diy solar

diy solar

Large DIY Battery Bank for a House

nick4673

New Member
Joined
Oct 14, 2019
Messages
11
I’m looking to put together a large 48v system for my house to take it off grid. I am going to use LiFePO4 prismatic cells. I’m just wondering which size cells to use. Each of my 16 strings is going to be 2000ah. Yes I need over 100kwh battery bank and here is my reasoning. My average daily consumption in the summer months is 40kwh. For longevity purposes I’m only going to use the middle 50% of the battery capacity. That brings me to 80kwh battery capacity. Then I threw in an additional 20% for whatever. Back to my question. Which size ah rating cell should I use 100ah, 200ah, 300ah, higher or maybe looking at 50ah or 75ah? During Will’s Battle Born teardown video it seemed that he really liked the fact that they used the lower ah rated cells in parallel to make 100ah vs. the 100ah prismatic cells. I not looking at using cylindrical cell, that is to small, but the idea is still there. Use a lot of smaller ah rated cells in parallel to get the ah rating that you want just in case one of them goes bad.
 
if your battery are lithium, the 50% discharge is not relevant, and that is why lithium is choosen over lead.
if your average power is 40KWh that is HUGE !, there is no need to have a 80KWh battery.
if this is a house, you will prefer several battery packs powering each a converter , for example a 5000W for the kitchen, a 2400W for the ligthing, another 5000W for AC or play with phases. Usually home have not all socket wired on the same wire.
This will offer redundancy, and split the effort so each element stay reasonably sized.
I still wonder how you will charge such amount of battery.... it means each hour you need to find 800Amps at 48V.
a 300W 24V panel is 12A, so to get 48V ,you need 2 panels and to get 800A you need to parallel 66 pair of panel= about 130 panels.
 
Last edited:
if your battery are lithium, the 50% discharge is not relevant.
if your average power is 40KWh )that is HUGE !, there is no need to have a 80KWh battery.
if this is a house, you will prefer several battery packs powering each a converter ), for example a 5000W for the kitchen, a 2400W for the ligthing, another 5000W for AC.
This will offer redundancy, and split the effort so each element stay reasonably sized.
I still wonder how you will charge such amount of battery....
There have been many studies done on how to increase the lifespan of your lithium battery bank. Will has done a video in this has well. C-Rate, Charge Voltage and the Cutoff Voltage, and the temperature. With my current power company I can go back and look at 30 min blocks of time and see how much power I used in that time frame and the most that I have used in one 30 min block is 5kwh. I was looking at a 10kwh split phase inverter/charger for the whole house. If I break it up in the many small systems for each room then that will get very expensive.
 
lithium batteries do not wear out when you use them, they wear out when you reach both end of the full charge/full discharge cycle (when they heat the most)
so if you can keep your batteries at least 10% charged and do not overcharge (95% is ok) it means you loose only 15% of capacity.
with that you still have thousand of charge/discharge cycles for many years of use.
but anyway, batteries are consummable, they do not last forever and you will need to change some soon or later, so the extra precaution could not worth the loss.
the problem of oversizing batteries is you forget that whatever capacity you get, you will have to charge it.
So if you need to put a huge amount of solar panels, you could as well use them to power directly the house, and keep the battery small for the few hours where solar power is not available.
You can also install a 40 feet container full of batteries, but it still not solve the problem on how you will charge them off grid.
You have to keep in mind that if it takes you T time to discharge a battery, you always need to recharge them faster (about 2 or 3 time faster) than you have discharged them.
(except if you are using them only to fill occasional grid interruption), just because the fact that a 24 hours day is longer than the average 8 hours of sun available.
The easiest way to remember that is to consider a battery like a load, not like a source. A battery produce no energy, it just give you back the energy you put inside. Batteries are just an easy way to level spikes or differ energy consumption, in case your source of energy is not steady.
 
Last edited:
- Battle Born teardown
- Solar Batteries: How low can you go. Increase performance and lifespan of your battery bank.
 
sure the best way to not wear a battery is not to use it….
 
