Any ideas for improvement?

[AF_Bob]

I would appreciate any feedback, especially if you see something wrong or something that could be improved. I didn't go crazy with fuses since I thought the battery BMS would basically shut down if something went wrong, but I will gladly add fuses or anything else, if recommended.

With the exception of the 2 AWG battery cables (on order), I'm ready to start building this DIY power station in a RIGID 2.0 25" box on wheels.

Please let me know if you see anything wrong or anything you would do differently to make it safer.

Thanks in advance for your help.

Bob
Some questions I have are:
1. I believe the two AWG 2 cables from the batteries to the Inverter MUST be the same length, but can I put a shorter cable BETWEEN the two batteries in series since they will be literally 3 inches away from each other?
2. I didn't plan on a switch to turn off the panel voltages going to the solar charge controller since I plan on rolling out the cart and solar panel stands, and then connecting the MC4 connectors. I can always turn the panel stand away from the sun, connect the MC4s, and then position the panels if the 68V would cause a problem pushing the MC4 connectors together.
3. I purchased two LIFEPO4 battery meters that can be programmed to monitor each 12V battery or monitor both batteries in series. Would you suggest I monitor each separately? Or monitor them in series? The meters display the voltage and also display the battery capacity (e.g., 80%, 90%) based on the battery voltage (e.g., 13.2V=70%, 13.3V=90%, 13.6V=100%).

Attachment​

 

[timselectric]

If you are building a 24v system. Buy a 24v battery.
We don't recommend putting BMS controlled batteries in series.
They will drift apart and require re balancing every few months.


I would also recommend a shunt to keep track of the battery SOC.
I like the Victron Smart Shunt. But others are available.

 

[Rednecktek]

1. I believe the two AWG 2 cables from the batteries to the Inverter MUST be the same length, but can I put a shorter cable BETWEEN the two batteries in series since they will be literally 3 inches away from each other?

Common misconception on the cable length, it really only applies to multiple batteries. If you had 2 battery banks, then the POSITIVES need to be the same length, and the NEGATIVES need to be the same length, but the Positives DO NOT need to be the same length as the Negatives. I.E. you can have a pair of 18" positive cables and a pair of 22" negative cables, but you DON'T need to have 22" cables all the way around. With a single battery you can just use whichever length you need. As for connecting a pair of 12v batteries, best option is to use a native 24v battery, but a shorty cable from one to the other is fine. You'll run into some imbalancing issues and may have to re-balance every so often, but it'll work.

2. I didn't plan on a switch to turn off the panel voltages going to the solar charge controller since I plan on rolling out the cart and solar panel stands, and then connecting the MC4 connectors. I can always turn the panel stand away from the sun, connect the MC4s, and then position the panels if the 68V would cause a problem pushing the MC4 connectors together.

MC4 connectors really don't like to be moved around under load, the metal in there is pretty thin. Having a breaker between your panels makes it MUCH easier in the future so you don't have to go out, man-handle 4 panels, do your work, then man-handle the panels again. You can just shut it all off, do the work, and bring everything online with a simple switch.

3. I purchased two LIFEPO4 battery meters that can be programmed to monitor each 12V battery or monitor both batteries in series. Would you suggest I monitor each separately? Or monitor them in series? The meters display the voltage and also display the battery capacity (e.g., 80%, 90%) based on the battery voltage (e.g., 13.2V=70%, 13.3V=90%, 13.6V=100%).

Voltage based battery monitors are worthless for LFP batteries because the power curve of an LFP is so flat. With lead acid it's very linear so the difference between 12.8v and 12.7v is about the same as 12.7v to 12.6v and it can guesstimate from there. With LFP the voltage can be 13.8v at 100% full and 13.6v at 20% (as example numbers) so your meter will tell you the battery is 80% full as it dies.

You'll need a proper shunt to keep track of how many amps go in and out of the battery to get a real reading on how full your battery is. Even something as simple as the well liked Aili shunts will give you FAR more accurate readings than any voltage based meter. You'll be monitoring the bank as a single bank. As far as the shunt and system are concerned, it doesn't know if your system is a single 24v battery, a pair of 12v batteries, or a dozen 2v batteries, it's all the same to the system.

