Help with DIY 10kw residential solar battery

Nosferatu

New Member
Hey guys,

I am afresh noob and trying to figure this thing out. I come from an engineering (fabrication) back ground, so I can put things together but haven't got all the knowledge to design the system.

I am planning on building a lifePo4 system.
Cheapest batteries I found are 3.2v 420ah.
Im planning on running 8 of those in series giving me 24V and 10752w.

I need help with the following:
Picking a BMS - from what I worked out, the system is 448a, so im thinking a 500a BMS? Also, I am not sure about one thing - voltage. Since the system is running 24v when all batteries are connected, I will require 24v BMS right? not 3.2v (just making sure I understand correctly).

Next - solar charger - I got 2 banks of solar arrays, both coming into the Inverter. As far as I understand, the cabling from the solar banks will go into the charger, then to the battery via BMS, when battery is fully charged the charger will divert excess power to the inverter?
If so, what size cabling do I use? What size solar charger do I need?

Lastly, do I need anything else? From what I read, the above is the 3 basic components of a battery system, but - do I need fuses? additional safety devices?

Sorry for the dumb questions, but I really hope some of you brainiacs can help a noob :)

Igor.
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
Welcome to the forum.

Quick things:
You are building 24V so I'll address things in 24V terms.
3.2V X 8 Cells X 420AH cell capacity = 10,752 kWh.
* 420AH cells are NOT Common - something fishy there.

488A being drawn off 24VDC system is 11,712 Watts which is honestly LUDICROUS ! For Surge Capacity that is one thing as it is momentary but for constant draw, this IS a fail. @ 48V that translates to 244A draw which is reasonable.

Realize that, at 488A, your 420AH battery would last a MAX of 45 minutes ! It is ALSO above the Maximum of 1.0 C-Rate for Discharge.
LFP cells can be charged at a Max Rate of 0.5C and discharged at a Max Rate of 1.0C.
Example: 100AH Battery can charge @ MAX 50A and Discharge at MAX of 100A (1C)

12V systems can handle 3000W / 250A excluding Surge Capacity. Generally it is recommended NOT to exceed 2000W (166A) for 12V.
24V systems can handle 6000W / 250A excluding Surge Capacity. Generally it is recommended NOT to exceed 4000W (166A) for 24V/
48V systems can handle 10,000W / 208A excluding Surge Capacity. 12,000-14,000 Watts is usually the Max.
Stackable Inverters for the relative voltage can increase the deliverable power

BMS required for 24V system is an 8S. Fet Based SmartBMS typically MAX at 200A deliverable allowing for a bit of margin. Usually beyond 200A people use Relay/Contactor based BMS's which can handle higher amps by selecting the right Relay/Contactor.

Solar Arrays are configured into Series & Parallel to deliver the Voltage & Amperage to satisfy the requirements of the Solar Charge Controller.
The Battery Bank voltage & size determines how much SCC power is required to charge said batteries within the amount of Sun Hours available. For optimal Calculations, we use the Shortest Sun Hour Days (December for North America) to work that out. There is some trickery involved in this because if miscalculated or poorly planned things could go terribly wrong terribly fast.

Fuses, Breaker, DC Busbars, Cutoff switch and MORE are required. This is called "Balance of System" and is typically 25-30% of the cost of the rest of the gear (excluding batteries).

RULE OF K.I.S.S. Applied and a Budget Saver as well.
You can go with a "Component" based system, with separate SCC, Inverter/Charger and using several breakers etc which all add up in Dollars very fast, but you will have a modular system which can be upgraded, modified etc... BUT this is a costly proposition when all added up. We now have Excellent "All In One" units on the market from MANY Companies. Most favoured by our Membership runs like this from least to most expensive. MPP-Solar, Growatt, SolArk, Victron. These AIO's all incorporate the Solar Charge Controller, Inverter & Charger into one unit. The components are ALL Modular, replaceable & serviceable. They also provide fo a simpler & more streamlines software management interface, many have options fr BlueTooth / Wifi connectivity with Phone Apps or even PC Apps to manage & monitor the equipment & system. WILL's WebSite covers some of this here: https://www.mobile-solarpower.com/all-in-one-122448v-packages.html and he has several Videos on the MPP & Growatts.
Links:
MPP Solar USA:
https://usamppsolar.com/
Growatt America: https://www.growatt-america.com/
Sol-Ark USA: https://www.sol-ark.com/homeowner/
Victron USA: https://www.victronenergy.com/inverters-chargers *See EasySolar Models for AIO's.

