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diy solar

How many additional batteries should I get?

marcfest

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Mar 18, 2022
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I'm thinking of adding to the currently 4 Battleborn 100Ah 12V batteries I have in my system whose specs are shown below (has ten 345W panels). How many more batteries should I reasonably get? Is there some kind of formula or logic for that? As a thought experiment, adding thousand batteries would seem excessive, because the ten panels would never be able to fully charge them over the course of a day. So what total number of these batteries would make sense in this scenario?

Thank you.

250V / 100A Victron charge controller
4000W Giandel inverter
4 Battleborn 100Ah / 12V batteries in 2S2P 24V config

10 Mission Solar panels in 5S2P config:
Watts (STC) 345W
Max Power Voltage (VMPP) 33.37 V
Max Power Current (IMPP) 10.34 A
Open Circuit Voltage (VOC) 41.0 V
Short Circuit Current (ISC) 10.92 A
Max System Voltage (UL) DC 1000 V
 
2S2P is fairly stable and easy to keep balanced. Three strings is manageable and four strings starts to get difficult. Although it's not as important with lithium chemistries as it is with lead, it's still something to consider.
 
I'm thinking of adding to the currently 4 Battleborn 100Ah 12V batteries I have in my system whose specs are shown below (has ten 345W panels). How many more batteries should I reasonably get? Is there some kind of formula or logic for that? As a thought experiment, adding thousand batteries would seem excessive, because the ten panels would never be able to fully charge them over the course of a day. So what total number of these batteries would make sense in this scenario?

Thank you.

250V / 100A Victron charge controller
4000W Giandel inverter
4 Battleborn 100Ah / 12V batteries in 2S2P 24V config

10 Mission Solar panels in 5S2P config:
Watts (STC) 345W
Max Power Voltage (VMPP) 33.37 V
Max Power Current (IMPP) 10.34 A
Open Circuit Voltage (VOC) 41.0 V
Short Circuit Current (ISC) 10.92 A
Max System Voltage (UL) DC 1000 V
I am running 12 battleborn batteries. I started out with 2. More i bought the more circuits i dedicated to solar. It is a never ending project.
 
There is exactly ONE amount of batteries to buy:

You buy as many batteries as you need to provide backup power for your loads for the period of time you desire.

You may choose less at your peril or more at your wallet's peril.
Pure logic… America needs more of that…
 
Simple as many as you can comfortably afford, the more you have the longer you can go with bad weather.
 
I'm thinking of adding to the currently 4 Battleborn 100Ah 12V batteries I have in my system whose specs are shown below (has ten 345W panels). How many more batteries should I reasonably get? Is there some kind of formula or logic for that? As a thought experiment, adding thousand batteries would seem excessive, because the ten panels would never be able to fully charge them over the course of a day. So what total number of these batteries would make sense in this scenario?

Thank you.

250V / 100A Victron charge controller
4000W Giandel inverter
4 Battleborn 100Ah / 12V batteries in 2S2P 24V config

10 Mission Solar panels in 5S2P config:
Watts (STC) 345W
Max Power Voltage (VMPP) 33.37 V
Max Power Current (IMPP) 10.34 A
Open Circuit Voltage (VOC) 41.0 V
Short Circuit Current (ISC) 10.92 A
Max System Voltage (UL) DC 1000 V
Your question is good one. Just throwing more batteries into a system will indeed reach a point that your panels can not keep them charged if your loading during the day is too high. Batteries are not an infinite power source.

I would say the practical point for any panel battery ratio is that at minimum your panels delivered enough watt-hours per day to equal your daily load requirements. Anything less than this and you will eventually have to supplement the need, either with grid power or with a generator. Your batteries storage allows you to bank amp-hrs to supply the load during the time you get no power from the solar panels.

Due to the nature of weather and season you will have times that you get less output from the solar panels. By having battery storage you can supply your loads but you then must needs to catch back up on recharging them. At this point if your panels are not enough you have to cut back loading or supplement power.

Sadly I see far too many people not understanding the simple power dynamic here.

For your system to make any sense you would need to know your daily load. You would also need to know your real average developed power from your panels.
 
Ok, look at it this way…

You have a maximum power input of 3450W.

Average solar output is 5.5h or 18.5kWh

If your loads are less than this, getting two or three times the battery storage can help you get over low production days…

If your loads are more than this… no amount of batteries will help, you will need a generator or grid input to maintain the batteries.

To answer your question, you need to know what you use.
 
