Homemade Charge Controller Idea

[Gillesfizzog]

I have been working on an idea for a charge controller for some time now. I feel that i've nearly finalized the idea. If someone could tell me if they think it will work or not, that would be cool.
I've attached the schematic of of this idea to this thread. Ill run you through it step by step.
Just to clarify, this design is intended to show how a solar panel can charge a battery during the day while supplying power to a load if needed. Furthermore, The battery will be disconnected when it is fully charged, but the solar panel will still be able to supply the load.

In the circuit:
- There are 3 Push Button Switches, (S1, S2 and S3)
- S1 is normally open
- S2 is normally open
- S3 is normally closed
- There are 3 relays, (R1, R2 and R3)
- There is a small battery on the top left which is part of a separate circuit to affect R1 and R3.
- There is a solar Panel (Far left)
- There is a battery (Slightly right from the center)
- There is a Light on the bottom right to be used as a temporary load, (L)
- There is a receptacle on the bottom right for a potential Load
- The way the relay switches are in the picture are the way they are normally, (Coils are not energized in the picture)

Process:
1. S1 and S2 are pressed at the same time (they close), S1 provides a closed circuit with the light than S2 can energize the coil of R1.(which switches the switch in R1).

2. Now that R1 is switched the electricity is able to switch R2( which closes the circuit through the bigger battery) and switching R3 which closes the separate circuit permanently.

3. Now the Battery is being charged. From the research that i have done, as the absorption stage of the battery progresses, the resistance of the battery rises until the current drawn by the battery becomes very low. All my relays are set to energize at 0.5 amps. When the battery is charged it will draw less than 0.5 amps, which is less amps than the relays need to be energized.

4. You might be wondering what the push button S3 is for,, well, S3 is timed to open the circuit to the load every 10 minutes. the reason i want it to do that is because when the battery is charged and drawing less than 0.5 amps, i want the relays to DE-energize and the circuit to bypass the battery charging. (S3 makes sure that the battery is the only load so to DE-energize the relays.) This protects the battery of over-charging.

- Overnight the battery can be used down to 70% of its charge, ready to be charged the next day.

 

[KrisBlueNZ]

First, you should make a number of corrections to the schematic.

Relays are normally named K1, K2, etc or RL1, RL2, etc or RLY1, RLY2, etc. R1, R2 etc are used for resistors.

All components should have references. Batteries are normally BT1, BT2 etc. Diodes are D1, D2 etc. Switches and pushbuttons are SW1, SW2, etc. This may seem like pedantry but it really will make it easier for us.

There are standard circuit symbols for switches and pushbuttons, including normally closed pushbuttons. Also there are standard symbols for relays. You should use them.

You should also make it clear which contacts are normally open, and which are normally closed, on the relays. You've explained that in the text of your post, but it should be marked on the diagram.

The block with "VR" inside it... I guess that's a voltage regulator? Modules like that can be drawn as boxes, but it's good to use the full name, not just "VR". Also generally they are drawn with the input on the left (you have it at the bottom), the output on the right (OK), and 0V connection at the bottom (you've drawn this as a dotted line for some reason).


With all of that out of the way, here are the problems with the design.

The first big problem is that the way you're using the relay coils won't work. A relay coil is resistive (as well as inductive), and it needs voltage across it to cause current to flow in it, to make the relay pull in. You haven't specified anything about the relays, but no matter what type you choose, the circuit won't work.

Second, your battery is connected in series with the coil of K3. A relay with a 12V DC coil that draws 0.5A will have a coil resistance of 24 ohms. If you try to draw 3.5A via the "LOAD" sockets, you won't be able to, because the current is limited by the 24 ohm resistance of K3's coil. In fact, at a load current of only 0.25A, you will already have 6V dropped across K3's coil and only 6V remaining at the load. At 0.5A load, there will be no voltage at the load!

This 24 ohm resistance will also limit the charging current from the solar cell into the battery. Assuming the battery is very discharged, say 10.5V terminal voltage, and the solar cell is generating, say, 13.8V, you will have 3.3V across a 24 ohm resistance, so the maximum charging current will be 0.14 amps. For a 100 Ah battery, that will take over 700 hours (30 days) to charge - actually, much longer, because as the terminal voltage rises, the voltage dropped across the 24 ohm resistance will drop, and so will the charging current.

Also consider that relays do not activate and deactivate at exact voltages or currents. A typical relay needs about 80% of its rated coil voltage (9.6V for a 12V coil) before it's guaranteed to pull in, and the voltage typically has to drop to 20% or less of the rated voltage (2.4V or less) before it's guaranteed to drop out.

Also you should say what type of battery you want to use. With a 100 Ah rating, it's probably going to be a sealed lead-acid battery, right?

And do you really have a solar panel that delivers 12V at 30A?

So in summary, you need a lot more circuitry to make a circuit that does anything useful. I suggest that you confirm the specifications of your solar panel and your battery - give us manufacturers and part numbers so we can get detailed information on them - and we may be able to suggest a way to do what you want.