Circuit to pick a power source

[libhart]

Hi Everyone,

New here, but not new to circuits, but not really very experienced either.

I'm planning to run a 5V ucontroller with a 12V SLA battery and charge the battery with a solar panel. I'll have plenty of current from the panel on a sunny day and plenty of capacity in the battery for nights. What I'm really interested in is this: I'd like the load to use the panel when possible, and any extra from the panel would go into the battery. This seems straight forward except of course that between the panel and the battery will be a charge controller and between the battery and the load will be a regulator. The load is 5V, and most times will be no more than 30-40mA but could go up to 300mA for very short periods. So here's the secnario I'd like to see, values are just picked from a hat so I'm clearer than mud: It's night and the load is running off the battery. The sun starts to come up and the panel starts generating 4V, still running off battery. The sun comes up a bit more and the panel is generating 8V. Now the panel is generating enough to run the load but not to charge the battery. I'd like it to do that, run the load so I can stop using the battery. If it's a sunny day the panel will eventually start charging the battery and running the load. But if it's cloudy, maybe the panel will generate 11V all day. Still enough to run the load but not to charge. I don't want to be stuck using the battery all day when I have totally useable power coming from the panel.

My question is what's the proper way to do this. The panel will be connected to the charger of course, but what's a good way to make it power the load if it can, while allowing the battery to power the load when needed? I have some ideas but none feel very right. Thanks.

 

[erendesh]

Hi Everyone,

New here, but not new to circuits, but not really very experienced either.

I'm planning to run a 5V ucontroller with a 12V SLA battery and charge the battery with a solar panel. I'll have plenty of current from the panel on a sunny day and plenty of capacity in the battery for nights. What I'm really interested in is this: I'd like the load to use the panel when possible, and any extra from the panel would go into the battery. This seems straight forward except of course that between the panel and the battery will be a charge controller and between the battery and the load will be a regulator. The load is 5V, and most times will be no more than 30-40mA but could go up to 300mA for very short periods. So here's the secnario I'd like to see, values are just picked from a hat so I'm clearer than mud: It's night and the load is running off the battery. The sun starts to come up and the panel starts generating 4V, still running off battery. The sun comes up a bit more and the panel is generating 8V. Now the panel is generating enough to run the load but not to charge the battery. I'd like it to do that, run the load so I can stop using the battery. If it's a sunny day the panel will eventually start charging the battery and running the load. But if it's cloudy, maybe the panel will generate 11V all day. Still enough to run the load but not to charge. I don't want to be stuck using the battery all day when I have totally useable power coming from the panel.



My question is what's the proper way to do this. The panel will be connected to the charger of course, but what's a good way to make it power the load if it can, while allowing the battery to power the load when needed? I have some ideas but none feel very right. Thanks.

Simple diode like BAT54C can solve your problem. If you need extra current you migh have to use different Scotty Diode with higher current rating. If you are not aware of this, BAT54C has 2 Scotty Diodes with their cathode connected together which means, out of 2 didoes, wichever has larger voltage at anode will conduct. And thus could solve your problem.
Regarding 300mA current brust, you might want to keep Larger capacitor or Super cap across the output of your 5V regulator.

 

[libhart]

That's great! Thanks so much! I'll start reading.

 

[mneary]

Have you tested your solar cells? Over a fairly wide range, a solar cell's no-load output voltage is fairly high. It's the current that decreases with decreased light. It may not be necessary to "choose" the input, since the voltage might be high enough whenever there is any useful light.

See **broken link removed** for more detail.

 

[libhart]

I don't yet have the panel, but I think I understand your point. You're saying that the voltage might be there with panel on a cloudy day, but it might not have the current to drive the load, so I'd have to use the battery anyway, so there wouldn't be much use picking, yes? That's a good point.

 

[mneary]

From what I've read, that appears to be a possibility. Or it might not have enough to drive the load but it still could reduce the current needed from the battery.