Solar charge controller supplemented by mains/grid power?

[martinv]

Hi everyone.  I searched the web for a few hours but didn't find anything quite like what I'm looking for.  Maybe someone here has an idea if such a unit already exists.

Essentially I have a bank of 12V SLA batteries that I want to charge with solar panels.  There will be equipment powered by the batteries all the time.  The solar panels will not be able to provide enough energy to run the equipment so I want to "top up" the deficit with energy drawn from the mains/grid.

Think of it as a slow transition away from the grid.  As I add more and more solar panels, the amount I draw from the grid will keep dropping.  Eventually (perhaps) I'll reach the point where I can disconnect from the grid altogether.  Note that I'm not interested in an inverter based solution - I'd like something simple but I cannot find anything that does this. !?!

[ghurd]

How about a modified version of the ghurd controller?
G-

[martinv]

Thank you for the reply.  That is indeed an interesting idea/circuit.  When the battery gets low the mains kicks in and charges it up to a pre-determined voltage.   I can see where this would be useful.  Hmm.  I can see an issue or two however for my particular situation though.  Ideally I don't want to discharge the batteries; they are there more for use in power cuts rather that discharge on a daily (nightly) basis.  Secondly I'm grappling with the issues of running a mains charger and a solar charge controller on the same battery bank at the same time.  Each charger will interfere with the other to some extent - the more charge modes the more potential issues.  Ideally the solar power and mains power needs to combine into a single battery charger.  One that (additionally) still works well when there is a load (varying no doubt) on the battery bank it's charging.  I am still hopeful I can find such a device, but may have to in the end resort to building something custom-made.

Regards,

-Martin

[ghurd]

I do not see the problems you see, especially in a backup system.

"Ideally the solar power and mains power needs to combine into a single battery charger... hopeful I can find such a device"
You can.  They are large... and expensive.

Or can do some surgery on the (existing?) UPS to allow the solar to assist in charging the battery.

There are a couple of contradictions in what you are doing or looking for.
If the solar can't keep up, then it needs grid.
If there is equipment powered by the battery all the time, then the battery will be discharging at night.
If the battery is more for power cuts, then the battery should be maintained on float all the time.

[martinv]

Let me explain.

During the day I expect electricity from both solar PV and the mains to charge the batteries/power the load.  As much as possible from the PV and the remainder coming from the mains.

During the night I expect electricity from the mains to charge the batteries/power the load.

In the event of a power cut, whether during the day or certainly at night time, the batteries will discharge during the outage into the load.

The batteries will be maintained on float at a constant voltage whenever possible.

I do not see how the modified ghurd controller would achieve this - if I understand correctly it would turn the mains battery charger on when the battery/load voltage drops to a certain point, then turn it off again once the voltage has raised to a certain level.  That is correct, or have I got that wrong?

Regards,

-Martin

[dnix71]

A regular automobile battery charger is all you need for this. But you have to buy the dumb kind. The fancy ones with microprocessors will find fault with the battery if the voltage floats up and down.

Buy a charger that can handle the full appliance load and let it charge. Put a blocking diode between the solar panels and the charger so when the panel voltage is higher, they charge the battery bank. Otherwise the mains charger does the work.

http://www.harborfreight.com/10-2-55-amp-6-12-volt-battery-charger-engine-starter-66783.html

If your load is less than 120-140 watts, this Harbor Freight charger will do it.

[martinv]

Hi dnix71,

I see a couple of issues with this, but the simplicity is quite appealing.  Problems I see are

1) The dumb kind of charger, which I agree you need in order to avoid the issues with fancy multi stage chargers, tend to be linear supplies and hence rather inefficient.  I'm hoping to reduce my grid power consumption, and a linear supply might have the unfortunate effect of increasing it or at least requiring a few PV panels before I'm back to current grid usage.

2) If I just use the PVs with diodes and no solar charge controller then in the (unlikely, but possible) case where my load is disconnected the batteries will very likely over charge.  Charge efficiency also wouldn't be great compared to a MPPT charger.

For 1) maybe I can find a big cheap switch-mode PSU that I can set to the float voltage and then use the supplies current limit to limit the charge rate of the batteries+load.  Hmm.  A switch-mode lab supply where I can set (and not accidentally bump) the voltage (and limit the current to what I want too) would be a possibility, though certainly pricier than the fixed switch-mode supply.

For 2) could I use a simple solar charge controller, perhaps one that lets me set a higher voltage than the battery charger, so that if the load is disconnected the voltage at least doesn't go way high, maybe just 0.2V too high or something?  Though it would have to be enough to make sure the PVs never get disconnected under normal use.

[ghurd]

I still do not understand what the problem is.
Can do exactly what you are asking for with 2 ghurd controllers, or a solar controller and ghurd controller, or...?
Lots of people do it.

No reason to keep it exactly at float voltage 24/7.
Just want to keep it over maybe 12.8V?

There are switcher battery chargers.  I use some 'battery tender' units which seem to be constant current switchers.

Do not trust the cheap semi-smart chargers, ie: HF.
The #42292 ran one of my SLAs up >15V even though it states "13.2V".  Tried a couple more from different production runs and they do the same thing.  Funny, but if the thing is placed on a low car battery, it goes into overload and shuts down (no charging at all), and you can ask my 4-Runner with the dead battery about that!  LOL

[bob g]

another option

get a surplus APC ups, something on the order of a 1400kva if you are planning on 24vdc nominal or a 2200kva or larger for 48volt nominal

then put a relay between your battery bank and that of the ups internal batteries,

control that relay with a circuit that monitors the solar panel voltage, so that it energizes the relay and disconnects the battery from the inverters charging circuit when the sun is shining and the panels are able to charge the bank, and deenergizes the relay and connects the batteries to the ups charging circuit when the sun goes down.

might be one way to accomplish your goals, and give you clean uninterruptable power in the process?

maybe this will do what you want, maybe not, not real clear as to exactly what you want to do.

bob g