Charging small 7Ah battery from 120w panel question

[TruckTastic]

Hi all,
My first post here and pretty new to solar so please be gentle! I probably know just enough to be dangerous!
(I have searched but could not find anything similar - please point me in the right direction if this has been asked before)

I've just completed a solar thermal water system (which works very well) but I would now like to run the whole thing from a solar pv panel and small battery. The solar water system is all 12v, with the following component ratings:
Controller: 2.5 watts (on 24/7)
pump: 14 watts (on pretty much constantly from 0900 to 1830)
3 way valve: 1 watt (once activated, potentially on from 0900 to 1830)
Heat dump fans: 30 watts (potentially could be on from 0900 to 1830)


So, what I want to do is fit a 120 watt solar panel to run everything during the day. I would also like a small 7Ah battery so that when the sun goes down, the battery can continue to power just the solar thermal controller only. (I already have the 7Ah battery, which I would like to re-use).

I know I need a charge controller to condition and stabilise the day time load and to control the charge to the battery, but....how do I limit the charging current from the panel and charge controller to the small 7Ah battery. I am assuming that a charge controller rated to handle the 120 watt panel will allow too high a charge current to the battery if I just connect it directly to the charge controller? I think I would like to limit battery charging to about 500mA.

Am I on the right tracks or could you offer advice on a more suitable way to do it please?

Is there a suitable or recommended charge controller for my application?

If not, what else needs to go between the charge controller and the 7Ah battery to basically just trickle charge and then float charge it?

Many thanks in advance for any information and suggestions.
Steve

[Sunking]

So, what I want to do is fit a 120 watt solar panel to run everything during the day. I would also like a small 7Ah battery so that when the sun goes down, the battery can continue to power just the solar thermal controller only. (I already have the 7Ah battery, which I would like to re-use)

This is silly. First off with a 7 AH the max charge current it can accept is 2 amps. With a 120 watt panel and a cheap 10 amp PWM controller will pump 7 amps into the battery and destroy it in a week or less.

[russ]

Steve - Listen to Sunking - he is being a bit blunt but also very correct. He will be happy to help you get set up correctly I expect - you just are going to catch a bit of BS on the way.

[TruckTastic]

OK - Thanks both.

Maybe I didn't explain myself very well (possible), or maybe I am being completely stupid (probably).

The primary requirement was to power all of my sunny daytime requirements (12v, about 50 watts) from a PV panel.

The 120 watt panel was to selected to have about double the capacity of the expected load during the day (to allow for non-perfect angle of panel, and therefore lower panel output, throughout the day). Virtually all of my load is during the day and there will only be a demand for power when the sun is out. Essentially I would like the daytime load to come straight from the panel via suitable conditioning.
Can I connect my load directly to the solar controller (without a battery in the system at all)?

If yes, then I guess I could power the water controller separately from the mains (2.5 watts) and this will meet most of my system requirements.


The battery idea was me just trying to be clever and add a battery back-up type power source to power just the water controller throughout the night. The 7Ah battery will easily be able to power just the controller (2.5 watts) throughout the night. I do not really have the space and do not want the expense of having to purchase a battery that will cope with the full (potential) 10A charge current from the panel.

I know I need to limit the charge current to the battery and that was what I should have asked in the first place. Are there any charge controllers that allow a full load to be connected directly to the panel whilst also providing a secondary, current limited, output to trickle charge the battery.
If not, could I simply use a smart charger between the output of the panel controller and the battery, and then switch to battery power when the panel output drops?

Thank you once again for taking the time to read my ramblings and for any advice on my best way forward.

Kind regards,
Steve

[Sunking]

Steve anything you take off the grid is going to cost you 10 to 20 times more than buying power from the electric company for the rest of your life. Is that what you want?

[TruckTastic]

OK. I get it, but this wasn't about saving money, although I fully accept that I am tight!

It was about seeing if I could do it - A project to keep me occupied.

I reckon a panel and controller could be got for about £150 (and I have the other bits and bobs already) so it's a cheap project to keep me a bit busy now the water bit is finished.

Sorry to have bothered you all.

[Sunking]

OK. I get it, but this wasn't about saving money, although I fully accept that I am tight!

It was about seeing if I could do it - A project to keep me occupied.

I reckon a panel and controller could be got for about £150 (and I have the other bits and bobs already) so it's a cheap project to keep me a bit busy now the water bit is finished.

Sorry to have bothered you all.

Steve you can do it as an experiment, bu tif you do it with a 120 watt panel at least use 70 AH battery. You will burn a 7 AH battery up.

[green]

There is nothing wrong with having a bit more battery capacity than you will need , but having to small of a battery connected to a larger panel could prove more expensive if you burn up your equipment and need to replace.

[PNjunction]

If you want to experiment with a 120 watt panel and a small battery, one way to do it is with a pure-lead agm battery (NOT the typical lead-calcium ups-style agm's) that can handle that 6 to 7amps with ease:

An Enersys Odyssey PC680 or a PC535 or so would do fine mated to a quality 10amp or so pwm charge controller set to at least 14.4 to 14.7 volts absorb. The Genesis XP or XE line up to about 20ah or so would fine too.

Great fun especially if you have limited solar-insolation time - but be forewarned - since these batteries are not cheap, you really must do your homework figuring out your power requirements over time.