8s 280Ah 24V "Compact" Solar Battery Build

[Deleted member 23531]

I thought I'd share some details of my current project which successfully powered on today. I'm currently learning about all the BMS and MPPT settings. It consists of an 8s 280Ah battery bank with MPPT and all electronics in a very compact box. This build tested the limits of what I could wire up in a small space, and I would definitely do a lot of things differently on my next build; I'm happy to discuss the failures as well as the successes; it's about 80% done so a lot of performance details are yet to be determined. But anyway, here it is.

PV:
- Aluminum I-beam cantilever + 80/20 frame as a "solar awning" over my workshop entrance. Hinged for seasonal angle adjustment. South-facing. The picture was taken before one additional 80/20 "strut" plus "track" was mounted, which supports the weight when configured to a more upright angle.
- 4x 210W Newpowa 24V configured as 2S2P (purchased; not mounted yet. I'm planning to mount my panels tomorrow; weather permitting.)

Electronics Box:
- 8s 280Ah EVE from Basen
- 8s / 24V BMS from Overkill Solar
- 100V / 50A MPPT from Victron
- 1200VA Phoenix inverter from Victron (casing removed; minor modifications to the frame for mounting it; more major modifications to wire in the automatic transfer switch to the existing outlet)
- MidNite Solar 50A battery breaker, 20A PV breaker, 0.5A/63A GFP protection in a mini breaker box. Ignore the incorrect sharpie labels; that's been fixed since the picture was taken.
- Xantrex automatic transfer switch (removed from case) + RV power inlet on the side of the box (not shown)
- BlueSea MRBF dual fuse, although not actually mounted on the battery terminal for height reasons (I'm aware of class T fuses; just didn't use it since I think/hope MRBF + DC-rated breakers + BMS + fuse inside Victron inverter will be sufficient). This is mounted on an insulated standoff, which is itself mounted on two risers; see pictures.
- Battery compression fixture is TBD; it will probably be a rectangular hoola-hoop compressing from the exterior of the plastic box; probably using 80/20 aluminum
- Electronics all fit in ~13.25" x 7.5" x 7.5" (shoebox sized?). I think this may be one of the most compact builds on the internet. The box is the Quantum Storage RSO2415-9. If I were to do this again, I would definitely use that box for a 4s build but not an 8s build.

Things that didn't go so well:
- BougeRV PV cable, although it appears to have nice thick insulation, is a huge pain to deal with in a box. It's so stiff; may not even be copper, and was a huge mistake. I used it anyway for the PV since it's 10ga (30A) and I only need about 9.5A for the PV since it's such high voltage, and the breaker is 20A. But, in the future I would prioritize more flexible wire for a compact build like this
- 80/20 is great and extremely strong, but very pricey, and it's sort of a pain to assemble. Getting all the T-nuts aligned to thread the bolts into took some time. I did get all the 80/20 used on Craigslist though, so that was good, and I can see how every bit of it would be recyclable again and again for other projects, as long as the pieces are long enough.
- The box I chose doesn't have enough space for threaded rods between the two sets of 4s batteries, as well as all the electronics. So, I'm going "external" for cell compression; that hasn't been implemented yet. In a future build I would just oversize the box a lot more to make my life easier. However, I'm pleased with how little space everything took; it just required a lot of labor, drilling+tapping holes, polyimide tape, and wire ties to get there. I removed or modified the casings of a few components. In the end the heat-sinking is probably quite good as they're all mounted to a shared hunk of metal, and I used heatsink compound where appropriate. However, since it's so compact the energy density is higher. I plan to add an exhaust fan, as I mentioned above. It hasn't arrived yet.

Still to do:
- add a vent cover for air inlet + add the 24V exhaust fan + outlet. Until that arrives I'll just use this with the lid removed and perhaps a fan blowing at it. Realistically usage will be light until this thing is vetted for several weeks at light loads. The the plan is to run a ~1kW window AC over the summer off of solar power.
- mount the PV panels on the hinged 80/20 frame
- look at efficiency / calibrate the BMS (there's no shunt monitor so capacity and current are approximate, I would say); tinker with settings over bluetooth
- maybe do an IR image under load to look for high-resistance spots. I used abrasive anti-oxidant for the first time and have my doubts (ACE hardware generic of "NoAlox", I believe).
- cut a hole in the box lid for the breaker switches to stick through
- tape down or wire tie a few more cables to keep it nice and organized
- capacity testing; realistically I don't care and I'll measure this in "hours of air conditioning" before the ATS switches over to the grid

 

[snoobler]

Extremely clean and compact, but I can see the MPPT getting very warm.

