Pump Conundrum

[securespark]

Hi folks

I'm trying to work out how to run an air pump off a leisure battery.

I'm planning on buying a 75Ah 12V battery:

http://www.gooutdoors.co.uk/numax-75ah-sealed-leisure-caravanning-battery-p96547

Problem is, the air pump is run off 12V, but has to run via a PSU: it cannot run direct from the battery.

The PSU has the usual universal input of 100-240V 50/60Hz (1.2A) and an output of 4.16A at 12V.

The air pump is rated at 12V 3 Amps max.

I realise I need to connect it via an inverter:

http://www.gooutdoors.co.uk/maypole-inverter-500w-caravanning-electrical-set-p140175

to this battery to run the air pump, but, assuming a good charge, how many hours use will I get before the battery needs recharging?

Thanks.

 

[PaulWales]

Inverters are never 100% efficient, it may be as bad as 60%,or up to 90% depending on quality etc. Assuming 80% efficiency gives a current draw of around 5.2 amps.

For a 75 Amphour battery that gives a theoretical run time of about 15 hours, but no leisure batteries will manage this in my experience - you'll be lucky to get much more than half of this without permanently damaging the battery.

How will the battery be recharged? Some chargers, particularly those in caravans, where most of my information regarding this comes, manage to charge the battery to no more than 85-90%. Dedicated separate chargers will manage a full charge but at significant cost if this sort of load is going to be a regular occurence.

A better idea of requirements may be available on this site: http://www.tayna.co.uk/Leisure-Batteries-C45.html

Having personally used Elecsol batteries thay seem to have a better power/weight ratio than some other leisure ones, though at significant expense.

 

[securespark]

Thanks, Paul.

The battery will be used on camping trips and charged with a CTEK charger, which I already have.

They are very good at recharging and maintaining a charge.

 

[ricicle]

Why can't the pump run directly off the battery (with appropriate fusing) ?

 

[ericmark]

I have in the past used diodes at a 0.6 volt drop two on a float charge of 13.2 volts give a 12 volt output. Since the 1.2 volt drop is in heat maybe inverters use less power and clearly would compensate for the volt drop as the battery becomes discharged.

CB's with insulated chassis vehicles required a DC - DC power supply and when I ran packet ration again could not run modem off same power as radio so used a DC - DC inverter.

Cost is a major factor as more 12vdc - 230vac inverters are made but likely you will find a lap top car power supply again DC - DC.

Although in theory 15 hours that would be at a 20 hour rate so in real terms more like 12 hours. Using a timer so switched on and off at regular times could extend the time assuming for a fish tank.

The problem I found was battery not being fully charged in first place. But looked into proper UPS and the price was silly and I also decided on inverter approach.

 

[stillp]

Why can't the pump run directly off the battery (with appropriate fusing) ?

Just what I was wondering!

 

[securespark]

Rice / Still:

Because it's an EXTREMELY expensive piece of kit that I do not own: it is leased to me and I cannot bugger it up!

Also, there is no indication of polarity on the machine or the PSU.

So, I have to use it with the PSU to avoid damage.

 

[Lectrician]

You could work out the polarity easily - that's no excuse

Ideally, you would find a DC to DC switch mode power supply - Takes the battery voltage (which varies with temp, and as it discharges), and then depending on it's rating, gives you a stable 12v clean and crisp output, regardless of the input.

 

[stillp]

Rice / Still:

Because it's an EXTREMELY expensive piece of kit that I do not own: it is leased to me and I cannot b*****r it up!

Then shouldn't you be using the correct PSU for it?
Bear in mind that switch-mode power supplies and inverters can have a lot of harmonics and other electrical noise on their output, whereas a battery will provide pure smooth DC.

Go on, do tell, what is this piece of kit?

 

[ban-all-sheds]

It's an air pump.

For airbeds/pools/tents?

The thing is, if it's in any way aimed at the leisure/camping/motoring market, I can't believe that one option for powering it isn't from a vehicle cigarette lighter socket.

Why can't the pump run directly off the battery (with appropriate fusing) ?

Just what I was wondering!

Me too.

You could work out the polarity easily - that's no excuse

Secure - ask the maker or the lessor if it will run off a battery.

 

[ricicle]

And to coin a phrase off of a well known orange-skinned gentleman - they are 'cheap as chips' to buy....

 

[securespark]

Then shouldn't you be using the correct PSU for it?

Yup. That's why I want to use an inverter an NOT run it direct from the battery! It has its own PSU.

Do you reckon a supply from an inverter is not clean enough for it?

Any advance on 7.5 hours run time? ideally, I need 24 hours of run time but with no opportunity to recharge in between, I think I'm doomed: unless I buy a bigger battery, of course!

 

[securespark]

Secure - ask the maker or the lessor if it will run off a battery.

Yeah, it will. They have just released a lead. It's marginally cheaper to buy a 12V power lead than an inverter.

On one hand, with an inverter + battery you can power other stuff.

On the other, powering direct from the battery will reduce the losses.

Either way, I fear I will not get the run time I need!

Oh, well...

 

[ricicle]

Either way, I fear I will not get the run time I need!

Oh, well...

What the heck are you trying to inflate ???!!!

 

[stillp]

Do you reckon a supply from an inverter is not clean enough for it?

No idea, since you haven't told us what it is! A supply from a battery will be a lot smoother than that from an inverter though.