diy solar

diy solar

battery bank for off grid solar setup

D

dawgleader1

New Member
Joined
Mar 9, 2020
Messages
19
Hey all, I'll preface by saying that I'm brand new to this solar thing and came across this forum via Will Prouse on youtube. I have tried to do some reading and research but it seems the more I read, the more confused I've gotten. I have drawn up a plan and hope to get some 'good job, you're on the right track' comments or some 'you're way off base idiot, do this instead' ideas. Either way it will be hugely helpful.

So what I'm trying to do is set up a medium sized solar system that will be off-grid for a mid term power outage or could even be used in a longer term emergency situation. I'm trying to make this as easy for a newbie like myself as possible while keeping the cost as minimal as possible (yes i like to get my cake and eat it too, lol). I'll mainly be using this system as an emergency system to keep the upright freezer and our normal Samsung household frig/freezer running along with charging phones, laptops, flashlights, and using an occasional electric convenience such as a bread-maker, toaster oven, hot plate, crock pot, waffle maker, etc. We will manage our usage and disconnect certain appliances to control our usage within the 3kw of our chosen all-in-1 inverter below. My plan is to get a plug in play system like this all-in-one 48v, 3Kw Growett inverter system (https://www.ebay.com/itm/USA-Supt-4...492378?hash=item48e0d1d19a:g:sxgAAOSwlD5eVqbD). I will then hook up 10 305 solar panels and connect my battery bank to this for storage. I'm looking at doing these 24v FiFePO4 Powerwall batteries from Bigbattery.com (https://bigbattery.com/product/24v-lifepo4-power-wall/ref/8/?campaign=budgetvideosystem) as suggested by Will on his site. Since these are 24v battery packs I know I need to connect 2 into series to get my 48v for the inverter system. I would like 10-12k watts of battery storage for usage overnight and in the case of bad weather/ no sun to get us through a couple days of minimal usage. If I connect 2 of the powerwall batteries in series that will get me my needed 48v for the inverter, while that doubles the voltage, does that also retain the same capacity as the 2 individual batteries. So in other words, since each battery is listed at 3kw of storage, when I connect them in series, does that mean that each set of 2 at 48v would equal 6kw of battery storage? If that is correct then I would need 4 of these powerwall batteries, with 2 series connected pairs to reach my desired 48w for the inverter and 12k watts of battery storage capacity. This is the most confusing part for me since I've talked to 2 different battery suppliers and been told 2 different things. I don't want to spend $4k on batteries and then be wrong and SOL.

Any advise, clarification, and help would be greatly appreciated. Thanks so much.
 
dawgleader1 said:
Hey all, I'll preface by saying that I'm brand new to this solar thing and came across this forum via Will Prouse on youtube. I have tried to do some reading and research but it seems the more I read, the more confused I've gotten. I have drawn up a plan and hope to get some 'good job, you're on the right track' comments or some 'you're way off base idiot, do this instead' ideas. Either way it will be hugely helpful.

So what I'm trying to do is set up a medium sized solar system that will be off-grid for a mid term power outage or could even be used in a longer term emergency situation. I'm trying to make this as easy for a newbie like myself as possible while keeping the cost as minimal as possible (yes i like to get my cake and eat it too, lol). I'll mainly be using this system as an emergency system to keep the upright freezer and our normal Samsung household frig/freezer running along with charging phones, laptops, flashlights, and using an occasional electric convenience such as a bread-maker, toaster oven, hot plate, crock pot, waffle maker, etc. We will manage our usage and disconnect certain appliances to control our usage within the 3kw of our chosen all-in-1 inverter below. My plan is to get a plug in play system like this all-in-one 48v, 3Kw Growett inverter system (https://www.ebay.com/itm/USA-Supt-4...492378?hash=item48e0d1d19a:g:sxgAAOSwlD5eVqbD). I will then hook up 10 305 solar panels and connect my battery bank to this for storage. I'm looking at doing these 24v FiFePO4 Powerwall batteries from Bigbattery.com (https://bigbattery.com/product/24v-lifepo4-power-wall/ref/8/?campaign=budgetvideosystem) as suggested by Will on his site. Since these are 24v battery packs I know I need to connect 2 into series to get my 48v for the inverter system. I would like 10-12k watts of battery storage for usage overnight and in the case of bad weather/ no sun to get us through a couple days of minimal usage. If I connect 2 of the powerwall batteries in series that will get me my needed 48v for the inverter, while that doubles the voltage, does that also retain the same capacity as the 2 individual batteries. So in other words, since each battery is listed at 3kw of storage, when I connect them in series, does that mean that each set of 2 at 48v would equal 6kw of battery storage? If that is correct then I would need 4 of these powerwall batteries, with 2 series connected pairs to reach my desired 48w for the inverter and 12k watts of battery storage capacity. This is the most confusing part for me since I've talked to 2 different battery suppliers and been told 2 different things. I don't want to spend $4k on batteries and then be wrong and SOL.