 

[Deleted member 23531]

but I can see the MPPT getting very warm.

Yeah me too. The BMS has two temperature probes so I think I'll tape one of them onto the heatsink of the MPPT to keep and eye on it, and the other one onto a battery cell. It's bolted and thermal greased to the frame of the inverter, which is heavy gauge steel, but the steel is also powder coated so may not be the best way to get the heat out, compared to some airflow on the MPPT heatsink fins.

Actually, now that I think about it there is a gap between the breaker box and the inverter frame, on the side opposite the batteries. That is directly above the fins of the MPPT heatsink, so maybe if I draw some air up through those fins it would help get some heat out of the MPPT. I'll report back once the PV is installed and I can see the temperature while charging.

 

[Deleted member 23531]

One other negative of this design (so far):

When the inverter is switched off, my vacuum I was using as a test load turned off for about half a second before switching to the grid power, so it's not "seamless" transfer. That Xantrex switch is designed for RVs that would use shore power when it's available, but obviously I want to use solar power when it's available so I reversed the two inputs from what they label them as. I wonder if it is only its claimed 80ms failover time in one direction (ex: A to B but not B to A). Or else, my fancy shop vacuum has a fast reaction time.

I would probably pick a charger/inverter with an integrated automatic transfer switch in a future design.

 

[snoobler]

Most integrated ATS seem to be <20ms.

 

[Deleted member 23531]

Most integrated ATS seem to be <20ms.

Yeah, same with the Victron switch. But it's about 6x more expensive than the Xantrex one. Fortunately the ATS is [just about the only] serviceable part in this box, since it's the thing wrapped in tape and mounted on standoffs near the top. I could probably swap it out for something else, even one that mounts to the outside of the box, pretty easily.

 

[Deleted member 23531]

If I were to do this again, I would definitely use that box for a 4s build but not an 8s build.

Or, if I put any of the electronics outside the box (particularly the inverter) there would be a lot more space available for 8s.

 

[Deleted member 23531]

I collected some initial load data. I was chatting with some folks in another thread about how I seem to be getting about 80% efficiency with my ~900W shop vacuum in a quick 2 minute smoke test yesterday. Tonight I did a 20 minute heat-gun load test; here's the details if you want to chime in to help me out on efficiency debugging.

- I have a 1200W wagner heat gun and the most basic Kill-A-Watt. Heat gun draws roughly 1168W. After 20 minutes, its own cord gets a bit warm and not from the hot air.
- With nothing on, there is no leakage anywhere and BMS reports 0A; the MPPT and BMS draw basically zero for their bluetooth and logic
- With the inverter on but no load, BMS reports 19W = 0.66 - 0.72A. The zero load power spec is 8W. I suppose the automatic transfer switch could be dissipating a few watts though.
- With the heat gun on, the battery measures 26.0V on my cheap multimeter at the grubscrews, 26.10V measured by BMS, and 25.8V on my multimeter at the inverter wire jacks. The BMS reports: ~57.4A, ~1506W, 7-10mV cell delta (3.26 - 3.27), max 26.7v, min 26.09V, max 58.05A, max 1516W for a 20 minute session. I assume the 26.7V battery session voltage was before the load was turned on.

- I measured bus bar drops grubscrew-to-grubscrew while it was drawing the heat gun load: 2.1mV, 1.9mV, 1.8mV, 2.5mV, 1.8mV, 2.0mV, 2.0mV. The middle one is my temporary hacked bus bar with less contact area; I'm waiting on a 4 AWG cable for that one since it needed to be a bit longer than the others. That one however allows me to probe the EVE terminals directly since it doesn't cover them fully. The terminals measured 2.8mV from one terminal to another under the hacked bus bar, so not too much worse than the 2.5mV measured at the grubscrews.
- Kill-A-Watt reports 0.38kWh, in agreement with the constant instantaneous watts and 1/3 of an hour
- BMS temperatures are fine (65F on battery, 83F on inverter chassis); not sure how much of that is just due to airflow or actual heating that happened in 20 minutes.
- positive lead to inverter is a bit warmer than negative lead; I might tighten the fuses down bit more, but I already did follow the recommended maximum torque from BlueSea instructions by torquing it to maybe 80% of that value.

Long story short: 1168W / 1506W ~ 78% efficient. Yesterday I saw roughly ~81% at a lower wattage load ~900-950W load (a vacuum).

Other thoughts/questions:
- this is right at the maximum 1200VA for the inverter. The heat gun power factor is probably 1.0; the vacuum may not be.
- I might re-test at half-max (~600W) if I can find something I own that consumes that amount of power.
- Victron inverter is rated for 91% efficiency max; I'd assume without any other info that it hits its peak efficiency at about 50% load.
- this might just be how efficient that inverter is at maximum rated load. Even assuming 250mV of drop from the battery grubscrews to the inverter input, that's 0.25V * 57A = 14.25W dissipated in the BMS mosfets, the wires, the fuses, the bus bar, and the wire jacks put together. Even if that were 0W the efficiency would still be ~78% (after rounding).

- Another possibility could be an issue with my soldering on the fins of the output receptacle, or the $58 Xantrex ATS is pretty inefficient. But, since those both run at 120V, the current is only 9.7A so the resistance would have to be pretty high for that to be the issue. Also the Kill-A-Watt would show a lower voltage at load than it does at no load, if it were some high resistance in the automatic transfer switch, for example. I'll check that tomorrow.

One other detail:
- the bus bar / grubscrew resistance might be irrelevant in this test, since the BMS would see a lower voltage and also higher current drawn by the inverter. In other words, any power "lost" in the battery connections does not show up in the Kill-A-Watt / BMS efficiency calculation, I think, since it just looks like a lower battery voltage for BMS power calculation purposes. I also did not see any cells diverging from the group, so I guess my Ox-Gard terminal scrubbing worked ok. None of the bus bars were warm in the slightest after 20 minutes.

One unrelated question/issue about the BMS:
- The Overkill / Xioxiang App shows ~65% capacity but I think that is just using a lookup table based on voltage, right? As soon as I paid for the "full" version of the app and entered the 280Ah capacity in the config page, it dropped from ~98% full to ~65% full; I'm not sure if that's resettable or what, or if it's just an inaccurate voltage-based lookup, since the app has you configiure 20/40/60/80% voltages and they're just generic round numbers now like 3.2, 3.3, 3.4V.

 

[Deleted member 23531]

This Victron document shows the efficiency at max load for the Phoenix 24V / 3000W. I have the 24V / 1200W which is a different form factor and probably an older design. The plot is reproduced below.

https://www.victronenergy.com/upload/documents/Output-rating-operating-temperature-and-efficiency.pdf

ping @Gazoo who was talking to me about this in another thread.

This shows about 86% efficient at its "nameplate" output watts (3000 in this case). It's worth noting that 1600VA and above are a different / newer design though. The 24V 3000VA Phoenix is 94% max efficiency, and mine is apparently 91%. In any case, it looks like it only meets that efficiency at roughly 1/4 of nameplate load.

Edited.

 

[snoobler]

I've seen similar charts for 25 year old Trace SW4024 with "peak" efficiencies in the low 90%s. This seems to be fairly typical of all quality LF inverters.

 

[Deleted member 23531]

Great, well unless any of my wires start getting really warm I'm not going to worry about 80% efficiency then.

 

[snoobler]

My Quattro is more often running in the 65-70% efficiency range because my "idle" loads are typically less than 80W.

 

[Deleted member 23531]

Yeah, I don't mind bad efficiency at low load because that's less power as heat. I'd rather they skewed their design to get good efficiency at 50-80% load, where it matters for thermal reasons. I'm fine with very low efficiency at low loads.

I work more in this arena:

https://www.analog.com/media/en/technical-documentation/data-sheets/LTM4671.pdf

You can see those are most efficient at 50%+ load.

 

[Deleted member 23531]

Here's an update now that I have the panels mounted and hooked up for the first time today!

I'm producing about 650W of solar power on a mostly-clear day (there are a few scattered cirrus clouds high in the sky). I'm running 2 refrigerators: a ~155W full-size side-by-side type (Kenmore, probably 10 years old) and a smaller minifridge, as well as some miscellaneous stuff like sprinker system standby current, gate motor standby current etc. About 400W is going into the battery, depending on how much load is being used directly from the MPPT.

