Component fail-over

[BroomJM]

When designing a DIY LiFePO4 battery, is there an order or logical arrangement of the components, in terms of which device "cuts off" first?

Do you want the BMS to be rated lower than the inverter, so it always shuts down before the inverter reaches its highest possible setting? Do want the BMS to be rated higher than the potential output of the inverter and count on the inverter to cut off voltage?

I'm building a pack with 230AH EVE cells, in a 4S-12v arrangement. I am using a Renogy 1,000w inverter. This is for outdoor and off-grid recreation where I want a fair amount storage capacity, but won't have any heavy loads. The BMS I'm looking at is rated for 120 amps, so around 1,500 watts. The inverter can't sustain that, but I would like to be able to start the compressor on a small chest freezer, if needed.

Thanks in advance for any feedback.

 

[Rossman]

As I understand it, the BMS is your last resort protection, so your inverter low and high battery settings should trigger before the BMS ones do. The BMS is there to protect the cells in case your inverter doesn't do the right thing.

 

[MisterSandals]

Do you want the BMS to be rated lower than the inverter, so it always shuts down before the inverter reaches its highest possible setting? Do want the BMS to be rated higher than the potential output of the inverter and count on the inverter to cut off voltage?

1000W / 12.8V = 78A BMS needs to handle

Factoring in inefficiencies and low battery possibility
1000 / .85 / 12 = 98A bare minimum, plus other DC loads

Most inverters cut off around 10V which is dangerously low for lithium. So some low volt disconnect is best before BMS disconnect (battery saving last resort).

 

[kenryan]

I would size the BMS to the battery specifications. That way, you get maximum performance from your battery. Then, you can size your inverter such that it does not exceed the capabilities of your battery.

 

[rmaddy]

I'm building a pack with 230AH EVE cells, in a 4S-12v arrangement. I am using a Renogy 1,000w inverter. This is for outdoor and off-grid recreation where I want a fair amount storage capacity, but won't have any heavy loads. The BMS I'm looking at is rated for 120 amps, so around 1,500 watts.

This is the correct approach. Your inverter can pull up to 100A from the battery so having a BMS rated at 120A is good. This lets your inverter run at full capacity, if needed, and the battery will purr along just fine.

This is just half of the setup though. As mentioned earlier, do make sure your inverter is setup to shut itself down at a reasonable low voltage. This should be higher than the BMS trigger voltage. This assumes the inverter can be set to user selected low voltage disconnect.

 

[John Frum]

When designing a DIY LiFePO4 battery, is there an order or logical arrangement of the components, in terms of which device "cuts off" first?

Do you want the BMS to be rated lower than the inverter, so it always shuts down before the inverter reaches its highest possible setting? Do want the BMS to be rated higher than the potential output of the inverter and count on the inverter to cut off voltage?

I'm building a pack with 230AH EVE cells, in a 4S-12v arrangement. I am using a Renogy 1,000w inverter. This is for outdoor and off-grid recreation where I want a fair amount storage capacity, but won't have any heavy loads. The BMS I'm looking at is rated for 120 amps, so around 1,500 watts. The inverter can't sustain that, but I would like to be able to start the compressor on a small chest freezer, if needed.

Thanks in advance for any feedback.

I check both renogy 1000 watt inverters and their low voltage disconnect is 10.0 volts +-.5.
So it could be as low as 9.5 volts.
That means the bms will very likely disconnect before the inverter.
1000 watts / .85 conversion factor / 12 volts(bms cutoff) = 98.039215686 amps
Opening the FETs on the discharge path of the BMS at ~100 amps is a brutal stressor for the BMS and is something I would try hard to prevent.

FETS can fail closed as this video demonstrates.

You may be able to control the BMS using a Victron battery protect.
Warning: don't put the battery protect in the high current path.

 

[mikefitz]

You may want to control the inverter using Victron battery protect to disable the Inverter via its control inputs, assuming it has control inputs.

Mike

 

[rmaddy]

You may want to control the inverter using Victron battery protect to disable the Inverter via its control inputs, assuming it has control inputs.

And just to make it clear to anyone that may not know, do not directly wire the DC power of a Victron Battery Protect to any inverter. A Battery Protect has a "remote" connection to indirectly control a device such as an inverter (assuming the inverter has a remote on/off feature). That is what Mike is referring to here.

