This site has an excellent article giving information on battery Ah ratings and how it affects discharging and charging however I need to know how to rate a solar charger under a given set of circumstances. I have a lead acid battery powering an electric fence energiser. (I know it would be better with a deep cycle battery).

The problem is this. The battery is a rather large 12V 120 Ah. The energiser takes 160 mA. The solar charger I want to use is a 12W unit delivering a peak O/P of 650 mA. I will use a controller to stop over charging. I am taking the simplistic view that the load is small in comparison with the top up current, Am I right or should I lower the battery Ah or could I get away with a smaller solar panel. What obvious point have I missed.

Any help gratefully recieved

Steven

 

That may not be enough solar panel to keep the battery charged.

Peak power is almost never seen, once a panel has been installed for a few months.(Initial readings over the first week or two can be extremely good/high)
Peak power also only occurs for one or two hours a day(sunny ones).

Figure 8 hours of useable output with a majority of it being about 1/2 power(say 4-5 hours) SO you would get 300mA charge current for 5 hours(good sun). That's only 1.5 Amp hours of power. Meanwhile the circuit has used almost 4 amphours of power in a 24 hour period.

You need a much larger panel and a circuit to energize/de-energize it based on battery voltage under load.

 

For more reliable service, a larger panel or an array with a charger would do the trick.


Will the fence unit run directly off the panel?

I would calculate the drain from the battery versus the recharge time with available sun.
http://www.batteryuniversity.com

Here is good info on charging.

That will give you the info you need to size a charger for the battery.

Remember to figure in the hours of sunlight you get a day.

 

Keep in mind that charging lead-acid batteries is only about 75% efficient. For every 1A you put in, you only get 750mA back out; the rest of the power is dissipated as heat due to the chemical activity.

Automotive-type batteries will quickly fail if used in a deep-cycle application.