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This is what you need to know to work how many solar panels you need to recharge your 100Ah battery:
- What type of battery are you using – lead-acid or lithium-based?
- What is the battery’s state of discharge?
- What kind of solar charger will you use – MPPT or PWM?
- What size solar panels will you use?
A fully discharged 12V 100Ah auto battery will need 3 solar panels each rated at 100 watts to fully recharge in one day, assuming average 4 peak-sun-hours. A 100Ah lead-acid deep-cycle battery discharged to 50% of it’s capacity will need 2 solar panels each rated at 100 watts to fully recharge in one day.
The starting point for any solar panel sizing begins with the load. How much energy is needed to fully recharge a 100Ah battery?
For convenience, I often convert amp-hours (Ah) into watt-hours (Ah) so panel watt-hours can be matched to the battery watt-hours needed to fully charge it.
The conversion is pretty simple:
watt-hours = amp-hours/battery voltage (12V)
Page Contents
- How to test battery capacity with multi-meter
- How low can you discharge a lead acid battery?
- How low can you discharge a deep-cycle battery?
- How Many Solar Panels Does It Take To Charge A 100Ah Car or Truck Battery?
- How many solar panels to charge a 12 volt 100Ah deep-cycle lead acid battery?
- How long will a 100w solar panel take to charge a 100ah battery?
- How long will a 300W solar panel take to charge a 100ah battery?
- How long will a 100w solar panel take to charge a battery?
- How long will a 100 Ah battery last?
- How long will a 100Ah battery run a fridge?
- How many solar panels do I need to charge a 100ah battery in 5 hours?
Video – Setting up a 100W solar panel with a 100Ah battery
How to test battery capacity with multi-meter
You need to know how far your battery is discharged.
For lead acid it doesn’t matter if it’s a straight auto battery for starting cars and trucks, or a deep-cycle battery for leisure use in RVs – the procedure is the same.
An accepted method of estimating how much energy is left in a lead-acid battery is to measure the terminal voltage with a multi-meter.
Readings can be anywhere from over 11 volts to almost 13 volts. Note that a ’12 volt’ battery indicates nominal voltage.
Nominal 12 volts just means the battery is intended to supply devices designed for 12 volts supply for optimum operation i.e. such as the equipment found on cars and boats.
The terminal voltage gives a rough indication of the amount of energy left in the battery. The test must be done when the battery has been idle for several hours.
Table 1 – 12 volt battery discharge chart
Depth Of Charge % (12 V Pb) | Battery volts |
100 | 12.73 |
90 | 12.62 |
80 | 12.50 |
70 | 12.37 |
60 | 12.24 |
50 | 12.10 |
40 | 11.96 |
30 | 11.81 |
20 | 11.66 |
10 | 11.51 |
How low can you discharge a lead acid battery?
This depends on the battery type. There are two broad kinds of lead acid battery – car or truck, and deep-cycle, sometimes called leisure batteries.
Car batteries shouldn’t be discharged more than about 15%. They are designed to deliver hundreds of amps to crank cold car or truck engines.
After the engine has started, the car’s alternator immediately gets to work charging the battery back up. It’s rare for a car battery to sit in any state of discharge.
How low can you discharge a deep-cycle battery?
Deep cycle lead-acid batteries can be discharged up to 80% without damage but only 50% is recommended for regular discharge.
This is because deep discharge affects the internal structure and shortens the battery life.
Leisure batteries are good for delivering medium currents for many hours and not for cranking engines, although a hybrid battery called a marines battery can fulfill both functions.
The different depth of discharge for both batteries is important, as it determines how much energy needs to be put back into the battery when sizing solar panels.
How Many Solar Panels Does It Take To Charge A 100Ah Car or Truck Battery?
I can imagine two scenarios with a 100Ah auto battery – either it’s discharged to 15% as recommended, or it’s been left to totally discharge. Maybe it’s been used to power a home appliance through an inverter.
Auto battery at 15% discharge
Energy needed to fully recharge:
100Ah x 15% = 15Ah
15Ah in watt-hours = 15 x 12 volts = 180Wh
With an irradiance of 4 peak-sun-hours/day (an average value used by solar installers):
100 watts solar will generate 100 x 4 = 400 watt-hours/day
So a 100 watts solar panel will easily recharge this battery in a day. In fact, at a rate of about 5 amps solar output, the battery will be charged back up in 3 hours.
Auto battery 100% discharged
Energy needed:
100Ah x 12 volts = 1200 watt-hours
In this case at least 3 solar panels, each rated at 100 watts would be need to recharge a fully discharged 100Ah battery in one day.
Note: If an auto battery is deeply discharged regularly it will be ruined very quickly.
How many solar panels to charge a 12 volt 100Ah deep-cycle lead acid battery?
Assuming that 50% depth of discharge is respected, then:
Energy required = 50Ah x 12 volts = 600 watt-hours
In this case, 200 watts of solar panels would be adequate for recharging comfortably in one day. (About 5 hours in full sunshine.)