# Battery bank size, configuration, and charging

Registered Users Posts: 11 ✭✭
Hello everyone,

I just have a quick question regarding battery bank design and was wondering if someone can clear up my confusion.

My maximum usage for any day will be 1.7 kW hours between two off-grid cabins. Occupancy is on a full-time basis. I would like to have 3 days autonomy due to lots of cloudiness in the winter months.  System is 24 Volts

This is my understanding of sizing a battery bank:

1,700 watts / 50% DOD x 70% load fraction x 3 days autonomy = 7140 watts

7140 watts / 24 volts = 297.5 amp hrs of storage.

I am going to using 6V AGM SunXtender batteries, 4 in series to achieve 24 volts. 3 strings in parallel to achieve the desired autonomy.

1) Is my thinking correct?

2) How do I figure recharging time for this battery bank?

I know this depends on sun hours, wattage of the array, DOD of the battery bank. But I am not sure how to make this calculation.  Does someone have a formula for this calculation.  Or is there another post someone could direct me to.

Patrick

The math equation needs some adjustments (missing parens). The way I write it:
• 1,700 WH per day * 1/0.85 inverter eff * 3 days of storage * 1/0.50 max discharge * 1/24 volts = 500 AH @ 24 volt battery bank
I am not sure what you mean by 70% load fraction... I suggest that you don't plan for more than 65% to 75% as "required base loads" to allow for solar panels not performing well under cloudy weather/etc. (i.e., the hours of sun are long term averages--You can have a day or even a week of dark cloudy skies).

Now, there is charging the battery bank... AGM's have less self discharge and are more efficient at charging--But for normal rules of thumb, suggest 5% rate of charge for weekend/seasonal cabin, and 10%-13% or so for a full time off grid home (so you don't have to manage loads vs sun every day). Note that larger battery banks suggest a larger solar array--2 days of storage usually meets most folks needs--If you go 3 days of storage, you should have a larger solar array. Based on 3 days of storage (50% larger battery bank than 2 days of storage and 50% max discharge):
• 500 AH * 28 volts charging * 1/0.77 rate of charge * 0.05 rate of charge = 909 Watt array minimum
• 500 AH * 28 volts charging * 1/0.77 rate of charge * 0.10 rate of charge = 1,818 Watt array nominal
• 500 AH * 28 volts charging * 1/0.77 rate of charge * 0.13 rate of charge = 2,364 Watt array "cost effective maximum"
Now, if you ran the above based on 2 days of storage:
• 1,700 WH per day * 1/0.85 inverter eff * 2 days of storage * 1/0.50 max discharge * 1/24 volts = 333 AH @ 24 volt battery bank
• 333 AH * 28 volts charging * 1/0.77 rate of charge * 0.10 rate of charge = 1,211 Watt array nominal
And then there is sizing the solar array based on where you live... More sun, you can use a smaller array. Don't know where the system will be located--But taking Los Angeles as an example:

### Los AngelesAverage Solar Insolation figures

Measured in kWh/m2/day onto a solar panel set at a 56° angle from vertical:
(For best year-round performance)
 Jan Feb Mar Apr May Jun 4.50 4.82 6.05 6.78 6.83 6.80 Jul Aug Sep Oct Nov Dec 6.69 6.67 6.40 5.85 5.07 4.41
This location has lots of sun, especially during the winter (relative to many other locations). So, we can use December's 4.41 hours of average sun (break even month--you may need to use a genset during an average month, and certainly during a month with bad weather):
• 1,700 WH * 1/0.77 panel+controller derating * 1/0.90 AGM batt eff * 1/0.85 inverter eff * 1/4.41 hours of sun = 654 Watt array nominal (Dec break even month)
Notice what happens here--If you have an "extra large" battery bank (3 vs 2 days of storage), lots of winter sun, and choose a 10% rate of charge--The battery bank "drives" the battery bank to 1,818 Watt array vs you only needing a 654 Watt array deep into winter.

