Making a portable battery pack instead of using propriatary batteries

[Gordon Henderson]

No, it's not that efficient, but it's convenient - especially if the appicance requires a regulated external power source to charge internal batteries. If the appliance comes with a 12V lighter-socket adapter, then by all means use that instead, but it might be cheaper to use the invertor than buy lighter-adapters for every bit of kit you want to power.

They aren't that innefficient either, and the one I have claims to switch off if the voltage drops to below 11V, but I've never been able to run anything that long to find out.

For battery calculations, you need to know the rated AmpHour capacity. But note that they are usually stated at a particular amp rating. Eg. a typical car battery might be 40 Amp Hours at 10 Amps. That means it will last for 4 hours (down to some set voltage, maybe 10V, I don't know, would have to check spec.) at a current drain of 10 amps, but it wouldn't last one hour at 40 amps (usually a lot less!)

So drawing 5 amps, you might expect it to last 8 hours. Most car batteris will have their rating printed on the labels.

Gordon

[Gordon Henderson]

Sure - one minor difference is the supply hertz - it's 50 here and 60 in the US. Make sure the transofrmers are capable of handling this. You also might have a problems with motors too - they'll run a bit faster on 60Hz.

Another problem is weight. A hairdryer might be up to 1KW (or more for some of the fancier ones these days) A transformer to handle that is f'ing heavy. I don't fancy putting that into my luggage...

I do a lot of scuba diving, and a lot of the boats these days are moving away from having gas (cookers) on-board to electrics. There are some nice

12/24V electric boilers and even microwave ovens avalable these days. You are probably looking at at least a 1KVA model to power a cat. D microwave though, and these aren't cheap!

Gas is convenient, but LPG is heavier than air, so you have to be totally paranoid about using it on a boat...

Not really - as long as you bought an invertor with the appropriate input rating.

P = I * V, or I = P / V

So 800/12 is 67. This isn't perfect for AC and inductive loads, but but it's good enough for a rough ball-park.

You are looking at a BIG battery bank and alternator run off the main engines to keep them charged. (And HEAVY cable to connect them all together!)

Gordon

[Andy Wade]

Except that 800 W is the _output_ of the magnetron in the microwave oven and you haven't allowed for efficiency of same in your calculation. The efficiency's probably about 70% so the input power is going to be getting on for 1.2 kW. At 100% inverter efficiency that would require

87 A from your battery (at 13.8 V, by the way, not 12 V, if it's on charge). With a practical inverter efficiency of say 85%, your poor battery's going have to deliver over 100 A! Forget it and use a primus stove instead :-)

[Dave Liquorice]

Quite a long time 12 hrs maybe longer but I doubt that a car battery would survive this sort of treatment for very long. They are designed for short high current use and immediate recharge. Not for long slow discharge to almost nothing.

(Mr Plowman will be along shortly saying his car battery based UPS is fine thank you very much...)

You should be using a "leisure" battery for deep discharge use. These are designed to provide all their charged over a period of time and then be recharged.

[raden]

Most laptops run of a higher voltage, e.g. looking at my laptop PSU, it's 19.6V output

[Dave Plowman (News)]

Err, no, Mr Liquorice. (gawd we're a polite lot). It's not a UPS as such - merely an emergency backup for the central heating if we should ever have a power cut. And it uses my spare car battery, which I wouldn't run anywhere near flat - if it were a prolonged power cut I'd swap them over in plenty time.

If you can be sure you'll not anywhere near fully discharge a car type, it might just be the cheaper option, given the difference in cost.

[Dave Liquorice]

It's a "style" thing. B-) I'm sure you've seen me use it before...

Sounds like a UPS to me, thought it had a relay or two to automatically change over and drop out on low batt volts.

[Gordon Henderson]

I have several UPSs at home and I can run the CH and cooker off them if required. Very handy things UPSs are.. (Although they are normally connected to my computer systems, but I know what I'd rather have during an extended power cut!)

Gordon

[Dave Plowman (News)]

Putting it that way, then yes it is a UPS. Mind you, a UPS has to have the battery connected to work, and I don't keep the 'spare' in the same room. ;-)

[Dave Plowman (News)]

You can run a cooker off a UPS? Good grief...

[Dave Liquorice]

Well my small APC one will chuck out 700W but not for long about 3 or

4 mins from it's internal 7A/hr batteries. 1.5kVA UPSs aren't that much more expensive, the limitation is the energy storage, I've somtimes thought about getting a couple of good sized leisure batteries for my UPS just to up the runtime, it's calculation of runtime will be up it but I could live with that.

[Gordon Henderson]

It's a gas fired range cooker thing. It has a fan blower thingy to make it hot quick. (not a fan oven)

Gordon

[Gordon Henderson]

I've used and installed various UPSs over the years (in my IT capacity) and the general rule seems to be that on a standard unit you'll get 15 miuntes at half load. It's more or less linear at less loads than that (eg. 30 miuntes at 1/4 load) but more than half load it's non-linear. Down to 5 or 6 minutes at full load. This seems to be a reasonable general rules I've seen with different makes (although I prefer APC ones)

Of-course the extended range ones offer more run-time, but ...

Gordon