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BMS for charging, but direct connection for inverter with LVC for short usage time?

bkeiser

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In a Sprinter RV, I have Victron Orion-Tr Smart 12-Volt DC to DC 30 amp charger, SunGoldPower 4000 watt 12 volt inverter with adjustable low voltage cutoff from 10.5 to 12 volts connected to 4 Eve LF280K batteries in 4s configuration with supplimental active balancer. Purpose is to be able to cook for 5-15 minutes (microwave and induction burner at around 2900 watts + losses) when I don't want to fire up my generator. Question: What are comments on purchasing a 60-120 amp bluetooth BMS for everything EXCEPT connecting my inverter directly to the battery terminals (through fuses of course)? I'm thinking the only risk would be if a component failed and inverter didn't shut off and I ignored the BMS app or I fell asleep cooking breakfast. Thanks
 
Do you have 4cells in series for 12v or 4 batteries in series for 48v? I think you are using a 12v system. And if you are 2900watts at 12v is 241amps on a single battery. I wouldn't advise that much current on one battery.
 
I'm thinking the only risk would be if a component failed and inverter didn't shut off and I ignored the BMS app
If you are monitoring everything, including cell level voltages, then YOU are the BMS. I was the BMS on my system until I got a real BMS. It can be done but you need to be attentive, expecially near high and low voltage extremes.

Getting a monitor with audible alarms for battery low voltage and cell level low voltage really helps. A Battgo monitor connected to balance leads is one such monitor.

https://www.amazon.com/ISDT-Battery-Meter,LCD-Capacity-Balancer/dp/B07797N9BG/

Has quite a few other features too.

But NVS is right about drawing this much from a (or 4?) 12V batteries.

Assuming inverter has 10V led, the wiring and fusing needs to handle a LOT of amps:

2900W / 10V / .85 inverter eff. x 1.25 headroom = 426A. Yow!
 
Do you have 4cells in series for 12v or 4 batteries in series for 48v? I think you are using a 12v system. And if you are 2900watts at 12v is 241amps on a single battery. I wouldn't advise that much current on one battery.
Completely 12 volt system, the 4 LiFePO4 are in 4s and rated for 280 amp continuous discharge. 560 amps for 20 seconds. Was thinking I would be running them around 90% of specification for 10 minutes a day.
 
If you are monitoring everything, including cell level voltages, then YOU are the BMS. I was the BMS on my system until I got a real BMS. It can be done but you need to be attentive, expecially near high and low voltage extremes.

Getting a monitor with audible alarms for battery low voltage and cell level low voltage really helps. A Battgo monitor connected to balance leads is one such monitor.

https://www.amazon.com/ISDT-Battery-Meter,LCD-Capacity-Balancer/dp/B07797N9BG/

Has quite a few other features too.

But NVS is right about drawing this much from a (or 4?) 12V batteries.

Assuming inverter has 10V led, the wiring and fusing needs to handle a LOT of amps:

2900W / 10V / .85 inverter eff. x 1.25 headroom = 426A. Yow!
The top of the line Overkill BMS is the one I want and can manage equalization during charge with high voltage cutoff and everything. It’s 120 amp can run my lights, fans and fridge just fine. My issue is that they don’t make one high enough in current, then realized many BMSs only balance during charge anyway, so that BMS would do the trick. My 12 volt inverter would pull 2900 / 13 / .88 = 253 not counting headroom. They will always be near near high state of charge because I drive a lot. Inverter claims 88% efficiency at higher loads. Inverter also has adjustable alarm and separate adjustable low voltage cutoff. My concerns are #1, too many people complaining about poor reliability of Daly BMS and #2, if I pull power directly from the battery, would my BMS go nuts?
 
I just got what you meant by using 10 volts in your equation meaning if I consume 3kw for long enough to drain my battery to the low voltage cutoff, by that time, current will go up over my battery bank rating. I see. Since we only run two devices for a few minutes, not sure I would ever drop my voltage that low. If use case changed, I get your point.
 
The top of the line Overkill BMS is the one I want and can manage equalization during charge with high voltage cutoff and everything. It’s 120 amp can run my lights, fans and fridge just fine. My issue is that they don’t make one high enough in current, then realized many BMSs only balance during charge anyway, so that BMS would do the trick. My 12 volt inverter would pull 2900 / 13 / .88 = 253 not counting headroom. They will always be near near high state of charge because I drive a lot. Inverter claims 88% efficiency at higher loads. Inverter also has adjustable alarm and separate adjustable low voltage cutoff. My concerns are #1, too many people complaining about poor reliability of Daly BMS and #2, if I pull power directly from the battery, would my BMS go nuts?

4000 ac watts / .88 converter efficiency / 12 volts low cutoff = 378.787878788 service amps
378.787878788 service amps / .8 fuse headroom = 473.484848485 fault amps

There is a way to rig a solid state relay in series with the power switch and control the inverter/charger power on/off function using the BMS.
But the amount of current you are drawing is well beyond spec for 280ah big blue prismatic cell.
 
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If you are monitoring everything, including cell level voltages, then YOU are the BMS. I was the BMS on my system until I got a real BMS. It can be done but you need to be attentive, especially near high and low voltage extremes.

Getting a monitor with audible alarms for battery low voltage and cell level low voltage really helps. A Battgo monitor connected to balance leads is one such monitor.

https://www.amazon.com/ISDT-Battery-Meter,LCD-Capacity-Balancer/dp/B07797N9BG/

Has quite a few other features too.

But NVS is right about drawing this much from a (or 4?) 12V batteries.

Assuming inverter has 10V led, the wiring and fusing needs to handle a LOT of amps:

2900W / 10V / .85 inverter eff. x 1.25 headroom = 426A. Yow!
Who invented this 1.25 factor for "headroom?"

Seems excessive if rules regarding ampacity and voltage dropped are properly observed. Throw out that 1.25 "adjustment" and the number is 341 which works with 4/0 cable and 400 amp Class T inverter fuse.
 
As long as battery current NEVER exceeds Eve's published continuous(1C) discharge of 280 amps (especially if only 10 minutes) I am not convinced I have a current issue. My question is whether connecting a BMS normally and charge through 30 amp DC to DC charger normally (which includes balancing) , what happens to a BMS if you connect your inverter directly to the battery terminals; essentially bypassing the BMS for discharge on this one device? You should still get BMS controlled charging, balancing and cell voltage Bluetooth monitoring. Of course, I have to keep my eye on battery current through shunt myself at all times which is worth it to have fresh coffee and eggs in an all electric RV when generator noise would be undesirable.
 
The risk with only protecting the charging phase of a pack is how well matched the cells are. The typical top balanced pack could have some imbalance at the bottom and one cell could go to zero or way below its minimum and the pack Low Voltage Cutoff would not trigger a shut down. The only way to mitigate that risk would be to watch the individual cell voltages as the pack discharges and pick a conservative value for LVC.
 
I probably learned it from smoothJoey along with quite a bit more.

I noticed this in your class T fuse link. Looks like they are at least thinking about 125% and 150%.

View attachment 93595
Call me uneducated, but it sure looks to me like they already thought about it and implemented it, and maybe there is no need to implement it again.
 
I probably learned it from smoothJoey along with quite a bit more.
The concept was around before my time.
I may have come up with or at least popularized the term headroom though.
My intent was to avoid misspelling nuisance.
 
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