Temporary PowerWall setup

[CU17]

I am moving closer to the final installation and operation of my 14s 160Ah PowerWall.

I have built all my packs and capacity tested them individually, I have (partially) charged and discharged the whole wall and now I am doing some more work before the final install. The wall is currently set up temporarilyin my shed but once I have finished my work I will move it to the garage and connect it to the inverter.

Here is a picture of a part of my testbench. I used a mix of Tp4056s to charge the cells and ZB2L3s to test the cells powered by a 5V 40A LED supply. I also have some Lii-500s to verify the results.





Below is a picture of part of the temporary setup of the wall:





I used the axial glass fuses (1.5A) to fuse my packs. I tested them and they won't blow immediately at 1.5A but will blow pretty much immediately at 2.3A. Not my prettiest pack but functional.




After capacity testing each pack individually I did not charge up each pack individually but instead made a 6s connection (while still monitoring the remaining packs) and charged these 6 packs using an old laptop charger connected to a boost converter.





I have also used a bunch of power resistors to discharge the pack and verify the Batrium's safety settings.





My electrical safety layers beyond the Batrium's monitoring capabilities include the cell fuses, a main 125A circuit breaker andan EV200AAANA relay controlled by the Batrium under the Critical Bat Ok functionality. This works great and gives me both a manual (circuit breaker) and a digital (relay) line of safety. If the breaker pops I will have to manually reset it and should anything happen, e.g. a cable gets cut or disconnects in any way, I don't need to worry because the relay is of the normally open type.
I had to build a simple PMOS switching circuit because the Batrium's output FETs can only handle 3A and the relay has a pickup current of 3.8A - so to avoid anything blowing up I am switching the PMOS with the Batrium which then switches the relay.





I am also making a PCB right now to include a pre-charge circuit and some status LEDs and manual switches and fusesfor the control logic components. Here is the quick and dirty working version:




As I mentioned, the wall will move and thus, I am building an enclosure of stone, steel, and Fermacell plates for it to have a (God forbid) fire safety line of defense.

 

[ChrisD5710]

What have You used for bus bars. How do You hold the packs together? (I see no straps, so how do You do it?)

best of luck with Your project.

ChrisD

 

[CU17]

What have You used for bus bars. How do You hold the packs together? (I see no straps, so how do You do it?)

best of luck with Your project.

ChrisD

Hi Chris,

Thank you.
I used 3x 2.5mm2 stranded wire and 1x 2.25mm2 solid wire (for stability) twisted together. I used this combination mainly because that was what I had at the time. It turned out to make solderingonto the busbar a lot easier when compared to just solid copper.

The packs hold together just from static friction between the cells and cell holders and the busbars ontop alsoprovide cross-joints that prevent the packs from coming apart.


Best,
MPM

 

[OffGridInTheCity]

>I am moving closer to the final installation and operation of my 14s 160Ah PowerWall.
Exciting! Mine has been operational for a while now and its a lot of FUN to just pluck power out of the air!

For my own education - what is'pre-charge' all aboutand why is it important for your system?

 

[CU17]

>I am moving closer to the final installation and operation of my 14s 160Ah PowerWall.
Exciting! Mine has been operational for a while now and its a lot of FUN to just pluck power out of the air!

For my own education - what is'pre-charge' all aboutand why is it important for your system?

I absolutely believe you. I can't wait for that feeling.

Precharge:
Pretty much all modern inverters have a large input capacitance to stabilize the DC bus. When you close the circuit between your battery and your inverter (i.e. close the breaker / relay), you are essentially shorting your battery across the input capacitance. A capacitor is an energy storage element and it is empty when you first connect it. The current charging your capacitor from your battery is only limited by your parasitic resistances (e.g thewire resistance). These usually range in the micro- tomilliohm. If you, for example, have a resistance of 100 milliohms on your 14s battery, the inrush current can be as high as 14*4.2V/0.1Ohm=588A. This can trip your breakers and is not good for the battery, other electrical components, or the inverter.
Below is a quick LTspice simulation showing the inrush current for a 58V battery and 0.1Ohm wire resistance. I modeled the main relay with a switch which is closed at 1s. You see a very short but very high inrush current.