I run 24V in Northern Ontario Canada (Ogg-Grid, Full-Time, Remote & Rural)
1190AH of LFP in Parallel which gives me about 7 Days of storage due to Winters (Gets crappy).
My Sun-Hours are (Facing due South BTW) Panels are set to 45Deg for optimal yr round use.
Between Mid Feb to Mid Oct my batteries are fully charged & in float usually between 12:00-2 pm. daily. (sunny day of course)

1658665374836.png1658665630100.png It is very important to know how much your array will actually generate monthly and what the averages are.

You have to work out what your Daily Usage is from Solar / Battery and then how days reserve you "Need" and then "Want" to determine how much battery you need.

The TABLES in the link provided below are Most Helpful & Simple to use.
http://www.solarelectricityhandbook.com/solar-calculator.html

Hope it Helps, Good Luck.
 
Thanks all for your input here. Many pointed out, rightly, that I failed to indicate to what I want to supply the power. Sorry about that. I'm basically talking about a bunch of fridges, electronic equipment, maybe one portable AC. It averages out to probably 1500W in terms of continuous load. The panels provide more than that during most daylight hours (I'm in South Florida). I'm playing with the thought of going from my current four to eight batteries.
 
I was just noodling through this today. Pretty close to your numbers.

First thing, give a lot of thought to a 48 volt, 100 amp hour server rack battery. See currentconnected.com and read up on EG4 threads to get an idea of some concerns.

I have a couple of battle born 12 volt/100 amp hour batteries also. They come in handy for stand-alone 12 volt stuff. But stacking more and more of these doesn’t make sense to me. For bigger loads, I think you will find the server rack batteries are a better value. Or diy batteries but that’s a whole other project.

If an example helps, I just cut and pasted this from my word doc. Still polishing so take it with a grain of salt. I usually have a calculation or unit error.

Solar calculations

2000 watts x 24 hours=48 kWH daily use estimate
20.5 kWH storage (4x SOK 48 volt/100 amp hour batteries)-In the neighborhood of $7,000 usd.
18 kWH useable (after inverter)
Need 400 amp hours at 48 volts to fill up batteries. Aka 20,000 watt hours.
This requires minimum 3,333 watts solar (net) for 6 hours. Therefore, 4,000 watts in panels.
Midnite 200 charges at 3000 watts. Approx. 3000 watts x 6 hours=18,000 watts
Midnnite 150 charges at 4000 watts. Approx. 4000 watts x 6=24,000 watts (bingo!)

Therefore, to charge existing bank:
-4,000 watts panels (approx 14 panels)
-Midnite 150
-4x-SOK 48 volt/100 amp hour batteries

Plus-to run 2000 watts of stuff during about 6 solar production hours:
-4,000 watts panels
-Another Midnite 150
-3,000 watt (ish) inverter

Therefore, to run 18 hours of 2,000 kWH load from mid afternoon until the next solar production period, I need 36 kWH of battery capacity. I have 18 kWH-useable.

If I reduce my load to 1,000 kWH (strictly air conditioning), I need 18 kWH Useable battery capacity.
 
Please Consider these two articles BEFORE proceeding:


Hope it helps, Good Luck
Steve.
 
It averages out to probably 1500W in terms of continuous load.
36 kWh/day on average on the AC load side.

Currently you have approx 3.45kW of solar PV and 5 kWh of batteries. 24 V system.

There is just insufficient information to say more on what you should do.

What are your objectives?

What are your alternative sources of power?

Is this completely off-grid, grid supplemented or grid-tied?

What load management options do you have?

What is the peak power draw?

Location?

Capacity for system expansion?

PV array orientation?

Seasonal demand and PV production variance?

Budget?
 
I was just noodling through this today. Pretty close to your numbers.

First thing, give a lot of thought to a 48 volt, 100 amp hour server rack battery. See currentconnected.com and read up on EG4 threads to get an idea of some concerns.

I have a couple of battle born 12 volt/100 amp hour batteries also. They come in handy for stand-alone 12 volt stuff. But stacking more and more of these doesn’t make sense to me. For bigger loads, I think you will find the server rack batteries are a better value. Or diy batteries but that’s a whole other project.

If an example helps, I just cut and pasted this from my word doc. Still polishing so take it with a grain of salt. I usually have a calculation or unit error.

Solar calculations

2000 watts x 24 hours=48 kWH daily use estimate
20.5 kWH storage (4x SOK 48 volt/100 amp hour batteries)-In the neighborhood of $7,000 usd.
18 kWH useable (after inverter)
Need 400 amp hours at 48 volts to fill up batteries. Aka 20,000 watt hours.
This requires minimum 3,333 watts solar (net) for 6 hours. Therefore, 4,000 watts in panels.
Midnite 200 charges at 3000 watts. Approx. 3000 watts x 6 hours=18,000 watts
Midnnite 150 charges at 4000 watts. Approx. 4000 watts x 6=24,000 watts (bingo!)

