How Long To Charge A 12 Volt Deep Cycle Battery With A 100 Watt Solar Panel? (Part 3 of Series)

How Long & Often Should You Charge A 12 Volt Deep Cycle Battery With A 100 Watt Solar Panel?

Well, we have a good estimation guide for you below that should save you a lot of time if you’re trying to figure all this stuff out!

This is Part 3 of our Solar System Estimation Series – you can read Part 1 in ‘How Much Power Does A 100w Solar Panel Produce?‘, and Part 2 in ‘What Can A 100 Watt Solar Panel Power?‘.

Below we will show you how you can estimate what how long to charge, and how often to charge your 12 volt deep cycle battery with one or multiple 100 Watt solar panels in your system.

In Parts 1 and 2 we showed you how to estimate how much power a 100 Watt panel produces, and how to estimate power loads of the items you are using.

Let’s jump into it …


How Long To Charge A 12 Volt Deep Cycle Battery With A 100 Watt Solar Panel?

To figure how long and how often to charge a 12 volt deep cycle battery, you can make the following considerations and calculations:


1. Consider and estimate the capacity of your battery

Ultimately, you may have 1 or several batteries you need to charge.

Several batteries creates a bank of batteries, but to keep this simple, we will estimate the capacity of 1 x 12 volt deep cycle battery.


How to estimate the capacity of your battery?

You’ll need to find out how many amp hours your battery provides, and multiply that number by 12 volts (the voltage of the battery).

To do this you’ll need check the label on the battery, or you can check the battery manufacturer website for the information.

Let’s say you have a deep cycle battery that provides 100 Amp hours – we get 100 Ah x 12 volts = 1200 Watt hours.


2. Cross check battery capacity, with solar panel production, and power usage

This is the last estimation you need to make.

In Parts 1 and 2 of this Solar System Estimation Series, we estimated:

  • that our 100 Watt panel would produce 400 Watts of power per day with 5 hours direct sunlight available
  • That we would run a TV, Laptop & LED Light for 3 hours a day at 113 Watts and hour, which puts our power usage at 339 Watts a day

Now, there are several reason why, but you should look to keep you battery above 50% total charge, which is 600 Watt hours for our battery above.


How to cross check battery capacity, with solar panel production, and power usage?

With your current setup, you are using 339 Watts a day from your 1200 Watt capacity battery, but you are replacing 400 Watts every day – so that setup works as long as you are charging every day.

However, if for example we choose to not charge for 2 days, or it’s cloudy or rainy for 2 days and we get close to zero charge – we subtract 339 x 2 (678) from 1200, and we find that our battery is now at 522 – which is below 50% of 1200 Watt hours battery (600 is 50%).

So, what you need to do is make sure you charge long enough each day, and often enough weekly, to keep your battery above that 50% mark – while also taking into account your power usage.


3. Summary

So, that’s how often and how long to charge a 12 volt deep cycle battery with a 100 Watt Solar Panel…

Estimate the Watt hour capacity of your 12 volt battery

Cross check battery capacity with solar panel power production, and your own power usage

Charge often and long enough to make sure you keep the charge of your battery above 50% of the total capacity


4. What To Do Next?

Read through Parts 1, 2 and 3 of the Solar System Estimation Series to make sure you understand the estimations.


A few things to note and be aware of:

You can get 12, 24 and 48 volt solar systems. These estimates are based on a 12 volt system, because it’s the cheapest and it’s what a lot of people start with. If your battery bank changes to 24 or 48 volt – you’ll need additional panels, and suitably rated solar controllers and converters.

When using solar charge controllers (for DC charging) and/or solar inverters (for AC charging), make sure they are rated for the power and voltage you have running through your system

How your solar panels are wired or connected up i.e. whether they are stacked together, and/or if they are connected to individual or combined controllers, inverters and batteries effects charging setup

Always consult a professional, and the relevant authorities before installing and using a solar system. These guides are for estimation purposes only.

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