Confused about watts to AH ratio

[Explorenzo]

I've purchased just about everything I need for my solar and it should all be arriving today! I purchased 200w of 12v Renogy solar panels (wired in series) and a nice MPPT controller with adjustable settings. The last piece to the puzzle I need is the battery. I realize I did this kind of backwards and probably should have gotten the battery first but this is where I'm at. 

I see Bob and others recommending 1:1 watts to AH. So with that formula I should get a 200AH 12v AGM battery for my 200w of solar. On the other hand, I see others recommending 2:1, so they say to get about a 100 AH battery for my 200w of solar. I currently have no plans to hook up the battery to my alternator, so this will be purely solar. There is a huge difference between these recommendations so I'm a bit confused.

This is my consumption calculation below

Laptop Charger x 2, 1.5 hrs/day each = 15 amps/day
Maxxair Vent (med-low setting), 8 hrs/day = 6.67 amps/day
Led Lights, 3 hrs/day, = 9 amps/day
Compressor Fridge (Dometic CF-025 or similar), assuming running 50% of day = 30 amps/day

This comes to a total of 60.67 amps/day at the very maximum. I doubt we will be using the LED lights or the laptop chargers this often. I also think I overshot the fridge by a lot.

Can someone point me in the right direction here? I think I'll have enough power no matter which battery I go with but I want to be efficient and keep my battery happy. 

My spreadsheet calculation if it helps

 

[LeeRevell]

Go with the 2:1. More solar is always good, and will allow you to recharge quicker. Also better on lower sunlight days when the panels are not operating at full output.

 

[B and C]

This comes to a total of 60.67 amps/day at the very maximum. I doubt we will be using the LED lights or the laptop chargers this often. I also think I overshot the fridge by a lot.

Can someone point me in the right direction here? I think I'll have enough power no matter which battery I go with but I want to be efficient and keep my battery happy. 

My spreadsheet calculation if it helps

60 amps a day with a 50% battery capacity use has you at a minimum of 120 amp hour battery.  This being said, the bigger the amp hour capacity of the battery bank, the shallower it will be discharged.  Shallower discharges leads to longer battery life.  To me it sounds like 200 amp hour's worth of batteries and 200 watts of solar would be a good match.  Remember it is not always bright sunshine.

Myself, I have 250 amp hours worth of battery and only discharge to about 80% overnight.  I have 200 watts of PV panel permanently mounted with another 200 watts of portable PV panels that I can put out if I park in the shade or it is overcast.  Each 200 watt array is wired in series and then these two are wired in parallel when the portable array is set up.  If more than two days goes by with minimal solar charging, I have a Progressive Dynamics battery charger/converter to use with my generator/shore power or I can drive some to use the alternator for charging.  You cannot have to many charging sources.

Nothing is perfect for solar recharging with 1:1 being good and 2:1 being better.

 

[DannyB1954]

I think rarely will a solar panel output it's maximum. A lot of environmental factors reduce the output. Also depends where you live. The Pacific Northwest will not give as much power as Southern Nevada.

 

[Ticklebellly]

Collyn Rivers is the author of several books on Van and Motorhome electrics and Solar setups in RVs. He typically recommends much more solar, in watts, then battery capacity, in AH. His rule of thumb is that what ever battery capacity you have, the battery should be going into float by late morning. Essentially, start with the battery capacity you think you will need based on what you think you will draw on a typical day, and about twice the solar in watts. It is often easier to add solar rather than more battery after some real world and real use experience. When you put things on charge also comes into the decision. If your battery is on float already, put everything on to charge with confidence that the batteries are still likely to be full towards the end of the day's sunshine.

There does not seem much point in having deep cycle batteries if you plan to only use the top 15 to 20% of the stored charge.

 

[drysailor]

I've purchased just about everything I need for my solar and it should all be arriving today! I purchased 200w of 12v Renogy solar panels (wired in series) and a nice MPPT controller with adjustable settings. The last piece to the puzzle I need is the battery. I realize I did this kind of backwards and probably should have gotten the battery first but this is where I'm at.

Perhaps I misread the first line of your post but, if you have (2) 100 watt / 12 volt panels wired in series, don't you really have 100 watts @ 24volts?