A couple other things to note on your drawing:

You have the solar panels listed by their Vmp rating, you need to calculate with VoC as that's the voltage you're putting into your charge controller.

The 15a SCC is just barely enough to handle that 400w of panel, if you ever want to expand in the future you'll have to replace your SCC with a higher amperage version.

People are going to tell you that you need about 744 more fuses in there, and to replace the ANL fuse you have on the battery with a Class-T fuse. Good luck finding a 100a Class-T fuse and holder nowadays.

 

[AF_Bob]

If you are building a 24v system. Buy a 24v battery.
We don't recommend putting BMS controlled batteries in series.
They will drift apart and require re balancing every few months.


I would also recommend a shunt to keep track of the battery SOC.
I like the Victron Smart Shunt. But others are available.

Thank you very much for responding. Since I already purchased 2 Li Time 12V 100A batteries and the 24V inverter, I'm stuck with putting the batteries in series. I'll remember a single 24V battery if/when I have to replace the batteries. The 2 batteries were purchased together so hopefully they're pretty closely matched. I also used a LIFEPO4 battery charger and charged each battery separately, so they measure the exact same voltage and the charger read fully charged. I read someplace that I could put them in parallel and leave them connected for a few days and that they would balance themselves. Since I can't use the cheap 12V battery meter, I'll go ahead and buy a shunt and a meter that will work. I included some pictures of the box which show how far the batteries are apart and the inverter will sit on top. I'll use 2 fans for ventilation. I should have purchased the box AFTER I bought all the components but I really like the Rigid 2.0 box on wheels and it was on sale and I'm determined to make it work as best I can.

I'll also put a breaker between the panels and I just ordered a MC4 fuse gizzmo to put in the positive line of the solar panels. I don't plan on using more than 400 watts of solar and by putting 2 panels in series, connected in parallel to another 2 panels in series, my current shouldn't exceed 12 Amps. If I put all 4 panels in series (which is my plan), I'll have about 68V at about 6 Amps (~400 watts). The solar charge controller supposedly handles 115V at 15 Amps so I should be well under the limits of the controller for now. I'm hoping that 400 watts of solar panels would give me up to 2,000 watts of power a day (400 watts times 5 hours). Since each battery has 1,280 watts (2,560 watts total), I'm hoping 2,000 watts of solar would pretty much charge it back up each day. I'm not planning on using the power station on a daily basis, but rather for when the power goes out for things like a 700 watt coffee maker, perhaps a microwave for 20 minutes a day, and some power for LED lights. I also have a VTOMAN Flashspeed 1500 and a River 2 Pro 800 watt power station for things as well. I'm actually building this for more for the knowledge and experience in case we ever lose the grid for a longer period of time.

Thanks again for all the help. I do sincerely appreciate it and will let you know how things turn out.

 

[Hedges]

Do we know that this BMS tolerates 24V system use? With a quick search I found a listing but it didn't address the topic.
I've read some 12V lithium batteries allow 48V use and some do not. It would depend on FET voltage rating, don't know if enough headroom for 24V system.

 

[AF_Bob]

Common misconception on the cable length, it really only applies to multiple batteries. If you had 2 battery banks, then the POSITIVES need to be the same length, and the NEGATIVES need to be the same length, but the Positives DO NOT need to be the same length as the Negatives. I.E. you can have a pair of 18" positive cables and a pair of 22" negative cables, but you DON'T need to have 22" cables all the way around. With a single battery you can just use whichever length you need. As for connecting a pair of 12v batteries, best option is to use a native 24v battery, but a shorty cable from one to the other is fine. You'll run into some imbalancing issues and may have to re-balance every so often, but it'll work.


MC4 connectors really don't like to be moved around under load, the metal in there is pretty thin. Having a breaker between your panels makes it MUCH easier in the future so you don't have to go out, man-handle 4 panels, do your work, then man-handle the panels again. You can just shut it all off, do the work, and bring everything online with a simple switch.


Voltage based battery monitors are worthless for LFP batteries because the power curve of an LFP is so flat. With lead acid it's very linear so the difference between 12.8v and 12.7v is about the same as 12.7v to 12.6v and it can guesstimate from there. With LFP the voltage can be 13.8v at 100% full and 13.6v at 20% (as example numbers) so your meter will tell you the battery is 80% full as it dies.

You'll need a proper shunt to keep track of how many amps go in and out of the battery to get a real reading on how full your battery is. Even something as simple as the well liked Aili shunts will give you FAR more accurate readings than any voltage based meter. You'll be monitoring the bank as a single bank. As far as the shunt and system are concerned, it doesn't know if your system is a single 24v battery, a pair of 12v batteries, or a dozen 2v batteries, it's all the same to the system.

A couple other things to note on your drawing:

You have the solar panels listed by their Vmp rating, you need to calculate with VoC as that's the voltage you're putting into your charge controller.

The 15a SCC is just barely enough to handle that 400w of panel, if you ever want to expand in the future you'll have to replace your SCC with a higher amperage version.

People are going to tell you that you need about 744 more fuses in there, and to replace the ANL fuse you have on the battery with a Class-T fuse. Good luck finding a 100a Class-T fuse and holder nowadays.

Thank you Solar Wizzard for all the responses. With the batteries, I was hoping I could put them side-by-side but the Rigid 2.0 box on wheels only allowed me to put each battery on opposite sides. Those Li Time MINI 100A batteries are actually taller than the normal 100A batteries. I have a piece of wood on the top of the batteries to allow for the inverter to be placed on top in the middle. I wound up using the supplied 26" long cables (came with inverter) for each of the batteries and a 2' cable between each battery since they're on opposite sides of the box.

 

[Rocketman]

For the shunt look at a Victron Smartshunt or BMV712. Then because you have 2 12v batteries- look up the mid-point voltage setting on the shunt - the aux input. This will track the two batteries together as a 24v battery and track the midpoint- each 12v battery- so you know when it is getting off and you need to recharge the batteries separately.

 

[AF_Bob]

Common misconception on the cable length, it really only applies to multiple batteries. If you had 2 battery banks, then the POSITIVES need to be the same length, and the NEGATIVES need to be the same length, but the Positives DO NOT need to be the same length as the Negatives. I.E. you can have a pair of 18" positive cables and a pair of 22" negative cables, but you DON'T need to have 22" cables all the way around. With a single battery you can just use whichever length you need. As for connecting a pair of 12v batteries, best option is to use a native 24v battery, but a shorty cable from one to the other is fine. You'll run into some imbalancing issues and may have to re-balance every so often, but it'll work.


MC4 connectors really don't like to be moved around under load, the metal in there is pretty thin. Having a breaker between your panels makes it MUCH easier in the future so you don't have to go out, man-handle 4 panels, do your work, then man-handle the panels again. You can just shut it all off, do the work, and bring everything online with a simple switch.


Voltage based battery monitors are worthless for LFP batteries because the power curve of an LFP is so flat. With lead acid it's very linear so the difference between 12.8v and 12.7v is about the same as 12.7v to 12.6v and it can guesstimate from there. With LFP the voltage can be 13.8v at 100% full and 13.6v at 20% (as example numbers) so your meter will tell you the battery is 80% full as it dies.

You'll need a proper shunt to keep track of how many amps go in and out of the battery to get a real reading on how full your battery is. Even something as simple as the well liked Aili shunts will give you FAR more accurate readings than any voltage based meter. You'll be monitoring the bank as a single bank. As far as the shunt and system are concerned, it doesn't know if your system is a single 24v battery, a pair of 12v batteries, or a dozen 2v batteries, it's all the same to the system.

A couple other things to note on your drawing:

You have the solar panels listed by their Vmp rating, you need to calculate with VoC as that's the voltage you're putting into your charge controller.

The 15a SCC is just barely enough to handle that 400w of panel, if you ever want to expand in the future you'll have to replace your SCC with a higher amperage version.

People are going to tell you that you need about 744 more fuses in there, and to replace the ANL fuse you have on the battery with a Class-T fuse. Good luck finding a 100a Class-T fuse and holder nowadays.

Solar Wizzard, regarding the solar panel Vmp vs Voc rating, I looked at the Voc rating on the Ecoflow website and it shows 20.3V. With 4 in series, that would give me 81.2V. Would I multiply 81.2V the maximum operating current of 5.9A or the short circuit current of 6.3A? If I go with the worst case (81.2V X 6.3A), it seems like I'd be OK since the Victron SCC can handle 100V at 15A. Do you think I have enough margin of safety? I don't plan on adding any additional panels and I could always try a series/parallel combo if that helped.