Battery Cables !
That all depends on the length of the wire run, the voltage being used, the Amperage that it has to handle continuously and with surges. I use ONLY Southwires Royal Excelene Fine Strand Copper Welding Cables and SelTerm Tin plated lugs. See Table below with regards to capacities.
Excelene-Wire-Info.JPG

I hope this information helps you to make a plan which you have to develop. Also have a look at the Links in my Signature... About My System will show you how my 24V Offgrid Residential system is setup. There are other links which will help you on your Path along with LFP Voltage Charts & Battery Assembly info.

Hope it Helps, Good Luck.
Steve
 

Nosferatu

New Member
Welcome to the forum.

Quick things:
You are building 24V so I'll address things in 24V terms.
3.2V X 8 Cells X 420AH cell capacity = 10,752 kWh.
* 420AH cells are NOT Common - something fishy there.

488A being drawn off 24VDC system is 11,712 Watts which is honestly LUDICROUS ! For Surge Capacity that is one thing as it is momentary but for constant draw, this IS a fail. @ 48V that translates to 244A draw which is reasonable.

Realize that, at 488A, your 420AH battery would last a MAX of 45 minutes ! It is ALSO above the Maximum of 1.0 C-Rate for Discharge.
LFP cells can be charged at a Max Rate of 0.5C and discharged at a Max Rate of 1.0C.
Example: 100AH Battery can charge @ MAX 50A and Discharge at MAX of 100A (1C)

12V systems can handle 3000W / 250A excluding Surge Capacity. Generally it is recommended NOT to exceed 2000W (166A) for 12V.
24V systems can handle 6000W / 250A excluding Surge Capacity. Generally it is recommended NOT to exceed 4000W (166A) for 24V/
48V systems can handle 10,000W / 208A excluding Surge Capacity. 12,000-14,000 Watts is usually the Max.
Stackable Inverters for the relative voltage can increase the deliverable power

BMS required for 24V system is an 8S. Fet Based SmartBMS typically MAX at 200A deliverable allowing for a bit of margin. Usually beyond 200A people use Relay/Contactor based BMS's which can handle higher amps by selecting the right Relay/Contactor.

Solar Arrays are configured into Series & Parallel to deliver the Voltage & Amperage to satisfy the requirements of the Solar Charge Controller.
The Battery Bank voltage & size determines how much SCC power is required to charge said batteries within the amount of Sun Hours available. For optimal Calculations, we use the Shortest Sun Hour Days (December for North America) to work that out. There is some trickery involved in this because if miscalculated or poorly planned things could go terribly wrong terribly fast.

Fuses, Breaker, DC Busbars, Cutoff switch and MORE are required. This is called "Balance of System" and is typically 25-30% of the cost of the rest of the gear (excluding batteries).

RULE OF K.I.S.S. Applied and a Budget Saver as well.
You can go with a "Component" based system, with separate SCC, Inverter/Charger and using several breakers etc which all add up in Dollars very fast, but you will have a modular system which can be upgraded, modified etc... BUT this is a costly proposition when all added up. We now have Excellent "All In One" units on the market from MANY Companies. Most favoured by our Membership runs like this from least to most expensive. MPP-Solar, Growatt, SolArk, Victron. These AIO's all incorporate the Solar Charge Controller, Inverter & Charger into one unit. The components are ALL Modular, replaceable & serviceable. They also provide fo a simpler & more streamlines software management interface, many have options fr BlueTooth / Wifi connectivity with Phone Apps or even PC Apps to manage & monitor the equipment & system. WILL's WebSite covers some of this here: https://www.mobile-solarpower.com/all-in-one-122448v-packages.html and he has several Videos on the MPP & Growatts.
Links:
MPP Solar USA:
https://usamppsolar.com/
Growatt America: https://www.growatt-america.com/
Sol-Ark USA: https://www.sol-ark.com/homeowner/
Victron USA: https://www.victronenergy.com/inverters-chargers *See EasySolar Models for AIO's.

