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What do you think?...7200Wh solar / DIY lifepo4 setup

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mr_ed

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Hi All,

My wife and I own a shack in country Victoria, Australia with my in-laws. It's currently set up with 2 x 160W panels, a Renogy 40A MPPT solar charge controller and a 120Ah 12V Blue River AGM battery. At the moment we just use the shack for weekends away and the set-up runs the LED lighting, 12V fridge and 12V shower. We don't run an inverter.

However, my father-in-law is just about to retire and is also turning 60 in October. They're talking about living at the shack for a few months a year from now on, so I'm planning to upgrade the system as a surprise for his 60th to make the shack a bit more live-able full time.

At the moment I'm looking at adding another two 160W panels and building an 8 cell 7200Wh 24V battery bank with a 1500W inverter as well as the existing 12V loads. I've put together a schematic of how I'm planning to build it - let me know what you guys think! What would you change?

Cheers.
 

Attachments

  • Battery design.pdf
    189.3 KB · Views: 45
7200Wh 24V is 300Ah cell. common size is 280Ah and 320.
I am not sure why you would need that power.
Anyway , if you get 7200W in a battery, you should get at least 7200W/5 = 1440Wh of panels. (and reality says closer to 2000Wh)
with 4x 160w you are not even half of that .
either you battery is to big, your panels to small and inverter matches neither one or the other.

the risk is you will slowly drain you battery until it is empty, then it will take forever to charge it again.
and a 1500W inverter should not be able to power device with high current start, like fridge.

you should make an audit of how much power your really need, cut everywhere you can (replacing standard bulbs with led, put motion detectors on lights, use emergency lights with integrated battery, or can run DC directly from the battery.
basically every important device should have its own battery (just in case you burn the inverter, so you can still survive a few hours with lights, music, phone, radio, gsm/wifi access point ) or add a small gas generator just for emergency.
 
Last edited:
8 awg is too small for the inverter.
1500 watts * 1.15 conversion factor / 24 volts = 71.875 amps.
71.875 amps * 1.5 fuse factor = 107.8125 fuse amps.

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Assuming pure copper rated at 105 celcius, 6 awg would be the minimum.
I have a 1500 watt inverter@24volts and I'm using 2 awg.
 
Hey bud. I'm also building an off grid setup in country Victoria, upper yarra. And I'm using similar 280ah Lifepo4 batteries in an 8S configuration. As previously mentioned, your solar power is unfortunately woefully underpowered for that size bank. As a comparison, I'm using 6x 370w panels, with a Victron 150/85 SCC. I'd also recommend a larger Inverter as also previously mentioned startup power draw is a killer in so many applications. An inverter that your can adjust low voltage shutoff (2.4kw mpp solar) would be ideal, and a victron battery protect 65a for the dc loads.
 
Awesome - thanks for the input all.

Sorry - I should have provided some more context to the idea. I know the proposed panels are way undersized for the battery, but there is some method to the madness:

1) The 280Ah cells provide the best value I can find by a decent margin. I'm keen to run a 24V system, so need 8 or 16 cells. If I were to drop to say 105Ah cells, with 8 cells I'd feel like my battery size is a bit light for potential future loads, so would probably end up going for 16 cells and end up at a similar cost. Therefore for a 24V system, the 280Ah cells are where I see the best value;

2) Our current loads are relatively low (<1000Wh per day), so 4 panels is fine for the moment, but the shack is an ever evolving beast and I see our loads going up a lot in the future once there is an inverter in place and the in-laws spend more time there and want more creature comforts. It's very easy to add panels in the future (and can be done very quickly - same day from decision to upgrade) where-as adding batteries is quite a faff with the lead times from China and then having a mix of battery ages etc in the set-up. Therefore I'd rather future proof the battery size now and add panels as needed. This will also obviously keep the DoD very low, so battery life should be extended, which helps the value proposition.

My post was more getting any tips/thoughts on the layout/circuit set-up, which you've provided, so thanks!

Regarding your replies (and thanks again for taking the time):

Nosys: The 7200Wh is based on 8 x 280Ah cells x 3.2V = 7168Wh. Thanks for the note on the inverter - will look at upsizing.