 

[AF_Bob]

For the shunt look at a Victron Smartshunt or BMV712. Then because you have 2 12v batteries- look up the mid-point voltage setting on the shunt - the aux input. This will track the two batteries together as a 24v battery and track the midpoint- each 12v battery- so you know when it is getting off and you need to recharge the batteries separately.

Thanks, Rocketman! I had no idea these smart shunts could do all this stuff. I read about the Victron Smartshunt and saved one of the pictures, showing the hookup to monitor 2 batteries in series. It's amazing how these things work! I looked at the BMV712 and that $191 price scared me off . Since I have a Victron SCC, I'll get that one and wire it up like the attached picture shows. I guess I should have known that the $9 battery capacity meter wasn't going to work! I was hoping to build my entire project for $1,000 or less, but I can see that's not going to happen. I probably should have waited for the VTOMAN Pro 3000...but I guess I'll learn a lot more doing it DIY and it will be much easier to repair/replace individual components rather than an entire power station. These days, it's difficult and expensive to even ship the power stations purchased online. Thanks again for that info on monitoring 2 batteries in series. I'm learning more and more on this website and sincerely appreciate everyone's help.

 

[AF_Bob]

Do we know that this BMS tolerates 24V system use? With a quick search I found a listing but it didn't address the topic.
I've read some 12V lithium batteries allow 48V use and some do not. It would depend on FET voltage rating, don't know if enough headroom for 24V system.

I checked with Li Time prior to purchasing and they said I can definitely put 2 Li Time 100A MINI batteries in series. I would have put 2 in parallel but all the reading I did regarding inverters stated that 24V inverters are much more efficient and can use smaller diameter cables so I went with the 24V Giandel pure sine wave inverter and decided to go with the 2 batteries in series.

 

[AF_Bob]

Do we know that this BMS tolerates 24V system use? With a quick search I found a listing but it didn't address the topic.
I've read some 12V lithium batteries allow 48V use and some do not. It would depend on FET voltage rating, don't know if enough headroom for 24V system.

According to Li Time, it should work.

 

[SupraSPL]

I know it's too late, but If you're anything like me I build a system and then justify changing it or building a better one.

Anyway once I got my hands on a battery with a bluetooth BMS I realized I wouldnt want to be without one again, sold all my prefab batteries and built new ones with BT BMS ($60 each from JBD). That way you can keep an eye on the actual coulomb counter SOC and capacity of each battery from your phone, keep an eye on each cell voltage, battery temp, adjust the balancing settings as needed/if needed and check to see if loads are balanced between batteries. Also they will have low temp cutoff and sometimes even heating option.

I assume the extra cost is probably not much different than the cost of a good shunt and with BT in each battery you get a lot more data and cell level data. Anyway if you end up building another system at some point, I can vouch for the BT being an awesome feature and hopefully that will become the standard because it wont add much cost for manufacturers so why not.

 

[AF_Bob]

I know it's too late, but If you're anything like me I build a system and then justify changing it or building a better one.

Anyway once I got my hands on a battery with a bluetooth BMS I realized I wouldnt want to be without one again, sold all my prefab batteries and built new ones with BT BMS ($60 each from JBD). That way you can keep an eye on the actual coulomb counter SOC and capacity of each battery from your phone, keep an eye on each cell voltage, battery temp, adjust the balancing settings as needed/if needed and check to see if loads are balanced between batteries. Also they will have low temp cutoff and sometimes even heating option.

I assume the extra cost is probably not much different than the cost of a good shunt and with BT in each battery you get a lot more data and cell level data. Anyway if you end up building another system at some point, I can vouch for the BT being an awesome feature and hopefully that will become the standard because it wont add much cost for manufacturers so why not.

Thanks for the info. I'll be sure to do that for any future builds. I didn't know the batteries with bluetooth could do that and eliminate the need for a shunt. I was hoping that the inexpensive voltage capacity meter would have worked, but I can see now that an extremely small voltage change can make a large capacity change. I have a lot more appreciation now for the commercial power stations, but it has been a good learning experience so far. I'm surprised the new Li Time MINI didn't come with bluetooth. They claim it has a revolutionary new BMS and solid state pouches versus cells. And the battery isn't cheap so they should have added bluetooth. Thanks again, I appreciate the information.