Battery Cables !
That all depends on the length of the wire run, the voltage being used, the Amperage that it has to handle continuously and with surges. I use ONLY Southwires Royal Excelene Fine Strand Copper Welding Cables and SelTerm Tin plated lugs. See Table below with regards to capacities.
View attachment 48743

I hope this information helps you to make a plan which you have to develop. Also have a look at the Links in my Signature... About My System will show you how my 24V Offgrid Residential system is setup. There are other links which will help you on your Path along with LFP Voltage Charts & Battery Assembly info.

Hope it Helps, Good Luck.
Steve
Thank you for the reply. Wow, thats a lot of information to sift through. I will have a read over all the links and post back if I got questions.

Thanks again!
 

Nosferatu

New Member
Yea, I'm with Steve_S, this does not sound right. Care to share your numbers?
Well, as far as I worked it out its 3.2v * 8 batteries = 10.752 / 24v = 448.

But I suppose as Steve_s said, it's probably not going to work.

As far as I gathered, building a 48v system would be better, which means I need to recalculate all the available batteries out there and re design the system.

I will go though Steve's links and try to work out more detail and gain more understanding before getting to it though. General gist is that I want a 10kw battery system to hook up to my existing 5kw system, but I don't want to spend $7800 - $10,000 (What I've been quoted and is considered cheap in AU) on it.
 

Steve_S

Offgrid Cabineer, N.E. Ontario, Canada
Ask a Question and you'll get replies.
Only Stupid Question, is the one Not Asked ! So ASK whenever in doubt !
Answers can often be something you do not like, Be prepared for that ! so don't be upset.
Everyone has opinions, Just like Noses. Sometimes some pretty cruddy stuff blows out of there. Welcome to the Web LOL !
Critical Thinking, PATIENCE and understanding are essential.
Never Ever Panic Buy or Impulse buy ANYTHING ! HAVE A PLAN, STICK TO IT ! Money can vanish extremely fast without a plan.
There are NO "Once in a LifeTime deals you hafta buy NOW" There are always deals and Patience Rules that Game !
IF ANY Vendor, Seller or Agent pushes to get you to "sign the dotted line" or buy into something, RESIST & PUSH BACK... Hi Pressure = 99% trouble Potential and a mistake to be realized later. DIVORCES HAVE RESULTED IN NOT THINKING 1st!

A Tip that is a Result of Pet Peeves.
Many folks give advice, some good some not. Ces't la vie mon ami ! Critical Thinking on all advice given is essential, for peace of mind, a happy wallet and subsequently happy home life. Many folks dish out advice but have no system or not one that is seen by anyone... IF a person does NOT have an "About My System" or similar link in their signature about what they run and their build, approach with an extreme critical eye ! Some folks can really "Talk it up" but have nothing to actually show for it...

3 Important RULES to be happy with your project.
1) KISS ! Keep It Simple Silly ! More complex and involved, the more "angst potential", simplicity is really the best solution.
2) CONSERVATION is FAR CHEAPER than Generation and/or storage. Get rid of all PowerHog devices & appliances. Hot Water Tanks, Electric Dryers & Stoves, Electric Heating = ALL NASTY CRITTERS on Power Usage.
3) TAKE YOUR TIME ! Make a Plan, triple-check it to finalize it and then stick to it.

Never forget this, 85% of folks install a Solar System based on what they believe they estimated for usage & storage that is sufficient. They often UPGRADE within 24 months because they under estimated OR they added new stuff that requires more. KNOW THIS AND CONSIDER IT ! When designing the system realize that an Upgrade later is quite likely, so there are "preemptive" measures that can be done to save you a heap of cash later. Even simple things like making sure the Battery Cables are big enough to handle higher amperage loads.

Case in point Example: I built my system around a 3kw Inverter system and wired according to my calculations and the math, using 2/0 gauge Fie Strand Copper cable. 18 Months later, I upgraded to a 4kw System BUT had to change up my wires to 4/0 gauge and THAT alone was an extra $350 for me ! and now I have a heap of 2/0 lying about (finding uses for it). Also changing the Inverter over cost me another 2 Grand ! OK, so Lessons Learned Dept. got something added as a result.

My own system details can be seen on my "About my System" page link in my signature.
24V System, 30kWh of LFP + 22kWh of Heavy Solar Lead setup as two independant banks.
2080W of Solar Panel, Midnite Solar Classic-200 SCC and Samlex EVO-4024 Inverter/Charger.

