Cell phone Li-ion batteries and charging them / General Science and Electronics / Forums

Forums

4hv.org :: Forums :: General Science and Electronics

« Previous topic | Next topic »

Cell phone Li-ion batteries and charging them

Move Thread

LAN_403

Zum Beispiel

Thu Dec 10 2009, 05:13PM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

How does one go about chargin cell phone Li-ion batteries (outside their intended application)? I would like to use a solar cell to charge a standard Nokia battery (type BL-C5, to be precise).

I have two questions and here they are:

-As I understand it, these batteries have internal protection circuits to prevent over charging. Can I rely on that and just plug it to a power source, or will this lead to an explosion?

-Does the voltage applied to the battery have to be highly regulated, or will it accept charge as long as the input voltage is somewhat above cell voltage? Obviously I won't be applying gross over voltage to the thing. Or will it just ruin the battery?

Knowledge in battery technology is one of my major shortcomings...

Steve Conner

Thu Dec 10 2009, 05:24PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Hey Zb

Lithium-ion batteries need to be charged with a current limited constant voltage supply. You can actually do it with a bench power supply: set the voltage to 4.2V per cell and the current limit to about half of the battery's capacity (2A for a 4Ah battery) Charging should be done in three hours. Don't leave it unsupervised in case it decides to go wrong and start a fire.

Yes, the voltage should be regulated. The most important thing is that it never exceeds 4.2V per cell. And you shouldn't really let it discharge below 3.0V/cell either. If using solar panels, you could fit a 4.2V or 8.4V shunt regulator, say based on the TL494.

You're not supposed to use the protection module for over/undercharge termination. It will kind of work, but it's intended as a second line of defence. Also, its limits are usually too wide to give the battery a long, happy life. And, it protects by opening the circuit completely, and you need to unplug the battery from the circuit before it'll reset. It's a circuit breaker, not a limiter.

Bjørn

Thu Dec 10 2009, 05:33PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

In general don't trust the battery to have a working safety mechanism, it is an added safety and in some cases it is not reliable.
Have a look here:

This is how I charge mine (with and without solar cells):

If you charge to 4.1 V you will not have a lot less capacity but you will get a longer lifetime or you can use higher charging currents. If you limit the voltage to 4.1 - 4.2 V and have a temperature sensor that cuts off the battery if the temperature starts to rise you should be quite safe.

Conundrum

Thu Dec 10 2009, 07:07PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

i'd add a timer in there just to be safe, in series with the 7805

Bjorn's approach is pretty safe for <300mAh cells like you might find in model helis and small MP3 players, as long as there aren't any board faults (test with dummy load such as three seriesed NiMH cells first and confirm correct performance)

the main problem with lipo or li-ion cells is that they can be dangerously unstable if overcharged, so a simple voltage regulator is extremely unwise.

the cheap chargers built into MP3 players etc also work, however a lot of them aren't designed for larger cells and can go "strange loop" causing overcharging.

additional "gotcha" - if li cells are very old their internal resistance can rise such that the charger never shuts down. this can result in overcharging which is why many laptop batteries simply shut down hard if used for long enough.

regards, -A
Who has a pile of very large format Lipo cells here that he still needs to build a proper failsafe charger and output regulator for...

klugesmith

Thu Dec 10 2009, 08:39PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Conundrum wrote ...
Who has a pile of very large format Lipo cells here that he still needs to build a proper failsafe charger and output regulator for...

Just for fun, look at this forum

which aggregates over a hundred reports of LiPo battery-induced fires.

Steve Conner

Thu Dec 10 2009, 09:01PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

When people use LiPos in RC models, they typically leave out the protection module, because the internal resistance of its FETs wastes power. So one of the two lines of defence recommended by battery makers is gone, and that's why they blow up more often. The protection module might not stop your dog sinking its teeth into a LiPo, but it would have caught all of those "wrong cell count" explosions.

I recently worked on the design of a product that uses a 60 watt-hour Li-Ion battery. We started out with six canned 18650 cells, but later we changed to a Chinese made "brick" of shrink-wrapped LiPo cells. I used a charging circuit based on the MAX1737 chip. None of them have exploded so far. :|

More recently, we went back to the original 18650 pack which came from a respected US battery manufacturer. Well, I respected them until they sent over a batch of the things with the terminals wired backwards! To give them their dues, when installed into the product, the batteries again failed to catch fire or explode. The protection module just cut out.

Mates

Thu Dec 10 2009, 09:16PM

Registered Member #1025 Joined: Sun Sept 23 2007, 07:53PM
Location: Czech Rep.
Posts: 566

Hi I'm using this voltage restrictors for my LiPO (4.2V) and LiFEPO4 (3.6V) batteries

Sorry the page is in czech but you can read the scheme.
If any questions, let me know...

Mates

Zum Beispiel

Fri Dec 11 2009, 04:34PM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

Thanks to everyone for the info and suggestions.

I'll put this project on hold untill I get a chance to whip up a proper charger. I was anxious to get this done before I lose my hair (=go do my military service) and was hoping I could use the internal protection as a sort of 'made in China' (or rather North-Korea) charger.

When it comes to electronics and me, it's usually been the way shown in my signature. But this time I'll let the lack of knowledge stop me (for now).

I'll be sure to post more details when and if I ever get this thing done.

Thanks again!

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob