Low battery voltage trigger for a relay. (Solar battery bank mains charge)

[Handmade Matt]

Hi everyone.
I've just created an off grid solar system for our Mongolian yurt and wood cabin. You can see the project on my blog (address below.) We do have the grid as well, the solar system is to provide an uninterrupted power supply (UPS) if the mains goes down. Additionally I've done it as an exploration into self sufficiency.

It has a 24volt battery bank.

I would like to design a system where a relay can be triggered when my battery bank falls below a specified voltage. The relay will turn a mains charger on so that my battery bank automatically gets charged up again... Ideally switching the relay off again at a high specified target voltage.

I can solder, am familiar with most basic concepts and am a good learner.
Do you think this is feasible? I don't have to "build" the whole thing, I'm happy to buy components that could work together to achieve my goal for example.

I'd love anyones advice and/ or support if you're game.
Thanks everyone for providing this resource.

All the best.
Matt
[MODNOTE]Deleted link[/MODNOTE]

 

[Handmade Matt]

Does anyone have any advice on this? It would be very much appreciated.

[MODNOTE]It was removed by Moderation[/MODNOTE]

 

[Handmade Matt]

[MODNOTE]It was removed by Moderation[/MODNOTE]

I understand that. I was wondering why.

 

[ronv]

If you check under site issues you will find the forum rules. I suspect it is this one:

- Advertising or Spamming

Electro Tech does not allow spamming in any forum, if we find spam it will be removed and a warning or ban will be issued on your account. We also do not allow any form of advertising (or deliberate types of promotion to benefit a third party site) unless it is in the "Buy, Sell and Trade" forum and prior arrangements have been made. Please also note that while we do publish all links to external sites without using "nofollow" tags, this is considered to be a privilege to members and not a right, in severe violations or abuse, this privilege maybe revoked.

So on to your question. Do you really need a circuit to turn it off? Won't the charger do that?

 

[Handmade Matt]

Thanks Ronv.
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Do you really need a circuit to turn it off? Won't the charger do that?

A good start would be a circuit that turns it on. However I would need to turn the charger off at some point or the solar panels would never do any work. The idea is that the charger is just a back up... I could always use such a circuit to fire up a generator as well if needed.

 

[ronv]

Battery switch

Here is one that should work. About 50% discharged and 80% charged.

 

[panic mode]

i would add additional resistor between U4 and R7 so Vgs does not get excessive.

 

[ronv]

Good idea! That will teach me not to just tweek a 12 volt circuit.

 

[Handmade Matt]

Wow, thank you very much.
A few questions if you don't mind:
The first is I've never seen a diode with a squiggly line infront of it, only straight. What is that?
Is "vbat" and "vload" negative battery and negative load?
I don't really understand "Pulse... etc." written by the battery.
Niether do I understand what is happening at "S13 443DV etc."
Nor how it's joined at "Rtot 1k... etc."

Basically, perhaps I'm in over my head already. HaHa

I have had a second thought though... My charge controller cuts power to my loads at 50% power all ready. It cuts both a 24volt and a 12volt supply.
Could this dropping out of power trigger a relay to turn on the charger? That might be more simple if a 240v AC relay can be made to make a circuit when a 12v ro 24v DC power drops out..?

The only problem being that I wouldn't want the relay to break the circuit again when the CC detects the batteries back up to 51% (theoretically) because then the charger's not really done much. It would be best that the charger stays on until the batteries are full in this case (or I manually turn it off.)

The batteries will rarely reach this state but it will happen sometimes in the winter.

 

[panic mode]

squigly line is one of representations for zener diode (which is basic voltage regulator). this element is actually not a zener diode, it is a small IC that integrates few components (voltage reference which is zener, voltage comparator and output transistor). you can tell there is difference by additional terminal used for control.

i suggest to download datasheet and read it:
https://www.electro-tech-online.com/custompdfs/2012/07/TL431A.pdf

V2 is your battery (he just simulated circuit and chose DC source as symbol for battery). vbat is positive terminals, GND is negative.

forget about vload if you are using relay.

