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A 300ah battery is an ideal companion for solar panels. No matter how much energy your system produces, it needs a battery to store the energy for later use. A 300ah battery is a good choice as it offers capacity and efficiency. The question though is, how long will it take to charge one of these? And how many solar panels you will need? That is going to be answered in this post.
It takes at least 8 x 100W solar panels to fully charge a 12V 300ah battery in 5 hours. If the battery is only 50% discharged, it will be ready in about 2.5 hours. Lithium deep cycle batteries have a discharge rate of 85-100% and are more efficient.
How Many Solar Panels to Charge a 300ah Battery?
Battery capacity is measured in amp hours (ah) while solar panels use watts (w). To find out how long the battery will take to charge, you have to convert amp hours to watts and find out how many peak sun hours are available in your area.
If you want to charge an empty 12V 300ah battery in 5 hours, you need 8 x 100W solar panels. The formula is: battery amp hours x volts / available sun hours = watts needed per hour
Using our example again:
300ah x 12V = 3600W
3600W / 5 sun hours = 720 watts per hour
Your solar power system must produce at least 720 watts an hour: 720 x 5 = 3600 watts
With 8 x 100W solar panels, your system can generate up to 800 watts an hour. Because solar power is not 100% efficient (more on that later), you should have additional power available.
If you need at least 720 watts as in this example, it is better to have an output of up to 800 watts an hour. You can even bump that up to 900 or 1000 watts to be sure.
If your battery is 24 or 48V, use the same steps as above but replace 12V with 24 or 48V. So if you have a 24V 300ah battery and it is completely empty, you will need 10 hours to charge it with 8 x 100W solar panels. Or you can get 16 x 100W or 8 x 200W to charge it in 5 hours or so.
A good choice is the Renogy 12V 100W solar panels, as it is efficient and optimized for charging batteries. Once set up, the panels will start charging to power the battery.
The two other variables are the sun hours available and battery capacity. The more peak sun hours available the more time you can charge it. And if the battery is only 50% discharged, charging time will be 2.5 hours.
Why are My Solar Panels Not Charging at Full Power?
One of the things you quickly learn about solar panels is you should have more power available than what you currently need. The reason is solar panels cannot produce their rated output continuously.
If you install 300W solar panels on your RV, you might get an average of 285W an hour. This is not a defect but has to do with how solar energy works.
A 100W solar panel for instance, will only generate 100 watts under optimum sunlight conditions. Since the sun changes position in the horizon, this will not be constant. Peak time will be around noon and early afternoon before dropping off.
This is why you have to calculate how many peak sun hours you get, so you can estimate how many watts a solar panel generates.
If you get 5 hours of sunlight a day, a 100W solar panel, in theory, can produce 500 watts a day. But because its output varies, you will probably get 485 watts or so per hour.
This brings us back to charging a 300ah battery. You need 720 watts an hour to charge the battery in five hours. But as shown here, solar production can fluctuate so you should have greater capacity present.
How Long Does a 300ah Battery Take to Charge?
The depth of discharge (DOD) determines how much time it takes to recharge the battery. It also depends on how many amps your solar panels produce. 8 x 100W 12V solar panels can charge a 12V 300ah battery at 50% capacity in about 2.5 hours.
If the battery is 24V, the charge time will be cut in half. You can also use a higher voltage solar panel for charging, a 24V solar panel for a 12v battery for example.
If you have a 300ah 12V AGM battery, the rule of thumb is never let capacity drop below 50%. So that means you recharge the battery when capacity goes down to 150ah.
A 100W 12V solar panel can generate 8.3 amps an hour:
100 / 12 = 8.3
How many 100 watt solar panels would you need to reach 150ah and fill the battery? It depends on how fast you want to charge.
If you want to fill the battery in less 5 hours, you will need 8 x 100W 12V solar panels.
800W / 12V = 66.6
800 watts can charge the battery up to 66.6 amps an hour.
66.6 x 5 = 333.3
In five hours the solar panels can produce 333 amps, more than enough to charge the entire battery. Since you only need 150 amps, charge time will be around two and half hours.
What Charge Controller Size Do I Need?
Aside from solar panels, you will also need a charge controller. This device ensures the battery gets the maximum charge possible from the solar panel without damaging it.
The charge controller size depends on the voltage of the solar panels. Divide the total solar panel watts by its voltage and add at least 20% to the total, and you have the charge controller size.
If you are using 12v solar panels to power a 300ah battery, you will need a 100a charge controller.
100W / 12V = 8.3 amps
If you have 8 x 100W solar panels, that is 66.6 amps. The rule is add 20% or more to the charge controller capacity, so you will need 100 amps.
If you are using 24V solar panels, the total amps drop to 33.3. You can use a 60A charge controller like Renogy 60A MPPT solar controller. MPPT controllers are more efficient than PWM so they are better suited for this purpose.
What is Battery Depth of Discharge?
There are many types of batteries for solar power but the most common are lithium and AGM. AGM and other lead acid batteries have a DOD of 50%. Lithium, AGM and gel are deep cycle batteries. Unlike car batteries, it discharges slowly which is suitable for solar panels.
To calculate battery DOD, multiply its capacity by the discharge percentage. A 300ah battery with a 90% DOD means it has 270 usable amps (300 x .90 = 270). This is the same formula you can use to find out how long a 500ah battery lasts.
A 300ah lead acid battery has a 50% depth of discharge (DOD) rate, so you can only use 150 amp hours (ah).
A 300ah deep cycle lithium battery has a DOD of 85-100%. If the DOD is 100% you can discharge the battery completely.
If the DOD is 85%, 95% etc., use this:
Battery amp hours x DOD = usable amp hours
Convert the percentage into decimals first. If the DOD is 95%, use .95. For example:
A 300ah lithium battery has a 95% DOD. That will look like this:
300 x .95 = 285
It has 285 usable amp hours. Put it another way, you can discharge it until there is 15ah left.
How Many Hours Will a 300ah Battery Run?
The faster a battery is discharged, the fewer amps become available. A 300ah battery for example, will not run a 300 amp load for an hour. It will lose power before the hour is up.
This is due to Peukert’s Law which states that the faster you drain a battery, the less amps remain. The fewer amps discharged, the longer the battery lasts. This sounds as if you are using what power is left, but it is not. The higher the amp load, the faster the actual capacity diminishes.
To avoid confusion, batteries use a 20 hour amp rating to determine capacity. A 200ah battery with a 20 hour amp rate means the battery was fully discharged for 20 hours with a load of 5 amps.
So a 300ah battery discharged over an hour will have z seemingly lower amp hour rating compared to a 300ah battery discharged over 100 hours. You can use this calculator to figure out how much will be left.