 

[SupraSPL]

Yeah I'm kind of baffled why bluetooth and low temp cutoff isn't just a given because it really doesn't add much to the cost of the BMS. I've been ordering 24V 100A JBD smart BMSs w BT and low temp direct from their site for $60 ea.

For some reason I only see the auto heating option on the 12 volt BMSs but it would be nice to see that become more common also.

 

[AF_Bob]

For the shunt look at a Victron Smartshunt or BMV712. Then because you have 2 12v batteries- look up the mid-point voltage setting on the shunt - the aux input. This will track the two batteries together as a 24v battery and track the midpoint- each 12v battery- so you know when it is getting off and you need to recharge the batteries separately.

Rocketman, I'm looking at both the Victron Smartshunt and the Fusion Energy TBD-Smartshunt 500A (much less expensive but rated high) and both seem to have the ability to monitor the Mid point (1 small red wire to POS of battery 1 and 1 small red wire (Aux) to POS of battery 2). I've tried to watch videos on Youtube that show people using the shunt to monitor 2 batteries in SERIES, but I couldn't find any (found only parallel or single battery hookups).
When using an App for the shunt, will the screen show me the total (26.6V) plus EACH of the 12V batteries (e.g., 13.2 and 13.4) so I can see that the batteries are not matched? Or will it just show 26V for the 2 in series and the midpoint at 12.7 and then I'll need to measure them both with a voltmeter to see which one is lower? I'm assuming that anytime the midpoint is not exactly 1/2 of the total, that I'll need to disconnect the batteries and either charge them separately or in parallel perhaps to get them matched again?

 

[Rocketman]

You asked a question that I didn’t know - how it shows midpoint. Mainly because I had seen the setting but never used it. If you asked about the temperature sensor- I would have just told you.

One really cool thing about Victron is if you download the app - VictronConnect (even if you are just considering a Victron item). They have a Demo library - (in iPhone) touch the three horizontal lines - select Demo Library- then select the item you are considering buying.

You can play with all the settings. I opened the demo - saw it was setup for starter battery - went and changed the aux setting to mid-point and the screen below is what popped up.




Evidently it gives the mid-point as a percentage of deviation. I learned something new. ?

One thing that doesn’t make sense on the screen above is the time remaining- no way it is 5min - maybe 5hrs… but then it just a demo screen.

 

[AF_Bob]

For the shunt look at a Victron Smartshunt or BMV712. Then because you have 2 12v batteries- look up the mid-point voltage setting on the shunt - the aux input. This will track the two batteries together as a 24v battery and track the midpoint- each 12v battery- so you know when it is getting off and you need to recharge the batteries separately.

You asked a question that I didn’t know - how it shows midpoint. Mainly because I had seen the setting but never used it. If you asked about the temperature sensor- I would have just told you.

One really cool thing about Victron is if you download the app - VictronConnect (even if you are just considering a Victron item). They have a Demo library - (in iPhone) touch the three horizontal lines - select Demo Library- then select the item you are considering buying.

You can play with all the settings. I opened the demo - saw it was setup for starter battery - went and changed the aux setting to mid-point and the screen below is what popped up.

View attachment 184623


Evidently it gives the mid-point as a percentage of deviation. I learned something new. ?

One thing that doesn’t make sense on the screen above is the time remaining- no way it is 5min - maybe 5hrs… but then it just a demo screen.

Rocketman, I can't thank you enough for doing all that! Ironically, I had downloaded the VictonConnect App earlier today to test out my Victron 100-15 SCC. I just connected 2 wires from the series batteries to the SCC, downloaded the App, and looked at the settings. I had connected the batteries directly to the 24V inverter earlier to make sure the inverter worked since I've had it for several weeks. I plugged in a single-serve coffee maker into the inverter and it drew 780 watts and made 16oz of coffee! I didn't try to connect anything that drew more power, but I know the inverter works so that's a relief! I never got to try the solar panels connected to the SCC because it was 4:30PM by the time I made up the cables and the sky was cloudy and getting dark. I'll try the panels tomorrow to make sure the SCC starts to charge the batteries (if I get some sun).