SmoothJoey: Thanks for that - I was sizing the wires based on the biggest wire gauge that comes with the BMS (8AWG). Do you have 2AWG wires soldered to your BMS, or do you bypass the BMS with a relay or similar? Following your point - I think maybe I'll keep the wiring on the BMS in line with what it comes with (8AWG) and run the inverter through a relay (as shown attached). Also, have you found startup loads to be an issue for typical appliances with your 1500W inverter?

Wanderingthomas: Nice - our place is in Merrijig. Thanks for the tip on the inverter. Will look at going bigger. If I run the inverter on a relay from the BMS, then the low voltage cut-off on the BMS will cut the inverter I think. Where did you get your 280Ah cells? Good experience?

Rev 2 thoughts attached. I'm a bit confused about how to wire the relay though - I've asked in another thread but if you guys have any thoughts it would be appreciated. My issue is that when Will described this in a separate thread, he shows the negative line from the SCC connected to the B- side of the BMS, but this seems to bypass the BMS and is against the instructions I've seen elsewhere. However with the wiring attached, the SCC could continue to provide load to the relay even when the BMS cuts due to low voltage. Not sure where to go with this.

Cheers.
 

Attachments

  • Battery design_Rev2.pdf
    207 KB · Views: 16
from what you said about your panels, i think you do not have " current loads are relatively low (<1000Wh per day) ", but more a 1000Wday
that is 1000/12= about 80Wh.
1000Wh is a big load, like running an AC 8 hours a day.
a 280A 24V battery gives you 6720Wh that is 560Wh (i count a day like 12 hours)
 
Hi Nosys - It looks like you’re confusing watts and watt-hours?

Our total energy usage is about 50Ah per day at 12V, therefore 600Wh. So averaging 25 watts over the day (24hrs).

An AC (reverse cycle for example) would use about 1000-1500W (mine at home is 1350W), so more like 30000Wh per day if running constantly.

Our 4 panels at 160W x 4 = 640W easily replaces our typical usage even on a crappy day.

And yes- if you take the 280Ah battery x 24V you get 6720Wh (although I believe the Ah rating is based on the nominal cell voltage of 3.2V, so the total nominal battery voltage is 25.6V and total capacity is 7168Wh as previously stated). I’m not sure what you’re getting at with the 560Wh though - are you just saying that if the system is active for 12hrs per day, then you can average 560W? If so, then yes I agree I guess, though this would fully deplete the battery every day.
 
@mr_ed the solar charge controller system side does not appear to have over current protection.
suggest you put a quality 60 amp breaker close to the scc system side positive terminal.
2000 ac watts * 1.15 conversion factor / 24 volts = 95.833333333 dc amps
95.833333333 * 1.5 fuse factor = 143.75 fuse amps.
A fet based bms rateed for 100 amps is the weak link here.
 
Last edited:
You have a fet based bms in the high current path, what is the relay for?
nitpicking but on the bms that should be p- not c-, confirm?
 
mr_ed said:
Awesome - thanks for the input all.

Sorry - I should have provided some more context to the idea. I know the proposed panels are way undersized for the battery, but there is some method to the madness:

1) The 280Ah cells provide the best value I can find by a decent margin. I'm keen to run a 24V system, so need 8 or 16 cells. If I were to drop to say 105Ah cells, with 8 cells I'd feel like my battery size is a bit light for potential future loads, so would probably end up going for 16 cells and end up at a similar cost. Therefore for a 24V system, the 280Ah cells are where I see the best value;

2) Our current loads are relatively low (<1000Wh per day), so 4 panels is fine for the moment, but the shack is an ever evolving beast and I see our loads going up a lot in the future once there is an inverter in place and the in-laws spend more time there and want more creature comforts. It's very easy to add panels in the future (and can be done very quickly - same day from decision to upgrade) where-as adding batteries is quite a faff with the lead times from China and then having a mix of battery ages etc in the set-up. Therefore I'd rather future proof the battery size now and add panels as needed. This will also obviously keep the DoD very low, so battery life should be extended, which helps the value proposition.

My post was more getting any tips/thoughts on the layout/circuit set-up, which you've provided, so thanks!