With a 3 day battery bank and 909 Watt array, you are at 5% rate of charge... OK if you have daytime charging and night time loads (all of the day time charging goes to battery bank--Flooded cell really work nicely with 10% rage of charge and some day time loads).

Also--When you are at 50% state of charge with a 3 day bank and 5% rate of charge--It is going to take you quite a bit of time to recharge... Basically:
• 500 AH * 28 volts charging * 0.50 max discharge = 7,000 Watt*Hours to "recharge"
• 909 Watt array * 0.77 panel+controller derating * 4.41 hours of sun (Dec) = 3,087 WH of charging per day (Dec)
• 7,000 WH charging * 1/3,087 WH per day (Dec) = 2.27 Days of "bulk" charging
• Plus add 4-6 hours of "absorb" charging time (lead acid batteries taper off charging current as they near 100% State of Charge)
So--With a 5% rate of charge and a 3 day battery bank, you are looking at pretty much 3 days of average December sun and NO LOADS to recharge the battery bank.

In general, I would suggest a 2 days bank and a larger solar array (10%+) for a full time off grid system... Put the battery savings into the solar array (batteries are expensive, solar arrays are pretty cheap these days). And add a Honda eu2000i genset to help during cloudy weather--Roughly:
• 1,600 Watts * 0.25 small loads * 9 hours per 1.1 gallons of gas = 3,272 WH per 1 gallon of gasoline (gensets are less fuel efficient at light loads--More or less worst case planning here).
Or, your genset will supply 1-2 days of cabin usage/battery recharging per ~1 gallon of gasoline during bad weather.

Note that AGM batteries are not cheap:
https://www.solar-electric.com/concorde-sunxtender-pvx-5340t.html
12 cells (AGM ~534 AH @ 2 volt) * \$308 each = \$3,696 + shipping for Sun Xtender AGM batteries (plus 4-6 weeks lead time) (24 volt @ 534 AH)

https://www.solar-electric.com/concorde-sunxtender-pvx-2240t.html
8 batteries (AGM ~224 AH @ 6 volts) * \$318 Each = @2,554 + shipping (24 volt @ 448 AH)

I am not a big fan of paralleling batteries (why I suggested using 2 volt @ 534 AH cells in the first example)... You can save approximately 33% on batteries if you go with a ~333 AH battery bank--And use that "extra money" to beef up the solar array and/or add a genset (you should have a backup genset anyway).

If this is your first off grid system--You may want to try for a 2 day system + flooded cell golf cart batteries (6 volt @ ~200 AH)--8x of them would be something like \$800 or so (~\$100 each). Most people "murder" their first battery bank or two as they work out the bugs and learn how to manage the off grid system. Hurts a lot more when you pay \$2-\$4k for your battery bank.

Anyway--Some suggestions based on my guesses. Any questions/comments?

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Registered Users Posts: 11 ✭✭
Hello Bill,

Mucho thanks for your detailed answer and all the info.  That was great.

I do not think I have any questions right now.  If so I will do another post.  Although I am new to the forum, I have done a lot of reading and such on solar so I completely understand the math and logic behind your answer.  Obviously, I still have a lot to learn due to the fact this is my first off grid system.

Thanks again,

Patrick
• Solar Expert Posts: 5,183 ✭✭✭✭
For the dead of winter I , based on my local conditions , need to increase my PV input to ~ 1600w to 2000ww, as at best I only get between 3 and 4 hours of sunlight hitting the panels due to mountains and trees.  A new location for the array  is in the works for next summer....

Tell us something about the 'sky view you have and what is the immediate terrain like?

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
• Registered Users Posts: 11 ✭✭
My cabin sits in a saddle of sorts on top of a ridge southwest of Denver, Colorado at about 10,000 feet elevation.  Fortunately, the highest terrain around me is mostly northeast and northwest.  My property is thickly forested and in order to get the best southern exposure I have had to remove some really big trees.  The added benefit to this is fire mitigation, which truly needed to be done anyway.

Mid winter I loose sun on the array by 2:30 pm.  Despite this, because of my elevation I still get 4 to 5 sun hours/day.  I can easily extend this at least another 2 to 3 hours by installing a second array in a different location, facing west to catch late-day sun, and have a split system.

