Li-ion "poor man's" PICAXE powering.

manuka

Senior Member
In case it's escaped your notice, the average life of a cell phone is ~a year before upgrading pressures set in. Nearby teenagers here in NZ say " A YEAR- as long as THAT !".

As the Li-ion battery in the obsolete phone is usually good for several years beyond this, it's feasible to use the slim battery pack in space/weight constrained PICAXE projects, & keep the phone handy just to use as the smart Li-ion charger! Although such power house Li-ions typically fetch ~US$20 new, you may thus get one for nothing by putting your hand up at phone discard time.

I'd explored this with a 3.7V 750 mAh "AB043446BE" (the standard battery for many Samsung phones), which weighs just 17g (encased total) & measures ~50mm x 33mm x 4mm. Single AAA NiMH with such a capacity weigh ~12g each & being cylindrical & bulky, pack together poorly.Battery holders for 3 NiMH cells would mean ~40-50g total weight for the same energy.

Before you get TOO excited however, recall NiMH are pretty charge tolerant compared with picky Li-ion, with the likes of even a hacked solar lamp being suitable. However,aside from the phone itself, all manner of DIY smart Li-ion chargers abound of course.

N.B. At "show off time" such a slick Li-ion battery can REALLY enhance overall project presentation. AA/AAAs look pretty cheesy when you've a $$$$ client to impress... Stan
 

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evanh

Senior Member
Before you get TOO excited however, recall NiMH are pretty charge tolerant compared with picky Li-ion, with the likes of even a hacked solar lamp being suitable. However,aside from the phone itself, all manner of DIY smart Li-ion chargers abound of course.
Actually, charging them is not that big a deal. Treat them as a lead-acid with higher internal resistance, ie: Voltage regulated but the current limit is lower.

Chargers usually have thermal sensing as a backup in case of fire risk - though doesn't work as has been demonstrated in recent months. So, as long as the battery doesn't get hot you're fine really.

Future Li-ion cells will have much lower internal resistance, so much so they'll be able to do the job of a lead-acid. Be prepared for lead-acids to be banned as "toxic".
 

andrew_qld

Senior Member
I recently bought a USB stick sized camera at Jaycar in their throwout box to use as an SMD Picaxe 08M powered "Cat Cam". The idea was to strap the thing to the family moggy and take pics at (say) 5 minute intervals. I found the 100mAh Li-on battery was dead, and found a replacement 110mAh battery from Sparkfun.com for $7 US. This thing is REALLY small. That got me thinking. I am now starting to build a picaxe powered flashing LED necklace using these.

They have 2AH packs for $13 US and 6AH packs for $40. If you grew up with 700mAH AA NiCaD's like I did then these are almost magical!

Cheers,
Andrew
 
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hax

New Member
In my recent project, I used a MAX1811 lithium poly charger IC, and an LT1308 boost DC converter. I also needed a LTC1477 high side switch to turn everything off when battery low. That's three (surface mount) chips minimum to ensure the battery is happy. I would not use anything but dedicated chips designed for lithium poly cells to do this stuff because the results can be catastrophic if you get it wrong. (even if you get it right!!)

A really simple chip to wire up is the MAX1555 chip, available in SOT23 package. It charges from a USB port or any 5v source, but it is still very important to prevent discharging past 2.7V so you still need extra circuitry.

If you use two cells (in series) instead of one, things get more complicated as you need to ensure that they remain balanced. You can use two cells in paralell, but the internal resistances need to match, and you still probably need a step-up converter.



@Evanh, charging and using a lithium poly cell is NOT at all like a lead acid. You need strict cutoff voltages (both charging and discharging) and lithium batt temperature can only ever be used to detect gross errors in charging or over-current conditions, and should only be a last resort to detect an error in charging.

A little known fact about lithium poly cells is that they can lose as much as 25% capacity each year from new, irrespective of how they are treated.

They should be treated with care because they have a lot of energy for their size. They can quickly explode and "flame out" if treated poorly. Not for the beginner, but good to play with...
 

evanh

Senior Member
@Evanh, charging and using a lithium poly cell is NOT at all like a lead acid. You need strict cutoff voltages (both charging and discharging) and lithium batt temperature can only ever be used to detect gross errors in charging or over-current conditions, and should only be a last resort to detect an error in charging.
Glad to see you agreeing on the thermal issue. As for the voltage, I've not looked up what the exact values should be, clearly not the same as lead-acid though.

A little known fact about lithium poly cells is that they can lose as much as 25% capacity each year from new, irrespective of how they are treated.
Actually, that one is so commonly stated I'm a little surprised by your assertion that it's not commonly known. and it was the standard thing to say about Ni-Cads also. But Ni-Cads turned out to be just fine when treated roughly enough to stop them from crystallising.

I read somewhere that the biggest problem with capacity loss in Li-Ion is the charger not knowing how to recharge the cells fully. I didn't look for the details though.
 

demonicpicaxeguy

Senior Member
what you'll find there is that the bloke was actaully welding at the time, what you'll also find is that the phone was probably subjected to a few of the old sparks from the welder which btw can be quite hot and usaully have no trouble getting through things like cotton,nylon,thin plastics considering they are several hundred degrees in tempreture
 

WHITEKNUCKLES

New Member
Granted,
But the first item is a fire while charging.
I liked the third story best and thought it was a better way to draw attention to the possibility of danger rather than suggest folks should search the electric model flight forums for details and photos of li-pos on fire, burnt out model aeroplanes, a couple of family cars and a garage.
In those cases incorrect charging was the most common cause of the fire which must be suffocated by soil or sand.
No intention of scaremongering, there are millions, billions? of them in daily use.
Charging at a current of one "C" (the mA in the stated capacity) and terminating the charge at 4.1V for a li-ion cell and 4.2V for a li-poly cell has proved safe for me.

Dave
 

manuka

Senior Member
Solar Challenge !

Such woes should not occur if you continue to use the original phone to charge the original Li-ion battery! This was really the whole point about my initial "poor man" posting...

Extra: For extreme Li-Ion applications, & for those of you facing a northern winter, cheer yourself up with the Solar Challenge race => http://www.wsc.org.au . This ~3000km PV (& this year also a "Greenfleet") event across Australia has just been held. The Dutch Nuna entry,with ~6 sq.m PVs, again won the event, at speeds averaging ~90km/hr. Inspirational!
 
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