Bigger battery ok?

[tab a]

I have a solar charged battery operated electric fence for keeping critters out of various crops. The solar panel was broken by hail and the battery is ancient and needs replaced. It was 6v 9 A.H.

I have several 12v 17A.H SLAs that don't do well powering an electric mower but I thought might be fine for the fence application. My thought was to use an LM317 to get the 6v for the fence, then swap the battery with a fresh(er) one, as needed.

Any problems you see with this idea, or things to watch out for?

Thanks.

 

[audioguru]

The LM317 will waste the same amount of power as the circuit uses.
Therefore the LM317 will get hot and will need a pretty big heatsink.

 

[dknguyen]

You'll also be wasting 50% of the energy from the battery with that setup. It won't last any longer than a 6V 17Ah battery.

EDIT...which audioguru pretty much said already.

 

[Hero999]

You could use a switching regulator to increase the efficiency.

 

[mneary]

A 6V battery of similar cost/size/weight will last twice as long as a 12V battery with a linear regulator. If you don't already have a 12V battery just shop for 6V, although 6V chargers might be harder to find.

First place I would check is all electronics.

 

[rezer]

Although using a 12V battery will cause it to be 50% efficient, you could use your 12V 17AH battery which should last about as long as your 6V 9AH. I'm just thinking economics here. You might as well use what you have if you have no other use for them.

 

[Hero999]

How much current do you require?

Probably not much, I recommend a Black Regulator, execept use a 6.2V zener diode to give just over 6V.

https://www.romanblack.com/smps/smps.htm

 

[Oznog]

You have a lot of worries about quiescent current.

For example, the fence charger might only use a 20mA average current, but a switching power supply might need 50mA to just stay on with no load. Varies a lot, depends on the supply. A cap switching scheme might be 10mA and some random high-power switching device might use 300mA.

 

[Hero999]

According to the author, the input current is about 1.1mA with an output of 0.3mA, where it acts as a linear regulator, but he didn't test the quiescent current, it's probably 0.8mA assuming it's stable with no load.

https://www.romanblack.com/smps/a00.htm