# Solar Panel Battery Charger Calculator

Hi. this is amy from the alte store. well, winter is finally over, spring is here, the sun is out, and so I’m finally going to be able to show you all of those demos that I’ve been promising you all winter. So what we are going to do is, we are going to answer a lot of those questions that we get all the time. And instead of just telling you, we’re going to actually show you. So we are going to put together a whole list of them, and we’ll do a playlist, and we’ll have it here, so you can actually go and watch all of the.

Common questions being answered. alright? so. the first question that we get all the time is, I bought a great deal on a 20 or 24V solar panel, and I want to use it to charge my 12V battery. What do I need to do it? Well, the answer is you need an MPPT charge controller to reduce the voltage down to the right voltage, about 14V to charge the battery. But also, increase the current on the output. So because watts, or power, equals volts times amps, if you drop the volts down to the right voltage for your battery bank, what’s going.

To happen is if you use just a pwm charge controller, the volts are going to drop, the amps are going to say the same, and so you’re going to lose about half of the power. So what you can get with an MPPT charge controller, is it will drop the volts, but raise the amps. And so you’re going to get more of that power from that solar panel to your battery. So, let’s take a look here. What I’ve got, is I’ve got two identical 100W 24V solar panels. And their short circuit current is about 3A, but their Imp, their maximum power current.

Output is 2.78a. and i did measure them both, and they are both getting right about that amount of output. The Vmp is 36V, so that’s the max power voltage that it will be putting out, and trying to charge a 12V battery. But you’re not going to want to put 36V into a 12V battery. So what’s going to happen is it is going to drop that voltage down to around 13.5 or 14V, depending on how you’ve got your charge controller set. And let’s take a look at what happens. Alright, so first I wanted to show you, this amp meter is actually measuring.

The output between the charge controllers and the battery. and you see i’m at 0.26a. What that is showing you is that when the charge controller is actually up and running and working, it will actually draw a little bit of current. So the PWM really doesn’t take much power at all, the MPPT, when it’s active, is going to be using about 1/4 of an amp. Now that does shut down fairly quickly if it senses there’s no solar coming in, it does shut down pretty quickly. But just keep in mind, that the MPPT charge controller will.

Actually use a little bit more power than the pwm. so, keep that in mind, if you’re considering using it on a very very small solar panel, the increase of output may not be worth it if you are talking only about 1/4 amp. OK? So, let’s actually turn off the MPPT charge controller. We’re going to turn on the input to the PWM, so again, my Imp, my maximum power current is about 2.78A, and I am actually seeing 2.83A output from the charge controller, the PWM charge controller. That’s great. But my voltage, again, has dropped.

From 36v down to 13.2v, because i’m charging that 12v battery. so that’s really cut the output of that panel down dramatically. Now, let’s go the other way. Let’s actually turn on the MPPT charge controller, I’m just turning that on, and you see now that it’s actually dropped down to 0.03A. That is the MPPT charge controller dropped back into standby mode. So you know that it’s not going to be drawing 1/4A all the time, but again, just kind of keep in mind that when it is actively working, actively charging, you are going to see a.

Higher current draw than with the pwm. so, we’ve got the mppt charge controller turned on, I’m going to turn on the input, and we now are seeing 5A coming out. So that voltage is 14.1 volts, it’s actually putting out a little bit higher voltage that the PWM one was. 14.1V and it’s dropping down to 4.6A, it’s kind of going all around, but anywhere between 4.6 and 5A output. That’s a pretty significant difference from the 2.8A we were getting from the PWM. So you can see how having a MPPT charge controller really does increase.

## How to Size Your Solar System

I’m going to talk about how to truly properly size your solar system how much power batteries can really store and the way of solar system really works because it’s pretty amazing how many people that have solar systems don’t truly understand how much power they have and it’s important to know because if you end up with a situation where your modules break your solar panels aren’t functioning.

For a period of time or you have a series of cloudy days it’s good to understand exactly how many kilowatt hours you can actually pull out your battery safely so how many days and how you can ration your power and also that really helps you sighs your system properly without with what appliances you’re using so I’m.

Going to start by explaining how the batteries are set up that first one to say the caps are off the batteries right now because I’m equalizing explain that in another tutorial but hydrogen gas is escaping right now and you don’t want these caps on while that’s happening so right here I have eight Deka LT16 batteries now these are 350 amp hours batteries.

I’m going to explain as part of what i can explain today what that means but before I do that these are set up in two strings parallel together so we have four batteries a group of four batteries that are in series which means that they’re wired from the positive to the negative positive to the negative positive to the negative and then positive to the positive and negative to the negative.

Over to the next string when you wire for batteries in series every time you do that the amperage stays the same but the volts double so each of these batteries is 6 volts so when I put 4 in series i end up with essentially one bigger 24 volt battery the amps are still the same though so what i have here are two 24 volt batteries and then they’re parallel together negative 2 negative and positive to positive and when you parallel.

Two batteries together you double the amprage so i have sized my battery system to make this 24 volts with double the amperage of a single battery so each of these batteries is 350 amp hours ok so first i want to clarify that when your solar panels on the roof the electricity that they generate you’re not using that electricity directly.

You’re never using that electricity directly any time you pull power from solar system you’re always pulling from the batteries the solar panels send electricity to the charge controller and the charge controller reads what the voltage of the battery system is and maybe even a couple other metrics and it determines how it should charge these batteries so if you’re using power during the day the pleading power from these batteries when.

The sun’s out that electricity from the panels is actually directed into the batteries to charge them but it’s not actually but the the power that you’re using your pulling power from the batteries and the solar modules through the charge controller are charging the batteries back up that’s the best way to look at this so that being said you only have as much power to use as what’s stored in the batteries.