# Battery bank sizing question



## cowboy joe (Sep 14, 2003)

A bit confused...think I'm reading too much into this. From what I know (or think I know), sealed lead acid batteries should be charged at a max of 10-15% of rated capacity (C) meaning a 50 amphour (AH) battery should be charged at no more than 5A (10%) to 7.5A (15%) during the bulk charge mode. Please note, there doesn't seem to be a consensus whether is should be 10 or 15%.

Reason I'm asking is I'm trying to size a battery bank. I'm starting small with only a single 45W (12V) panel right now. For discussion sake, let's assume that the panel can supply round numbers 3A in full sunlight...OK, so 3A is very optimistic as my measurements show it closer to 2.25A. But I digress.

Using 3A for the example would imply a 30AH battery at a 10% bulk charge rate or 20 AH capacity at 15%. So, basically, the battery bank should be between 20-30 AH for max charging speed which is fairly important as I live in a cloudy area. I understand that the bank could have a higher capacity but this means it would take longer to charge. Are my assumptions to conservative? 

Is there a better method to size a battery bank based upon panel size ? I've seen systems advertised with 60W panels and 100AH battery banks which seem oversized so I'm a bit confused. Any suggestion / comments sincerely appreciated.


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## wy_white_wolf (Oct 14, 2004)

Not ment to be personal:

1. Key word *"MAX"* charging rate. That has nothing to do with ideal rate. The max rate is set by the manufacturer of the battery not some consensus because of internal battery design and plate thickness.

2. You are trying to calculate from the wrong end of things. You need to work from the daily load end. Charging rate only sets the upper limit for the array size compared to the battery bank. Why supply a bunch of amps if your batteries are already fully charged? 

3. Trying to use array size to determine battery size will only result in an undersized battery bank if the array is properly sized for the daily load.


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## Jim-mi (May 15, 2002)

Once you realize with your own equipment in hand as to the fact that *solar really does work* . . . . .you will start increasing your 'load' and then realize that a 100ah battery is very small.

Think bigger.......

reread w white wolf's post.


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## cowboy joe (Sep 14, 2003)

WWWâ¦No offense taken as I never take the knowledge of others, or my lack thereof, to be a personal attack. Obviously learning as I go so I thank you for your remarks and advice. Youâre absolutely right regarding âmaxâ versus ideal. Boy, talk about being out in left field. Until your comment, for whatever reason, 2+2 equaled 8, 347 as I was considering max charge rate to be ideal.

I probably should have explained that the system is more of an experiment than anythingâ¦a way for me to get me feet wet if you will. Self installing with lots to learn. Rather make the mistakes now than when I go to put in the main system.

The planned load on the system will be very small at first. Most of the requirement is intermittent, low current with the intent to automate both the greenhouse and the aquaponics system. As example, the DC timer I just finished will power a relay to cycle the AC circulating pump for the aquaponics tank ~2 minutes twice a day. The current draw for the timer is ~1mA during idle and 20mA for the few minutes that the relay is energizedâ¦yes, thatâs milliamps. The AC pump will run off the grid until the inverter shows upâ¦I built my own inverter but the idle current is a killer due to a very inefficient design on my part (200W inverter with a 1A idle currentâ¦Fail!). Hereâs a case where it will be more cost / time effective to buy rather than build. This is a work in progress with the eventual intent of switching to a low current DC pump. Even then, the average current will be very small with only short intervals of higher draw. I also plan to intermittently power a few, low power (5W) DC lamps for ambient light (~15 minute a day) and a solenoid attached to a gravity feed from a rain barrel to automatically water the greenhouse once a day. Based upon your comments & guidance, I feel comfortable in adding in the second 18 AH battery which will boost the battery bank to 36 AH. The batteries are freebies so itâs not like it will be an added expense or money misspent. Now Iâm thinking that there will be enough capacity to also power the 12V, 60W lighting system in the kitchen. Hmmmâ¦Jim-Mi is right...this is kinda being a kid in a candy store.

So far, the panels are up on the roof. Iâll need to add lightening suppression if I leave them there but still deciding whether or not to move the panels to a ground mounted system. We get a fair amount of snow, +100â, during an average winter and itâs difficult to remove the snow with them up on the roof. Lessons learned. The final system will be ground mounted. Also learning to pay attention to items such as the electrical separation of the PV panels from the battery bank (and loads) via the isolation provided by the charger controller. This was a chin scratcher which came to light while designing the remote volt / ammeters. Took me more time than I care to mention to figure out why the analog meters mounted on the panel box worked just fine but the really nifty looking LCD remote display that I designed for the main living area was wacked. The problem was finally resolved by isolating the meterâs power supply from the PV system. Having fun putting this all together and learning lots. Guessing my excitement right not is probably nothing compared to when the system is up & running. Thanks again for the patience and guidance with my noob questions.


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## artificer (Feb 26, 2007)

cowboy joe said:


> The planned load on the system will be very small at first. Most of the requirement is intermittent, low current with the intent to automate both the greenhouse and the aquaponics system.