Hope it helps, sorry for the extra reading.
PS: Everyone gets the headache of Info-Overload within week-1 by week-3 it will all start to settle into the grey matter.
It is a Heap of stuff to learn and understand AND worse, DC is not like AC and some stuff translates differently between the two and that's where some serious troubles can occur.
 

400bird

Solar Enthusiast
I think the missing detail on BMS sizing is that you size the BMS (amperage) based on your load.

You can have a 1,000,000 ah battery bank, but if you load is only 50 amps, you need a BMS that is rated to 50 amps. Maybe double that (100a) for surge and the expected exaggerated manufacturer rating.

Edit: you might also need to consider your charge current. In the above example, if you charge at 120 amps, you would need a 120 amp or larger BMS.
 
Last edited:

Higher_wire

New Member
Never forget this, 85% of folks install a Solar System based on what they believe they estimated for usage & storage that is sufficient. They often UPGRADE within 24 months because they under estimated OR they added new stuff that requires more. KNOW THIS AND CONSIDER IT ! When designing the system realize that an Upgrade later is quite likely, so there are "preemptive" measures that can be done to save you a heap of cash later. Even simple things like making sure the Battery Cables are big enough to handle higher amperage loads.
Step 1 is to measure and/or calculate the typical and max loads for your system. You can then determine PV/battery/inverter wattage, then calculate cable sizing + current protection.
 

Roqm

Solar Enthusiast
Hey guys,

I am afresh noob and trying to figure this thing out. I come from an engineering (fabrication) back ground, so I can put things together but haven't got all the knowledge to design the system.

I am planning on building a lifePo4 system.
Cheapest batteries I found are 3.2v 420ah.
Im planning on running 8 of those in series giving me 24V and 10752w.

I need help with the following:
Picking a BMS - from what I worked out, the system is 448a, so im thinking a 500a BMS? Also, I am not sure about one thing - voltage. Since the system is running 24v when all batteries are connected, I will require 24v BMS right? not 3.2v (just making sure I understand correctly).

Next - solar charger - I got 2 banks of solar arrays, both coming into the Inverter. As far as I understand, the cabling from the solar banks will go into the charger, then to the battery via BMS, when battery is fully charged the charger will divert excess power to the inverter?
If so, what size cabling do I use? What size solar charger do I need?

Lastly, do I need anything else? From what I read, the above is the 3 basic components of a battery system, but - do I need fuses? additional safety devices?

Sorry for the dumb questions, but I really hope some of you brainiacs can help a noob :)

Igor.
Welcome
your math on your battery capasity is off...if your using 8 x 420a cells in series you will have a 420ah 25.6v nominal bank(24v), when sourcing your bms it has to be lifepo4 (3.2) 8s (24v) the amp capacity will depend on the loads you intend to power, this is what you need to calculate to know the appreciate bms amperage to order if you need to do more than 100a constant you should consider increasing the battery bank size ,also the bms amperage should be twice what you will be running constantly (fets wont be working at thier max for extended periods and should extend thier life).
 

fopoku2k2

New Member
Hey guys,

I am afresh noob and trying to figure this thing out. I come from an engineering (fabrication) back ground, so I can put things together but haven't got all the knowledge to design the system.

I am planning on building a lifePo4 system.
Cheapest batteries I found are 3.2v 420ah.
Im planning on running 8 of those in series giving me 24V and 10752w.

I need help with the following:
Picking a BMS - from what I worked out, the system is 448a, so im thinking a 500a BMS? Also, I am not sure about one thing - voltage. Since the system is running 24v when all batteries are connected, I will require 24v BMS right? not 3.2v (just making sure I understand correctly).

Next - solar charger - I got 2 banks of solar arrays, both coming into the Inverter. As far as I understand, the cabling from the solar banks will go into the charger, then to the battery via BMS, when battery is fully charged the charger will divert excess power to the inverter?
If so, what size cabling do I use? What size solar charger do I need?

Lastly, do I need anything else? From what I read, the above is the 3 basic components of a battery system, but - do I need fuses? additional safety devices?

Sorry for the dumb questions, but I really hope some of you brainiacs can help a noob :)

Igor.
How did you get the 448a for your system? I thought the BMS current was selected based on your maximum charge current
 

Isla

New Member
It’s important to learn how to use a voltage meter for these solar batteries, since they should be monitored every few weeks, ensure that the charger you use is matched to your pack and use a BMS with your charger. BMS also incorporates a low-voltage cut off ( LVC).
 
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