Regarding your replies (and thanks again for taking the time):

Nosys: The 7200Wh is based on 8 x 280Ah cells x 3.2V = 7168Wh. Thanks for the note on the inverter - will look at upsizing.

SmoothJoey: Thanks for that - I was sizing the wires based on the biggest wire gauge that comes with the BMS (8AWG). Do you have 2AWG wires soldered to your BMS, or do you bypass the BMS with a relay or similar? Following your point - I think maybe I'll keep the wiring on the BMS in line with what it comes with (8AWG) and run the inverter through a relay (as shown attached). Also, have you found startup loads to be an issue for typical appliances with your 1500W inverter?

Wanderingthomas: Nice - our place is in Merrijig. Thanks for the tip on the inverter. Will look at going bigger. If I run the inverter on a relay from the BMS, then the low voltage cut-off on the BMS will cut the inverter I think. Where did you get your 280Ah cells? Good experience?

Rev 2 thoughts attached. I'm a bit confused about how to wire the relay though - I've asked in another thread but if you guys have any thoughts it would be appreciated. My issue is that when Will described this in a separate thread, he shows the negative line from the SCC connected to the B- side of the BMS, but this seems to bypass the BMS and is against the instructions I've seen elsewhere. However with the wiring attached, the SCC could continue to provide load to the relay even when the BMS cuts due to low voltage. Not sure where to go with this.

Cheers.
Click to expand...
Hey bud, it's a good idea getting the larger batteries, they're good value. Your calculations are based on 100% depletion. When you probably shouldn't go past 85% if you want decent longevity, going too low on the lifepo4 is a death sentence. This is why it's important to use the victron battery protect for ELV and adjust the inverter to shut off at the 85% level. It wouldn't be so much of an issue if you had robust solar power but those panels will never pull the full rating, especially if they're not perfectly positioned and have cloudy conditions for multiple days in a row. The concern is slowly depleting the batteries over a week of bad victorian weather. One other solution would be to get 4 cheap second hand 250 - 270 w panels and max out your solar straight away. My experience is always that, Peace of mind is worth the little extra cost and effort. With regards to the BMS,it's usually used as the final failsafe. Does it have provisions for a relay, and what type of relay, alot of relays use a huge amount of power. does the bms automatically switch back on or require manual reset when power is restored? Does the inverter automatically switch on when it shuts down in low voltage situation? Sorry for all the points. Just going through the motions I've already been through
 
@smoothJoey - no worries on the nitpicking - I came for advice and happy to be getting it.

Thanks for the note on the SCC breaker - will add.

Yes, agree that the 100A BMS is the weak link, but I can't find a much higher rated 8S BMS - hence trying to get around it with the relay but I think the relay set-up would only work with a separate port BMS (i.e. so I could have the SCC connected to the C- port and the relay connected to the P- port, and therefore maintain the full protection of the BMS). However, I understand that the charging capacity on separate port BMS is usually very low, so not ideal in my situation either where I expect in the future I'll be wanting to charge at up to say 80A. The way Will Prowse shows to get around this is to connect the SCC negative direct to the battery negative, and then run the relay on the C- port. His argument seems to be that this is fine for charging voltage so long as your SCC is good quality, but I don't like this approach as it cuts out the low temp disconnect protection from the BMS and in my location we often have sunny mornings at say -3 degrees C where I would want the low temp cutoff to activate.

Regarding P- / C-, I'm just going with the BMS labelling on the OverkillSolar site, which is the BMS recommended by Will, so is what I am planning to use. The BMS is common port, so yep, C- = P-. Link: https://overkillsolar.com/product/bms-100a-8s-lifepo4/

Do you have any suggestions on a cost effective way of getting around this? One option seems to be to install a Sterling Prolatch-R, but at higher ratings these get very expensive. Wanderingthomas suggests a Victron battery protect, but there's a thread linked below which discusses that these shouldn't be used for an inverter (@Wanderingthomas - have you seen this?) - https://diysolarforum.com/threads/3...tion-to-bms-battery-question.1303/#post-10976.

@Wanderingthomas: Yep - that's the beauty of the MPPT - can keep the amperage in the panel cable, which is the longest cable in the whole system, very low. Saves a lot of copper. When we add another bank of panels, planning to put an additional SCC in parallel. I think this should work pretty well.

