Cheap 9 volt solar battery charger?

[travelover]

I'm sure I'll regret this before it is over but here goes...


I have a sprinkler controller in a remote location that is powered off 6 AA batteries. It is kind of a pain to replace them and it seems like this could be operated by a small solar panel and battery. Experimentally, the latching type sprinkler valves will run off a 9 volt battery.


Hunter makes a solar adapter for $100+, but given how cheap solar panels are, I'm wondering is I can just re-purpose something mass produced to provide the necessary 9 volts for a lot less.


Any suggestions?





https://www.hunterindustries.com/sites/default/files/BR_SolarPanel_dom.pdf

 

[Sunset]

You will need to run the device off a battery, and the solar panel will charge the battery.
To run directly off the solar panel would require a huge solar panel.

Here is how to do it: 9V Solar Battery Charger

 

[ncbill]

How long do the batteries last?

Long enough to just use rechargeable AA batteries and swap them out manually?

 

[travelover]

You will need to run the device off a battery, and the solar panel will charge the battery.
To run directly off the solar panel would require a huge solar panel.

Here is how to do it: 9V Solar Battery Charger

Right - I know I need a battery. I guess the key is whether I can get by without a controller. If it was 12 volts, it would be a piece of cake, as controllers and panels are cheap and readily available. This seems do-able with Dollar Store panels and a rechargeable NiMh 9 volt battery.

How long do the batteries last?

Long enough to just use rechargeable AA batteries and swap them out manually?

I thought of this, but the goal is to not have to service it.

 

[samclem]

If you need 9-ish volts, could you just get a bunch of cheap solar yard lights and wire them in series/parallel to get the voltage/storage capacity (cloudy days, etc) \ you need? Disconnect the LEDs from them, output wires from their battery packs to the sprinkler controller. Take advantage of whatever onboard voltage regulation scheme they already have to keep the cells from overcharging. You'd definitely need to test this out to assure you aren't putting out too much voltage during periods of strong illumination.

 

[ERD50]

Right - I know I need a battery. I guess the key is whether I can get by without a controller. If it was 12 volts, it would be a piece of cake, as controllers and panels are cheap and readily available. This seems do-able with Dollar Store panels and a rechargeable NiMh 9 volt battery. ...

Why not just 6 AA rechargeables (like Eneloop - Nimh)? That's about 9 V.

Since those cheap solar lights recharge a single cell, 6 of them should recharge 6 cells. I wouldn't go through the work of pulling the panels out of their little holders like in that link, but I guess it depends in how aesthetic this needs to be. I'd just hook each solar cell and the battery it came with in series, and connect the ends to the sprinkler controller (remove the LED and associated circuitry from each solar garden light - you can probably get by just removing the LED).

edit/cross-post: Or, just take samclem's advice!

-ERD50

 

[RobbieB]

You could go with a 12 volt system and just make a series diode assembly to drop 3 volts off the 12. Should be 4 to 6 diodes rated about an amp or 2. Cheap ebay parts.

 

[travelover]

You could go with a 12 volt system and just make a series diode assembly to drop 3 volts off the 12. Should be 4 to 6 diodes rated about an amp or 2. Cheap ebay parts.

I like this idea, as it seems a little more robust that the tiny solar lights which seem to die after a year or so. I forgot about using diodes to drop voltage as opposed to resistors, whose voltage drop would depend on the load.

 

[ERD50]

I like this idea, as it seems a little more robust that the tiny solar lights which seem to die after a year or so. I forgot about using diodes to drop voltage as opposed to resistors, whose voltage drop would depend on the load.

I like it too!

You could also use a 9V regulator, but diodes are simple, cheap.

-ERD50

 

[travelover]

I like it too!

You could also use a 9V regulator, but diodes are simple, cheap.

-ERD50

I'm not sure how much of a surge current there is when the sprinkler valve kicks - it might be too much for a small DC-DC inverter, which I thought of initially. Example Would a regulator be more robust to a current spike ?

 

[ERD50]

I'm not sure how much of a surge current there is when the sprinkler valve kicks - it might be too much for a small DC-DC inverter, which I thought of initially. Example Would a regulator be more robust to a current spike ?