 

[toms]

Ideally the BMS will control all loads and charge sources.

 

[time2roll]

BMS is to protect the battery. Nothing else.

Inverter can shut down on its own accord if the load is too high.

 

[toms]

BMS is to protect the battery. Nothing else.

Inverter can shut down on its own accord if the load is too high.

Disagree completely, the BMS is best placed to do the following:

- taper current to allow effective balancing
- switch on additional loads at high SOC (hot water etc)
- reduce charge current at high temps / low temps
- shed non-essential loads at low SOC

That’s just for starters

 

[BroomJM]

The Renogy inverter has a remote on/off switch, so I'm guessing that means it's capable of working with the Victron battery protect device. The manual says the inverter beeps when battery voltage reaches 10.5v and cuts off at 10.0 volts. I think I can live with that, but will look into the battery protect option as well.

My next question is about the output from the inverter. It has two outlets built in, each rated for 8.7 amps (on this 1,000w model) and then the high-output terminals (hidden away under a cover) where you can wire in an outlet capable of drawing more current. I have seen Will buy a heavy-duty extension cord, cut off the end and wire that into this part of the inverter. Should that extension cord be rated for 15 amps, 20 amps, 50 amps? The reason I ask is this is a 1,000 watt inverter, but even if I got a 50 amp extension cord (which is kind of expensive) that's only going to let me safely draw around 600 watts, right? Maybe closer to 700w at the inverter, due to efficiency losses?

 

[rmaddy]

The Renogy inverter has a remote on/off switch, so I'm guessing that means it's capable of working with the Victron battery protect device. The manual says the inverter beeps when battery voltage reaches 10.5v and cuts off at 10.0 volts.

10.0V for LiFePO₄ is 0% SOC. The BMS will likely shutdown before it lets the battery get to 10.0V. You don't want that. 10.5V is only 2%. Even 12.0V is under 10% SOC.

 

[rmaddy]

My next question is about the output from the inverter. It has two outlets built in, each rated for 8.7 amps (on this 1,000w model) and then the high-output terminals (hidden away under a cover) where you can wire in an outlet capable of drawing more current. I have seen Will buy a heavy-duty extension cord, cut off the end and wire that into this part of the inverter. Should that extension cord be rated for 15 amps, 20 amps, 50 amps? The reason I ask is this is a 1,000 watt inverter, but even if I got a 50 amp extension cord (which is kind of expensive) that's only going to let me safely draw around 600 watts, right? Maybe closer to 700w at the inverter, due to efficiency losses?

50A at 120VAC is 6000W, not 600W. Your 1000W inverter can only support about 1000W / 120V = 8.3A. I'm not sure how there are "high output" terminals when the supplied 8.7A outlets are the most you can get out of a 1000W 120VAC inverter.

 

[time2roll]

Get a cord with #14 wire is fine with a 1000 watt inverter. I would still prefer NMB (romex)

 

[BroomJM]

50A at 120VAC is 6000W, not 600W. Your 1000W inverter can only support about 1000W / 120V = 8.3A. I'm not sure how there are "high output" terminals when the supplied 8.7A outlets are the most you can get out of a 1000W 120VAC inverter.

OK, the mistake I was making is I was calculating the amperage at 12 volts, but the A/C output of the inverter is 110-120 volts. Thank you for clarifying that. This means the 15 amp waterproof outlet I bought to connect to that "high output" line is more than enough for anything I can safely power with this inverter.

 

[rmaddy]

Using 12V in the calculation is good for determining how much current will be pulled from the battery. And of course it will be more watts from the battery than the inverter due to inverter inefficiencies. 1000W output from the inverter requires 1000W / 85% = about 1176W pulled from the battery. You get 8.7A of 120VAC out of the inverter while the battery sees roughly 98A of 12VDC. Your BMS should be rated for at least 100A continuous discharge current to support a full 1000W at the inverter.

If you are going to wire a 15A outlet on the "high output" line, make sure there is an appropriate breaker. I don't know how your inverter will handle trying to pull 15A when it can only provide 8.7A.

Quick nitpick:
AC = alternating current
A/C = air conditioner