With your other points -
1) Yep - understand on DoD vs. life cycles. As I understand, this is the purpose of the low voltage disconnect on the BMS. Interesting point on the BMS only being a last line of defence - most people seem comfortable relying on them for low voltage cut-off but I will look at adding a second line of defence. However, the thread above is very strong on not using a Victron BP with an inverter.
2) From a read of the OverKill solar support reddit - looks like the BMS automatically reactivates when recovering from a low voltage disconnect. In terms of the inverter re-activating, I expect not, but not too worried about this as I would be aiming never to hit the low voltage disconnect anyway. Should just be an emergency fail-safe and manually reconnecting wouldn't be a big deal in that event.

Thanks again both - much appreciated!
 
also @Wanderingthomas - do you already have your cells? Where did you get them? I've been talking to a supplier on Alibaba who seems pretty legit, but would be good working from some experience.
 
mr_ed said:
@smoothJoey - no worries on the nitpicking - I came for advice and happy to be getting it.

Thanks for the note on the SCC breaker - will add.

Yes, agree that the 100A BMS is the weak link, but I can't find a much higher rated 8S BMS - hence trying to get around it with the relay but I think the relay set-up would only work with a separate port BMS (i.e. so I could have the SCC connected to the C- port and the relay connected to the P- port, and therefore maintain the full protection of the BMS). However, I understand that the charging capacity on separate port BMS is usually very low, so not ideal in my situation either where I expect in the future I'll be wanting to charge at up to say 80A. The way Will Prowse shows to get around this is to connect the SCC negative direct to the battery negative, and then run the relay on the C- port. His argument seems to be that this is fine for charging voltage so long as your SCC is good quality, but I don't like this approach as it cuts out the low temp disconnect protection from the BMS and in my location we often have sunny mornings at say -3 degrees C where I would want the low temp cutoff to activate.

Regarding P- / C-, I'm just going with the BMS labelling on the OverkillSolar site, which is the BMS recommended by Will, so is what I am planning to use. The BMS is common port, so yep, C- = P-. Link: https://overkillsolar.com/product/bms-100a-8s-lifepo4/

Do you have any suggestions on a cost effective way of getting around this? One option seems to be to install a Sterling Prolatch-R, but at higher ratings these get very expensive. Wanderingthomas suggests a Victron battery protect, but there's a thread linked below which discusses that these shouldn't be used for an inverter (@Wanderingthomas - have you seen this?) - https://diysolarforum.com/threads/3...tion-to-bms-battery-question.1303/#post-10976.

@Wanderingthomas: Yep - that's the beauty of the MPPT - can keep the amperage in the panel cable, which is the longest cable in the whole system, very low. Saves a lot of copper. When we add another bank of panels, planning to put an additional SCC in parallel. I think this should work pretty well.

With your other points -
1) Yep - understand on DoD vs. life cycles. As I understand, this is the purpose of the low voltage disconnect on the BMS. Interesting point on the BMS only being a last line of defence - most people seem comfortable relying on them for low voltage cut-off but I will look at adding a second line of defence. However, the thread above is very strong on not using a Victron BP with an inverter.
2) From a read of the OverKill solar support reddit - looks like the BMS automatically reactivates when recovering from a low voltage disconnect. In terms of the inverter re-activating, I expect not, but not too worried about this as I would be aiming never to hit the low voltage disconnect anyway. Should just be an emergency fail-safe and manually reconnecting wouldn't be a big deal in that event.

Thanks again both - much appreciated!
Click to expand...
Yep, have read that re. Low voltage disconnect. Is why I mentioned to have it only on the ELV setup (24v, 12v etc). The victron has a better cutoff voltage than my bms, and reconnecting is automatic. I'm more comfortable with Victron reliability than the bms. going to use the mpp solar 2400w 24v inverter, which comes in around $400 landed. It has adjustable low voltage disconnect. As a bonus it has an SCC in case I wanted to use it elsewhere or in place of my SCC.
 
Ah, cool - makes sense! Think I'll go with a similar set-up. Thanks!
 
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