Determining whether an external battery can deliver enough power

[ZrednaZ]

Hi,

I need to power my HTC Desire cell phone for 4-5 hours on high load (running GPS navigation software on max screen brightness in an aircraft), so I'm looking into external battery packs.

I want to avoid buying something too weak which will struggle to deliver enough power.

The internal battery says: 3.7V --- 1400 mAh 5.18 Whr
The AC adapter's output says: 5V --- 1A

So I should ideally get a battery that can deliver 5 watts, yes?

I've been looking at:
http://www.batterygeek.net/Batterygeek-100-Hr-iPhone-iPod-iPad-Battery-Pack-p/geekpod_110_.htm (press the 'Technical Specs' tab)
and
http://luckypacks.com/EXTERNAL-IPHONE-4-3G-3GS-BATTERY-I-UP-5400-LP5400-5400-mAh-p5.html

The first battery's specs says "5V" and "2A (max)", but I suppose that doesn't necessary guarantee 2A @ 5V! Is it possible to determine in advance whether these batteries have enough oomph for the job?

 

[JaredJames]

I find myself asking the question, why would you need to do such a thing and why is the phones battery not sufficient?

OK, I can probably answer the latter as I have a desire myself - but what could you possibly need GPS at full brightness for lasting up to 5 hours?

The first battery's specs says "5V" and "2A (max)", but I suppose that doesn't necessary guarantee 2A @ 5V! Is it possible to determine in advance whether these batteries have enough oomph for the job?

That would be perfect, just check the rating for Amp-hours - this will tell you how long it can deliver the required values.

 

[rcgldr]

That would be perfect, just check the rating for Amp-hours - this will tell you how long it can deliver the required values.

Depends on where the original poster live. Amp hour ratings for batteries are common in Europe, but not common in the USA, depending on the battery type. You'll need to look at the specs for a portable charger to see if it list the amp hours. Apparently the charge port on smart phones is setup for 5 volts. I found a 2 amp hour, 5 volt "portable charger", with a max current rating of 1/2 amp, that is good for one charge on a smart phone. I don't know what the current draw of a smart phone in GPS mode is, but you can estimate this based on how long it takes to run down a fully charged internal battery, knowing that it takes 2 amp hours to fully recharge the internal battery after the losses involved in recharging.

Although a bit heavy, an alternative would be to use a medium sized lead-acid battery, a DC to AC inverter, and then plug in the phones AC charging adapater into the inverter, assuming that the phone can run continuously off the phones charger. This wouldn't work well if you plan on doing aerobatics in the aircraft. In that case the entire setup would need to be secured.

 

[ZrednaZ]

but what could you possibly need GPS at full brightness for lasting up to 5 hours?

Well, because I need to power a GPS unit in a glider with no power outlet for the full duration of a 5-hour flight.

@rcgldr I live en Europe, and my phone is bought here, but I plan on buying the external battery in the US. I'm guessing that y'all haven't clicked the links I posted.. Both of the batteries I've found state the mAh value (6600mAh and 5400 mAh respectively). They just don't seem to state whether or not they can deliver 5 watts.

I would rather not go down the DC to AC inverter road. I've tried that once in a car, and the rig would tend to get rather hot .

 

[sophiecentaur]

You could actually measure the current requirement with a meter. All you'd need to do is to make up a lead with the right connectors and a break-oput box in it and then hook up a multimeter. Really smart engineers would then do some datalogging - but you could just read the current now and again. If you are a gliding man, then you will have loads of money (~no?).

The actual consumption is difficult to determine from what's written on the side. ~A lot will depend upon how often you hop from cell to cell. I had a phone which ran down in no time when it was sitting in the bedroom. It must have been just on the boundary between two cells and it kept using power saying hello to each one in turn - could have been traffic outside the house affecting the propagation of the signals. ALso, if you are on the boundary of a service area, your transmitter power will be higher than when you are near the base station.
An experiment is called for, I think.

 

[russ_watters]

You don't need power, just capacity. Why not just test the longevity of the internal battery and ratio that against the time you need?

Oh, and Jardenjames: GOLF!

 

[ZrednaZ]

You could actually measure the current requirement with a meter. All you'd need to do is to make up a lead with the right connectors and a break-oput box in it and then hook up a multimeter. Really smart engineers would then do some datalogging - but you could just read the current now and again. If you are a gliding man, then you will have loads of money (~no?).

You would be surprised. It's actually a very inexpensive hobby ($100 USD a month for unlimited use of the club's 20 shared gliders, plus a small fee per launch to cover the fuel consumed by the towplane or winch).

