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Small off grid system with battery charger as a backup


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Hi all.


In a few weeks I'm going to put together a small solar powered battery backup system for 1 or 2 PC's.

I bought one of the kits from sustainable.co.za to make the process a bit easier for myself.

This is my planned layout:



I might have omitted a few breakers and horrified you with my artist skills but over looking that, are there other configuration problems?

Now on to my main question.

My panels might not be in the best location (sometimes partially shaded and I can't do anything about that for now), so I'm worried that I might not be able to keep the batteries at the 50% level.

So in this case I want to be able to use a battery charger to top up the batteries.

I was looking at the Omnitech : HT-C-4012, which is a 40 amp charger and hopefully smart enough not to over charge my batteries.

But now I don't want to have to do things manually every time my batteries head towards 50% charge.

Does anyone know of a design where I can automatically start the battery charger when the batteries start getting flat and then turn it off sometime later?

I don't want to leave the battery charger on/connected the whole time because then surely the charge coming from the solar panels is just wasted because the batteries are already full.

Is there an inexpensive way to do this?


If I need to go and flip a switch and turn on the battery charger manually, I should at least have some notification system so I know when to do so.

Is that the way to go?



Thanks in advance for any feedback.

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Hi Avatar33,


No problem with your drawing.

Your 3 x 120W solar panels matches your 30Amp MPPT controller perfectly.


Does your MPPT controller have an extra output for Load?

The smaller 10 and 20Amp units have that and it can come in handy!


I would however reccomend you connect the 600W invertor directly to the battries via a fuse. 75Amp should be fine.

The MPPT load side output can only do 30Amps, so you will overload it with the 600W invertor.


Now, if your MPPT controller have the extra load output relay, you can use that output to switch a 12V relay/contactor.

Now you wire your battery charger AC via this relay, using the normally closed contact (N/C).

The logic works like this now....

So long your battries is ok, the load relay will be energised.

While the relay is energised, no AC power can go to your battery charger.

Only once your battries run low, will the load relay switch off.

With the load relay off, AC power now goes to the battery charger, thus charging your battries.

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Hi Wekit,


Thanks for your response.

The charge controller does indeed have an load output.

You can also select the battery voltage at which load should be cut off.

10.0V, 10.5V, 11.5V. I still need to determine what this means in terms of percentage charge. 


Ok, I never thought about the charge controller not providing enough current for the inverter.

So it makes sense to connect the inverter to the batteries directly, but  doesn't this increase the risk that I

damage the batteries by discharging them completely (if for example there is a mains power failure and I've left a PC on)?


I love your normally closed relay idea :)

I'm also thinking of adding a timer/capacitor so that once battery charging starts, it stays on for a certain amount of time instead of the charger rapidly turning on and off.


Thanks again for the inputs.

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Just check the specs on the 600W invertor.

It should have a low voltage cut-off. So if your battery voltage drops below this level, the invertor should switch itself off.

Personally I would not go below 11.5V on the battery.

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