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I use solar battery charging every day for lead-acid car batteries, deep-cycle batteries and lithium iron phosphate batteries .
Running costs are zero once the solar panel kit is paid for and every little bit of renewable energy energy used in the world helps our environment.
One battery takes very little power to charge but multiply that by 2 billion! That’s how many cars are estimated to exist on the planet.
It’s a good idea to understand some of the basics about solar charging before connecting a solar panel directly to a battery.
Let’s assume that most people are interested in charging the average 12 volt car battery – in this post I’ll answer some basic questions relating to charging batteries with solar power.
- Solar Battery Charging Questions:
- What is nominal voltage of a battery?
- Battery depth of discharge chart (lead acid)
- Difference between deep cycle and regular battery
- Solar panel voltage and current output
- Can you charge a 12V battery directly from a solar panel?
- Battery Charging With Solar Panel Direct Connection Vs Solar Charge Controller – Test Results Compared
Can I charge a battery directly from a solar panel?
A solar panel can be connected directly to a 12 volt car battery, but must be monitored if it’s more than 5 watts. Solar panels rated higher than 5 watts must not be connected directly to a battery, but only through a solar charge controller to protect against over-charging.
In my experience theory never quite stands up to actual real-live tests so I’ll hook up a solar panel directly to a partially discharged deep-cycle lead-acid, comparing the voltage and current with a solar charge controller. Jump to test results.
Before that, I’ll go through some theory – it’s good to learn, it clears things up!
Solar Battery Charging Questions:
- Why is battery voltage important?
- What is the battery Depth of Discharge (DoD)?
- What is the open circuit voltage of a solar panel (Voc)?
- How do you find the maximum current of a solar panel?
- What does solar panel Maximum Power Point (MPP) really mean?
- Can you ever connect a solar panel directly to a battery?
- Do I need a solar charge controller?
- What does Maximum Power Point Tracking (MPPT) and Pulse Width Modulation (PWM) mean? Which is better and does it matter?
- How to choose a solar charge controller
- How do you connect a solar charge controller to a battery?
I think of the term ‘nominal‘ as almost the same as ‘designed for‘. When you buy an appliance rated for 12 volts, this is an indication that it’s designed to operate at 12 volts and will draw a certain amount of power.
The same appliance could operate on 11 volts or 13 volts, which is quite in order if the volts doesn’t deviate more than 10 to 12%.
If the voltage is too low, then the speed and power will suffer. If it’s too high, the appliance pulls too much power and could overheat.
Car batteries also have a nominal voltage, which is 12v. Like an appliance, it’s terminal voltage can be below or above 12 volts.
In fact the terminal voltage of a 12 volt lead-acid car battery is a rough indication of it’s state of discharge, also known as Depth of Discharge. The chart below shows this relationship:
Percentage State Of Charge (12 V Lead-Acid Battery)
Battery terminal volts
So you can see that the battery is fully charged at 12.73 volts and 90% charged when the voltage reads 12.62 volts.
Once the voltage drops down to 12 and under, a lead acid battery is pretty much discharged and should be charged up as soon as possible to avoid damage.
Note that the voltage reading should only be measured after the battery has not been used for at least 3 hours, so there is no chemical activity within the cells.
Regular, or auto, lead-acid batteries are primarily designed for one thing; to provide a lot of power to crank a cold car engine until it fires. Current draw is hundreds of amps for a short time.
After the engine has started the alternator quickly recharges the battery and maintains it fully charged.
Deep-cycle batteries are designed to deliver light to medium currents over long periods. Most batteries are rated over 20 hours.
The performance of both types goes down significantly if the current draw is high. Neither type will supply it’s rated capacity. Auto batteries should never be discharged more than 15% and deep-cycle batteries between 50% and 80%.
A deep-cycle lead-acid batteries may last 5 times longer if regularly discharged to only 50%. This means that when you buy a 100Ah deep-cycle battery, you’re only getting 50Ah to use!
Lithium iron phosphate batteries are inherently deep-cycle. They can be discharged up to 95% without damage, but the number of charge-discharge cycles reduces, shortening its life.
This is important because you only need to buy a lithium iron phosphate battery with an Ah capacity of 60Ah to benefit from 57Ah of usable capacity.
Also, if you discharged a LiFeP04 to only 80%, the battery would last half a life-time! For many applications, a lithium phosphate battery is a once-a-life-time purchase.
Solar panel voltage and current output
Solar panels are unusual devices with special operating features. The 4 most important operating parameters are:
- maximum power (Pmax)
- open circuit voltage (Voc)
- short circuit current (Isc)
- maximum power current (Imp)
- maximum power voltage (Vpm)
Solar panel specifications – Windynation solar panel spec. label
What is solar panel Voc?