PULSE(0 28 ...)
is a command in circuit simulator telling it what signal shape the V2 should produce for testing purposes (this is signal shape that you see in the screenshots). you don't need to worry about that....

TL431 is just checking supply voltage. if over certain limit, it conducts and in this case triggers mosfet. mosfet turns on relay. the downside is that when battery is ok, relay would be on. normally this is not an issue as most relays have variety of contacts (NO/NC) but this is battery application so it would be good to not have relay drawing current all the time.

R1 and R7 are voltage divider to reduce trigger voltage to reasonable range (gate of mosfet should never go to large voltage such as 20V compared to source terminal - which is here connected to vbat). same goes for idea of using AC relay.

 

[Handmade Matt]

I think I am definitely in over my head. HaHa

I have had a second thought though... My charge controller cuts power to my loads at 50% power all ready. It cuts both a 24volt and a 12volt supply.
Could this dropping out of power trigger a relay to turn on the charger? That might be more simple if a 240v AC relay can be made to make a circuit when a 12v ro 24v DC power drops out..?

 

[DJAE]

Have a look at this form another post i was getting help on. It is a work in porgress but is very simple and made up of readily available parts
**broken link removed**

The ouput is taken from the normall closed contact of the relay meaning the relay is activated when the battery is fully charged. This is a better was than trying to activate the relay to start charging.

Hope its some use!
BTW the unnamed diode on the battery ouptput is any capable of handling the required current!

NOTE: Circuit shouldn't be used 'as is' see this post for further development and description of the issues i am having:

https://www.electro-tech-online.com/threads/basic-battery-monitor-charger-help-me.128701/

 

[dougy83]

I think I am definitely in over my head. HaHa

If the circuit above does everything you want, I'm sure it wouldn't be hard to find someone that could knock one up on protoboard/stripboard for you. What sort of charger are you using anyway? I assume it has no regulation function of its own.

I have had a second thought though... My charge controller cuts power to my loads at 50% power all ready. It cuts both a 24volt and a 12volt supply.
Could this dropping out of power trigger a relay to turn on the charger? That might be more simple if a 240v AC relay can be made to make a circuit when a 12v ro 24v DC power drops out..?

Didn't you say that wouldn't work because it turns back off at 51%?

Have a look at this form another post i was getting help on. It is a work in porgress but is very simple and made up of readily available parts
**broken link removed**

The ouput is taken from the normall closed contact of the relay meaning the relay is activated when the battery is fully charged. This is a better was than trying to activate the relay to start charging.

Hope its some use!
BTW the unnamed diode on the battery ouptput is any capable of handling the required current!

I would steer clear of that circuit unless it's been modified to include some hysteresis in the switching threshold voltages. Won't that relay chatter like crazy when it hits the threshold voltage?

 

[DJAE]

I would steer clear of that circuit unless it's been modified to include some hysteresis in the switching threshold voltages. Won't that relay chatter like crazy when it hits the threshold voltage?

OH GOD YES>>>> DO NOT USE THAT CIRCUIT AS IS<<<<<< You must modify it to your requirements. My point was about using the relay in that configuration (normally closed) and activate to off!

Dougy83; Perhaps you could help me with that exact issue; how would you apply hysterisis in that circuit? Sorry, not hijacking this thread, please go here and help me!:

https://www.electro-tech-online.com/threads/basic-battery-monitor-charger-help-me.128701/

 

[dougy83]

OH GOD YES>>>> DO NOT USE THAT CIRCUIT AS IS<<<<<<

You may want to remove the link to the incomplete circuit or add a disclaimer on the post then or just leave the link to the thread showing your progression.

Matt, are you wanting to switch the 240V to the charger or the 24V output of the charger?

 

[DJAE]

You may want to remove the link to the incomplete circuit or add a disclaimer on the post then or just leave the link to the thread showing your progression.

Done! Thanks, sorry.

 

[colin55]

Basically this is the simplest circuit. You will have to adapt it for 24v:

**broken link removed**