 

[blars]

Perhaps I misread the first line of your post but, if you have (2) 100 watt / 12 volt panels wired in series, don't you really have 100 watts @ 24volts?

Watts are volts times amps. Watt-hours are watts times hours. Amp-hours are amps times hours. (An amp is an instantaneous measure.) A 100 Amp-hour 12v battery holds about 600 usable watt-hours. 200 watts of solar panels will take about 3 hours of noon sun with the panels tilted to replace this, assuming there are no system losses.

Some people call an amp-hour an amp and wonder why this is confusing.

 

[skyl4rk]

Get two or three batteries.

 

[SternWake]

I too am limited to 200 watts of solar.


I however would not go with less capacity than my current 90Ah AGM battery, and I certainly cannot keep my battery happy on only Solar, and I am in the Sunny SW.

Your 60Ah per day estimate is on the high side, but certainly not impossible to consume especially on rainy days where one might decide to be inside on the laptop.

I would recommend you definitely consider hooking to alternator.  If you do not, then get the cheapest group 29/31 marine battery, 100 to 115AH( claimed) from Wal mart, and plan on having to use the warranty at the most inconvenient time to do so.

Many people think the solar controller dropping from absorption to float indicates a fully charged battery.  This is overwhelmingly false, and has led to the premature demise of many thousands of batteries and is doing so at this very moment.  To achieve a happy long lived reliable  battery when cycling them daily, The absorption voltage needs to be held until the specific gravity maxes out, and this is in the 4 hour range on a battery cycled to 50%.

Getting to absorption voltage by late morning is excellent advice, as it takes many hours in absorption to get near 100% fully charged, and the fridge compressor cycling on will pull voltage down below absorption early on in the absorption cycle.

Lesser capacity cycled less deeply versus lesser capacity cycled deeper, can be argued till the cows come home.  There are too many variables for either side to win this argument, yet opinions can be yelled/typed louder than others and often the loudest are by those still trying to convince themselves.

More capacity is a bigger buffer in less sunny weather, but then there is just more capacity that is less than happy being recharged at a less than ideal rate and not being recharged fully.  Lesser capacity approaches the desired charge rate, but less buffer for those high consumption days with lesser sun.

Ultimately it comes down to $/per cycle, and the happiest Lead acid battery is the one that is never discharged at all and kept 100% full the whole time.

Obviously this is impossible.  

Not being able to take advantage of the Alternator, in my opinion, is unwise.  The alternator can get the depleted battery back upto 80% charged quickly, and then the solar at least has a chance to hold the battery at absorption voltage for the time required to get it to 100%.

The 80% to 100% is important, and the more cycles accumulated where 100% is not achieved, the longer 80% to 100% ultimately takes, and on solar alone, the day is simply not long enough.  Without the 100% achieved every so many deep cycles, battery capacity declines much more rapidly.  The morning hour long drive on a sunny day almost ensures 100% is at least possible. 

While many rely on voltage, or their solar controller, to tell them when the battery is full, these are extremely misleading.  The wet/ flooded battery polygraph, is the hydrometer.

Ideally, with a new battery, one would fully charge it, and take and record specific gravity levels on each cell.  Then later after a deeper cycle, after the solar controller goes to float, one would dip the hydrometer again.  Does it read the same as those initial recorded readings?  If not, it is not being fully recharged.  This simple fact cannot be argued, even if it is an unpleasant one.

With a programmable solar controller, consider pushing the absorption voltage upto 14.8v, and holding it for longer.  If this still cannot get the specific gravity back up to the original readings, then consider putting the float voltage upto the mid to high 14s as well.  If this still cannot do it, then obviously the discharge cycles need to be shallower, or other recharging sources, or more solar is/are required.

14.8v is at 77F, in colder temperatures over 15 volts can be set too.  I had one battery which I set to 14.9v then floated at 15.3v for the rest of the afternoon, and this still was barely adequate.

Or one can simply just ignore it and replace the battery when it no longer can power their overnight needs.  this might be 2 months, 6 months or 2 years.  Impossible to say.

 

[wayne49]

Scratch my previous post. Forget you wasted your time reading it.

I am reading up on MPPT controllers now.

I screwed up the Renogy spec, it is 100W @20VDC 5 amps. Commonly called a "12V panel", just to drive people like me crazy.

I will defer to those who have more experience than I have at this time.

