteries means a 24 volt system including 24 volt inverter/charger.
Also since your jacks will use considerable current you will need a beefy converter.
Those are not cheap.
The ones on amazon are not beefy.
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