Are you suggesting hooked up a multimeter between the internal battery and its connectors? I don't think I have the nerve, but maybe I can find an app on the Android marked to measure the comsumed power... Anyway, I assume that the comsumption is around 5 watts based on:
a) the AC adapter's output is 5V --- 1A = 5 watts
b) it takes roughly the same time to recharge the battery as it takes to consume the battery running GPS software.

Sure I could measture it more accurately and reach some watt value, but as far as I can tell, it wouldn't answer the question of how much power the external batteries can deliver...

 

[ZrednaZ]

You don't need power, just capacity. Why not just test the longevity of the internal battery and ratio that against the time you need?

That's what I was unsure of and why I came here. I figured I might accidently buy a battery with plenty of capacity and not enough power to actually keep the phone on. Please explain why only capacity (the mAh value I suppose) matters.

 

[russ_watters]

The ratings tell you whether the new battery can power the phone at its peak draw, but the longevity is a function of average power, which you don't have.

 

[sophiecentaur]

@ZZ
I meant to measure the Ah consumed from your charger over an extended period of running. You can be fairly sure that the internally battery's not contributing whilst you're connected to the charger.
No, I wouldn't suggest you got inside the phone with a soldering iron. Even gliding types have to draw the line somewhere. Sounds pretty good value actually. Is your gliding club subsidised by some government or other organisation? I know someone who uses a UK Civil Service yacht club facility. That's good value for him, too.

Now, re power / capacity. I think most batteries will have a current output commensurate with their Ah capacity. If the external battery you are using has significantly more capacity than your internal one then there should be no problem with the instantaneous current it can supply - but it wouldn't matter if it failed on occasional peaks because the internal battery copes in any case.

 

[ZrednaZ]

Thanks for the info SC and Russ. It looks like both batteries I've linked to should easily do the job.

@ZZ
I meant to measure the Ah consumed from your charger over an extended period of running. You can be fairly sure that the internally battery's not contributing whilst you're connected to the charger.
No, I wouldn't suggest you got inside the phone with a soldering iron. Even gliding types have to draw the line somewhere.

Sounds pretty good value actually. Is your gliding club subsidised by some government or other organisation? I know someone who uses a UK Civil Service yacht club facility. That's good value for him, too.

I suppose we get some government subsidies for every young member under 25 (the Danish government's way of saying thanks for keeping them out of trouble, I suppose ), but that age group comprises less than 10% of all 150 members, so the club is mostly self-sustaining.

 

[JaredJames]

Oh, and Jardenjames: GOLF!

Hmm, didn't consider that, I suppose a sport such as golf could demand it - but I was thinking more about the OP mentioning within an aircraft.

 

[sophiecentaur]

Thanks for the info SC and Russ. It looks like both batteries I've linked to should easily do the job.





.

Are you planning to use a car charger lead?

 

[ZrednaZ]

Are you planning to use a car charger lead?

No. The charging cable plugs into a standard USB socket. The external batteries also come with USB sockets, so that's what I will be using.

 

[rcgldr]

No. The charging cable plugs into a standard USB socket.

Note that some smart phones, such as the iPhone, require more of the USB pins to be supplying current than an iPod. You need to make sure the external battery USB connector will work with your smart phone.

Also I looked at a few links, and could not find maximum current specifications on the external batteries you mentioned. I did see a maximum current of 1/2 amp specified for a smaller model that only had 2 amp hours insted of the 5.4 amp hours of the ones you linked to. I don't know if there's a recommended current limitation on the USB connector on smart phones. The capacity is not a good indicator for peak current, since there is additional circuitry to limit the voltage to the 5 volts or what ever the smart phone want on it's USB port.

 

[ZrednaZ]

Note that some smart phones, such as the iPhone, require more of the USB pins to be supplying current than an iPod. You need to make sure the external battery USB connector will work with your smart phone.

Also I looked at a few links, and could not find maximum current specifications on the external batteries you mentioned. I did see a maximum current of 1/2 amp specified for a smaller model that only had 2 amp hours insted of the 5.4 amp hours of the ones you linked to. I don't know if there's a recommended current limitation on the USB connector on smart phones. The capacity is not a good indicator for peak current, since there is additional circuitry to limit the voltage to the 5 volts or what ever the smart phone want on it's USB port.

Ok, I'll keep that in mind.
This battery http://www.batterygeek.net/Batterygeek-100-Hr-iPhone-iPod-iPad-Battery-Pack-p/geekpod_110_.htm offers some specs if you scroll down a bit and click the 'Technical specs' tab.
The 5.4 Ah battery - as you noticed as well - doesn't offer much information.

 

[sophiecentaur]

Note that some smart phones, such as the iPhone, require more of the USB pins to be supplying current than an iPod. You need to make sure the external battery USB connector will work with your smart phone.

I don't think that will be an issue here. The limitation is in the driving capability of the computer / hub and a battery will have oodles of current available.