Batteries should not be added to a bank that is already in use. After several months (year?) it causes problems and can shorten the life of the batteries. If your batteries are cheap/free, then its less of a problem. You can add solar panels at any time. The more panels, the faster the charging.




cowboy joe said:


> The AC pump will run off the grid until the inverter shows upâ¦I built my own inverter but the idle current is a killer due to a very inefficient design on my part (200W inverter with a 1A idle currentâ¦Fail!). Hereâs a case where it will be more cost / time effective to buy rather than build.


12watts in standby/idle mode is fairly typical of what an inverter uses, at least for sine wave ones. Even small modified sine wave inverters use 4-6 watts, typically.

Michael


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## wy_white_wolf (Oct 14, 2004)

cowboy joe said:


> .....The planned load on the system will be very small at first. Most of the requirement is intermittent, low current with the intent to automate both the greenhouse and the aquaponics system. As example, the DC timer I just finished will power a relay to cycle the AC circulating pump for the aquaponics tank ~2 minutes twice a day. The current draw for the timer is ~1mA during idle and 20mA for the few minutes that the relay is energizedâ¦yes, thatâs milliamps. The AC pump will run off the grid until the inverter shows upâ¦I built my own inverter but the idle current is a killer due to a very inefficient design on my part (200W inverter with a 1A idle currentâ¦Fail!). Hereâs a case where it will be more cost / time effective to buy rather than build. This is a work in progress with the eventual intent of switching to a low current DC pump. Even then, the average current will be very small with only short intervals of higher draw. I also plan to intermittently power a few, low power (5W) DC lamps for ambient light (~15 minute a day) and a solenoid attached to a gravity feed from a rain barrel to automatically water the greenhouse once a day. Based upon your comments & guidance, I feel comfortable in adding in the second 18 AH battery which will boost the battery bank to 36 AH. The batteries are freebies so itâs not like it will be an added expense or money misspent. Now Iâm thinking that there will be enough capacity to also power the 12V, 60W lighting system in the kitchen. Hmmmâ¦Jim-Mi is right...this is kinda being a kid in a candy store.....


You need to design out the final system and base all the calculations on that. Calculate what the *max daily load* will be in *watt-hours*. Then we can figure the solar system.

Working from the direction you are will either result is a system too small for your needs or spending way more money than needed.

Your right, you do need to get rid of that inverter. The 1 amp idle current amouts to 24AH a day if you do nothing. That is already way over your batteries AH rating. Couple that with that you should not use more than 20% of the batteries capacity in 1 day. So you would need over 180AH battery bank just to cover an idle load when you calculate in system ineffiecencies (sp?). Get the DC pump as it will start out cheaper working with it.


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## cowboy joe (Sep 14, 2003)

Thanks for the input Michael. The batteries are all the same age and were obtained at the same time. The one that was installed has only been in place for a month or two so I'm thinking this is not a big issue but I can see where this would be a problem if a new battery was added to a bank which were a few years old. Thanks for the comment...I'll add that one to my project book for future reference.

interesting...didn't realize that the inverters drew so much idle current. The one I designed uses the transformer primary windings and a pair of power transistors to form an oscillator...very much a brute force method. The output has very little resemblence to a sinewave other than the fact that it is cyclic. Not to mention that the inverter weighs a good 25 lbs and could double as a boat anchor in a pinch. Aside from being much lighter, I was under the impression that the newer, commerical inverters have a much more elegant, efficient design. I have an el cheapo 400W inverter on the way...guess I'll find out soon how the homebrew unit compares. If nothing else, it may due as a spare in a pinch.


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## 12vman (Feb 17, 2004)

You are limited to the amount of panel that you have. Doin' it simple..

45 watts of panel during good conditions.. 3.75 amps

3.75 x 4 = 15 aHr. (4 being peak charge time during a day in hours)

15 aHr. x 4 = 60aHr. (Size of battery)

Keep your loads within the 15aHr. limit in a 24 hr. period for your daily loads and stay in it. With a 60 aHr. battery, draining it with 15 aHr. would be 25% of it's capacity, which is safe.

The small amount of charge before 10 am and after 2 pm usually covers the losses within the battery without all of the fancy mumbo-jumbo.


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## artificer (Feb 26, 2007)

Since you are familiar with electronics, why leave the inverter running? If its driven from the timer, put your relay in the DC input, instead of the AC output. It'll take a bigger relay, and SSR are out, unless you have a DC rated one, but just think of all the power savings. 

Your timer sounds similar to one I purchased for the off-grid chicken coop:

Michael


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## Jim-mi (May 15, 2002)

Since you seem knowledgeable about this electric *stuff* . . .you might well consider getting a Good battery monitor . . .?!?!
Such as a Tri-Metric..........

It will tell you of your In's & Out's to what ever battery 'bank' that you end up with. . . . .and if your "load" is to much for what ever system you have.
It might seem like a Big expense for such a small system . .But . . the info you get from it will tell you what you need to know about your "load" so as to let your 'system' run properly . . longer.

