Down converting voltage

[mynameismud]

Hello everybody, noob here!

In process planning my system and finding myself in a bit of a dilemma.

I got a good deal for two mono 360W panels and purchased a 12v/24v 40a mppt thinking it would be suitable for a 24v battery bank in my workshop. Now, thanks to my ignorance, and impatience, I miscalculated the actual cost vs ah needed to store energy from the panels.

My idea was to use 8 x 3.2v 100ah lifepo4 in series/parallel somehow thinking, great, that would give me 400ah in 24v (above 300ah needed) in the system. Later on realizing my mistake the actual ah will be only half of what I planned, I thought (without blowing my budget) perhaps I should go for 12v instead?

Would down converting with the mppt to 12v be an energy efficient and cost effective solution to this, or is it simply wasting energy from the panels into thin air?

Appreciate any guidance here.

 

[rmaddy]

8 3.2V 100Ah cells is 2560Wh whether you put them in 8S for a 24V 100Ah battery or 2P4S for a 12V 200Ah battery. FYI - the only way you get a 400Ah battery from those cells is if you make a 6V battery (4P2S) which you certainly don't want to do.

You have 720W of solar. With a 12V battery you need a 60A SCC. With a 24V battery you only need a 30A SCC. Stick with 24V and your SCC will be fine.

Why do you think you need 300Ah or more?

 

[Bud Martin]

Hello everybody, noob here!

In process planning my system and finding myself in a bit of a dilemma.

I got a good deal for two mono 360W panels and purchased a 12v/24v 40a mppt thinking it would be suitable for a 24v battery bank in my workshop. Now, thanks to my ignorance, and impatience, I miscalculated the actual cost vs ah needed to store energy from the panels.

My idea was to use 8 x 3.2v 100ah lifepo4 in series/parallel somehow thinking, great, that would give me 400ah in 24v (above 300ah needed) in the system. Later on realizing my mistake the actual ah will be only half of what I planned, I thought (without blowing my budget) perhaps I should go for 12v instead?

Would down converting with the mppt to 12v be an energy efficient and cost effective solution to this, or is it simply wasting energy from the panels into thin air?

Appreciate any guidance here.

Eight 3.2V 100Ah cells in series (8s configuration) will make 25.6V 100Ah battery pack.
2p4s configuration will make 12.8V 200Ah battery pack.
12V system, then 40A x 12V = 480W of PV input power (it will be actually higher since the charging Voltage will be higher than 12V).
24V system: 40A x 24V = 960W of PV input power (it will be actually higher since the charging Voltage will be higher than 24V).
12V will require bigger gauge wires than the 24V system.
What is the spec of the panels?
What is the max PV Input Voltage of the SCC?

 

[mynameismud]

8 3.2V 100Ah cells is 2560Wh whether you put them in 8S for a 24V 100Ah battery or 2P4S for a 12V 200Ah battery. FYI - the only way you get a 400Ah battery from those cells is if you make a 6V battery (4P2S) which you certainly don't want to do.

You have 720W of solar. With a 12V battery you need a 60A SCC. With a 24V battery you only need a 30A SCC. Stick with 24V and your SCC will be fine.

Why do you think you need 300Ah or more?

Thanks for the answer.

Yes, 24V seemed most logical in this situation.

My load will be at most some Indoor and outdoor LEDs, a chest freezer, a router and surveillance camera and charging battery driven tools. In my estimation I would need at least 300ah battery to keep those things going over night. I guess a got something confused with the ah/wh? Could 2560wh be enough for my needs even in 8S?

 

[rmaddy]

When you did your estimations, what was the total watt hours that you determined you needed each day? That's the important number. Forget voltage and amp hours. Do everything in watt hours to make it much simpler to compare.

For example, you LED lights. Let's say they are 30W and you want to run them 2 hours a day. That's 30W x 2 hours = 60Wh.
Determine how many watt hours your freezer uses in a day. Once you get the watt hours of each item, add them all up to get a total.

Once you know you have enough battery capacity in watt hours to meet your needs, then you can decide on the best system voltage. You also need to factor in the size of the inverter you need and the amount of solar you need.

If you haven't actually done a formal energy audit, take a look at the following:

System Energy Audit and Sizing Spread Sheet

To get the spreadsheet, click on the orange button at the top of this page. This sheet is intended to help you plan your system. Fill it out down to the last decimal place, look at the results..... and then make your best guess;) Notes...

diysolarforum.com

 

[mynameismud]

Eight 3.2V 100Ah cells in series (8s configuration) will make 25.6V 100Ah battery pack.
2p4s configuration will make 12.8V 200Ah battery pack.
12V system, then 40A x 12V = 480W of PV input power (it will be actually higher since the charging Voltage will be higher than 12V).
24V system: 40A x 24V = 960W of PV input power (it will be actually higher since the charging Voltage will be higher than 24V).
12V will require bigger gauge wires than the 24V system.
What is the spec of the panels?
What is the max PV Input Voltage of the SCC?

SCC. 100V.

PV.
Voc, 44.4
Vmpp, 36.1
Impp, 99.

