Possible to put back 280ah in 7-10 days?

[offgridcab]

Hey all,

Planning my first system.

It is for my off grid cabin I visit for 2-3 days every other week or so. So it doesn't get much usage.

It is in Upstate NY.

Before I explain the reason for the setup I am planning (24V, 400AH agm, 2000w inverter) I have a technical question / estimate.

Is it possible to have a large enough solar array to bring batteries from about 100ah (75% DOD) to about 380ah (5% DOD) in 7-10 days in upstate NY?

I know the general recommendation is to not go under 50% but at my usage level, cycles wise, this means it will take me 12.5 years to reach the graphs cycles at 50%... while AGM only last what 7-10 years in general... so it doesn't make sense in my set up.

Then the question is: is there a large enough array to fully charge the batteries while I am away for those 7-10 days (no usage while I am gone) ?

 

[rmaddy]

Fun with math:

380Ah - 100Ah = 280Ah * 24V = 6720Wh. Figure an average of 4 hours of solid sun per day (more in summer, less in winter). 6720Wh / 4h = 1680W/day. But you have 7-10 days. Guess 4 good days in that period to be conservative. So 1680W/day * 4 days = 420W of solar needed. That's a lot of guessing and hand waving but it gives a starting point. Given these numbers you need a bit over 400W of solar if you want to recharge from 100Ah to 380Ah in a week. Some weeks you'll recharge in 3 days. Some weeks it will take the full 10 days. More solar will help ensure you can do a recharge during a long bleak winter week. If you have a nice unshaded view of the southern sky from the roof of your cabin you could easily have enough solar to charge the battery in one good day. But no amount of solar will help if the panels are covered in snow the whole time you are away.

Do keep in mind that a 24V 400Ah (using only 280Ah) battery will last a bit over 3 hours if you fully load the 2000W inverter. So obviously you need to keep your loads a lot lower to last 2 or 3 days.

 

[offgridcab]

Fun with math:

380Ah - 100Ah = 280Ah * 24V = 6720Wh. Figure an average of 4 hours of solid sun per day (more in summer, less in winter). 6720Wh / 4h = 1680W/day. But you have 7-10 days. Guess 4 good days in that period to be conservative. So 1680W/day * 4 days = 420W of solar needed. That's a lot of guessing and hand waving but it gives a starting point. Given these numbers you need a bit over 400W of solar if you want to recharge from 100Ah to 380Ah in a week. Some weeks you'll recharge in 3 days. Some weeks it will take the full 10 days. More solar will help ensure you can do a recharge during a long bleak winter week. If you have a nice unshaded view of the southern sky from the roof of your cabin you could easily have enough solar to charge the battery in one good day. But no amount of solar will help if the panels are covered in snow the whole time you are away.

Do keep in mind that a 24V 400Ah (using only 280Ah) battery will last a bit over 3 hours if you fully load the 2000W inverter. So obviously you need to keep your loads a lot lower to last 2 or 3 days.

Excellent! Thank you for that. That is nothing! I was thinking the math will show like 2000 Watts haha. I'll be happy to put 600-800 Watts just to be safe.

My cabin is wired for 120v AC. I have a breaker box that is fed by my 2000eu Honda. Been doing it for a few years like this. I bought a killawatt kinda a device a few weeks ago and plugged it into my generator to see my cabin's draw when I'm at the cabin. My total usage over 3 days is between 5-6 kwh. Highest draw is 1400watt at any given time (microwave for 2min) So I don't use much power.

I'm looking at the renogy 12v 200ah agm Batts on home depot. Spec sheet shows about 450-500 cycles at 75% dod. Please check my math -

450 cycles / 3 cycles (going for 3 day weekend) = 150 cycles. 150 cycles / 24 (24 times a year I go there) = 6.25 years. So assuming life of agm is 7-10 years right? I'm right at the bottom. If I have a 600-800 watt array then I might even have a good nice sunny day or two while I am there and never go to 25% dod... And only reach 50%.. Increasing life of battery.

Am I looking at this correctly?

 

[rmaddy]

I have no experience with AGM batteries so hopefully someone else can chime in on those numbers.

But you are correct about it being sunny while you are at the cabin. During peak sun hours the solar might run your daytime loads without using the battery at all. The battery would only get used in the morning, evening, and while dark.

 

[offgridcab]

I have no experience with AGM batteries so hopefully someone else can chime in on those numbers.

