Solar Panel recommendations please

[Mark_B]

So I am building 2 12v Lifepo4 battery banks with prismatic cells.
I have a Victron 100/50 controller.
I'm having a hard time having confidence as far as what total wattage I can set up.
I like the mission panels. How many watts can I set up to match my controller?

 

[Ampster]

What are the current and voltage limits of your controller? You need that detail. Watts is the product of Amps and Volts.

Watts and Watthours are good metrics to figure out how much capacity you batteries have and how fast you want to charge them.

 

[Mark_B]

These are the specs I think you are asking for on the Victron:
Features and Specifications:

Ultra-fast Maximum Power Point Tracking (MPPT)
Flexible charge algorithm
Extensive electronic protection
Internal temperature sensor
Rated charge current: 50 Amps
Max. PV short circuit current: 60 Amps
Maximum PV open circuit voltage: 100 Volts

These are my battery specs:

 

[Ampster]

So 50 Amps and 100 volts are the two constraints. That is plenty of room for plenty of panels or even one big panel.
The next step is how big are your batteries?

You now have the tools to figure this out.

 

[MisterSandals]

<deleted what Ampser just said ... he pre-repeated what i was typing, slowly>

2 12v Lifepo4 battery banks

You mean a single battery bank with 2 12v LiFePO4 batteries? Presumably in parallel for 12v?
Any particular size (in amp hours) you have in mind?

 

[Mark_B]

I believe 4p4s in the Navitas prismatics gives me 100AH banks. Two of those.
I'm not sure whether I am going to use them in series or parallel yet.
Will that matter as far as choosing panels? My controller will handle either 12 or 24v.
Recommendations on that?

 

[Mark_B]

A picture and the Bhookup add where I got them.

NEW 3.2v 25ah Lifepo4 Topband Navitas Prismatic​

Regular price $12.50 Sale price

The first batch has sold out. The remaining will be listed in about 2 weeks. Be sure to sign up for our email list at the bottom where it says "Be in the know" to get notified when we restock.
Here are brand new 3.2v 25ah prismatic lifepo4 cells. These are Topband cells manufactured for Navitas Systems. All cells were manufactured in 2019 and they are all fully tested to meet the most strict standards.
These are not grade B or used cells being sold as new. This is the real deal! These were cells that were 3rd party verified to meet the most strict specs by the US military and they are priced way below any other 25ah battery. This is one of the best deals we ever offered and when they are sold out we don't expect to get more! This was a one time deal. Easily build a 12v 100ah battery with these in 4s4p configuration.

Dimensions: 7 1/8 x 2 3/4 x 1 1/8
4 cells in series for 12v
8 cells in series for 24v
12 cells in series for 36v
16 cells in series for 48v

 

[Ampster]

Will that matter as far as choosing panels? My controller will handle either 12 or 24v.

No it should not affect panel choice. What will affect panel choice is the inverter you are going to use and how much power you are going to use. Whether you buy a 12 or 24 volt inverter would once again be determined by loads and that is where most people start. What are your anticipated loads?
There are lots of resources on this forum to help you with that question. It is a question that only you can answer. Is this a home, RV and how are you going to be using it?

 

[MisterSandals]

4p4s in the Navitas prismatics gives me 100AH banks. Two of those.

So each 4P4S battery will be 100ah (25ah x 4 (aka 4P)) and 12.8v (3.2 x 4 (aka 4S).

Two of those (12.8v batteries) in parallel will make a 200ah 12.8v battery bank.

Two of those in series will make a 100ah 25.6v battery bank.
This could also be accomplished as 4P8S as a single battery with a single BMS.

 

[Mark_B]

No it should not affect panel choice. What will affect panel choice is the inverter you are going to use and how much power you are going to use. Whether you buy a 12 or 24 volt inverter would once again be determined by loads and that is where most people start. What are your anticipated loads?
There are lots of resources on this forum to help you with that question. It is a question that only you can answer. Is this a home, RV and how are you going to be using it?

Really, this is just emergency power for CPAP, coffee maker, and laptops and whatnot like that. I don't want to shell out for something that will run my entire house. I just want to have something in case the grid goes down or whatever.
So probably a 2000 watt Inverter is probably going to do me.
Being in AZ we get plenty of sun. So just looking for panels enough to charge my two batteries, and maybe a bit of overage just because.
I guess what I would want to know is what is the max my Victron controller will handle, and will that be enough to charge my two batteries?
Many thanks for all the input here.

 

[Mark_B]

So each 4P4S battery will be 100ah (25ah x 4 (aka 4P)) and 12.8v (3.2 x 4 (aka 4S).

Two of those (12.8v batteries) in parallel will make a 200ah 12.8v battery bank.

Two of those in series will make a 100ah 25.6v battery bank.
This could also be accomplished as 4P8S as a single battery with a single BMS.

I think I could go 24v with this setup. I can't think of a reason why not. 4p8s.

 

[Ampster]

........... this is just emergency power for CPAP, coffee maker, and laptops and whatnot like that. ............. just looking for panels enough to charge my two batteries, and maybe a bit of overage just because.
I guess what I would want to know is what is the max my Victron controller will handle, and will that be enough to charge my two batteries?............

I have no idea what a CPAP machine would take. I can guess that coffee and laptop might take 1 kWh. Do you want to look at your CPAP machine to figure that out and maybe double the hours that it would be on for a healthy reserve? All of that data would go into how much your battery would be depleted overnight and that would answer the question about how much would be needed to charge your batteries.

