# Thread: 7.5 amps coming in from solar panels VS battery bank size

1. Frequent Poster
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May 2011
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## 7.5 amps coming in from solar panels VS battery bank size

I have over 1200 amp hours in battery bank but since i'm only bringing in 7.5 amps right now i only hooked up 2 6volt 200amp hour AGM batteries to get 12 volts at 200amp hours

I've been talking to other people into solar and they are telling me that your battery banks will be limited to the amount of Amps your panels pull in.

He said if i had all 1200 amp hours hooked up that the battery discharge would be over 35 amps, and bringing in 7.5 amps is a waste of time.

My first question is, is this true? My 2nd question is their a battery discharge calculater that will give you the amount of discharge on batteries depending on the size?

2. BB.
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## Re: 7.5 amps coming in from solar panels VS battery bank size

Let's say your AGM's are old and have 2% discharge per week:

• 1,200 AH * 2% per week / 7 days per week = ~4 AH per day

If you get 4 hours of sun per day (average):

• 4 AH per day / 4 hours of sun = 1 amps of solar array needed to keep bank "floating"

So, from a simple point of view--your array appears to be large enough to keep your bank floating... Although, you should go out once a month and check the battery bank voltage and make sure the AGM state of charge is >75%... If the batteries fall to 75% or less state of charge, you need to get a larger charger (grid, generator, or solar) on them to get them back >90% state of charge--then start the whole cycle over again.

Note that your AGM's are just being stored--and they will last around 3-5 years (maybe 8 years tops?) before they eventually age out/fail to hold a charge (a guess)...

The reason we recommend a 5% minimum rate of charge is that once you discharge your bank below 75% state of charge, a smaller array will take a long time to get back above ~75% state of charge--The longer the bank sits below 75% state of charge, the more they will sulfate (try to get >75% state of charge within ~24 hours maximum or so for longer battery life).

AGMs tend to have lower self discharge than flooded cell batteries, plus they do not need a 5% rate of charge to mix the electrolyte (flooded cells do need mixing, particularly "tall" batteries).

-Bill

3. ## Re: 7.5 amps coming in from solar panels VS battery bank size

7.5 Amps to a 1200 Amp hour battery bank isn't much:

7.5 * 100 / 1200 = 0.625%

It is recommended that battery charging be at a rate of at least 5% of the Amp hours, and maintenance (float) can be less so long as it stays ahead of the self-discharge rate. That last item will vary according to the type of battery and its age. 0.625% won't meet any of these criteria.

I don't know where that "35 Amps" number came from, as that's about 3%. Self-discharge is usually only about 1% per month (as mentioned, depending on battery and condition).

A 1200 Amp hour battery bank would want at least 60 Amps of peak charge current net - in other words, without any loads.

That's a pretty darn big battery bank at any system Voltage. Normally you size the bank to handle loads demands and size the solar to recharge it properly. It would be more accurate to say the size of the battery determines the size of the array, rather than the array size limits the battery capacity.

Always a good read, the battery FAQ's: http://www.windsun.com/Batteries/Battery_FAQ.htm

4. Frequent Poster
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## Re: 7.5 amps coming in from solar panels VS battery bank size

Thanks man, i wasn't sure what he was talking about..
The reason I have such a big battery bank is most deals on 6volts batteries come in groups of 6, the AGMs were 300 for all 6, then i got the T105 for \$85.00 for all six. Sometimes a good deal will dicate what and when I buy, if i bought everything i needed when I need it i'd be broke :(

I started on my solar system back in late Jan beginning of Feb, and just pulled in my first 7 amps this weekend :) outside of my battery bank, total cost so far is less than \$150.00 this includes my \$29 25 amp charge controller :)

5. Noob with a plan
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## Re: 7.5 amps coming in from solar panels VS battery bank size

The 10% rate is the one I see most commonly for good battery health. With only using those two batteries(total 200ah) you would want a 20a charge to be able to go to them. 5% as the minimum would put you at 10a. Your 7.5 is a bit below that at 3.75%

Question. Have those batteries just sat there, or have you done some kind of charging to them since you got them?