@snoobler I stuck one of the BMS leads between the heatsink fins of the MPPT and the steel frame of the inverter to which the MPPT is bolted / thermal-greased. I don't think it is in contact with the MPPT heatsink fins directly; I think it's taped to the steel frame 1/4" away but it's a bit hard to tell looking down that crack. It's sitting at about 37.5 degC, which seems fine to me. It probably helps that this MPPT is about 2x oversized for my current solar production; it's also apparently ~98% efficient so there shouldn't be a huge heat problem, I'm thinking. The probe on the battery is at about 21.3 degC. The lid of the battery box is off but there's basically no airflow in that cavity right now as I don't have my fan installed yet and it's on the backside of the inverter frame from the inverter's own fan.

Here's a picture showing a 90% complete PV mounting including the track that allows the angle to be adjusted (the panels are currently resting at the lowest setting, ~5 degrees, on "legs"). I put the power and ground wire in a cable drag chain like you might see in a CNC machine, to allow the strut to slide up and down and the cables should stay neat. I have some clean up to do this weekend, as you can see on the ground.

 

[snoobler]

Glad to hear it's keeping cool.

I like the mount!

 

[Gadgetnerd]

I thought I'd share some details of my current project which successfully powered on today. I'm currently learning about all the BMS and MPPT settings. It consists of an 8s 280Ah battery bank with MPPT and all electronics in a very compact box. This build tested the limits of what I could wire up in a small space, and I would definitely do a lot of things differently on my next build; I'm happy to discuss the failures as well as the successes; it's about 80% done so a lot of performance details are yet to be determined. But anyway, here it is.

PV:
- Aluminum I-beam cantilever + 80/20 frame as a "solar awning" over my workshop entrance. Hinged for seasonal angle adjustment. South-facing. The picture was taken before one additional 80/20 "strut" plus "track" was mounted, which supports the weight when configured to a more upright angle.
- 4x 210W Newpowa 24V configured as 2S2P (purchased; not mounted yet. I'm planning to mount my panels tomorrow; weather permitting.)

Electronics Box:
- 8s 280Ah EVE from Basen
- 8s / 24V BMS from Overkill Solar
- 100V / 50A MPPT from Victron
- 1200VA Phoenix inverter from Victron (casing removed; minor modifications to the frame for mounting it; more major modifications to wire in the automatic transfer switch to the existing outlet)
- MidNite Solar 50A battery breaker, 20A PV breaker, 0.5A/63A GFP protection in a mini breaker box. Ignore the incorrect sharpie labels; that's been fixed since the picture was taken.
- Xantrex automatic transfer switch (removed from case) + RV power inlet on the side of the box (not shown)
- BlueSea MRBF dual fuse, although not actually mounted on the battery terminal for height reasons (I'm aware of class T fuses; just didn't use it since I think/hope MRBF + DC-rated breakers + BMS + fuse inside Victron inverter will be sufficient). This is mounted on an insulated standoff, which is itself mounted on two risers; see pictures.
- Battery compression fixture is TBD; it will probably be a rectangular hoola-hoop compressing from the exterior of the plastic box; probably using 80/20 aluminum
- Electronics all fit in ~13.25" x 7.5" x 7.5" (shoebox sized?). I think this may be one of the most compact builds on the internet. The box is the Quantum Storage RSO2415-9. If I were to do this again, I would definitely use that box for a 4s build but not an 8s build.

Things that didn't go so well:
- BougeRV PV cable, although it appears to have nice thick insulation, is a huge pain to deal with in a box. It's so stiff; may not even be copper, and was a huge mistake. I used it anyway for the PV since it's 10ga (30A) and I only need about 9.5A for the PV since it's such high voltage, and the breaker is 20A. But, in the future I would prioritize more flexible wire for a compact build like this
- 80/20 is great and extremely strong, but very pricey, and it's sort of a pain to assemble. Getting all the T-nuts aligned to thread the bolts into took some time. I did get all the 80/20 used on Craigslist though, so that was good, and I can see how every bit of it would be recyclable again and again for other projects, as long as the pieces are long enough.
- The box I chose doesn't have enough space for threaded rods between the two sets of 4s batteries, as well as all the electronics. So, I'm going "external" for cell compression; that hasn't been implemented yet. In a future build I would just oversize the box a lot more to make my life easier. However, I'm pleased with how little space everything took; it just required a lot of labor, drilling+tapping holes, polyimide tape, and wire ties to get there. I removed or modified the casings of a few components. In the end the heat-sinking is probably quite good as they're all mounted to a shared hunk of metal, and I used heatsink compound where appropriate. However, since it's so compact the energy density is higher. I plan to add an exhaust fan, as I mentioned above. It hasn't arrived yet.

