Solar Cat Flap

Thread Starter

Wildgoose99

Joined Jul 31, 2021
6
Hi all,

I have an electronic cat flap that reads the chip in my cat's neck and only opens for her. It takes 4 AA batteries which last for about 6 weeks. I don't like using so many batteries so I've switched to standard rechargeable AA ones. They're a fairly good brand (Eneloop) but only last for about 10 days.

I'm planning on buying a 6V lead acid 5AH battery which I'm hoping will last a lot longer.

Then I had the crazy idea of hooking up a small solar panel I already have to trickle charge the battery and that way I'd never have to charge it again :)
With the panel positioned just above the cat flap it gives 9V open circuit on a dull day going up to 10.5V with the kitchen lights switched on. I did think of putting the panel on the outside of the door but it's quite a dim porch area so no real benefit.

So my question is do I need anything more complicated than a 6V regulator between the solar panel and the battery?

If the whole idea of using indoor solar is mad and doomed to failure then please feel free to say so!


It would look something like this with the battery tucked in below the cat flap. Hopefully with fewer dangling wires!

IMG_4251.JPG
 

dl324

Joined Mar 30, 2015
13,307
Welcome to AAC!
So my question is do I need anything more complicated than a 6V regulator between the solar panel and the battery?
What are the solar panel voltage and current specs?

The nominal voltage of a fully charged 6V SLA battery will be around 6.35V so the charging voltage should be more like 7V. But, it's unlikely that your solar panel would generate enough current to hurt the battery by connecting it directly. I'd add a diode to prevent the battery from discharging through the solar panel when it's dark.
 

Dodgydave

Joined Jun 22, 2012
10,063
Put a Shockley diode in series with the panel and connect it directly to the sla, as long as the solar panel can give out 500mA it should charge the battery.
 

ericgibbs

Joined Jan 29, 2010
13,850
With the panel positioned just above the cat flap it gives 9V open circuit on a dull day going up to 10.5V with the kitchen lights switched on.
hi W99,

Remeasure using a 220R resistor test load.

E
 

Thread Starter

Wildgoose99

Joined Jul 31, 2021
6
Hi Dennis,

Thanks for your reply.

I have no idea what the specs are, there's no writing on it and I don't even remember buying it, it was that long ago. In direct sunlight it reads 19V open circuit. Judging by the size of it I'd guess the current at about 200ma max. In low light I'm sure the current will be way less.
 

Thread Starter

Wildgoose99

Joined Jul 31, 2021
6
Ok I've measured with a 220 ohm resistor and the voltage across it reads 0.2V. With the multimeter in series with it the current reading is 1.4mA. That doesn't sound so good to me.
 

ericgibbs

Joined Jan 29, 2010
13,850
Hi W,
I suspected that, if you have selection of resistor values, keep increasing the value until you read 7v, that will indicate how much current you will get into your 6v battery.

E

Update:

ESP_ 649 Jul. 31 17.32.png
 
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dl324

Joined Mar 30, 2015
13,307
I've measured with a 220 ohm resistor and the voltage across it reads 0.2V. With the multimeter in series with it the current reading is 1.4mA. That doesn't sound so good to me.
Your meter is adding hundreds of ohms in series with the load resistor. You'll get a better measurement if you just use the resistor and measure the voltage drop across it. Then calculate current with Ohm's Law; I = V/R.

It's unlikely that that panel will be able to damage the battery.

I'd just use connect the solar panel to the battery with a diode to prevent the battery from discharging through the solar cells.
 

ronsimpson

Joined Oct 7, 2019
1,548
Datasheet:
Non-rechargeable AA batteries. 1 to 2.5mAH (some down to .75mAh) 1.5V
Rechargeable AA batteries. .5 to 2mAH. 1.2V
I think you should be able to get better rechargeable batteries.

Because the rechargeable batteries start out at 1.2V each the cat door may stop working before all the energy us used up.

rechargeable: If you can only get 1.4mA from the cell then we do not have to worry about over charging the batteries. But at 100mA there will be a over charge problem. With 10 day batteries you can go a week with out Sun. This might be enough.

Non-rechargeable: I have good luck recharging non-rechargeable batteries. (but you can't let them discharge all the way down) With high end batteries I take 10 to 20% of the power out of a battery and then trickle charge it back up and I get 50 cycles. I know you are told not to recharge but...... I think if you can really get 1mA to charge you should get over a year out of your 1.5 month batteries.
 

Thread Starter

Wildgoose99

Joined Jul 31, 2021
6
I think you're right about the rechargeable AA batteries only being 1.2V causing the cat flap to stop working sooner than non-rechargeable.

I stuck a 220K potentiometer across the panel and adjusted it to get a voltage reading of 7V. I then carefully disconnected it and measured the resistance as 130K. So with your ohms law equation does that make it 0.05mA?!
 

ericgibbs

Joined Jan 29, 2010
13,850
hi W,
Sounds about right, much too low a current.
E

Why not use a 6Vdc Wall wart, from the mains, drop the batteries

See update in post#7
 

Thread Starter

Wildgoose99

Joined Jul 31, 2021
6
Yeah doesn't sound like it's a goer :-(

Mains power isn't really an option, there's no mains power point anywhere near the door.

Thanks everyone I've had a fun afternoon! I've learned some things, found my long lost box of resistors and only burned myself once with my soldering iron.:)
 

dl324

Joined Mar 30, 2015
13,307
Yeah doesn't sound like it's a goer
I haven't seen any showstoppers. A 5Ah SLA should last several times longer than the Eneloop batteries (assuming they're around 1500mAh). If the solar panel can't keep the battery charged, worst case is that you need to charge it once a month.

Have you tried connecting the panel to the battery to see what the charging voltage will be? I have some solar chargers to trickle charge batteries in vehicles I don't use very often. I happened to measure the no load voltage of one last week, and it was around 18V.
 

crutschow

Joined Mar 14, 2008
27,939
Edit: There's likely not enough ambient light to charge the battery with the solar cell you have.
It needs to be in direct sunlight.


You could use five 1.2V rechargeable NiMh batteries which would give you 6V when charged, the same as 4 new alkaline batteries.
That should then likely power the door until the batteries are fully discharged.
For 2000 mAh NiMh's, they should last about as long as the alkalines.
 
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Thread Starter

Wildgoose99

Joined Jul 31, 2021
6
> Have you tried connecting the panel to the battery to see what the charging voltage will be?

I've already got the panel but I've not bought a battery yet. I'm probably going to go for one of these

https://cpc.farnell.com/leoch/lp6-5-0/battery-lead-acid-agm-6v-5ah/dp/BT06396

When it arrives I'll have a go with the solar panel but either way it will be an improvement.

If the solar panel can charge the battery in full sunlight I may have a go at putting it on the garage roof and running some cable through the porch to the back door.

Adding an extra 1.2V AA sounds like it would help but I think I prefer the one big battery strategy rather than fiddling around with lots of little ones.
 
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