 

[mynameismud]

When you did your estimations, what was the total watt hours that you determined you needed each day? That's the important number. Forget voltage and amp hours. Do everything in watt hours to make it much simpler to compare.

For example, you LED lights. Let's say they are 30W and you want to run them 2 hours a day. That's 30W x 2 hours = 60Wh.
Determine how many watt hours your freezer uses in a day. Once you get the watt hours of each item, add them all up to get a total.

Once you know you have enough battery capacity in watt hours to meet your needs, then you can decide on the best system voltage. You also need to factor in the size of the inverter you need and the amount of solar you need.

If you haven't actually done a formal energy audit, take a look at the following:

System Energy Audit and Sizing Spread Sheet

To get the spreadsheet, click on the orange button at the top of this page. This sheet is intended to help you plan your system. Fill it out down to the last decimal place, look at the results..... and then make your best guess;) Notes...

diysolarforum.com

Ok. I can't remember exactly how I did my estimation but I roughly calculated for, lights 4x20W=80W. Router + camera 50W. Freezer 100W. Charging tools only in daytime. Around 230W load for minimum 15 hours = 3450wh needed.

Thanks for link.

 

[mynameismud]

12V will require bigger gauge wires than the 24V

Sorry if this sounds like a stupid question. With gauge wires you mean from panels to SCC, from SCC to battery and inverter, or all wires including from inverter to lights and such?

 

[Don B. Cilly]

The wire, from battery to inverter mainly. Which inverter, depending on what it is, will probably draw some 10-30W just sitting idle.
Watts are watts, whether it's 12V or 24V you get the same power.

Still, for that consumption, 100Ah at 24V should be OK. Consider, you won't use all the lights for 24 hours, the freezer will probably run 20 minutes in the hour, and you'd only be discharging the battery when there is no sun.

[EDIT] Now, of course, if you live in Tierra del Fuego or Antarctica - which is where I have to assume you live since you don't state your location - you're heading fast into winter, and sun hours may look bleak in the near future ;·)
-

 

[mynameismud]

The wire, from battery to inverter mainly. Which inverter, depending on what it is, will probably draw some 10-30W just sitting idle.
Watts are watts, whether it's 12V or 24V you get the same power.

Still, for that consumption, 100Ah at 24V should be OK. Consider, you won't use all the lights for 24 hours, the freezer will probably run 20 minutes in the hour, and you'd only be discharging the battery when there is no sun.

[EDIT] Now, of course, if you live in Tierra del Fuego or Antarctica - which is where I have to assume you live since you don't state your location - you're heading fast into winter, and sun hours may look bleak in the near future ;·)
-

Thanks. But you still need thicker wire longer the distance is from panels to SCC in 12V compared to 24V? Edit: Lower voltage PV as opposed to higher voltage PV.

Unfortunately I'm still a bit confused. If there's no difference running, in my case (assuming I had a 60A SCC) 720W, through the SCC to 12V 2P4S, compared to 8S 24V. What then would be the main drawback using 12V instead of 24V?

Hahaha, my situation is fortunately not that bad.

 

[Don B. Cilly]

You can still use the two panels in series for ~88V Voc regardless of battery voltage. The SCC will take care of that - just connect the battery first and the panels second. SCC to battery, yes, 12V would need thicker wire than 24. Not much of it hopefully, just put it close to the battery.

For that sort of load, 12V should be fine. The main drawback would be needing a bigger SCC - and not having much room for growth.
-

 

[rmaddy]

I can't remember exactly how I did my estimation but I roughly calculated for, lights 4x20W=80W. Router + camera 50W. Freezer 100W. Charging tools only in daytime. Around 230W load for minimum 15 hours = 3450wh needed.

That's not the correct approach since your freezer won't run at 100W all 15 hours. Will the lights be on all 15 hours? Why 15 hours? you need to work out how many watt hours you will use in a day. I presume the router (and camera?) will run for 24 hour per day. The freezer cycles on and off several times per hour all day. Ideally you should measure the real use over several days and get an average daily use. A Kill-a-watt meter is great for this. You still need to know how much power the tools charging will use even if during the day. There will be days with no useful sun.

SCC. 100V.

PV.
Voc, 44.4
Vmpp, 36.1
Impp, 99.

With a Voc of 44.4V and an SCC with a max input voltage of 100V, keep in mind that with 2 panels in series you would exceed the 100V limit if the temperature ever got below roughly 2ºF/-16ºC. If it ever gets that cold where you are then you will need to put your two panels in parallel.

Unfortunately I'm still a bit confused. If there's no difference running, in my case (assuming I had a 60A SCC) 720W, through the SCC to 12V 2P4S, compared to 8S 24V. What then would be the main drawback using 12V instead of 24V?

720W on a 12V battery requires a 60A SCC. 720W on a 24V battery only needs a 30A SCC. You stated you already have a 40A SCC. If you setup your battery cells for a 24V battery then you can use your existing SCC.

The main drawback of 12V versus 24V is that with 12V your 40A SCC only support 480W of solar while running at 24V the same SCC supports 960W of solar. Higher voltage means lower amps means smaller wires and less voltage drop.