Open circuit voltage is the value of DC volts measured across solar panel leads. For a typical 36 cell ’12 volt’ panel it’s between 21-22 volts.
What is solar panel Isc?
Short circuit current is measured by connecting the panel leads together through an ammeter. This isn’t dangerous as the circuit voltage falls to zero, so there is no power to cause overheating or damage.
How much current can a solar panel produce?
The label above shows that a current of 5.75 amps flows when the panel voltage is 17.4 volts. If you multiply these together you get the rated power of the panel, which is 100 watts.
These two values are very important and is known as the Maximum Power Point or MPP.
What is the MPP of a solar panel?
The Maximum Power Point is the point at which the voltage and current combine to deliver the maximum power (Pmax) that a solar panel can deliver.
This occurs when the load resistance matches the internal resistance of the solar panel, called the panel’s Characteristic Resistance. A typical value for a 100 watt solar panel may be about 3 ohms.
Can you charge a 12V battery directly from a solar panel?
The internal resistance of a lead-acid battery fluctuates according to its state of discharge, but let’s assume it’s about 1 ohm.
We know that maximum power flows when the load resistance equals the panel’s Characteristic Resistance. In this example the battery may charge, but very inefficiently.
Also, we have no idea what the voltage is across the terminals, which determines how much current is flowing into the battery. It had better not be left alone for long!
Unless the solar panel is very small,like a trickle charger, a solar charge controller should always be used for battery charging. Modern controllers have settings for all types of batteries, lead-acid and lithium.
What kind of solar charge controller do I need?
Remember I talked about the Maximum Power Point of a solar panel? An MPPT charge controller adjusts its internal resistance to match that of the panel, thereby drawing maximum power from the panel safely.
The chart below shows results of a MPPT vs PWM solar charger tests I carried out on a lead-acid deep-cycle battery – MPPT shoed an increase in power of 22%:
Solar Panel Configeration
PWM Power Transferred
MPPT % Gain
2 x 100 Parallel
2 x 100 Series
MPPT solar charge controllers are proven to be about 30% more efficient than PWM, so will charge your battery 30% quicker.
PWM controllers are more primitive and cheaper. It depends if you’re looking for maximum efficiency or if you have plenty of time to charge your batteries.
Check out this page on DIYSolarShack to read a detailed comparison – MPPT vs PWM Test.
Battery Charging With Solar Panel Direct Connection Vs Solar Charge Controller – Test Results Compared
The proof of the pudding is in the doing; I always like to run tests to verify solar theory and sometimes the theory comes down in flames – let’s see:
- I measured the terminal voltage of a partially discharged 90Ah deep-cycle lead-acid marine battery to determine its State of Charge.
- First, I hooked up a 100 watt solar panel directly, and measured the voltage & current.
- Next, I connected the panel to the battery through an MPPT solar charge controller, and measured volts & amps.
The table below compares the battery charging readings in both cases:
Solar Charging 90Ah Varta Deep-Cycle Lead-Acid Battery.
Volts = 12.31V = State Of Charge 65%
Battery Charging Voltage
Battery Charge Current
Estimated Time To Recharge
100 watt solar panel direct connection
100W panel + MPPT
Surprisingly, there isn’t a great deal of difference. The battery was 35% (31Ah) discharged and pulled a healthy current as would be expected.
It’s interesting that the MPPT controller is outperforming the raw solar panel output. This is because the controller is adjusting its internal resistance searching for the Maximum Power Point.
The question is: If the charge currents are similar, why bother with a solar charge controller at all?
The answer is to be found at the end of the charge cycle and the battery is fully charged. I’ll leave it on the MPPT controller and then connect up the solar panel direct again to check voltage and current.
See you in 11 hours time!
When the battery was fully charged by the MPPT controller the volts reading was 13.5V and current was under 0.5A. The voltage would eventually creep up to 12.73V with hardly any current going into the battery.
I disconnected the MPPT and connected the 100W solar panel directly to the battery. Here’s what I found:
- Terminal battery voltage = 15.5V
- Charge current = 2.5A
So even though the battery was fully charged, a directly connected solar panel will still push current into it. If left connected, the battery will overheat and become damaged.
DIY home made battery maintainer – buck-boost converter
If you just want to maintain a battery in full charge condition, there is a way to connect any high-power solar panel to your batteries without danger of over-charging and damaging it internally.
A buck-boost converter accepts DC voltage input and the user can limit the DC output voltage. This buck-booster model (I’m not an affiliate) takes up to 30 volts, so perfect for a single solar panel.
If you set the output volts to a typical float voltage 13.5V the battery will stay in tip-top condition in complete safety.
Wind the output up to 14 volts and the device will increase the charge current if you need it – but remember to monitor it and don’t leave it connected indefinitely.
What size solar panel do I need to charge a 12V battery?
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