Making a portable battery pack instead of using propriatary batteries

[dg]

Faced with a number of battery operated portable items I wondered if it was possible to make a battery pack that would plug into the units' mains adapter socket, to power the item.

I presume that mains adapters just output the necessary voltage and current to power the unit, so If I made up a pack consisting of a number of 1.2v NiCads to acheive the correct output voltage and use and a standard connector, then this could power the unit?

Is it this simple or would I need some electronics to regulate to output voltage?

If its not just a case of batteries, box and a connecting lead, any pointers to suppliers of the necessary kit would be appreciated.

dg

[mrcheerful]

You can run low power things off an adapter, but don't expect to run battery operated drills and the like.

mrcheerful

[Gordon Henderson]

If the unit is designed to run off a mains power supply, then you could use (eg) a car battery and a 12V to 230V invertor. I do it this way when "in the field" and need to do things like charge mobile phone, cameras, Laptop, etc.

Eg. See

have the basic 150W model. Seems to work fine for my needs.

It could - IF it's just designed to power the unit.

Regulating voltage isn't a problem, but if the units also charge their (internal, rechargable) batteries then you must be carefull how you regulate it. NiCd rechargables have different requirements to NiMH for example. Some devices do have internal regulators and aren't so fussy though, but it's hard to tell which does and which doesn't, and unless you are absolutely sure, it's best to not try.

I've found it easier to use the invertor, then just plug the devices standard mains adapters into the invertor to run/charge them.

Gordon

[John Rumm]

For devices that have a DC in socket, it is pretty straight forward. Make up an external pack with a flylead a suitable connector on the end.

Most devices that expect to run from an external DC power source will have a regulator in them anyway since the external "wall wart" type power bricks are typically unregulated.

If the device expects mains in only then an inverter may work, but chances are it will work out bigger and heavier than just taking a few sets of replacement batteries.

[Dave Liquorice]

In principle yes but you'll probably find that every single one of your devices has a different input voltage requirement, some may not be a multiple of the (nominal) 1.2v and have quite strict limits. You need to go through the manuals of each device to find what they will live off from the "DC" input.

Parts available from Maplin, Rapid, CPC, RS, Farnell etc.

[Set Square]

It *can* be done, but bear in mind that there are a lot of size variations in these little concentric plugs which feed the appliances - so one size definitely *doesn't* fit all. Add to that the range of voltages required and polarity differences (some have +ve on the outside and some on the inside!) - and you need to design a pretty flexible bit of kit!

I think I would go with the car battery plus inverter suggestion, in order to provide "mains" to the original adapter. You're then sure of the right fit, and the right voltage and polarity.

[Ian Stirling]

There are adaptors and adaptors. Some will run things like battery drills, but are of course comparatively expensive.

[raden]

In message , Set Square writes

CPC do an excellent little power supply which comes complete with 8 or

16 connectors which plug into another connector either way around to give you about every possible permutation

That's that problem solved for the OP'r

[N. Thornton]

Right. Get voltage right, and polarity. Also put a fuse on the nicads, as they can deliver firestarting amounts of current if the connector touches what it shouldnt.

Regards, NT

[Dave Liquorice]

Until he takes the device from a device needing 12v tip +ve to a device needing 3v tip -ve without remembering to change the connector round...

[Set Square]

But does that require a mains input? AIUI, OP wants it to run off a 12v battery.

[Dave Plowman (News)]

Certainly possible.

Assuming the voltage is correct, then yes. If size isn't a problem, one of those car starter packs that are complete with charger would be a very cheap source of 12 volts - much cheaper than building one.

It would depend on the individual item. Some have internal regulation, some have this in the supplied wall wart.

You could build in internal regulators for all the voltages you need - although you'd have to start out with somewhat higher a voltage than the highest you require. IC regulators are cheap and require few external components.

[raden]

Ah, the human interface problem

If he does this, he has nobody to blame but himself

[raden]

In message , Set Square writes

I was pointing him to a source of connectors - I was expecting that he was useful enough to be able to use a pair of cutters on the lead

[Paul Mc Cann]

Thats an interesting site. I would be utterly at sea in this area but two things seem interesting.

Firstly they sell a compact USA 110v to UK240v transformer

are various ratings (100va,300va,500va,) available. Would any of these be suitable to run things like hair dryers, curling tongs etc. (For SWMBO, who moans when across the pond that none of her collection of feminine tools work over there)

And secondly are the ratings on their 12v to 240v inverters a reliable way of determing their suitability for a particular task. I have in mind running a microwave on a boat.

As an addendum to this last would 24v to 240v be any different to 12v to

240v ? (The boat has a bank of 12v batteries for the domestics)

What sort of draw would an 800w microwave have on a 12v battery ?

[Dave Plowman (News)]

Assuming 100% efficiency of the invertor, some 67 amps. But invertors aren't 100% efficient, so you could add perhaps 20% to that.

A 1000 watt invertor will cost serious money.

[dg]

LOL. I looked on their site and noted a multi purpose LiOn battery pack for £250 !! I thought you meant this.

dg

[dg]

On the subject of inverters, isn't it very inefficient to convert 12v to 240 v, and then back to say 12v for the appliance?

If say, I wanted to power a laptop this way (mains transformer = 12v and 4.16A) how long could I expect the car battery to last? Would it just be better to get a 12v cigar adaptor - or would the current drain to the car battery be the same either way?

dg

[dg]

Thanks for all the replies.

I used to have a battery belt for a video camera ( from the days when they used to be power hungry!), and I was thinking that something similar would be useful for some of the devices I carry around today.

Something like a few high capacity D cells may be more portable than a bigger square pack.

An extended runtime 7.4v battery for a TabletPC I have to use for extended site visits is nearly £100, so I thought why can't I just use a pack of normal batteries - not quite as glamorous but probably would keep me going all day.

That got me thinking about the other devices and equipment etc I have to frequently carry and recharge.

One query regarding amperage - if I did make up a battery pack to replace the mains adapter, is it the appliance that draws the current or is it the pack that supplies the current? ie if the pack supplied say 'x' volts at 5000ma, but the appliance stated that it used 'x' volts at 4000ma, would this cause a problem or just mean it would run for longer?

Sorry if thats a bit OT DIY.

dg

[John Rumm]

You could - although remember the current drain on a tablet PC may be quite high. Hence running costs on ordinary "dry" (i.e. non rechargeable) batteries could be high. Some decent (i.e. 4Ah or better) D cell NiCds or NiMh cells may work out cheaper.

Generally the appliance will dictate what current it requires. The supply simply needs to be able to supply enough.

No problem - and it will run longer. Batteries are usually given a capacity rating in either Amp Hours (Ah) or Milli-Amp Hours (mAh). So a

2Ah cell could in theory supply 2A for 1 hour before being exhausted. It could also do 1 amp for two hours, or perhaps 4 amp for 30 mins.

Not OT really...