But you are correct about it being sunny while you are at the cabin. During peak sun hours the solar might run your daytime loads without using the battery at all. The battery would only get used in the morning, evening, and while dark.

No problem. Thank you.

A super novice question, since I have not been really looking into solar panels up until now (been planning to just always charge with generator):

Let's say I want to go to 600-700w array: this means that if I see a 325w panel and I buy two of them then I have 650w array or I then need to, like with batteries, wire in series to reach 24v or something so in reality I need more panels?

 

[rmaddy]

If you use an MPPT solar charge controller then the voltage of the panels in relation to the voltage of the battery is mostly irrelevant. The charge controller will take the power from the panels and convert into whatever volts and amps are needed by the battery. As long as the charge controller can generate 5V more than the battery voltage then it can provide power to the batteries. Since you have 24V (really 25.6V), the controller needs at least 29V (really 30.6V) to be able to charge.

Two 325W panels will be 650W whether you wire them in series or parallel. A 325W panel will have a Voc of around 40V. If you wire in parallel it will still be 40Voc. But if you wire in series it will be 80Voc. A charge controller has a critical specification of max input voltage. You must never go over that value. That is the Voc of your panels. And Voc actually increases a little in colder temps. In this example, if the two panels are wired in parallel you could use a charge controller with a max input voltage of around 50V or more. But if the panels are in series you need one that can handle 100V or more.

So you don't need more panels due to the voltage. You might need more panels to deal with shading, clouds, or just ensuring it can fully recharge a 400Ah battery. Let's say you use 300Ah. On average you might get 5 hours of solid sunlight in a day. 300Ah x 25.6V = 7680Wh of power needed to recharge the batteries. 7680Wh / 5h = 1536W of solar panels. So clearly 650W isn't enough if you needed to put back 300Ah into your batteries each day. And this doesn't take into account a string of cloudy days.

Play with the math given your expected actual battery usage and how much sun you really get at your location. And take into account that depending on how the panels are mounted (flat or tilted, adjustable) you will not actually get 325W of power out of a 325W panel.

 

[mikefitz]

Based on my experience in the UK, further north than your location, expect 600 watts of solar to yield, on average, over 1000Wh per day, on a god day in summer expect 2000Wh per day. With several days between use this amount of solar would seem suitable. Of course more would give insurance for several days of poor weather and winter with shorter days.

With the AGM batteries the service life will be less than you expect, anything over 3 years will be a bonus.

Mike

 

[offgridcab]

wow great info here. Thank you!

Yeah after looking around at panels and mppt's today it looks like the way to go is 900w array.

I've seen some used panels on craigslist.. like a 305w for $85 (LONGi LR6-60HPB-305M) and I can pick it up. Is that a no no? are used ones trash? Can I test them to see if they are good?

I also found brand new ones locally as well. 420w for $210. Are these good prices? (Model q.peak duo l-g8.3/bgt 420)

I like the EPEVER Tracer 4215BN for MPPT.

Seems like I will be way under the 150v max (I am in upstate NY mountains so it could reach -30f) but not sure how much charging I will do in winter anyway.

Not sure if you are familiar with this charger.

I had some questions on it:

1. Does it have a low temp cut off? so it won't charge the batteries when it is too cold?
2. In my set up, since my usage is very low, I might only go to like 80-85% SoC before it gets fully charged back which is called "shallow charging" and is not so good... I read that you can program some chargers to not charge until you reach a certain SoC (I will set to a min of 75% or so...). Do you know if this charger can do that?
3. Does the charger and/or monitor MT50 run on battery power or via solar input?

I need to post my wiring diagram in a new post and have the help of this great forum with planning my solar and finalizing my battery bank.. I am not 100% sure about gauges and breaker I have there or if I am missing anything.

 

[IkkeDus]

With the AGM batteries the service life will be less than you expect, anything over 3 years will be a bonus.

That....

My 3.5y old AGM has difficulties now.
It is used in a small setup: 155W panel with an Epever 2215BN. Moving to LiFePo4 I think (anyway... different topic)

 

[offgridcab]

I'm hoping with my little usage in winter and frozen weather - I'll get more life out of them than 3 years. This is my first go with solar so I rather not kill lifepo4. But probably will move to lifepo4 on my next run

 

[IkkeDus]

1. Does it have a low temp cut off? so it won't charge the batteries when it is too cold?
2. In my set up, since my usage is very low, I might only go to like 80-85% SoC before it gets fully charged back which is called "shallow charging" and is not so good... I read that you can program some chargers to not charge until you reach a certain SoC (I will set to a min of 75% or so...). Do you know if this charger can do that?
3. Does the charger and/or monitor MT50 run on battery power or via solar input?

