Automatic battery backup

[viewer]

Bernardgreen wrote"Measuring the current to the pump will not be a reliable way to detect pump failure. The pump might even take more current if it stalls.

A flow switch in line with the primary pump would detect any type of failure in the main pump and that could then start the DC pump running on 12 volts from a trickle charged battery.

Put the DC pump in a pipe bypassing the main pump and with non return valves in both pumps ( or pipes ) and you have a near fail safe system."

Agreed. For my mains failure system I used the ring final as a source so no volts there tripped in the battery circuit. Not quite the same situation as the need for the CH was to prevent boiling with a wood burning stove. As a one time fish keeper, I found the biggest problem was flow through filters rather than pump failure, so the flow switch idea would kill two birds with one stone.

 

[imroberts]

What kind of fish do you keep? Coldwater or tropical?

Also how big is your tank?

If you're keeping coldwater I'd be surprised if a computer UPS wouldn't be able to provide enough power to run your filtration system for quite some time. Certainly for the duration of an average power cut, during your upcoming work would depend on what you class as an extended period

If you keep tropicals the power required to keep your heater(s) running may rule out the option of a UPS.

 

[bernardgreen]

A low cost flow switch is a spring loaded paddle on a pivot and placed so the water coming out of the pipe from the pump pushes the paddle back against the spring. A micro switch is operated by the dry end of the paddle or a magnet on the paddle operates a reed switch.

Or the water falls into a small cup with a small hole on a spring loaded arm. Only when the cup is almost full does the arm descend due to the wieght of water in the cup. If the flow stops then the cup empties via the hole and swings up closing a switch.

 

[sparkticus]

suggest that availability over here is fairly limited, and they are far from cheap (in the £30-£70 range from one source I found). Do you use them and, if so, where do you obtain them from?

I'm particularly intrigued by the fact that many of them have 'universal' input volltage (12V-240V, AC or DC).

Yes, they are incredibly flexible but as you say on the expensive side. They are readily available over here and I will post the distributor tomorrow. Having said that, there is a lower cost alternative but for the life of me I can't remember the manufacturer at the moment despite using them quite often. Again I will post that info tomorrow.

 

[sparkticus]

That's why I was going for 12V. The inverter will eat through the battery at a rate of knots.

Now I'm trying to source a sensibly priced flow switch or current sensing relay.

There is a much lower cost alternative but I can't remember the manufacturer. I will post info tomorrow.

 

[JohnW2]

Yes, they are incredibly flexible but as you say on the expensive side. They are readily available over here and I will post the distributor tomorrow. Having said that, there is a lower cost alternative but for the life of me I can't remember the manufacturer at the moment despite using them quite often. Again I will post that info tomorrow.

Many thanks - the 'lower cost alternative' might be of particular interest!

Kind Regards, John.

 

[SimonH2]

12v pump/lights heater supplied from a car or traction battery with a 12v charger no mains just the 12v but charged from the mains

I was going to suggest that, use a CTek or similar "intelligent" charger and you won't cook the battery ...

Actually you don't want that type of charger. They are great where the load doesn't run off the charger, but in this case you are effectively running a DC load off the charger/poser supply with the battery as a float backup. This means the charger will see the connected load as battery charging current, and thus the controls just won't work properly.

What you really need is a 13.5 to 13.8V supply which is the correct voltage for float operation of most 12V "dry" batteries. It needs to be rated for the load current plus enough to recharge the battery, and be current limited to protect itself when the power comes back on after an extended outage and the battery wants to take a significant bulk charging current. Note that the current limit must not be a foldback or shut off protection as this would mean it'll just keep turning off.

I had to look for one for a work project a couple of years ago, and unfortunately such devices are not nearly as common as you might think (at least in all the places I looked !). There are plenty of small (transformer, rectifier, linear regulator) devices with ratings up to about 1A for alarms and CCTV cameras etc, but after that they seem to be a bit thin on the ground - or expensive.

I ended up buying one from RS (http://uk.rs-online.com/web/p/products/0413976/?searchTerm=413-976) which is rated for 11.2A (a bit more than I needed), but which costs £68+VAT. You still need to stick it in a box etc as although it is cased, it's not adequately protected on it's own. Add in a few "dry" batteries (I test and keep any half decent ones from scrapped UPSs) and you've got a 12V power supply with battery backup limited only by the capacity of connected storage.

Edit: Most modern caravans are fitted with a similar type of power supply. These will run the connected 12V loads AND float charge the battery while you are hooked up to mains. Unfortunately they aren't generally "cheap" as my father found out when his let the smoke escape.