Therefore, to charge existing bank:
-4,000 watts panels (approx 14 panels)
-Midnite 150
-4x-SOK 48 volt/100 amp hour batteries

Plus-to run 2000 watts of stuff during about 6 solar production hours:
-4,000 watts panels
-Another Midnite 150
-3,000 watt (ish) inverter

Therefore, to run 18 hours of 2,000 kWH load from mid afternoon until the next solar production period, I need 36 kWH of battery capacity. I have 18 kWH-useable.

If I reduce my load to 1,000 kWH (strictly air conditioning), I need 18 kWH Useable battery capacity.
Thank you for sharing this!
 
36 kWh/day on average on the AC load side.

Currently you have approx 3.45kW of solar PV and 5 kWh of batteries. 24 V system.

There is just insufficient information to say more on what you should do.

What are your objectives?

What are your alternative sources of power?

Is this completely off-grid, grid supplemented or grid-tied?

What load management options do you have?

What is the peak power draw?

Location?

Capacity for system expansion?

PV array orientation?

Seasonal demand and PV production variance?

Budget?

Thanks for asking for the additional information. I've pasted some answers below:

What are your objectives?

Two-fold: have a minimum viable situation on my first (very sun-protected) floor during a prolonged power outage. This would require running a 12A portable AC unit and basic electronics and a few lights. Second, during normal times, when I have grid power, save a bit of money each month on my utility bill. I want my system to stay off-grid, so I've decided to simply plug a bunch of fridges and extension cords to electronics into my system to achieve this.

What are your alternative sources of power? Grid 110 and 240V

Is this completely off-grid, grid supplemented or grid-tied? Completely off grid.

What load management options do you have?

Not sure what that means.

What is the peak power draw?

Probably 1600W

Location? South Florida, Everglades. Very sunny.

Capacity for system expansion?
I am willing to spends a few more thousand dollars.

PV array orientation?
I know it's not optimal, but I have them lying flat down on a former pool deck. The sun shines almost straight down during summer days right now.

Seasonal demand and PV production variance?

Less need for AC in the winter.

Budget?

Up to $5K.
 
Up to $5K.
OK, well all that info gives people here more attuned to the costs of DIY systems in your neck of the woods a bit more meat to chew on when it comes to advice on what sort of system expansion you might consider.

Two-fold: have a minimum viable situation on my first (very sun-protected) floor during a prolonged power outage. This would require running a 12A portable AC unit and basic electronics and a few lights. Second, during normal times, when I have grid power, save a bit of money each month on my utility bill. I want my system to stay off-grid, so I've decided to simply plug a bunch of fridges and extension cords to electronics into my system to achieve this.

If you are happy with the rest of your system and it's only storage you are considering, then I would be investing in power/energy monitoring if you haven't already and run the system with the devices you want to see how it performs.

Simulate a grid outage. Simulate your otherwise regular off-grid operating mode.

Gather data on how the system performs. You have 4.8 kWh of storage now (call it 4 kWh effectively useable on the AC side). Nothing like hard data and actual use to know whether something is up to the task.

In regular off-grid operating mode it might be that if you experience poor solar weather conditions for an extended period then you choose to rely on the grid rather than expand your system to cope with multiple consecutive days of low PV output. IOW the off-grid system handles the chosen loads most of the time but on occasions you are using the grid for backup.

Only you can do the calculus on whether spending the $ on extra storage to cover for those less frequent occasions is worthwhile, or just accept that the system will do it's thing 80-90% of the time and the grid handles the rest, or whatever ratio makes sense for you. Local grid tariff regimes vary widely so it's hard to say from here in Australia.

For grid outage backup scenarios then you'll also need to have in mind for how long you want coverage for, and consider why backup might be required, e.g. is this mostly a storm/weather related grid outage scenario (ours are)? How long will they typically last for, or how long do you want to be able to cover? If it's weather related then that has implications for whether your solar PV is going to be sufficiently secured and also whether it will be able to supply all that much energy during inclement weather to recharge batteries. You might need generator as backup, at least to do a battery charge for periods during the daytime (might not be socially acceptable where you are to run a generator at night).

