How big is too big?

Rossman

Solar Addict
Joined
Dec 7, 2021
Messages
277
Hi folks,

Recently discovered this forum, so much great information here, and everyone seems so helpful!

Before I get into my question, some background about our setup:

About seven years ago we built an off-grid house in eastern Ontario (Canada). We DIY'd the solar setup, and have added to it over time, but we currently have:
Conext XW6048 Inverter (w/ the accessories: SCP, Combox, AGS, and Battery Monitor incl. shunt)
6500W Canadian Solar panels -> Conext MPPT80-600 (this array is south east facing)
6080W LG Neon Bifacial -> Conext MPPT80-600 (this array is south west facing)
48V Rolls battery bank @ 1445Ah (so ~700Ah usable)
Kohler 14kW (14RESA) propane generator - it's oversized but was what was available with a warranty for being used off-grid

We over-paneled each charge controller to try to eek out as much energy as we can in winter (Nov/Dev are the worst months), though eventually I would like to replace the charge controllers with the new Conext MPPT100's to capture more energy at the top end as well. We keep the panels fixed at 60' (winter angle) because at that angle in summer we still make way more power than we use, though they are adjustable (ground mounted).

We use about 16kWh / day for our loads.

My question:

Right now I am considering replacing our lead acid bank with lifepo4 batteries, like so many of you are doing (the 3.2V 280Ah cells look great!), as our lead acid batteries are aging. I was hoping to increase our usable capacity for more days of autonomy in winter, so that we run our generator as little as possible.

I am just curious how much battery capacity would be considered "too much" with our current hardware? Ideally I would assemble at least four (4) 280Ah, 48V batteries (4p 16s is I think how you put it here?) for a total capacity (@48v) of 1120Ah, of which we could use ~900Ah (80% of 1120Ah).

Assuming, on a good day, an average charge rate of 130A from our charge controllers, this would be charging the batteries at ~0.1C (i think??) Is this workable? Do I want too much capacity? Can I have more capacity?

Appreciate any opinions or insight anyone can provide!

Thanks!
 

DLTooley

Solar Enthusiast
Joined
Jan 12, 2020
Messages
143
Just a general comment.

Although starting to convert to LiFePo4 is a good idea doing so incrementally might be wise as prices may drop, there is a learning curve, and you may have additional life in the lead acid perfect for bad weather use.

Make sure you purchase from a known good vendor.

You might also check out the larger ready made batteries, the prices have gotten more competitive. For one, look at the rack mounted products recently reviewed by Will Prowse on YouTube.

Lastly, with order lead times you will be at end of winter before any Alibaba based cells would arrive.
 

sunshine_eggo

Happy Breffast!
Joined
Oct 26, 2021
Messages
3,375
Location
USA
My question:

Right now I am considering replacing our lead acid bank with lifepo4 batteries, like so many of you are doing (the 3.2V 280Ah cells look great!), as our lead acid batteries are aging. I was hoping to increase our usable capacity for more days of autonomy in winter, so that we run our generator as little as possible.

I am just curious how much battery capacity would be considered "too much" with our current hardware? Ideally I would assemble at least four (4) 280Ah, 48V batteries (4p 16s is I think how you put it here?) for a total capacity (@48v) of 1120Ah, of which we could use ~900Ah (80% of 1120Ah).

Assuming, on a good day, an average charge rate of 130A from our charge controllers, this would be charging the batteries at ~0.1C (i think??) Is this workable? Do I want too much capacity? Can I have more capacity?

Appreciate any opinions or insight anyone can provide!

Thanks!

Unlike the balance necessary between lead-acid and charge currents (.1-.2C typical) LFP cares little for the charge current provided it is below max rated. It would be perfectly happy with 0.1C

Having a ~1000Ah FLA 12V Rolls bank myself, if I were you, I wouldn't consider replacing it until it truly showed signs of degradation. I have been shocked at the abuse mine has tolerated and yet still keeps on performing well.

You'll likely either need to make provisions for battery heating or climate control as LFP can't be charged below freezing.
 

smoothJoey

Ding Ding!
Joined
Nov 30, 2019
Messages
11,797
3.2 volts nominal * 280 amp hours = 896 watt hours

64 cells * 896 watt hours = 57344 watt hours

57344 watt hours * 0.8 DOD = 45875.2 watt hours usable

16s4p is 4 16s batteries each with its own BMS.

1120 amp hours / 130 amps charge = 8.615384615 hours

That is ~.12c which should be fine.
The tail current for the popular big blue cells is 0.05c so you are ok there.

Even with 3 of the 4 batteries off line you would still be under .5c(140 amps).

This BMS https://overkillsolar.com/product-category/48v-bms/
Has charge and discharge rates of 100 amps.

For a bank of this scale you may want to look at some of the more advanced BMSs though.