And when you 'move on' to a bigger system--(and it sounds like you will)-- you can move the tri-metric to that system.


As a side bar; I wonder how many batterys are killed by users who had no clue as to the Over usage--under charged bat


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## cowboy joe (Sep 14, 2003)

artificer said:


> Since you are familiar with electronics, why leave the inverter running? If its driven from the timer, put your relay in the DC input, instead of the AC output. It'll take a bigger relay, and SSR are out, unless you have a DC rated one, but just think of all the power savings.
> 
> Your timer sounds similar to one I purchased for the off-grid chicken coop:
> 
> Michael


That's the plan, at least for the pumps. It's a bit more expensive in that individual inverters will be required for the two timers. Then again, there are some very inexpensive inverters on the market. They're not the greatest but they should suit this application just fine. The big picture includes a high quality inverter to go along with the main system later down the line. For now, the relay contacts will handle 2A (rated 220VDC / 210VAC) and so the load should be within that even with a small inverter and the pump which draws ~0.5A. 

The timer isn't nearly as nice as the one shown. No display. The cycle time is set via a bank of DIP switches and the 'on' time with a small potentiometer. Not fancy but meets my needs. Oh yeah, and they only cost me about <$8 each in parts. The schematic is done, the prototypes are built and tested...the PCB artwork needs a few tweaks like changing the spacing of a two components and the addition of mounting holes. 



Jim-mi said:


> Since you seem knowledgeable about this electric *stuff* . . .you might well consider getting a Good battery monitor . . .?!?!
> Such as a Tri-Metric..........


Thought about it but really want more design control...please read that as easy to design myself than have to reverse engineer. I built a monitor using a 200mV LCD meter similiar to this one:

http://www.martelmeters.com/pdf/DPM_125.pdf

I have the meter set up to monitor 
- current from the PV panels to the charger
- current to the batteries / charger output to the loads
- PV output voltage
- battery voltage

It's a manual setup right now meaning I have to toggle switches to view the values. This is a short term solution until I finish the analog to digital converter and the PC interface...the first version is done / working but requires too much current so back to the drawing board. Eventually, the inputs will be monitored real time for alarm conditions (i.e. low battery) and logged for later analysis (usage, etc). Perhaps overkill but it's going to be a long winter. I'm documenting the project as I go along and hope to share the details, if anyone is interested, as each stage is completed.

The real goal is to automate as much as possible. Hence, opting for inexpensive, upfront timers which will later be replaced by control from a small laptop. Trying to make each section as generic / transportable as possible so anyone can implement / use / modify. Figure that there are some very talented folks out there who can what I've done as a starting point, then add their own improvements. 

Right now though, taking the advice offered here and mapping out the system the right way. Thanks again to everyone for their advice & thoughts.


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## artificer (Feb 26, 2007)

cowboy joe said:


> That's the plan, at least for the pumps. It's a bit more expensive in that individual inverters will be required for the two timers.
> ...


Not really. You don't have to have an inverter for each circuit, just a main DC power relay to power the main inverter. Any time a device needing AC is used, the timer used for that device can also activate the inverter. If you're making your own timers, you can add a second relay in parallel to activate the inverters DC in relay. A cheap read relay for each timer output wouldn't add much cost, and saves getting a separate inverter.

... unless I'm totally off the wall, in which case just ignore me like everyone else does... 

Michael


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## cowboy joe (Sep 14, 2003)

artificer said:


> Not really. You don't have to have an inverter for each circuit, just a main DC power relay to power the main inverter. Any time a device needing AC is used, the timer used for that device can also activate the inverter. If you're making your own timers, you can add a second relay in parallel to activate the inverters DC in relay. A cheap read relay for each timer output wouldn't add much cost, and saves getting a separate inverter.
> 
> ... unless I'm totally off the wall, in which case just ignore me like everyone else does...
> 
> Michael


Good thought but I like your first suggestion better...using the relay to turn the inverter off until needed to conserve power. Unfortunately, the timers are very simple and can not be synchronized...oh, wait I think I see what you are talking about...same inverter but paralleling relay contacts from all the timers to turn on the inverter. The output of the inverter then goes to the relays second set of contacts so only the device on the active timer circuit is powered. (does my description make any sense???) Cool! Very interesting idea. Locating all the timers and the inverter in the same location would make wiring fairly easy and any timer could be removed at a later time if no longer needed. Great idea! THANKS!!!


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## 12vman (Feb 17, 2004)

Just remember that some inverters don't like starting with a load connected to them. I've had problems with having even a small load connected and switched the DC to my inverter and it would fail. I had to disconnect the load, fire up the inverter and reconnect.


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## cowboy joe (Sep 14, 2003)

12vman said:


> Just remember that some inverters don't like starting with a load connected to them. I've had problems with having even a small load connected and switched the DC to my inverter and it would fail. I had to disconnect the load, fire up the inverter and reconnect.


Hmmm...didn't know that. Thanks for the heads up.


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## Jim-mi (May 15, 2002)

Your getting way more complicated than necessary.


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