Based on your batteries you have 2.4kWhrs of storage. We could assume you use all of it for the CPAP machine and worse case you would make your coffee after the sun comes up. Assuming conservatively 4 hours of sun and good south exposure you would need 600 To 1000 Watts of panels.

 

[MisterSandals]

I don't want to shell out for something that will run my entire house. I just want to have something in case the grid goes down or whatever.

An 4P8S 25.6v battery bank is a good power reserve.
A Victron 100/50 is a nice meaty SCC (i have an 100/30 and its great).

How much will it handle? Lets do some math.

You seem to be undecided for panels so lets find some big cheap ones on craigslist (phoenix?):

SOLAR PANELS 330 watts - electronics - by owner - sale

AXIpremium HC BLK 330watts 120 cell monocrystalline High performance solar module German engineered – made for America Axitec-Soft-Grip-Seam aluminum frame Soft Grip Higher stability and easier...

phoenix.craigslist.org

330w for $165 - looks great!

Lets arrange an array to stay below 100v for your SCC.
Solar panel Voc: 40.49 (shown in 4th pic)

We can put 2 of these in series for 81v. This is very good, gives us a little leeway for cold weather which can increase the voltage. 2 in series is 660w.

660w / 25.6v (battery voltage) = 25 amps

2 panels in series connected to 2 panels in parallel would give 1320w and would use your SCC nicely near its charging limits. To be honest, 2x of the 330w panels would make a nice backup system in itself. If you want the max, 4 panels in a 2S2P config is the max usable.

So, locate some panels and see how the Voc works out for building an array to work within 100v SCC and maximize the 50amp (if you want to maximize that is).

Come back when you locate some panels, check back in with a few if you want opinions or want to check the math or array config options.

 

[Mark_B]

I have no idea what a CPAP machine would take. I can guess that coffee and laptop might take 1 kWh. Do you want to look at your CPAP machine to figure that out and maybe double the hours that it would be on for a healthy reserve? All of that data would go into how much your battery would be depleted overnight and that would answer the question about how much would be needed to charge your batteries.

Based on your batteries you have 2.4kWhrs of storage. We could assume you use all of it for the CPAP machine and worse case you would make your coffee after the sun comes up. Assuming conservatively 4 hours of sun and good south exposure you would need 600 To 1000 Watts of panels.

The CPAP has a heated humidifier, which is adjustable lower/higher/off. That is what I understand takes most of the power.
I've seen several youtube vids of folks camping with a pretty small battery pack claiming it will last a couple of days.
Probably not heated though. Just dry air, which can work in a pinch. But himidity is more comfortable.
I have a Kill-o-watt meter that I'll hook up tonight and see what the deal is.

 

[MisterSandals]

And of course with a "24v" battery bank, you will need a 24v inverter.
And if you have plans to power any 12v lights/fans/whaterver, you will need a 24v to 12v converter.

I have a Kill-o-watt meter that I'll hook up tonight and see what the deal is.

Perfect. If you can get the watt hours for an entire night of use we can do some more math!

 

[Mark_B]

And of course with a "24v" battery bank, you will need a 24v inverter.
And if you have plans to power any 12v lights/fans/whaterver, you will need a 24v to 12v converter.


Perfect. If you can get the watt hours for an entire night of use we can do some more math!

But I hate math. I like that you said we.

 

[Mark_B]

So each 4P4S battery will be 100ah (25ah x 4 (aka 4P)) and 12.8v (3.2 x 4 (aka 4S).

Two of those (12.8v batteries) in parallel will make a 200ah 12.8v battery bank.

Two of those in series will make a 100ah 25.6v battery bank.
This could also be accomplished as 4P8S as a single battery with a single BMS.

Just musing on the 4s 12v vs 8s 24v and using one BMS instead of two. Does 2 - 12v batteries make sense from the perspective of having two BMS's maybe doing a better job than one? Each monitoring 4 in series vs monitoring 8 in series? Might it be easier to find defective or dying cells?

I plugged in my CPAP and took a quick reading and it is cycling through 50 / 63 / 114 watts right now at the setting I usually use for humidifier. Seems like the heater might be on a switching power supply? The tank was also cold, so maybe there is a heat up spell it goes through until it reaches temp.

 

[MisterSandals]

Each monitoring 4 in series vs monitoring 8 in series? Might it be easier to find defective or dying cells?

No 4S vs 8S is not the issue.
The issues REALLY is that you have 4P which means your BMS has one reading for these 4 fells in parallel.

But look at Battleborn and Tesla batteries, they have quite a few cells in parallel and then tied together in series to make the desired voltage. Its a safety in numbers kind of thing. But with 4 cells in parallel and a single BMS lead going to that group of 4, you have no idea whether all 4 cells are equal or if 3 cells are propping up a 4th.

Make sense? You do get what 4P4S and 4P8S are right?

 

[Mark_B]

No 4S vs 8S is not the issue.
The issues REALLY is that you have 4P which means your BMS has one reading for these 4 fells in parallel.

But look at Battleborn and Tesla batteries, they have quite a few cells in parallel and then tied together in series to make the desired voltage. Its a safety in numbers kind of thing. But with 4 cells in parallel and a single BMS lead going to that group of 4, you have no idea whether all 4 cells are equal or if 3 cells are propping up a 4th.

Make sense? You do get what 4P4S and 4P8S are right?

OK, Yep, I get it. I had high school electronics and did a lot of tinkering. I know at least what series and parallel mean.
I appreciate the explanation though and it makes perfect sense. I'm leaning on 24v now.

 

[Mark_B]

So my Killowatt meter shows .26 KWH for my cpap. About 8 hours of use. So how do I work that out in relation to my battery storage?