Also https://www.signaturesolar.com is making pretty attractive offerings for ready made storage.
Although you might be the first Canadian customer.
 

Rossman

Solar Addict
Joined
Dec 7, 2021
Messages
277
Thanks all for the replies.

The lowest cost, ready made storage available from local suppliers (that I have found) appears to be the Volthium 200Ah rack mount unit which sells for about $7000CAD.

If i bought four of those for 800Ah capacity that brings me to $28kCAD, pre tax (add 13%). Seems like that is a lot when 64 280ah batteries is about $10kCAD from Amy Wan? (yes i need bms's, cabling, busbars etc but still).

I was going to re-purpose our existing battery bank for a standalone EV charger/garage power source. I am not worried about lead times because our battery bank still works (I understand the worlds supply chain issues right now). Also, our batteries are in our utility room, so they are in a conditioned space, not subject to freezing or other temperature extremes.

I will check out Will Prowse's reviews and see what he says, thanks again for the feedback (esp you @smoothJoey, coming in clutch with the math and details)
 

chrisski

Solar Boondocker
Joined
Aug 14, 2020
Messages
3,247
I think you have all the data you need by now. You know how much power, 16 kwh a day, and you didn’t mention it, but you know how many days you want to go without turning the generator on in case of bad weather.

I think a 14 kw generator is nice. With a 3 kw generator I can only push 24 volts 37.5 amps on a 120 volt, 15 amp circuit, which would be 19 amps at 48 volts. 19 amps is not reasonable to charge a large battery bank, and a 14 kw is so much more capable.

I am building 2 X 8S 280 ah batteries. It’s not as easy and cheap as some make it sound, especially if you don’t have the tools. I will not compete this project at 10% to 25% the equivalent cost of BattleBorns, but closer to 50%. I started this at the beginning of 2021, and by the end of the year, prices for pre-made batteries dropped significantly.
 

Rossman

Solar Addict
Joined
Dec 7, 2021
Messages
277
I think a 14 kw generator is nice. With a 3 kw generator I can only push 24 volts 37.5 amps on a 120 volt, 15 amp circuit, which would be 19 amps at 48 volts. 19 amps is not reasonable to charge a large battery bank, and a 14 kw is so much more capable.
We love the generator TBH, but it is somewhat oversized and therefore consumes a relatively healthy (unhealthy for mother earth, and my wallet) amount of propane. It is nice it can charge our batteries at 100A (~5000W) and still have plenty of power for loads at the same time. We are hoping that we could reduce our runtime even further if we had more battery capacity.

Kohler makes (made?) a 6kW generator specifically for charging 48V battery banks, which we probably could have lived with, except we wanted a generator our solar setup integrates with, and that smaller unit manages the batteries on it's own, which I didn't care for.

It’s not as easy and cheap as some make it sound, especially if you don’t have the tools.
Yeah, it's always like this. However, at a battery bank this size, 50% off is still a significant savings!! :D
 

chrisski

Solar Boondocker
Joined
Aug 14, 2020
Messages
3,247
However, at a battery bank this size, 50% off is still a significant savings!! :D
What I left out was there are now a bunch of batteries that are significantly cheaper than the BattleBorns. I’m afraid to do the math because I’m afraid to find out would have been cheaper to get those batteries than DIT!!
 

BarkingSpider

Carbon Lifeform
Joined
Apr 14, 2020
Messages
425
Location
Silicon Valley, CA
Look at the new all in one rackmount 48v batteries from signaturesolar, that is the future of storage. I see prices dropping quickly in the next year from competition.

For my off grid house, I currently use the new 6000 cycle EVE LF280K batteries from Docan Tech, I bought 16 for under $2000 USD shipped to California. I have 32 cells total.

16 x 280 cells equals roughly one Tesla powerwall of energy, if that helps your calculations.

Remember, lithium cells charge so much faster than old lead acid, it's a game changer.
 

Rossman

Solar Addict
Joined
Dec 7, 2021
Messages
277
Look at the new all in one rackmount 48v batteries from signaturesolar, that is the future of storage.
Yep, I have seen these, they are very nice and very similar to the Volthium rack mount products I mentioned above. They do look nice and surely are convenient in many ways but there appears to still be a bit of a price premium there.
 

sunshine_eggo

Happy Breffast!
Joined
Oct 26, 2021
Messages
3,375
Location
USA
Yep, I have seen these, they are very nice and very similar to the Volthium rack mount products I mentioned above. They do look nice and surely are convenient in many ways but there appears to still be a bit of a price premium there.

True. That was part of my motivation for you to keep the Rolls bank for awhile. DIY batteries are probably going to get a little more expensive, but finished batteries are likely going to come down quite a bit over time. In the couple years I've been paying attention even Battleborn is down 15-20%. As more and more competition enters the market, battery builders will see their margins erode, and the consumer will benefit.
 
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