Any advise, clarification, and help would be greatly appreciated. Thanks so much.
Click to expand...


Hi DL,
From what I've read on here, and there are a lot of REALLY smart people on here, you will have to spend more than you expect because:

Your panels may say 305W but you are only going to get max wattage out of them for a small fraction of the daylight hours, and that max wattage is likely to be maybe 70% of their rated wattage, tops

Your bats might say 3kw but you will likely only get 80% max of that, because they are not new.
And in even quite aggressive use of them you shouldn't charge them above 95% tops or discharge them below 10% minimum.

Others, way smarter than me, may give you different numbers, but after reading maybe 500+ posts this is my understanding.

So, you won't be SOL if you spend $4k - you'll be well on the way to an awesome solar system, but you should raise your budget (or lower your requirements) so you're in that happy place of under promising and over delivering.

Remember, most of us here are just looking for ways to get "free electricity". And most equipment is best used with a buffer rather than maxxing it out. I suggest you keep these thoughts in mind when you get the more "vehement" responses ;-)

Shine On!
 
Hey Craig! That's exactly what I was looking for. I figured that capacity on a battery storage system shouldn't change irregardless of how many you had or how they were connected. I thought that you just added up as many batteries as you had and boom, that's what you ended up, didn't matter series or parallel. What confused me so much was that I spoke with an employee at an online company that sells battery packs and he said that if you connect 2 of the 24v 3kw batteries to get my 48v for the panel inverter, then it doubles the voltage but the storage doesn't increase so I only get 3kw of battery storage. That threw me for a loop and screwed up all my plans.

Sounds like I should be good to go with my original plan now. I'm going to get 4 of the 24v 3kw battery powerwalls and connect 2 of them into series twice to get 2 pairs at the 48v and 6kw of storage then connect those 2 pairs into parallel to get a total of 12kw of battery storage.

Thanks again Craig, if you see anything wrong with my plan please let me know. I'm planning on buying the batts today so any information would be super helpful.



Thanks Kauai for the advice, sounds solid. If I plan on 12kw of battery storage and then figure in your 80ish% numbers that should still put me around 10kw. I'm comfortable with that, I was originally thinking of going with around 9kw but decided to build a buffer in there just as you've suggested since I don't know how many days I can run without any sun at all. If we run into a couple days of no solar collection we can drop down to bare minimum and just run the freezer and frig; grilling/gas for cooking and paper plates for eating. Then re-up the batts when I get some sun, at least that's the current plan. I have seen a 48v powerwall battery rated at 16kw that I really was interested in before I realized it weighed 650lbs. I think that going the smaller battery route may be better so that if a battery dies on me I'm not completely screwed, if I only have the 1 big mamma and it goes down I have nothing. And of course if I have to leave our home for an emergency, I'd have to be leaving the 650lb behind but I can grab the 4 smaller ones and load them up with us. But if you have any opinion please toss it to me, I'd appreciate it.


If anyone else has any advice or sees anything wrong with my thought process or plans please throw me a note!!
 