The batteries will provide current for the turn on spike. Your panels /supply just provide a trickle current to keep the batteries charged. But one of those little switching converters wouldn't be a bad idea anyhow. I bought a couple similar ones that I've used for a few projects (including 12V solar panels to drive a 5V fountain pump - worked well), they are kind of handy to have on hand. The ones I bought were a little fancier ~ $3, twice the price!), slow-boat from China prices:

https://www.ebay.com/itm/LM2596S-DC...-LED-Driver-/253706299488?hash=item3b1212e860

These have adjustable volt out, adjustable current limit (can be set to a constant current source), and a harder to explain current 'foldback' - I think it lowers the current once a voltage is reached - used to charge, then trickle a battery after it has reached full voltage. But it is only a step-down.

The current adjust will help make sure you don't overcharge a battery. At this price, any hobbyist should have a couple on hand.

edit - to be clear, you want it wired:

[12 V SOLAR PANEL] > [DC-DC 9V out] > [9V rechargeable battery] > [sprinkler controller]

-ERD50

 

[samclem]

The ones I bought were a little fancier ~ $3, twice the price!), slow-boat from China prices:

https://www.ebay.com/itm/LM2596S-DC...-LED-Driver-/253706299488?hash=item3b1212e860
. . . .
The current adjust will help make sure you don't overcharge a battery. At this price, any hobbyist should have a couple on hand.

Thanks, that's a great tip. They will be very handy to have around.

edit - to be clear, you want it wired:

[12 V SOLAR PANEL] > [DC-DC 9V out] > [9V rechargeable battery] > [sprinkler controller]

The converter handles up to 3A, if that's enough to run the valve solenoids/drives then I suppose the converter could be between the battery (which could then be 12V) and the sprinkler controller.

And just to be clear, we're not talking about using one of those pricey, tiny capacity 9v rechargeable batteries, right? We'd build up a robust 9v or 12V battery from 1.2v AA cells.

Having a battery pack with a lot of capacity will be useful in cutting down the chances of an inconvenient loss of clock and timer settings due to long periods of cloudy days.

 

[travelover]

Thanks, that's a great tip. They will be very handy to have around.

The converter handles up to 3A, if that's enough to run the valve solenoids/drives then I suppose the converter could be between the battery (which could then be 12V) and the sprinkler controller.

And just to be clear, we're not talking about using one of those pricey, tiny capacity 9v rechargeable batteries, right? We'd build up a robust 9v or 12V battery from 1.2v AA cells.

Having a battery pack with a lot of capacity will be useful in cutting down the chances of an inconvenient loss of clock and timer settings due to long periods of cloudy days.

I was thinking of about a 5 watt solar panel powering a 12 volt gel battery like this which would power the sprinkler through a DC-DC buck inverter, a regulator or a couple of dropping diodes. So, the current rush to operate the sprinkler valve solenoid would go through the voltage dropping device.

 

[ERD50]

...
The converter handles up to 3A, if that's enough to run the valve solenoids/drives then I suppose the converter could be between the battery (which could then be 12V) and the sprinkler controller.

And just to be clear, we're not talking about using one of those pricey, tiny capacity 9v rechargeable batteries, right? We'd build up a robust 9v or 12V battery from 1.2v AA cells.

Having a battery pack with a lot of capacity will be useful in cutting down the chances of an inconvenient loss of clock and timer settings due to long periods of cloudy days.

You are right - with a 3 A regulator, a 12V battery could go before the regulator, and that would actually probably be better, some 'droop' would be compensated by the regulator (assuming the solenoid surge is < 3A as you say, but that seems safe). I was just going along with him saying it came 9V battery ready.

I was thinking of about a 5 watt solar panel powering a 12 volt gel battery like this which would power the sprinkler through a DC-DC buck inverter, a regulator or a couple of dropping diodes. So, the current rush to operate the sprinkler valve solenoid would go through the voltage dropping device.

You need to make sure the battery doesn't drain below a certain voltage, ~ 10.2 for lead-acid, might be different for a gel-cell, but I think it's still a concern. When the voltage drops that low, a lead-acid battery gets permanently damaged.

OK, so now the complexity is growing a bit, but in the RC world, they sell small, cheap battery management modules that will disconnect the battery when it gets too low. Those might be hard-set for lithium batteries, not sure. I guess they have similar modules for lead-acid in the solar powered world.

edit - her's the first source I found:

http://www.kendrickastro.com/lvc.html

The truth is that any lead acid battery, be it a Gel Cell, AGM or flooded batteries such as DCBs, should be cut-off at 11.6 volts. Not doing so increases the risk of damage to the battery and if taken down deep enough into the batteries charge, will ruin the battery. Having said that, the absolute lowest level a lead acid battery can be discharged to, UNDER LOAD, is 10.8 volts but this is not recommended (more on this below).

