For the past three years I've been using car batteries, all cast asides that had reached the point they had trouble starting the car. This was sort of OK until I got a new fence charger this summer, with an average current draw of 120mA to 300mA. At the same time I also got a 20W solar panel and everything was pretty good during the summer (solar panel receiving enough sunlight to run the fence during the day and charge the battery for night. I did notice though that when I got out the animals early in the morning that the fence was blinking red (indication of low battery, output pulse only ever 15 seconds to 60 seconds, instead of once a second). Now its well past the equinox, the sun is low and often behind clouds, the solar panel often doesn't even contribute enough to run the fence during the day, and the car battery was giving up the ghost without any "new" used batteries coming.
I bit the bullet to buy a deep cycle battery (wet- waterable cells). It is the best decision I've made with that electric net fencing and my animals (other than eventually getting permanent electric fencing!). After a week the 105Ah battery was half discharged. I've got it now on the charger and a small 20Ah SLA running the fence till tomorrow morning when I put the big battery back. It could last even longer, but the deeper the battery is discharged the shorter its life. I bought this size of battery after a long comparison of different batteries, costs, weight, lifespan. As long as I charge it at 50% discharge (once a week) then it should last 9 years.
I recommend anyone who has got a 12V fence charger to make sure to get a deep discharge (traction) battery (unless you have a constant stream of free old car batteries). Don't make the mistake of buying a car starter battery for an electric fence.

My suggestion is to buy TWO!!! That way you switch them out once a week (or however long 50% takes for your fencer) so you don't have to rush the charge. Plus you have a backup.

I notice that you are posting from Slovakia.
Here in the U.S., there is a mandatory 'core deposit' when buying a new car battery. This encourages recycling.
It was $7 the last time I bought one.
Since everybody is getting back $7 when they turn in their 'dead' battery, it means that those 'free' batteries we used to get are nearly impossible to find anymore.

Car batteries are designed for high output for a very short period of time.
Deep cycle batteries are designed for lower output, but for prolonged periods of time.
The deep cycle batteries are much better for purposes like electric fences.
In the long run, a good deep cycle battery is cheaper than all of those 'free' automobile batteries.

From what I've been able to figure out, you don't get any core charge back here and for some reason scrap yards apparently can't pay you for batteries... but once or twice a month Hungarians drive through our village in large vans buying old batteries and other scrap metal. I can't remember how much I got for the last completely dead batteries I sold to them, it may have just been 2€ ($3) a piece-- better than nothing, especially as it isn't safe to melt battery lead to make bullets because of some of the additives in it...

I would get a second deep cycle battery, but its currently out of my budget- I got the 27TMX Trojan, as it was the best long term value on the market here, but at 180€... It is made in the USA, so as an American I "feel good" about the purchase, just can't afford a spare!

Big advantage of a second one though that I realize now (next morning)-- a smaller battery charger takes forever to charge the battery, so it may well be a question of-- do I spend a lot on a battery charger that can rapidly charge or just get a second battery? I have a cheap smart charger, but it has a max output current of 3.5 Amps, so this equates to at least 14 hours to deliver 50Ah of energy, and as it isn't 100% effecient, perhaps even longer. ugh!

Andrew Ray wrote:Big advantage of a second one though that I realize now (next morning)-- a smaller battery charger takes forever to charge the battery, so it may well be a question of-- do I spend a lot on a battery charger that can rapidly charge or just get a second battery? I have a cheap smart charger, but it has a max output current of 3.5 Amps, so this equates to at least 14 hours to deliver 50Ah of energy, and as it isn't 100% effecient, perhaps even longer. ugh!

Yup.