I am waiting on delivery of 6 Kyocera 270 watt panels to replace my present very wimpy array that I have limped along with for years.  That small array, which is 12 volt,  will then be re-purposed for some sort of emergency back up.  The new system will be 24 volt.

Come spring, I will have a good idea of how the new system functions, what my "real time" energy usage is and whether or not I need/want to expand.  Provided I don't "murder" parts of my system in the process. ;0))

Patrick
• Solar Expert Posts: 5,183 ✭✭✭✭
the amount of time you get sun, not just the total watts, is quite important, especially if you have tasks that will be done in the late PM....  there also is the completion of absorb and getting into Float to consider, a 2 array  setup can facilitate the achievement of that... and there is also the fact that when you get into Absorb, you CC starts to throttle back and there is unused energy losses

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
• Registered Users Posts: 11 ✭✭
I guess I should also mention that since I have had a wimpy array for years, and my place is off-grid, I do have a couple of generators that I use regularly.  The first one is a Honda 5500 watt with 120V/220V connections, which is about 12 years old and going strong.  I use this almost exclusively for running power tools and break it out for big projects.  It is a heavy beast and difficult to move. Not to mention noisy to operate.  The second generator is a 2000 watt inverter generator from Northern Tool and Equipment, only 120 Volt connections, super efficient on gas,light weight,  and can easily be picked up with moved one handed.  I use this for smaller projects and had the intention of using this one as the back up gen because of gas efficiency and quiet operation.
• Solar Expert Posts: 5,183 ✭✭✭✭
do you have a dedicated charger  yet or will you go for an Inver/charger?

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
• Registered Users Posts: 11 ✭✭
Inverter charger it is.  I felt this was the way to go so I could use the gen in case of emergency.  I will also still have my wimpy set up is case of total failure or loss for some reason.
• Solar Expert Posts: 5,183 ✭✭✭✭
Yes, for me '' Redundancy Rocks!''  Without it in an OG setup, we are well up a 6 hr creek for any parts and no paddle...

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
• Solar Expert Posts: 6,006 ✭✭✭✭✭
edited October 2016 #11
pkeith My maximum usage for any day will be 1.7 kW hours between two off-grid cabins. Occupancy is on a full-time basis. I would like to have 3 days autonomy due to lots of cloudiness in the winter months.  System is 24 Volt.
For me, it's as simple as 3 days autonomy, x 1700 watts/hours per day = 5100 watts/hours of storage.

Edit; so you would need 10,200 Kwh of lead acid, and about 6500 watts of Lithium.

Not sure what else you are calculating in. Be sure you understand that this is draw from the battery bank, not watt/hours the inverter will provide, as the inverter is only 85-90% efficient.

Use your wimpy12 volt panels in series to boast the input, if you can face them in a slightly different direction to increase the input over the day. You can put them on a cheap PWM charge controller.
Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Magnum MS4024, Prosine 1800(now backup) and Exeltech 1100(former backup...lol), 660 ah 24v Forklift battery(now 10 years old). Off grid for 20 years (if I include 8 months on a bicycle).
- Assorted other systems, pieces and to many panels in the closet to not do more projects.
• Solar Expert Posts: 9,583 ✭✭✭✭✭
AGM batteries in parallel banks is a poor choice.   Any teeny tiny difference of resistance in the parallel legs will cause bank imbalance and likely early death of the entire bank,
Flooded batteries in parallel are not as bad as AGM
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Solar Expert Posts: 9,583 ✭✭✭✭✭
Please double check the specs for your inverter charger, if it's a 240V output unit, it may require a 240V genest or step-up transformer to work with a small 120V genset
Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
|| Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
|| VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

solar: http://tinyurl.com/LMR-Solar
gen: http://tinyurl.com/LMR-Lister ,

• Registered Users Posts: 11 ✭✭
Well, please correct me if I am wrong here.  I am certainly not an expert.  But, here was my thought process:

I chose AGMs because of the less maintenance because I am gone from my place sometimes for several months.
I chose to use smaller batteries, 6V SunXtender 224 aH @ 24 hr rate, due to the weight in case I had to move them.

