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Charging 12 volt batteries with solar panels is a great way to use natural energy from the sun. However, there’s a few things we need to know before matching the correct solar panel to a particular battery.
When I first used solar to charge my car battery I was working in the dark, so to speak – I chose quite a large solar panel and just hooked it up. Bad idea!
Before thinking about charging batteries with solar you need to know the following:
- What is the fully charged battery capacity?
- How far is it regularly discharged?
- Where is your location?
- What kind of solar controller will you be using?
An average size 12v 50Ah car battery discharged 20% will take 2 hours to charge using a 100 watt solar panel. A deep-cycle 12v 50Ah battery discharged 50% will take 4 hours to charge with a 100 watt solar panel. Both examples assume a current of 5.75 amps and MPPT controller.
- 12 v car battery sizes – What are the car battery sizes?
- Video – Battery amp-hours explained
- What does amp hours mean on a car battery?
- How to determine car battery capacity
- How are solar panel watts measured?
- How long will a 100W solar panel take to charge a 12 volt battery?
- How many watt solar panel to charge deep cycle battery?
12 v car battery sizes – What are the car battery sizes?
Battery sizes can mean two different things – the electrical capacity or the physical dimensions expressed as types such as the most common designations 24, 24F, 25, 34, 35, 51, 51R, 52, 58, 58R, 59 and 65.
These are used when you need to fit the correct battery into your vehicle’s engine compartment. When taking about solar charging, we refer to the electrical capacity in Ah.
Modern vehicles have an amp-hour capacity of between 40 to 100Ah, while some bigger diesels can go up to 120Ah.
Video – Battery amp-hours explained
What does amp hours mean on a car battery?
Car batteries are quite special. They are constructed to deliver a lot of current in amps for a short period, just long enough to crank an engine until it fires.
For 10 or 20 seconds an auto battery might supply up to 900 amps. However, it can’t sustain this for a long period and will soon discharge.
A 12 volt car battery shouldn’t be discharged more than 15 to 20 %. Once the engine fires, the alternator quickly charges the battery fully and maintains it ready for use.
Theoretically, you would think a 50 amp-hour battery should be able to deliver 5 amps for 10 hours and 50 hours for 1 hour, but this isn’t the case.
As the current increases the capacity of the battery reduces and it will supply much less than the advertised 50Ah.
What’s the difference between an car battery and a deep cycle battery?
Deep cycle batteries are different from car batteries, This type of battery can deliver medium level currents for some hours without harming the battery.
As with auto batteries, the higher the current draw, then the less overall capacity can be drawn in total. Increase in temperature will also reduce the overall capacity.
What are deep cycle batteries used for?
Deep cycle batteries are ideal for applications that need sustained current draw for lighting, TV,etc and also be available for supplying things not used all the time, such as water pumps.
For this reason they are used for marine situations and RVs. RV deep-cycle batteries are also known as ‘leisure’ batteries.
Marine-type deep cycle batteries are a tougher breed and can also crank an engine, so they are a kind of hybrid between the two types.
How to determine car battery capacity
A useful guide to the DOD, or Depth of Discharge, of a 12 volt lead-acid battery can be found by measuring the terminal voltage with a simple multi-meter.
Table showing terminal volts vs percentage capacity of a lead-acid battery
State Of Charge % (12 V Lead-Acid)
Battery terminal voltage
How to measure car battery health
I’ve read on some auto blogs that a healthy battery will have a terminal voltage of around 12.6 volts after standing for a couple of hours. I think this is a bad way to determine your battery health.
Using the table above, you can see how the voltage can be useful to give some indication of state of discharge, but there’s one missing factor – load.
The real test of a good battery is how the terminal voltage reads when the battery is delivering current. If one of the cells is damaged or worn out, the volts will drop when the engine is cranked.
The easiest way to test your battery is to ask at your local garage or service station. They have a tester which clips across the terminals, draws a load and an indicator shows battery state.
How are solar panel watts measured?
‘Solar panel watts’ is quite a tricky subject and there’s a lot of ‘rule-of-thumb’ estimates applied.
Basically, a 100 watt solar panel has an open circuit voltage (Voc) of about 22 volts and a short circuit current (Isc) of around 5.75 amps – a 100 watt panel specification label from Windynation is shown below:
The main takeaway is that this solar panel produces maximum power when Voc = 17.40 volts, at the Maximum Power Point of the panel – more of this later.
How many watts per day does a solar panel produce?
People often talk about how much current a solar panel delivers, but this value is often not very useful. For example, if a cloud pass over panel output will drop significantly, while at other times it may enjoy bright sunshine.
What does irradiance mean and how is irradiance measured?
A sure way of measuring solar panel output in watts is to use an average irradiance value throughout the day, measured in kWh/m2/day or year. This value is also known as peak-sun-hours.
If you multiply a solar panel wattage rating by the this value, the result is watt-hours. An average value of 4 for peak sun hours is often used for a rough guess at panel sizing.
100 watt solar panel output = 100W x 4 peak.-sun-hours = 400 watt-hours (Wh)
How many peak sun hours in your area? How to find peak sun hours
All geographical locations have different levels of sunshine energy, or irradiation. The easiest way to assess the energy available in your city is to use historical data such as found in sites like Global Solar Atlas – see image below:
How long will a 100W solar panel take to charge a 12 volt battery?
Table – 100 watt panel vs battery Ah
12 volt battery size in Ah
Minimum time to fully charge with 100 watt solar (hours)
Car battery discharged 20%
Deep-cycle discharged 50%
Do I need solar charge controller?
You most definitely do need a solar charge controller when using a 100 watt solar panel to charge a 12 volt battery. If not, there is a danger of overcharging and damaging it.
Which charge controller is better MPPT or PWM?
As a general rule it isn’t worth the expense of using an MPPT charge controller in low-power applications, even though they can be 35% more efficient than PWM models.
That said, if you in a location where irradiance is low or subject to sporadic cloud cover, MPPT may be worth it. How do they work?
Simply put, all solar panels have a characteristic resistance, typically about 3 ohms for a 100 watt solar panel. Maximum charging current flows when any connected load has the same resistance i.e. they are matched.
An MPPT charge controller adjust it’s own internal resistance to match the solar panel in order to maximize the current output of the panel and charge the battery faster.
The answer depends on two things:
- What is the deep-cycle battery capacity?
- What is the battery Depth of Discharge (DoD)?
Batteries are normally rated in ampere-hours (Ah), but I find it convenient to convert this to watt-hours (Wh). In this way energy produced by ther solar panels relates directly to the energy required by the battery.
Battery watt-yhours is found by multiplying th Ah by the battery voltage. For example:
Watt-hours for a 100Ah deep-cycle battery = 100 x 12 volts = 1200 watt-hours
Lead acid deep-cycle batteries can be discharged to 80%. However, 50% is recommended in the interests of a long battery life.
In this case, 50% of the total battery capacity needs replacing with solar.
Solar energy required to charge deep cycle battery with 50% DoD = 1200Wh/2 = 600Wh
The energy that a solar watt can produce depends on the irradiance (the sun’s energy) for any given location. It’s measured in kWh/m2/day and is also called ‘peak-sun-hours‘.
This value can be used to estimate solar output – 1 solar watt can generate 4 watt-hours per day.
Solar panel rating needed to charge 600Wh battery energy in one day = 600/4 = 150 watts.
If a 100Ah battery were discharged 80% then 960/4 = 240 watts of solar panels would be needed.
If completely discharge, a 100Ah battery would require 300 watts of solar panels to recharge in one day.
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