6. Frequent Poster
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## Re: 7.5 amps coming in from solar panels VS battery bank size

Re: 7.5 amps coming in from solar panels VS battery bank size
Question to Bill from your info on July 5th 2011 at 11:07 PDT.....
You say............. "AGMs tend to have lower self discharge than flooded cell batteries, plus they do not need a 5% rate of charge to mix the electrolyte (flooded cells do need mixing, particularly "tall" batteries)."

My question is, what is the absolute minimum an AGM requires for a charge rate. thanks.....Simagic

7. BB.
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## Re: 7.5 amps coming in from solar panels VS battery bank size

Technically, if you are not loading the batteries and are only storing them--You just need enough current to keep them >75% state of charge--Or around 12.5-12.6 volts or higher at night (resting voltage measured with accurate volt meter and no charging/discharging current for 3 hours or more--i.e., measured at night with meter).

Check your battery documentation and see if you can find a state of charge vs resting voltage online table somewhere (note that resting voltage is also based on battery bank temperature of ~77F too--cold batteries will have a higher voltage, hot batteries will read lower).

Using the previous calcuation of 2% rate of discharge every week--that came out to ~1 amp at 12 volts--Since "12 volt" solar panels are rated for Imp at ~17.5 volts -- 1 amp would require a minimum of an 18 watt "12 volt panel" to keep your 1,200 AH of 12 volt AGM batteries "floating".

The original calculation was based on an average of 4 hours of sun per day--If your area is particularly cloudy or near the poles--you might average 2 hours of sun per day or less--Requiring at least 36 watts of 12 volt panels.

I would still check the battery bank nighttime resting voltage around once a week to once a month to insure that everything is OK. Although, AGM's are supposed to be good for ~3 months, or more, of storage without recharging.

You need to monitor the batteries because there is always a chance of cell failure over time--And a single shorted cell could discharge your entire battery bank (if all batteries are connected in parallel)...

You could break your bank into 1/2 or 1/4 and put a fraction of solar panel on each bank--or even connect to the banks via diodes to a common panel. The diodes will prevent one fractional bank from discharging any other bank (assuming you do not install a solar charge controller--I do not think one would be required for this application anyway).

If you are cycling (with electrical loads) the batteries, you will need to add more solar panels.

If you have a backup genset, you can always come in once or twice a year and bring the batteries back >90% state of charge if the panels cannot keep up for some reason.

A 36 watt panel with battery bank broken up into 4 sub banks and a 5+ amp diode and fuse/mini-breaker per sub bank in the positive lead--Probably what I would do if I need to store a bunch of AGM batteries. Check resting voltage once a week to once a month is >12.55 volts. And use a genset/grid power to add 10% of rated Amp*Hours (~120 AH) if the bank voltage reads low (10 amp charger for 12 hours, 100 amp charger for 1.2 hours, etc.).

-Bill

8. ## Re: 7.5 amps coming in from solar panels VS battery bank size

i have to chime in again on that 75% soc stated as a crossover point for sulfation as that is a misnomer that if you keep it above 75% soc that a battery will not sulfate because it will sulfate. if you, for instance, keep it at 90% soc it will still sulfate as any deficit charge will result in sulfation if prolonged over a long time. only a 100% soc will prevent sulfation.

as to float charging a battery as little as 1% of the battery capacity can do that, but i often recommend a bit higher like 2% or 3%. agms are more efficient so 1% to 2% is a good range and i'd probably say 2% for possible losses foreseen or not. with fla types then it would be at 2% to 3% for floating.

remember that a battery has to be 100% charged or 100% soc before it can be thought of going to either a prolonged, but diminishing, absorb charge and then to float charge. you should never float charge a battery without it having a full charge given to it first and then it could be maintained by the float charge indefinitely from there on. if you drain your battery, as stated, to still be above 75% and then only bring it up with a float charge you will find yourself with a bad battery fairly soon down the road so forget that 75% soc point that was mentioned as sulfation occurs to any battery that does not get a 100% soc charge and is allowed to stay below that 100% soc for a prolonged period. if my 90% soc example is used then that does limit the amount of sulfation that the battery can get to that 10% undercharged area of the battery, but that is still 10% of the battery to go bad. it is time with less than 100% soc that creates sulfation and not an arbitrary number of 75% that would limit the damages to the battery to 25% from sulfation. of note, many battery manufacturers consider a battery as no good at 80% of capacity.

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