You may also want to consider how much storage capacity to keep in reserve. This will be a bit of a subjective assessment on your part, and it might be an adjustable target. e.g. in storm season you might choose to always have a minimum capacity of X kWh on hand, but during times of the year when outages are less likely you are OK with letting the battery state of charge drop to a lower level. Reserve capacity costs more as much of the time it is not being cycled and so is not helping to reduce grid import costs.

e.g. I will have two storages - the 18 kWh of lead acid I currently have for grid outage backup, and the 10 kWh of LiFePO4 I am adding for regular off-grid cycling for running the regular power and lighting circuits for our two homes (but not the large power draw devices such as ducted aircon, electric oven, induction stovetop etc). I don't expect the home to be able to run off-grid the whole time, so at times energy demand will be supplemented by the grid supply.

The energy required to keep lights, refrigeration and basic power devices, e.g. phone charging, computer, internet connection, maybe a TV, running isn't a difficult sum to do but it would probably help you to do some power monitoring of the more energy hungry of those such as the fridge and any other devices you want to operate during an outage. A plug in power monitor is pretty cheap.

Some older devices might be worth updating if they suck too much power. A while back I sold off an older Sony TV and got a new replacement. The new one uses 100W less power and is way less of an energy drain. Fridges are another - modern units (and inverter driven) are way more energy efficient than older fridges.

I made the assessment I needed to cover in the vicinity of 500-600 W on average, with peaks over 3 kW, and to have coverage for > 12 hours.

By load management I meant will you be actively managing loads so as to ensure you keep them within the capacity of the system to deliver power, as well as shifting loads to times of day when your PV array has excess capacity to burn? There are various ways to do this, from changing habits, to automation and smart devices. e.g. my hot water system uses a smart power diverter which controls the power the water heater consumes such that it avoids importing energy from the grid.

Primarily the point being to think not only about the energy supply side but also the energy demand side. By getting demand under control you may not need to spend nearly as much $ on the supply side.
 
OK, well all that info gives people here more attuned to the costs of DIY systems in your neck of the woods a bit more meat to chew on when it comes to advice on what sort of system expansion you might consider.



If you are happy with the rest of your system and it's only storage you are considering, then I would be investing in power/energy monitoring if you haven't already and run the system with the devices you want to see how it performs.

Simulate a grid outage. Simulate your otherwise regular off-grid operating mode.

Gather data on how the system performs. You have 4.8 kWh of storage now (call it 4 kWh effectively useable on the AC side). Nothing like hard data and actual use to know whether something is up to the task.

In regular off-grid operating mode it might be that if you experience poor solar weather conditions for an extended period then you choose to rely on the grid rather than expand your system to cope with multiple consecutive days of low PV output. IOW the off-grid system handles the chosen loads most of the time but on occasions you are using the grid for backup.

Only you can do the calculus on whether spending the $ on extra storage to cover for those less frequent occasions is worthwhile, or just accept that the system will do it's thing 80-90% of the time and the grid handles the rest, or whatever ratio makes sense for you. Local grid tariff regimes vary widely so it's hard to say from here in Australia.

For grid outage backup scenarios then you'll also need to have in mind for how long you want coverage for, and consider why backup might be required, e.g. is this mostly a storm/weather related grid outage scenario (ours are)? How long will they typically last for, or how long do you want to be able to cover? If it's weather related then that has implications for whether your solar PV is going to be sufficiently secured and also whether it will be able to supply all that much energy during inclement weather to recharge batteries. You might need generator as backup, at least to do a battery charge for periods during the daytime (might not be socially acceptable where you are to run a generator at night).

You may also want to consider how much storage capacity to keep in reserve. This will be a bit of a subjective assessment on your part, and it might be an adjustable target. e.g. in storm season you might choose to always have a minimum capacity of X kWh on hand, but during times of the year when outages are less likely you are OK with letting the battery state of charge drop to a lower level. Reserve capacity costs more as much of the time it is not being cycled and so is not helping to reduce grid import costs.

e.g. I will have two storages - the 18 kWh of lead acid I currently have for grid outage backup, and the 10 kWh of LiFePO4 I am adding for regular off-grid cycling for running the regular power and lighting circuits for our two homes (but not the large power draw devices such as ducted aircon, electric oven, induction stovetop etc). I don't expect the home to be able to run off-grid the whole time, so at times energy demand will be supplemented by the grid supply.

The energy required to keep lights, refrigeration and basic power devices, e.g. phone charging, computer, internet connection, maybe a TV, running isn't a difficult sum to do but it would probably help you to do some power monitoring of the more energy hungry of those such as the fridge and any other devices you want to operate during an outage. A plug in power monitor is pretty cheap.

Some older devices might be worth updating if they suck too much power. A while back I sold off an older Sony TV and got a new replacement. The new one uses 100W less power and is way less of an energy drain. Fridges are another - modern units (and inverter driven) are way more energy efficient than older fridges.

