Solar to battery ratio

[JRRNeiklot]

I recently purchased two Sams Club Gc2  Duracell golf cart batteries.  I'm charging them with 300 watts of solar that only gets a couple hours of peak sunlight - if that.  They are not enough to run my freezer.   I'm thinking of expanding.  I can get a good deal on two 250 watt panels.   So, one array of 500 watts and another of 300 feeding the same small 215 amp hour battery bank.  Will that damage my batteries?

Thanks.

 

[tx2sturgis]

Nope. 

The solar charge controller is called a 'controller' because it will do exactly that: control the amount of charging.

I assume you will have two separate controllers.

 

[Trebor English]

The cheapest solar charge controller, like the one I have, keeps my battery at 14.4 volts until the sun goes down.  With flooded batteries you can add water to make up what water gets lost to electrolysis.  The corrosion of the positive plate structures will eventually kill the battery in several years.  800 watts is a lot for a 215 amp hour battery.  When you compare charge controllers pay very very very close attention to the description of how the controller decides to turn off charging and go to float mode.  There's at least one that keeps track of how long it took to get the battery to 14.4 volts then holds the absorb mode charge, 14.4 volts, that long.  Very high speed, 800 watts, could distort that algorithm.  There are other algorithms.  With 800 watts you do want a clear understanding of how the controller decides to stop the charging.  Usually failure to hold 14.4 volts long enough causes undercharging, sulfation, and death.  Only 2 hours of charging may not be enough time.  The additional panels might be better if you can put them in a place where the sun is available at a time that is different from the original panels.

 

[jonyjoe303]

you can never have too much solar panels. You want to fast charge your batteries every day at max amps before the sun goes down. 800 watts is very good for your 215 ah battery, it will come in handy now that winter is here and the sun is lower in the horizon. If it goes to float too soon, just raise the float voltage, I use to have the float on my agm battery set to 14.3 volts.

Battery will only get damaged if you have it charging 24/7 at 14.4 volts, with solar the sun goes down before it even gets fully charged most of the time.

 

[tripper]

Let me start by saying, I charge my lead acid at a .4 C rate and it seems to work fine (about 40 amps on a 100 AH battery).  However most battery places say lead acid batteries should be charged slower. According to this website:  Golf Cart Battery Chargers    "A lot of traditional lead acid batteries of 250 amp hours and under (deep cycle marine, etc.), are recommended to be charged at a level of .1C, which is 10% of battery amp hour capacity. Some batteries have this printed right on them. For example, .1C of a 100 amp hour battery is .1 x 100 = 10 amp charge rate." 

 

[Larry-new]

I run two compressor fridges (one's a freezer) and furnace on 300w. / 200 ah.....just fine.
This winter, for solar replacement issues, ended up with 500w. in portable panels with the same two 100ah Renogy gel.
Nice to wake up at 9am and have power at 100%.

 

[Reducto]

Running your batteries all the way down will kill them much faster than charging them too fast. Assuming you have enough battery that you have at least 50% remaining when the next charging cycle starts you should be good.

 

[theBest]

Hmm...3 hours of sunlight x 300 watts = 900 watt hours per day.
This is where you start your calculations for how much energy is getting to the batteries.

Charge Controller charges batteries at 14.6volts.
300 watts / 14.6 = 20.54 amps x 3 hours = 61.5 amp hours to the batteries
This doesn’t account for the 15% conversion loss.
Inverters, if the freezer is AC, also has a conversion loss.
Freezer requires ___(V x I)__ watts to run per hour x 24 hours.
Let’s say the freezer is 120v x 3I = 360 watts per hour x 24.
This mythical freezer requires 8640 watt hours of energy per day.
8640 watt hours / V = Amp hours
Is the V 14.6v or 12v? Answer 12v
The inverter or the appliance will draw 12 volts from the batteries.

How many amp hours are the batteries?
If the batteries are 30 amp hour batteries and they are six volts each in parallel then you have 30 amp hours of maximum capacity, 15 amp hours are usable.

 

[bagabum]

Hmm...3 hours of sunlight x 300 watts = 900 watt hours per day.
This is where you start your calculations for how much energy is getting to the batteries.

Charge Controller charges batteries at 14.6volts.
3
Freezer requires ___(V x I)__ watts to run per hour x 24 hours.
Let’s say the freezer is 120v x 3I = 360 watts per hour x 24.
This mythical freezer requires 8640 watt hours of energy per day.
8640 watt hours / V = Amp hours

Wattage draw to high on freezer, I would assume it's a DC freezer, (55 watts x 12 hrs a day) draw might be correct for AC but I would adjust for 25% duty cycle.

 

[theBest]

Wattage draw to(o) high on freezer, I would assume it's a DC freezer, (55 watts x 12 hrs a day) draw might be correct for AC but I would adjust for 25% duty cycle.

I wrote my post to give the OP a basic idea of where to begin the calculations to size the battery system so they can have a general idea of where improvements can be made.
The “Freezer” I used in the calculations was AC and “Mythical”.
I didn’t account for the duty cycle, my bad. 8640wh/4 (25% duty cycle) = 2160watt hours / 12v = 18ah
Additionally, you can use 20% as a figure for the loss in converting the battery energy into 120volts AC.
Add the 20% to the amp hours because your “Mythical” device requires 18ah.
“Mythical” device — Inverter — 12v battery
______18ah__________20%________22ah

I’m not good at doing “Mythical” math, but I do my best.