 

[akrvbob]

You may be using 40-60 amps a day, but only some of that is coming off it at night. If you draw 20 amps over night the batteries should stay happy--as long as you can recharge them the next day mostly to 100%. If you can't, then get more power coming in. Here's how:

1. Ideally, you'll charge off the alternator and give the battery just that much more power.
2. Also ideally you'll add a third panel, that will give you nearly the ideal ratio in my mind.
3. Tilt the panels.
4. Learn to use power management to it's best advantage to minimize battery use at night. In other words, plan to use everything you can during the day and as little as possible at night. Force the fridge on in the morning and at mid-day so it runs little at night--add insulation to the fridge. Have all your devices fully charged at sundown so they are not charging at night.   Only use LEDS.

 

[Explorenzo]

Thanks everyone for the great responses! I decided to go somewhere in the middle and ended up with a 125AH battery. Hopefully that ends up staying charged most of the time but if our usage goes beyond expected then I will hook it up to the alternator.

 

[highdesertranger]

I would hook it up to the alternator anyway. highdesertranger

 

[Ticklebellly]

I would hook it up to the alternator anyway.  highdesertranger

Seems excellent advice to me.

 

[ArtW]

Yup, do it before you find out the hard way you should have
Even if your solar is optimal, a series of cloudy days could ruin rain on your solar setup

 

[Oneleggedcowboy]

Yup, do it before you find out the hard way you should have
Even if your solar is optimal, a series of cloudy days could ruin rain on your solar setup

Why hasn't anyone mentioned 6 volt batterys?

 

[jimindenver]

Always have a back up if you can.

 

[29chico]

Why hasn't anyone mentioned 6 volt batterys?

Probably the ever popular 6v golf cart batteries have not been mentioned because folks in this thread are discussing 12v systems and 12v battery banks. Since most folks that are into solar are aware that two 6v batteries can be wired in series to deliver 12v the option just never came up.

 

[DannyB1954]

Why hasn't anyone mentioned 6 volt batterys?

One reason I don't favor 6 volt batts is the max charge rate is usually lower than two 12v batteries in parallel. If each battery will take 30 amps, then with 12v system, you can charge at 60 amps. If batteries were in series, 30 amps would be your max charge rate. If you charge via generator, this is a factor.  

I can also use one 12 volt battery while I set the other one on charge. If a cell goes bad in a 6 volt battery, you are now paying to replace 2 batteries, (Having an old 6v and a new 6v in series, is less than optimal). Having a weak and a strong 12v battery in parallel is less harmful. The strong battery will continue to take a charge when the weak one stops. In series, current will only flow as much as the weak battery allows.

 

[SternWake]

A depleted pair of 6v batteries has absolutely no issues accepting 60 to 75 amps, and cell failure of 6v batteries is not nearly as likely as with a 12v marine or starting battery deeply cycled.

Cell failure probability increases exponentially once battery capacity declines to 80% of original capacity, and a 6v battery capacity will decline at a slower rate than 12v marine batteries due to thicker denser plates, all factors being equal.

When A pair of 12v batteries has one battery that shorts a cell, the human must be a close observer to know that something is wrong and quickly remove the failed cell battery from the string.

The time it takes 10.5v battery( failed cell) to damage the still good 12v battery and cause it to fail too, is no more than 10 days.

Will it be obvious to the casual user that a cell in a set of 12v batteries has failed? Well I suspect it took me several days. My other failed 12v battery still read 12.8v, until a small load was applied, then it dropped to 10.8v, then rebounded instantly to 12.8 once load was removed. I had to load each battery independently to figure out which 12v battery had failed.

6v vs 12v battery threads can go on for pages. however, No 12v 2 parallel marine batteries will out cycle a pair 6v gc-2 batteries in series. The 6v GC-2 is easier to recharge fully. The 12v batteries can support higher inverter loads for longer if that is a concern.

Either battery format still requires a minimum of 3.5 hours to reach 100% charged from 80% charged under ideal charge voltages. A hydrometer used regularly, will prove this time and again. When battery capacity starts declining as they age, the required time from 80% to 100% gets longer and longer and if this 'longer' is not applied the capacity then declines at an even faster rate and the 12v will decline faster than the 6v.

http://www.pbase.com/mainecruising/deep_cycle_battery