  1. I'd identify the available dedicated space allocation I'm intending for
    1. Cells
    2. Support equipment
    3. PV panels
  2. Next I'd seek to assign my loads into 3 categories
    1. Critical
    2. Essential
    3. Convenience
      1. Be brutal in this assessment so that critical is smallest & convenience is the largest.
    4. Provide a discrete source for each level & a means to swap up if a higher priority source fails.
  3. I'd procure my cells in the largest capacity readily available to my market. With this size system you will be looking to maintain it vs replace whole parts. Think military thoughts ie cell bypass/replace vs battery replace, what if this or that fails etc.
  4. Consider stepping up your voltage to 72 or 96 volts to minimize your amperage.
  5. I'd probably acquire 4 spare cells & dedicate them to an autonomous lighting function @ 12 volts these would become hot swap candidates for your primary battery.
 
With regards to SOC bandwidth 10-90% seems to be widely accepted as long term endurance bandwidth. 15-85% would definitely keep away from the SOC knees though I'd consider at an annual managed full discharge re-charge cycle to keep things fresh.
 
sure the best way to not wear a battery is not to use it….
If in my scenario I can get for than double to life of my battery bank then I’ll get my monies worth. With the research that has been done the method on which I am going to use my battery bank I should get many multiples of the standard 2000 cycles.
 
With regards to SOC bandwidth 10-90% seems to be widely accepted as long term endurance bandwidth. 15-85% would definitely keep away from the SOC knees though I'd consider at an annual managed full discharge re-charge cycle to keep things fresh.
I would agree that I would need to do a full charge/discharge cycle for maintenance purposes. I have seen the 85% to 15% level to increase longevity, also saw the difference in between that and 75% to 25% being better when I calculated cost vs. returned longevity. I’m not worried about space so I can scale up my battery size to off-set the more capacity required.
 
i doubt that by treating batteries nicely you can significantly increase their life beyond their "industrial" lifespan.
yes you can push them 30% longer probably but not 200%, especially in your case, where fast charging/discharging would be probably necessary.
 
For a battery that big, might want to consider buying an entire Tesla or leaf battery. I'd go 96v.

I have a similar 40-45kwhr a day load, so I too was looking at making a large 100kw battery bank. I'm not in a huge rush though, and I bet in a few years there will be more and more wrecked electric cars in the junkyards....edit: and more options in the solar CC/inverter market with maybe even higher battery voltage allowances. Ideally, I'd like to do about a 192v battery.
 
a tesla battery contain only 14 Kw of energy (48V about 280Amps), it is pretty small.
 
For a battery that big, might want to consider buying an entire Tesla or leaf battery. I'd go 96v.

I have a similar 40-45kwhr a day load, so I too was looking at making a large 100kw battery bank. I'm not in a huge rush though, and I bet in a few years there will be more and more wrecked electric cars in the junkyards....edit: and more options in the solar CC/inverter market with maybe even higher battery voltage allowances. Ideally, I'd like to do about a 192v battery.
I am actually starting to look at 96v equipment.
 
Thank you all for responding to my post, but I haven’t received replies to my original question. If you were asked to design an 100KwH battery bank using LiFePO4 cells, which size cells would you use? 100ah seems to be the most readily available, but I have also seen plenty of higher ah rated cells.
 
Thank you all for responding to my post, but I haven’t received replies to my original question. If you were asked to design an 100KwH battery bank using LiFePO4 cells, which size cells would you use? 100ah seems to be the most readily available, but I have also seen plenty of higher ah rated cells.

I'd use whatever worked out to be the cheapest.
 
You really need no more than 60kWh battery if your peak use is 40kWh only on worst days. This would account for 20% calendar aging and 20% reserve (10/90) for longevity. With your power needs <10kW you can still stay in 48V ( actually 51V with 16S LFP cells ) range.
So, you'd need no more than 1200AH at 51V, which can be done with either 200AH cells in 16S6P ( 96 cells ) or 300AH cells in 16S4P ( 64 cells ).
It is a massive battery and I would consider reducing the need for AC in some other ways if possible ( insulation, whole house fan, etc ) since AC is the killer app. However, this is doable if money is not a problem. I have seen such systems done a few times.
You will need enough solar panels to charge the battery, I'm guessing at least 10kW-15kW array, depending on your geo location.
 
Thank you all for responding to my post, but I haven’t received replies to my original question. If you were asked to design an 100KwH battery bank using LiFePO4 cells, which size cells would you use? 100ah seems to be the most readily available, but I have also seen plenty of higher ah rated cells.
Use largest prismatic cells you can buy to minimize parallel connections, 100AH is still doable, but a lot of unnecessary work when you can get 300AH cells easily these days.
 
Back
Top