I wanted a 24 volt system because some of my runs are long.  I will have less resistance at a higher voltage.
If I place 4 of these batteries in series that gives me 24 volts @ 224 aH.
The next thought was to have another string of batteries in series, connect the two strings in parallel to give me 448 aH @ 24V.

This was my understanding on how to build a battery bank.  If it is not such a great idea to place them in parallel, why is it that so many solar reference manuals say to do this?  Plus, if I use the same type of battery cords, at the same length, will that not address the issue of a difference in resistance?

Or, is there a better way to build a battery bank that I am not aware of??
Suggestions???
Patrick
• Solar Expert Posts: 5,183 ✭✭✭✭

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
• Solar Expert Posts: 5,183 ✭✭✭✭
and this link may be one of the best, lots of info; http://www.smartgauge.co.uk/batt_con.html

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
• Registered Users Posts: 11 ✭✭
Okay, mucho, mucho, mucho thanks for that info.
The science behind proper battery balancing makes perfect sense to me.  I read the articles carefully and nothing was really over my head.

The next obvious question is how can I properly balance a large battery bank.  Since I am going to have 12 batteries total, 3 strings of 4 (at least that was my plan) , in order to achieve 3 days autonomy.

I do know a master electrician that works in the solar industry.  Maybe a question for him?

Again, thank you very much for your help
Patrick
• Registered Users Posts: 11 ✭✭
Hello again,

I found the discussion below on the forum and just did a copy and paste.

Although I am using 6 V batteries to build a 24 Volt system, would this type of configuration yield a better balance since I am going to have a 12 battery bank?

Cheers,
Patrick

Posted on September 9 by Bryan:

Hey guys, I have a vacation home off grid and I am building a new battery bank.  The system will be 48v. My inverter will be 48v 3000 watts.  I got a good hook up on 12 volt AGM batteries they are 140ah at 20hr rate. I bought 24 of them.  I will make 6 strings of 4 batteries each. I know its overkill but I could not pass up the deal. We only go a few weekends a month so most of the time the bank will be at idle.
Here are my questions:

1. I have a buss bar on each end. Cable length from each string in 18". I will need to buy 12 cables for 6 strings. What guage cable do I need for this?

2. I would like to run breakers. Should I run a breaker on each string? Thats 6 breakers.

3. What size breakers do I need for each string?

4. Coming off the bus bars combing all the batteries what guage wire should I run to the inverter? These two cables will be 32" each.

5. I have a midnight solar 200. How critical is it for the output cables to be the same length?  Would 18" pos and 50" neg be a bad set up?

6 In the middle of each string I will run a 1" x 1/4" copper flat bar about 4" long. so the battery have a 1" space between them. It gets up to 100deg here. Is this copper bar sufficient?

Thanks
Bryan
• Solar Expert Posts: 1,147 ✭✭✭✭
edited October 2016 #19
Bryan,

My preference is to always use fewer, larger batteries. I sell a lot of single string battery banks. But I also have many, many off grid customers with three and four strings of PVX-2240T's or PVX-3050T's in parallel. Some of those systems are 10-12 years old.

The subject of parallel battery setups can become like a food fight in a cafeteria! There are those who swear that it can't work well and those who have been watching it work for years. Will I argue that a parallel bank of Concorde AGM's that died at 10 years, would have made it 15 years if it was a single string? Ummmm, no.

I WILL say that not all battery brands are equal - especially with AGM's, and that may be part of the differing experiences.

There are many other factors that are far more critical to achieve long battery bank life with your AGM's. Things like battery bank temperature, charging voltage temperature compensation, average daily DOD, protection against prolonged discharge below 20% SOC (for Sun Xtender) and the big one: ensuring that the battery bank is being fully charged. Most people do not understand what this means.

Well over 90% of early battery bank failures that I see, are the result of undercharging in one form or another.

Marc

I always have more questions than answers. That's the nature of life.