I need to post my wiring diagram in a new post and have the help of this great forum with planning my solar and finalizing my battery bank.. I am not 100% sure about gauges and breaker I have there or if I am missing anything.

1. You don't need that for an AGM (unless you want to switch to LifFePO4 but in that case the BMS should take care of that). A fully charged AGM is better protected against severe cold. Also charging will heat your battery a bit. My experience is with outside temperature upto -15C. It may be different when it gets very cold. Others may chip in here ;-) And checkout: https://batteryuniversity.com/article/bu-410-charging-at-high-and-low-temperatures
2. Epever has configuration options for your battery type but you would want to keep an AGM battery at the full SoC if possible.
3. MT50 is part of the solar charger which needs a battery to run on. Important note: you need to connect the charger and battery first before you connect the solar panels.

 

[A.Justice]

My cabin is wired for 120v AC. I have a breaker box that is fed by my 2000eu Honda. Been doing it for a few years like this. I bought a killawatt kinda a device a few weeks ago and plugged it into my generator to see my cabin's draw when I'm at the cabin. My total usage over 3 days is between 5-6 kwh. Highest draw is 1400watt at any given time (microwave for 2min) So I don't use much power.

Just a suggestion; if you have a generator, you can get a battery charger and use the excess power that's it's making to charge your batteries up.

An adjustable battery charger could divert extra power into the batteries for snowy or cloudy days, and if needed your batteries could be charged in just a few hours.

 

[offgridcab]

Thank you for that link dus.

Justice - yeah I do have and that's my plan but -

This is for my weekend cabin and batteries will be kept outside in a special insulated box.

I'm concerned about what to do when I get back to the cabin in winter and the batteirs are colder than -4f...

Then what do I do? I can't charge or discharge them right?

I guess I need to figure out how to warm them up first?

I could turn on genset when I arrive, power cabin directly from it, and also run a heater inside to warm them up. Once they are warm enough - shut off genset and run off batteirs.

If that is a good idea - is there a specific battery temp gauge I should use that will tell the inside temp of the batteirs? I'm guessing a simple wall temp gauge will not be accurate for the temp of the batteirs.

 

[DerpsyDoodler]

Hey all,

Planning my first system.

It is for my off grid cabin I visit for 2-3 days every other week or so. So it doesn't get much usage.

It is in Upstate NY.

Before I explain the reason for the setup I am planning (24V, 400AH agm, 2000w inverter) I have a technical question / estimate.

Is it possible to have a large enough solar array to bring batteries from about 100ah (75% DOD) to about 380ah (5% DOD) in 7-10 days in upstate NY?

I know the general recommendation is to not go under 50% but at my usage level, cycles wise, this means it will take me 12.5 years to reach the graphs cycles at 50%... while AGM only last what 7-10 years in general... so it doesn't make sense in my set up.

Then the question is: is there a large enough array to fully charge the batteries while I am away for those 7-10 days (no usage while I am gone) ?

If you consistently discharge them to below 50% you will kill them fast.

 

[IkkeDus]

Your solar charger supports a RTS (Remote Temperature Sensor) which you _should_ add to make sure the charger can compensate for the temperature when charging. The MT50 will display the temperature if available and as it should be close to/attached to your battery you will have an accurate idea of the temperature inside your box. I have no idea how cold it gets in your case. A truly frozen battery may need some special handling. But remember that AGM's are often used in cars which also need to start when freezing, if I remember correctly AGM should be fine upto -30C or something like that. In case of arctic conditions you likely have to look for battery heating or other solutions. No experience with that.

 

[offgridcab]

Sorry I changed my plan since this port. I'm planning on using lv2425 by solar mpp. It's an all in one. It doesn't have a temp sensor.

 

[oregonsun]

How deep is your frost line? You could dig a small basement out deep enough to where the earth keeps your batteries above freezing and insulate it so it stays above freezing year round just from the earths heat.

 

[offgridcab]

That's a great idea... It is quite deep. I think around 38 inch. I don't remember.. Also we get a lot of rain so it will be dangerous for the batteries if it gets flooded. But it is a great idea

 

[oregonsun]

Yes that could be a problem.
One day we will have sealed units that can be buried, like a barrel of batteries with just two power cords coming out and moisture and temperature meters.