I made the assessment I needed to cover in the vicinity of 500-600 W on average, with peaks over 3 kW, and to have coverage for > 12 hours.

By load management I meant will you be actively managing loads so as to ensure you keep them within the capacity of the system to deliver power, as well as shifting loads to times of day when your PV array has excess capacity to burn? There are various ways to do this, from changing habits, to automation and smart devices. e.g. my hot water system uses a smart power diverter which controls the power the water heater consumes such that it avoids importing energy from the grid.

Primarily the point being to think not only about the energy supply side but also the energy demand side. By getting demand under control you may not need to spend nearly as much $ on the supply side.
Wow, I'm moved by your generosity. Thank you so much. This is a treasure trove of information that I will carefully take in and learn from. Thanks for educating me on load management. And the usefulness of a watt meter. Framing my goals in terms of achieving 80-90% success and being OK with grid as a backup feels like an epiphany. Thanks so much again.
 
OK, well all that info gives people here more attuned to the costs of DIY systems in your neck of the woods a bit more meat to chew on when it comes to advice on what sort of system expansion you might consider.



If you are happy with the rest of your system and it's only storage you are considering, then I would be investing in power/energy monitoring if you haven't already and run the system with the devices you want to see how it performs.

Simulate a grid outage. Simulate your otherwise regular off-grid operating mode.

Gather data on how the system performs. You have 4.8 kWh of storage now (call it 4 kWh effectively useable on the AC side). Nothing like hard data and actual use to know whether something is up to the task.

In regular off-grid operating mode it might be that if you experience poor solar weather conditions for an extended period then you choose to rely on the grid rather than expand your system to cope with multiple consecutive days of low PV output. IOW the off-grid system handles the chosen loads most of the time but on occasions you are using the grid for backup.

Only you can do the calculus on whether spending the $ on extra storage to cover for those less frequent occasions is worthwhile, or just accept that the system will do it's thing 80-90% of the time and the grid handles the rest, or whatever ratio makes sense for you. Local grid tariff regimes vary widely so it's hard to say from here in Australia.

For grid outage backup scenarios then you'll also need to have in mind for how long you want coverage for, and consider why backup might be required, e.g. is this mostly a storm/weather related grid outage scenario (ours are)? How long will they typically last for, or how long do you want to be able to cover? If it's weather related then that has implications for whether your solar PV is going to be sufficiently secured and also whether it will be able to supply all that much energy during inclement weather to recharge batteries. You might need generator as backup, at least to do a battery charge for periods during the daytime (might not be socially acceptable where you are to run a generator at night).

You may also want to consider how much storage capacity to keep in reserve. This will be a bit of a subjective assessment on your part, and it might be an adjustable target. e.g. in storm season you might choose to always have a minimum capacity of X kWh on hand, but during times of the year when outages are less likely you are OK with letting the battery state of charge drop to a lower level. Reserve capacity costs more as much of the time it is not being cycled and so is not helping to reduce grid import costs.

e.g. I will have two storages - the 18 kWh of lead acid I currently have for grid outage backup, and the 10 kWh of LiFePO4 I am adding for regular off-grid cycling for running the regular power and lighting circuits for our two homes (but not the large power draw devices such as ducted aircon, electric oven, induction stovetop etc). I don't expect the home to be able to run off-grid the whole time, so at times energy demand will be supplemented by the grid supply.

The energy required to keep lights, refrigeration and basic power devices, e.g. phone charging, computer, internet connection, maybe a TV, running isn't a difficult sum to do but it would probably help you to do some power monitoring of the more energy hungry of those such as the fridge and any other devices you want to operate during an outage. A plug in power monitor is pretty cheap.

Some older devices might be worth updating if they suck too much power. A while back I sold off an older Sony TV and got a new replacement. The new one uses 100W less power and is way less of an energy drain. Fridges are another - modern units (and inverter driven) are way more energy efficient than older fridges.

I made the assessment I needed to cover in the vicinity of 500-600 W on average, with peaks over 3 kW, and to have coverage for > 12 hours.

By load management I meant will you be actively managing loads so as to ensure you keep them within the capacity of the system to deliver power, as well as shifting loads to times of day when your PV array has excess capacity to burn? There are various ways to do this, from changing habits, to automation and smart devices. e.g. my hot water system uses a smart power diverter which controls the power the water heater consumes such that it avoids importing energy from the grid.

Primarily the point being to think not only about the energy supply side but also the energy demand side. By getting demand under control you may not need to spend nearly as much $ on the supply side.
Great read. Thanks
 
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