Hey Steve, thanks very much for chiming in. You seem to be saying the same thing as the guy I spoke with on the phone and counter to what I thought and what I understand Craig to be saying. Again I'll reiterate I'm new and not really mechanically inclined, hell I pay someone to change my oil, so please excuse my hard headed ignorance on this subject. But the way I understand it is that W=V x A. If I look at the examples you give above with 2 of the 12v100AH batteries each battery has 1200W of storage independently. In the series set with 2 of the 12V/ 100AH batteries it is 24V(x)100A which = 2400W. In the Parallel example it's 12V(x)200A which is also = 2400W. So it seems like the capacity is double in either setup. I really appreciate the link you included and I'll read the whole thing later but just scanning over it I see this I see a diagram which leads me to believe that it does increase capacity if you link batteries in series OR parallel.


battery setup picture.PNG

In the picture above it shows an in-series setup calculated as double of the voltage and capacity over a single battery, which is the same Wh of capacity as the 2 batteries in paralel. If you put them both in parallel AND series then it quadruples the capacity of a single battery to 9600Wh. Don't get me wrong, I'm not arguing that you're wrong or anything, I'm just trying to understand and looking for help. Thanks again. Any other help would be appreciated.
 
Hi DL, The key is exactly as you said: W=V x A. And your drawings are 100% correct. Series gives you higher volts, parallel gives you higher amps. If you consider W as capacity, volts as volts, and amps as amps, you only get more capacity - W - by getting more batteries. How you get that additional capacity - W - depends on how you connect the bats: its more volts if you connect in series, more amps if you connect in parallel.

Have fun!
 
Hey No problem, your getting there... If this is all new it can most certainly be overwhelming. It is absolutely Information Overload ! and unfortunately many people hit that point and then confusion kicks in... only seen it once or twice or...……. ;-) Happened to me too and I AM a builder and worse as I was a Board Level designer for Control Data Corp (mainframes), then network design & engineering to Computer design & engineering ending up as a core engineer doing software & hardware implementations across government, military & business.... To keep my sanity when in the I sector I rebuild & Built show cars (64 Chevelle, Several Camaro's, 68 Beaumont SD396) and many more... so I can change oil too.

That Victron Doc will explain it all very well, they did a bang up job on making that straight forward and easy to get.... They even kept the "shop talk" minimized too.

Point to keep in mind when doing your math.... when we talk about 12V or 24V those are the numbers we work with (provides margin). We never calculate things like 12.6V X 100AH, it's always 12x100AH or 24Vx100AH. Exact precision is required when configuring SCC & Inverter for cut off points, charge points and so on but that opens another kettle of fish too... I wrote this up to help folks cope with that and get it right. Calibrating Voltage of your system to ensure Optimal Operations.
 
Hey again Kauai. So it sounds like I'm correct, it doesn't matter how I connect the 4 batteries, I will retain all of their battery storage capacity and so 4 batteries as I'm discussing would give me @12kw (4x 3kw). Since the 3kw batteries at 24v have about 125Ah (3k/24) then the Ah remain the same for the pair of batteries in series but doubles my voltage and when done in parallel, the 2 pairs keep the same voltage at 48v but doubles the Ah.

my solar diagram2.PNG

So this is my planned system; It sounds like connecting the 2 batteries in series gives me 48v at around 125Ah for 6kw and then I can connect 2 of those pairs in parallel and that gives me 48v at 12kw of total battery storage capacity. Am I correct?
 
You must log in or register to reply here.

Similar threads

S
DER storage meter wiring with solark
Replies
2
Views
119
DIYrich
D
N
Anyone have experience with the ECO-WORTHY 48V 50Ah 2560Wh Golf Cart LiFePO4 Lithium Battery?
Replies
3
Views
1K
millsan1
millsan1
S
General Battery advice
2
Replies
33
Views
1K
AmpD
A
unkyjoe
Multiple Inverters with one battery bank
Replies
5
Views
231
unkyjoe
unkyjoe
C
Dual battery inverter
Replies
5
Views
284
SeaGal
SeaGal
Back
Top