Since I have the App on my phone, I did what you said and it was a little tricky finding how to get to MidPoint option, but it was on the Misc option, which was the last one I tried. Although I wish it actually gave me the voltage reading at the midpoint, perhaps it's more important to give the variance between the 2 batteries. Now I need to research what variance is enough to break the series connection and charge each battery separately. There's a lot to learn about this stuff. Sometimes I think I should have just put the batteries in parallel and bought a 12V inverter but everything I read told me to go with the 24V system.

I wonder if the Fusion Energy shunt shows percent or voltage...I'll check that out. Thank you again for taking the time to do that reasearch. I had no idea the Victron app had that library feature. I've been watching videos and reading about the settings for the Absorbtion voltage (28.4 I think), and the Float setting (26.4, but I've read you don't need it for LIFEPO4). The Li Time battery folks say to charge it between 28.4-29.2V. There sure is a lot to learn about Solar power stations!

 

[AF_Bob]

You asked a question that I didn’t know - how it shows midpoint. Mainly because I had seen the setting but never used it. If you asked about the temperature sensor- I would have just told you.

One really cool thing about Victron is if you download the app - VictronConnect (even if you are just considering a Victron item). They have a Demo library - (in iPhone) touch the three horizontal lines - select Demo Library- then select the item you are considering buying.

You can play with all the settings. I opened the demo - saw it was setup for starter battery - went and changed the aux setting to mid-point and the screen below is what popped up.

View attachment 184623


Evidently it gives the mid-point as a percentage of deviation. I learned something new. ?

One thing that doesn’t make sense on the screen above is the time remaining- no way it is 5min - maybe 5hrs… but then it just a demo screen.

I forgot to comment on the 5 minutes remaining. If the total voltage was 25.46, each battery had an equivalent of 12.78V...I'm guessing 1 battery had more and 1 had less. At 12.78, I think LIFEPO4 would be less than 17% capacity. At 12.5V, it would be at 14% capacity and perhaps if the power is drawing 191 watts (about 8 amps at 25V), maybe it wouldn't take too long to get into that danger curve. I'm just guessing at this point.

 

[Rocketman]

If you have Victron mppt - get the Victron shunt!!!

One reason is you can set up a Bluetooth network and the shunt will send the voltage, current at the battery to the mppt. This helps with voltage losses and current draws.

It is well worth it keeping the software working as it should.

Also, if your inverter is a Victron that has the VE.Bus (looks like a network RJ45 cable), at some point look at getting a Cerbo or other GX device (or roll your own w/a Raspberry Pi). You can use the Cerbo to control the on/off/charger only/inverter only and input amp limit. But it’s real job is to talk to all the other Victron items and summarize them on its screen (and send the data to the internet (if you have it) - VRM.

 

[AF_Bob]

If you have Victron mppt - get the Victron shunt!!!

One reason is you can set up a Bluetooth network and the shunt will send the voltage, current at the battery to the mppt. This helps with voltage losses and current draws.

It is well worth it keeping the software working as it should.

Also, if your inverter is a Victron that has the VE.Bus (looks like a network RJ45 cable), at some point look at getting a Cerbo or other GX device (or roll your own w/a Raspberry Pi). You can use the Cerbo to control the on/off/charger only/inverter only and input amp limit. But it’s real job is to talk to all the other Victron items and summarize them on its screen (and send the data to the internet (if you have it) - VRM.

I realize I probably should buy the Victron shunt, but it's twice as much as the one which looks like it does the same thing. I'll give it serious thought before I make a purchase. I don't know if I have a phone issue but I couldn't even connect to the Victron SCC until I turned off my smartwatch. I wonder if my phone (Samsung S23+) can even connect to two bluetooth devices at the same time and setting up a network sounds like another challenge to conquer. If I had known about this shunt requirement to monitor the battery capacity, I would definitely have purchased batteries with bluetooth.
On another topic, I researched the midpoint variance battery issue and it's amazing how such a minor difference in battery voltages in series can result in a problem which reduces capacity by 20%...or worse yet--ruin the battery bank altogether. I put the formula into Excel and here's what I got (See photo). The 4% variance that your Demo App showed can be caused by a minute difference in voltages. You'll have to click on the photo to expand it fully.