-ERD50

 

[samclem]

I was thinking of about a 5 watt solar panel powering a 12 volt gel battery like this which would power the sprinkler through a DC-DC buck inverter, a regulator or a couple of dropping diodes. So, the current rush to operate the sprinkler valve solenoid would go through the voltage dropping device.

That's quite a battery (86 watt hours), this is a bigger "format" than I'd envisioned. I don't think those batteries like cold temps, would you bring it inside when temps are forecast to be low, or bury it deep enough to avoid any problems? I would have thought 8 AA NiMH cells (approx 18 watt hours, and about 1/2 the price of the lead acid battery) on a solar trickle charge would be plenty for this, given that 6 AA alkalines (approx 14 watt hours total) are getting the job done now for (I assume) weeks/months at a time with no recharging.

 

[samclem]

At the risk of being a wet blanket: would it be simpler, cheaper, and more reliable to just put in a big battery pack of NiMH cells and let it run for the 9 months you need sprinklers? Take it in at the end of the season and recharge the batteries in a conventional battery charger. Each AA NiMH cell has approx the same WH capacity as an AA alkaline (though at 1.2V instead of 1.5V). If six alkalines are now lasting for 3 months, get 18 rechargeables, wire them (in battery holders) series/parallel to give the voltage needed, and forget about the solar charging, voltage controllers, etc. Lots fewer things to go wrong, connections to get wet/corrode, panels to get cracked/knocked over, etc. 18 NiMH AA batteries (1.2V, 2000 MAH each) would cost about $27.

 

[travelover]

..............You need to make sure the battery doesn't drain below a certain voltage, ~ 10.2 for lead-acid, might be different for a gel-cell, but I think it's still a concern. When the voltage drops that low, a lead-acid battery gets permanently damaged.
...........

I actually have a solar panel controller that I bought for another project that has this function. They are designed for street lighting in developing countries

That's quite a battery (86 watt hours), this is a bigger "format" than I'd envisioned. I don't think those batteries like cold temps, would you bring it inside when temps are forecast to be low, or bury it deep enough to avoid any problems? I would have thought 8 AA NiMH cells (approx 18 watt hours, and about 1/2 the price of the lead acid battery) on a solar trickle charge would be plenty for this, given that 6 AA alkalines (approx 14 watt hours total) are getting the job done now for (I assume) weeks/months at a time with no recharging.

My drifting off to a small lead acid battery was driven by a desire to be able to use a real solar controller. I'm not sure if charging NiMh batteries with no controller would be reliable over the long term, whereas there are all kinds of 12 volt solar components available. Re freezing, we rarely get below 32F here (greater Portlandia) and unless the battery is dead, even lower temperatures shouldn't hurt it.

At the risk of being a wet blanket: would it be simpler, cheaper, and more reliable to just put in a big battery pack of NiMH cells and let it run for the 9 months you need sprinklers? Take it in at the end of the season and recharge the batteries in a conventional battery charger. Each AA NiMH cell has approx the same WH capacity as an AA alkaline (though at 1.2V instead of 1.5V). If six alkalines are now lasting for 3 months, get 18 rechargeables, wire them (in battery holders) series/parallel to give the voltage needed, and forget about the solar charging, voltage controllers, etc. Lots fewer things to go wrong, connections to get wet/corrode, panels to get cracked/knocked over, etc. 18 NiMH AA batteries (1.2V, 2000 MAH each) would cost about $27.

That is certainly a viable option. Ironically, what kicked this whole thing off was the battery holder of 6 alkaline batteries was causing the sprinkler control to work intermittently. I finally figured out that all the connectors at the ends of each battery had just enough corrosion to make the output voltage jump all over the place. The battery pack lives in a box, but it is very moist here.




All that said, I appreciate the great suggestions and now I need to mull them over and decide which way makes sense. Thanks again.

 

[travelover]

I finally finished this project and thought I'd share my solution. I ended up buying a 20 watt solar panel, a small solar controller, a DC-DC converter and using an old lawn tractor battery. The sprinkler controller operates on 9 volts DC so the converter was to drop down the 12 volt battery voltage to 9 volts. More money than I wanted to spend, but a fairly robust solution.



Solar Panel


Controller


DC-DC Converter