Questions about 12v deep cycle batteries and inverters

[PranK]

Hi All,

I'm hoping some of you can answer my non car related electrical questions.

I have a large fish tank which needs power to some components 24/7. I've thrown around the idea of buying a generator in the past but the need to be on my property when power goes out threw that out the window.

Now I'm thinking about a battery and trickle charger with an inverter in an attempt to ensure uninterrupted power for my pumps.

So, in my head, I'm thinking this;

3 pumps in my tank that I need to run are about 200w total (thats about 60w bigger than actual, but I'd like to over estimate than under.)

A 50AH deep cycle battery will run these pumps quite easily for 24hrs.

A 300w inverter will allow 240v access to the batter.

A good ol' trickle/maintenance charger will keep the battery charged. 

Power outage means that the fully charged battery will take over.

Is that sane? Am I missing anything? It seems too easy.

I guess the only thing I am really wondering about is maintaining the health of a battery when it is charged at 100% for 99% of the year. Should I make it work every month? Switch off the mains to it to force it to cycle? 

Would love any help.

Thanks,

Christian

 

[GTSBoy]

What you're talking about is effectively a UPS - which you could look at instead of a cobbled together bunch of bits.

The other thing you could do is work out if the tank can run for a length of time only say, only 1 of the pumps.  So when you have the rare power failure that you're protecting against, would the single pump keep it going for 3 hours?  12 hours?  2 days?  By rationalising how much stuff you need to keep going for a (short) time, you can either use a smaller power setup or get more run time for your capacity.

[PranK]

I looked at UPS options but found they were quite prohibitive in cost when trying to run these pumps for up to 24hrs.

So, 200w is what I'd like to run and my calculations have a 50ah deep cycle battery capable of running this for over 24h. That battery is only about $200 and the inverter only another $100. But that seems cheap and I'm worried I'm missing something.

 

[GTSBoy]

The biggest issue is that, as you suspect, you have no charge management in your proposed system at all.  And the thing will always be running both the charger and the inverter, so you will always be using a lot more power (due to all the inefficiencies stacking up) than if you just ran the pumps off the wall.

The next problem is that you seem to have a problem with your expected battery capacity.  If you want to pull 200W off of a 12V battery, then you will be pulling 200/12 = 16.7 Amps.  A 50 Ah battery will only support that sort of power draw for a few hours.  You would actually need a number of batteries that size to support your power requirement for 24 hours.  And that is probably the reason why UPSs look so expensive - because you can bypass the faulty calculation and go straight to their specs.  A typical small UPS can run 200W for only a few minutes.  That's using an 7Ah alarm battery.  If you want to support a typical 200W PC for 6 hours then you're looking at a UPS that will cost a couple of grand and will have at least 8 or maybe 12 of those batteries in it.  And it will be the size of a large PC case.  If you want to run for 24 hours, now you're looking at something the size of a small filing cabinet.  That's also how large a stack of car batteries would need to be.

[KiwiRS4T]

Can you not have a generator set up to start when the mains power is off (and vice versa)?

Generators are not that expensive:

https://www.ebay.com.au/itm/GenTrax-Inverter-Generator-3-5KW-Max-3KW-Rated-Portable-Silent-Camping-Petrol/153040875857?epid=14022448432&hash=item23a1f27151:g:5VAAAOSwwNtbem2v:rk:12:pf:0

The relays to start and stop it should not be that hard (too hard for me but not some electronically able person).

[PranK]

17 hours ago, GTSBoy said:

The biggest issue is that, as you suspect, you have no charge management in your proposed system at all.  And the thing will always be running both the charger and the inverter, so you will always be using a lot more power (due to all the inefficiencies stacking up) than if you just ran the pumps off the wall.

The next problem is that you seem to have a problem with your expected battery capacity.  If you want to pull 200W off of a 12V battery, then you will be pulling 200/12 = 16.7 Amps.  A 50 Ah battery will only support that sort of power draw for a few hours.  You would actually need a number of batteries that size to support your power requirement for 24 hours.  And that is probably the reason why UPSs look so expensive - because you can bypass the faulty calculation and go straight to their specs.  A typical small UPS can run 200W for only a few minutes.  That's using an 7Ah alarm battery.  If you want to support a typical 200W PC for 6 hours then you're looking at a UPS that will cost a couple of grand and will have at least 8 or maybe 12 of those batteries in it.  And it will be the size of a large PC case.  If you want to run for 24 hours, now you're looking at something the size of a small filing cabinet.  That's also how large a stack of car batteries would need to be.

Wow, I totally mis calculated my requirements. Suddenly the cost has blown right out. Thanks for putting me straight, I would have been pissed if I bought everything based off my calculations only to find I had a few mins of power! haha

16 hours ago, KiwiRS4T said:

Can you not have a generator set up to start when the mains power is off (and vice versa)?

Generators are not that expensive:

https://www.ebay.com.au/itm/GenTrax-Inverter-Generator-3-5KW-Max-3KW-Rated-Portable-Silent-Camping-Petrol/153040875857?epid=14022448432&hash=item23a1f27151:g:5VAAAOSwwNtbem2v:rk:12:pf:0

The relays to start and stop it should not be that hard (too hard for me but not some electronically able person).

Ok, this does seem like a good option but how on earth do you start a pull-start generator with relays?

[KiwiRS4T]

Ok sorry,  didn't look too hard at the generator but there are electric start versions about...

Many buildings have emergency generators set up to start automatically in a power outage so the technology must be fairly common. Obviously you only need a small version but the principles should be well understood.

[PranK]

13 hours ago, KiwiRS4T said:

Ok sorry,  didn't look too hard at the generator but there are electric start versions about...

Many buildings have emergency generators set up to start automatically in a power outage so the technology must be fairly common. Obviously you only need a small version but the principles should be well understood.

Yeah, this guy looks pretty good. I wonder what the range is on the keyring starters! ?

https://www.ebay.com.au/itm/GenTrax-Inverter-Generator-3-5kVA-Remote-Electric-Start-PureSine-Portable-Petrol/292370091074?epid=6007499700&hash=item44129d8c42:rk:1:pf:0