After that I did the same simulation with a simulated precharge circuit which basically works after the following principle:
0. Both relays are off at 0s.
1. Switch a secondary relay (precharge relay)with a power resistor (in this simulation 8Ohm)between the battery and the inverter while leaving the main relay open. Secondary relay switches on at 1s
2. After a few seconds (depending on the capacitor and resistor value - in this case, 2.5s on-time) switch off the secondary relay
3. Now (at 4s) switch on the main relay and commence normal operations.

You will see that the maximum inrush current here ~8A, which is not at all harmful. The voltage across the inverter input capacitance rises slowly over 2.5s to 58V.
You can also see the timing of the secondary and main relay switching and you will notice only a tiny current spike when the main relay is switched on.

 

[OffGridInTheCity]

>I am moving closer to the final installation and operation of my 14s 160Ah PowerWall.
Exciting! Mine has been operational for a while now and its a lot of FUN to just pluck power out of the air!

For my own education - what is'pre-charge' all aboutand why is it important for your system?

I absolutely believe you. I can't wait for that feeling.

Precharge:
Pretty much all modern inverters have a large input capacitance to stabilize the DC bus. When you close the circuit between your battery and your inverter (i.e. close the breaker / relay), you are essentially shorting your battery across the input capacitance. A capacitor is an energy storage element and it is empty when you first connect it. The current charging your capacitor from your battery is only limited by your parasitic resistances (e.g thewire resistance). These usually range in the micro- tomilliohm. If you, for example, have a resistance of 100 milliohms on your 14s battery, the inrush current can be as high as 14*4.2V/0.1Ohm=588A. This can trip your breakers and is not good for the battery, other electrical components, or the inverter.
Below is a quick LTspice simulation showing the inrush current for a 58V battery and 0.1Ohm wire resistance. I modeled the main relay with a switch which is closed at 1s. You see a very short but very high inrush current.





After that I did the same simulation with a simulated precharge circuit which basically works after the following principle:
0. Both relays are off at 0s.
1. Switch a secondary relay (precharge relay)with a power resistor (in this simulation 8Ohm)between the battery and the inverter while leaving the main relay open. Secondary relay switches on at 1s
2. After a few seconds (depending on the capacitor and resistor value - in this case, 2.5s on-time) switch off the secondary relay
3. Now (at 4s) switch on the main relay and commence normal operations.

You will see that the maximum inrush current here ~8A, which is not at all harmful. The voltage across the inverter input capacitance rises slowly over 2.5s to 58V.
You can also see the timing of the secondary and main relay switching and you will notice only a tiny current spike when the main relay is switched on.

Interesting - yes I'm aware (jumped at least a foot) the 1st time I connected my AIMS inverterto the system. I've seen folks (like @DavidPoz) use a big resistor as part of the initial hookup to avoid this spark (and potential damage).

BUT -after the initial connection I don't believe there is an ongoing issue. I have an off-grid system, with dual AIMS 12,000wattinverters that turn on when thebattery voltage rises to a certain level and then off at cutt-off voltage each day. The capacitors must hold their charge - at least for a couple of days -because I don't think this is an ongoing issue in my system else my inverter circuit breakers would trip.

So my thought is, why go to a lot of trouble for this when a manual 1st hookup process (with resistor)can safelycharge up the capacitors and then it shouldn't be a problem unless the inverter is inactive for such a long time (weeks? months?)that the capacitor discharges. I'm asking out of genuine interest - don't mean this to sound negative.

 

[CU17]

>I am moving closer to the final installation and operation of my 14s 160Ah PowerWall.
Exciting! Mine has been operational for a while now and its a lot of FUN to just pluck power out of the air!