Still to do:
- add a vent cover for air inlet + add the 24V exhaust fan + outlet. Until that arrives I'll just use this with the lid removed and perhaps a fan blowing at it. Realistically usage will be light until this thing is vetted for several weeks at light loads. The the plan is to run a ~1kW window AC over the summer off of solar power.
- mount the PV panels on the hinged 80/20 frame
- look at efficiency / calibrate the BMS (there's no shunt monitor so capacity and current are approximate, I would say); tinker with settings over bluetooth
- maybe do an IR image under load to look for high-resistance spots. I used abrasive anti-oxidant for the first time and have my doubts (ACE hardware generic of "NoAlox", I believe).
- cut a hole in the box lid for the breaker switches to stick through
- tape down or wire tie a few more cables to keep it nice and organized
- capacity testing; realistically I don't care and I'll measure this in "hours of air conditioning" before the ATS switches over to the grid

I did this slotted floating box design with a ratchet strap. Crank it up for compression, pop it loose and you can easily remove the cells for winter storage.

 

[Johnyguy]

Here's an update now that I have the panels mounted and hooked up for the first time today!

I'm producing about 650W of solar power on a mostly-clear day (there are a few scattered cirrus clouds high in the sky). I'm running 2 refrigerators: a ~155W full-size side-by-side type (Kenmore, probably 10 years old) and a smaller minifridge, as well as some miscellaneous stuff like sprinker system standby current, gate motor standby current etc. About 400W is going into the battery, depending on how much load is being used directly from the MPPT.

@snoobler I stuck one of the BMS leads between the heatsink fins of the MPPT and the steel frame of the inverter to which the MPPT is bolted / thermal-greased. I don't think it is in contact with the MPPT heatsink fins directly; I think it's taped to the steel frame 1/4" away but it's a bit hard to tell looking down that crack. It's sitting at about 37.5 degC, which seems fine to me. It probably helps that this MPPT is about 2x oversized for my current solar production; it's also apparently ~98% efficient so there shouldn't be a huge heat problem, I'm thinking. The probe on the battery is at about 21.3 degC. The lid of the battery box is off but there's basically no airflow in that cavity right now as I don't have my fan installed yet and it's on the backside of the inverter frame from the inverter's own fan.

Here's a picture showing a 90% complete PV mounting including the track that allows the angle to be adjusted (the panels are currently resting at the lowest setting, ~5 degrees, on "legs"). I put the power and ground wire in a cable drag chain like you might see in a CNC machine, to allow the strut to slide up and down and the cables should stay neat. I have some clean up to do this weekend, as you can see on the ground.

Hi how many panels and what brand etc.. are you using i am about to build the same pack its 24V right?

 

[Johnyguy]

Here's an update now that I have the panels mounted and hooked up for the first time today!

I'm producing about 650W of solar power on a mostly-clear day (there are a few scattered cirrus clouds high in the sky). I'm running 2 refrigerators: a ~155W full-size side-by-side type (Kenmore, probably 10 years old) and a smaller minifridge, as well as some miscellaneous stuff like sprinker system standby current, gate motor standby current etc. About 400W is going into the battery, depending on how much load is being used directly from the MPPT.

@snoobler I stuck one of the BMS leads between the heatsink fins of the MPPT and the steel frame of the inverter to which the MPPT is bolted / thermal-greased. I don't think it is in contact with the MPPT heatsink fins directly; I think it's taped to the steel frame 1/4" away but it's a bit hard to tell looking down that crack. It's sitting at about 37.5 degC, which seems fine to me. It probably helps that this MPPT is about 2x oversized for my current solar production; it's also apparently ~98% efficient so there shouldn't be a huge heat problem, I'm thinking. The probe on the battery is at about 21.3 degC. The lid of the battery box is off but there's basically no airflow in that cavity right now as I don't have my fan installed yet and it's on the backside of the inverter frame from the inverter's own fan.

Here's a picture showing a 90% complete PV mounting including the track that allows the angle to be adjusted (the panels are currently resting at the lowest setting, ~5 degrees, on "legs"). I put the power and ground wire in a cable drag chain like you might see in a CNC machine, to allow the strut to slide up and down and the cables should stay neat. I have some clean up to do this weekend, as you can see on the ground.

hi so you able to power a full size fridge and a mini fridge with this setup not bad is it a 24 v setup?
and only a 1200W inverter ?