12V battery for my solar rainwater pump....

[Justme]

A 12w panel @ 17v will only produce 8w @12v. So even with no system losses you need 4 hours of full peak power per 1 hour of running time. So each day at 15 x 2mins you need 2 hours full ouput. In winter you could be lucky to get 1 hour. Add in the system losses, charging losses, sensor & controller consumption, battery self discharge, cold temp reduction in capacity & you are miles out. Also all your usage is outside of charging times so all the power has to be stored & not used as made which is more efficient.

A 10.5v bat cut off if far far to low for a small load like your 30watts. You should never be taking your bat below 12.2v (off load, but your load is small so I would still use that value) which is about 50% SOC (State Of Charge) or its life time will be seriously reduced.

Then charge max voltage should be nearer 14.2-14.4v. Stopping at 14v increases the time it will take to fully charge the bat.

All values above are for open refillable FLA (Flooded Lead Acid) bats. Other bats have different values.

[EccentricAnomaly]

Martin just beat me to it with about the same conclusion. Here's my version anyway. And before I could post this Justme came along with the same thought again. Just in case you don't want to accept just one opinion. :-)

All sounds rather marginal:

the water pump (30w) ... runs for 2-3 mins ... 15 times a day

So 30 × (2 to 3) × 15 = 900 to 1350 W·minutes, 15 to 22.5 W·h per day (say 1 to 2 amp·hours per day).

The panel is a 12watt (max output 17.55)

I assume that's 12 W output at a maximum power point voltage of 17.55 V. So that would be 12 W × (12 V/17.55 V) so about 8.2 watts if constant current when charging a battery at 12 V. A the battery voltage the actual current will be a little more than at maximum power but 10 W would be a generous estimate of the practical output.

Average daily output of a panel is about an hour's worth in winter and about four hour's worth in summer. So there's probably just about enough, on average, power from the panel most of the spring, summer and autumn. Winter and any runs long of poor days will leave the battery pretty badly discharged though.

For any battery to have much hope of long term survival I think you need at least one and probably two more of those PV panels.

[Justme]

and a proper controller (look for the magic "pwm" - power width modulation) - which is affordable,

Whilst a PWM is the right choice for a low cost system like this a MPPT (Maximum Power Point Tracking) is a "proper" controller IMHO.

As I mentioned your 12w panel will only give 8watts when connected via a normal or PWM controller it would give its full 12watts if you used a proper MPPT controller.

[Justme]

Even though we all use different "ball parks" we have all come to the same conclusion.

You are killing your battery, you will kill the next one no matter what type you buy.

Unless you increase the solar panels or charge the bat by another method.

[AV1]

Brilliant..I have recovered the battery and put that back into the system and will buy into a battery that is the same are the classic cars I have/work on for customers (we all suffer from the same drain and non recovery with batteries esp. with brand new batteries that NEVER see any usage that we then expect to work) and manage the system AND recharge them accordingly, and look into the MPPT controller to get better management.
MANY thanks to the sums etc. I will looking into these, but come up with some actual factual figures to work with over the next few weeks along with a end solution (so that this is system works all the year oround without any of my input).
Adrian@

[AV1]

Lets see how the system fairs now...I replaced the battery with a 110Ah leisure battery, replaced the controller with a MPPT unit and added another 12w panel, and did the sums again ...