For my own education - what is'pre-charge' all aboutand why is it important for your system?

I absolutely believe you. I can't wait for that feeling.

Precharge:
Pretty much all modern inverters have a large input capacitance to stabilize the DC bus. When you close the circuit between your battery and your inverter (i.e. close the breaker / relay), you are essentially shorting your battery across the input capacitance. A capacitor is an energy storage element and it is empty when you first connect it. The current charging your capacitor from your battery is only limited by your parasitic resistances (e.g thewire resistance). These usually range in the micro- tomilliohm. If you, for example, have a resistance of 100 milliohms on your 14s battery, the inrush current can be as high as 14*4.2V/0.1Ohm=588A. This can trip your breakers and is not good for the battery, other electrical components, or the inverter.
Below is a quick LTspice simulation showing the inrush current for a 58V battery and 0.1Ohm wire resistance. I modeled the main relay with a switch which is closed at 1s. You see a very short but very high inrush current.





After that I did the same simulation with a simulated precharge circuit which basically works after the following principle:
0. Both relays are off at 0s.
1. Switch a secondary relay (precharge relay)with a power resistor (in this simulation 8Ohm)between the battery and the inverter while leaving the main relay open. Secondary relay switches on at 1s
2. After a few seconds (depending on the capacitor and resistor value - in this case, 2.5s on-time) switch off the secondary relay
3. Now (at 4s) switch on the main relay and commence normal operations.

You will see that the maximum inrush current here ~8A, which is not at all harmful. The voltage across the inverter input capacitance rises slowly over 2.5s to 58V.
You can also see the timing of the secondary and main relay switching and you will notice only a tiny current spike when the main relay is switched on.

Interesting - yes I'm aware (jumped at least a foot) the 1st time I connected my AIMS inverterto the system. I've seen folks (like @DavidPoz) use a big resistor as part of the initial hookup to avoid this spark (and potential damage).

BUT -after the initial connection I don't believe there is an ongoing issue. I have an off-grid system, with dual AIMS 12,000wattinverters that turn on when thebattery voltage rises to a certain level and then off at cutt-off voltage each day. The capacitors must hold their charge - at least for a couple of days -because I don't think this is an ongoing issue in my system else my inverter circuit breakers would trip.

So my thought is, why go to a lot of trouble for this when a manual 1st hookup process (with resistor)can safelycharge up the capacitors and then it shouldn't be a problem unless the inverter is inactive for such a long time (weeks? months?)that the capacitor discharges. I'm asking out of genuine interest - don't mean this to sound negative.

Yeah, you can ionize air with that. I basically use a big resistor and electrically switch it in-between the battery and inverterevery timeI start the system.

Once it is connected and remains connected you don't have a problem (hence disconnected the secondary relay and connecting the main one). That purely depends on the inverter setup and capacitors in use. Most capacitors will hold their charge for a couple of minutes to days. You might have seen the light on a laptop charger remain on for a couple of seconds after disconnecting it from power. There the cap discharge through the LED. I suspect your inverters will also discharge and the capacitance in a similar fashion.
Does your system ever disconnect the battery electrical after the cut-off voltage is reached?

My system will be inactive for weeks given that my PV doesn't even produce enough to just power the house loads in the winter. I think last December it managed 120kWh for the whole month and with 7.5h of (usually very cloudy) daylight, just the lights in my house and regular appliances use all the power.
In the summer months, however, the system will produce 1.1-1.5MWh. So my system will probably frequently disconnect between November and February.

The precharge system took me about 20min to design and pick the components for and I wanted to make a PCB for my low power 7 logic level routing anyways. So I spent a total of 45min and $15 on this (PCBs are cheap - you can get 5x for $2). It also prevents the messiness of having a bunch of wires hanging around. So I figured a long term system protection was well worth the time.
On a side note, a lot of breakers might not trip / fuses might not blow because the current spike duration is shorter than their trip time. Yetit is definitelynot great for a system over multiple cycles.
Best,
MPM