Powering Mobile Phone from Leisure Battery

Hi,

I'm trying to use an old LG GT540 Android for surveillance on my allotment (garden away from my house, for those who don't know the term), which is off-grid, so I've got solar panels charging a leisure (Lead acid) accumulator.

My first thought was to leave the Lithium Ion battery, and plug in a car charger and power the charger from the lead acid battery. However, that apparently ruins the lithium ion battery and rapidly flattens the lead acid battery.

So I turned to stepping down the voltage from 12 - 14 volts to the 3.7V required for the phone. Research suggested that a switch mode regulator would give far greater efficiency so I set up an L200C to do the job.

The mobile battery ("LGIP-400N") contains protection circuitry. One component of that is a "strap battery device VLD P 220 SLF". The datasheet is not clear to me but it looks like a thermal shutdown device to protect things if the battery gets too hot. There are also two ICs and 7 resistors - they have no symbology and I don't know what they do. I tried leaving all this in circuit by connecting the battery terminals to the mobile via soldered wires and connected the regulated 3.7V to the place where the lithium ion battery used to be. I reasoned neither the phone nor the electronics would know the battery was... not what it used to be. However, it doesn't work; the phone does nothing.

So I removed the battery circuit components, and routed the 3.7V power from the regulator straight to the two outer pads in the mobile phone; the middle pin is ignored. Well, it works, and the phone comes all the way up and works fine.

However - two things:

1. There is an 80mA current drain from the leisure battery, even with the phone switched off
2. With the phone on, the regulator is too hot to touch and about 300mA is drawn, implying a 4 Watt consumption!

This is not going to fly with the meagre PV charging I have going, and anyway - it suggests something is worryingly wrong.

I wonder if anyone can shed any light on what if anything I'm doing wrong. I'd appreciate any help you can offer.

Regards,
Chris

 

Switched off - so it should be drawing nothing. And when on, I know that network activity and other specific functions can be very heavy on power, but it sits there in airplane mode doing nothing.

Should the regulator get hot? I thought switch mode regulators were efficient.

 

Dave I am using the L200C which you can read about here http://www.hqew.net/datasheet/L200C-datasheet-pdf-download-197577.html. It quotes a max junction temp of 150c but my concern is not self destruction (it's got protection inside anyway) but rather efficiency.

No photo - it's just two wires soldered directly to the outer 2 pins in the battery compartment, which I feed from the regulator circuit output.

 

Thanks CocaCola. You are correct of course. The phone has a push-button which isn't a physical electrical cutoff switch. When you said "standby" I thought you meant the phone is on, but not in a call. So yes, 80mA in standby. Still seems vast. If the phone took that current continuously when normally powered, it would die after less than 19 hours on a full 1500mAH battery, whereas it doesn't.

And it's worse than that becuase this 80mA is being drawn from the Leisure battery side of things, so the power isn't 3.7 x .08 but 13 x .08 = 1Watt. And the fact that the regulator is so hot is the killer. Right there, that heat is a big dissipator of power I don't have to waste in the leisure battery.

 

Looknig at the phone spec here http://www.gsmarena.com/lg_gt540_optimus-3081.php they quote stand-by (which most people take to mean the phone is ON but the screen is off and it's not in any calls) of "up to 500 hours". Allowing for the bull factor, let's say 300 hours. For a 1500maH battery, that's FIVE milliamps, with the phone on. I'm seeing 80 with it off. So I think smething is clearly wrong. No?

 

Steve you're a star and I'm a moron. Thanks. The store I bought it from described it as a high efficiency switching regulator, and though I have been through the datasheet and the design guide a lot, I never spotted that it really isn't.

The leisure battery is just nominal 12V - fully charged it's around 13.5V, so it's dropping 8.3V.

Given the nature of my error I'm inclined to just junk this effort and start again with a (cough) switching regulator. Apologies for wasting anyone's time & thanks for your help.

Chris

 

You may be right Steve. The internal battery is removed, so it's not charging that, I have purchased a switch mode regulator from China for £4 and I'll see how that performs. If it works fine and draws sensible current, then job done. If not, then I may be forced to look at whatever this other issue may be again.

But until that's proved, I'm going to move on. Thanks again for putting me on the right track with this.

Cheers,
Chris