Solved! Rebuilding a laptop battery

[hankhsiao]

Hey guys, here's my predicament.

I hope I posted this in the right area.

Anyway, basically about half a year ago I bought a china brand netbook. Aside from the fake motherboard and terrible wifi card, it performed rather well. Anyway, the battery that came with it was a 7.4V 2900mAh battery.

The problem was, I've only used this laptop maybe less than 20 times and in that time range, the battery went from terrible to even more terrible. At a full charge, the laptop battery lasts a good thirty or so minutes.

Now, I thought this would be rather easy, you know. Pry apart the battery, unsolder the old cells, solder in new cells, boom. Done.

But after opening up the laptop battery, I realized that it was composed of two 3.7v 2900mAh Lithium polymer batteries. Still, not a problem. I made a diagram of how everything was soldered together, I just need to find replacements. The problem was, after a good two days of researching, I could not find any lithium polymer batteries that had the same voltage and milli amps which fit into the dimensions.

Each cell had the dimensions of: 100mmx30mmx10mm [length by width by thickness]

And there were two of them, side by side in the battery.

Through my extensive research I could not find any battery that could potentially replace these because they either had too low of mAh or they were too large in dimensions.

So in my desperation, I decided to consult you guys.

The laptop battery was manufactured by a company by the name of YOKU. After doing a search, it seems they are a chinese company that makes lithium ion batteries, but I don't believe they sell individual laptop batteries.

So my only option is to DIY.

I attached some pictures so you guys can get a look at this issue.

Anyhow, thanks for your time and I look forward to your input!

 

[poutinerocks]

after some digging around, I found that battery #493 here should fit, but once again, it doesn't look like they sell singles.
Good luck with your search

 

[hankhsiao]

after some digging around, I found that battery #493 here should fit, but once again, it doesn't look like they sell singles.
Good luck with your search

Hey,

Thank you very much for your response! Hypothetically that battery will fit, but the mAh is 2300. I'm not a battery expert but I think if you wanted to rebuild a battery, you can either do the same or greater mAh rating, but never lower? Can anyone shed some light on this? But thank you so much for your help! Great to know someone out there is willing to take some time out to help!

 

[acron1]

Have you considered looking for a single 7.4V 5800-6000mAh LiPo cell?
I believe there are many RC/Hobbies models using this type of battery....
BTW... you may also want to make sure that the internal resistance of the replacement battery is similar as the charging module may not be able to cope otherwise...

 

[poutinerocks]

regarding the mah number, it's basically how long the battery will last, so a smaller number will last less time than the original battery, but probably more than what you get now.

What you could do to see how long a battery would last is hook up a multimeter to the battery while it's in the laptop to measure the current draw and then divide the mah of the battery by the number you got. That would give you the number of hours it would last.

 

[hankhsiao]

Have you considered looking for a single 7.4V 5800-6000mAh LiPo cell?
I believe there are many RC/Hobbies models using this type of battery....
BTW... you may also want to make sure that the internal resistance of the replacement battery is similar as the charging module may not be able to cope otherwise...

Yeah, I've looked into those. It turns out that the dimensions of those battery packs are too large to fit.

Can you explain a bit more about internal resistance? Do you mean that the mAh should match or be greater or there would be overcharge?

 

[acron1]

Yes... just like any electrical circuit batteries have resistance("The resistance of an object is defined as the ratio of voltage across it to current through it" and is measured in ohms Ω). All I am saying is since the charging module is built-in the netbook's motherboard the resistance should be close or the motherboards charging module may not be able to tolerate it resulting in a breakdown...

 

[RetiredChief]

(1) don't sweat Internal resitance. Using a NiCad for example. To determine internal resistance you would take the open circit voltage ( 1.25 Volts), connect a load resistor to it say 1.0 ohms this should equate to 1.25 Amps current. Measure the Output voltage, Lets say it is now 1.24 Volts. That means that the Cell is dropping 0.01 Volts. 0.01 Volts/1.25 Amps = 8 milliOhms. Reason is this is fixed by the chemical composition and the state of charge, Charged = LOW internal resistance, Discharged = High internal resistance. And yes Internal resistance is affected by Amp/hr rating. A higher amp/hr rating has a lower internal resistance tha a lower Amp/hr cell, same chemical composition.

A simpler explaination.
(2) Again for Nicad (have not got upto speed on Li-ion) - but the principle is the same. The charging circuit is generally set for a max charging voltage (say 1.35 Volt)and cuirent limited to 1/10 the Amp/hr rating (this is ideal for Nicad, but a higher current is used to decrease charde time). At this current the output charging voltage is lower and as the battery becomes charged the current provided is decreased and charging voltage increases, up to the 1.35 Volts. Once the battery cell is charged, the output vcharging voltage is held at the 1.35 Volt (Approx 10% higher than cell voltage) and the current is reduced to say 50-> 100mAmps (Trickle current, enough to maintain charge, but not overcharge the cell.)

And Yes you can probably use a lower rated cell, but I would not go more than 20%->30% lower.

PS the example is for a much more expensive charging unit tha the cheap Bricks, but again it's similar in principle.

This is the reason that you should use = to Amp/hr, or sligthly geater and not less than the 20->30% lower than installed value. Note on Li-ion cells the output is more a function of the cathode material

Added: Of note, the same charger is used on the Toshiba 6 Cell (2 each 3 cells) and the 9 cell (3 each 3 Cells in Parallel) battery packs. (Ref is the A205 and A305 laptops - have both and have both battery pacs and use the same power brick)

 

[hankhsiao]

Oh, I see. Well, I was wondering, lets say if I rebuild the battery pack. But dropped the cell to a 2500mAh this time instead of a 2900mAh. Isn't the circuit board inside the battery programmed to stop charging at 2900mAh? So if I had a 2500mAh cell in there, wouldn't the battery continue to charge even when it is full, leading to overcharge?

Or is there some kind of resistance measure to this, I dont know. Thank you for your input guys!

 

[RetiredChief]

Should not be a problem overcharging.
A cell is determined to be fully charged by the current drawn at a given voltage. For ex. (NiCad. at the set voltage of 1.35 V, when the current drops to 100 mAmps ( picked this value out of the air) the cell is considered charged. If the Cell amp/hr rating is lower than the size used in the charging circuit, the main effect is that it will be charged quicker.

One cavet - There are several diff Li-ion Batteries which have a slightly diff open circuit voltage (primarily dependent on cathode material). This could be more critical than the amp/hr rating.
http/en.wikipedia.org/wiki/Lithium-ion_battery

A little on charging Li-ion batteries.
Seems they use a constant current, and "fully" charged when the voltage = a set value (Determined by the Li-ion chemical composition). So again, it's the output voltage that is the determining factor. The Batteries used in my Pac have a Eout per cell of 3.60 volts, not the 4 volts in referenced link.
http/www.powerstream.com/li.htm

 

[hankhsiao]

Okay, so basically the only drawback to having a lower mAh rating would be less battery life. And matching voltage would be more important than matching mAh basically. So using this logic, I could find a smaller mAh cell with the same 3.7v and it would work properly?

If this is possible, then it'll be a piece of cake to rebuild the battery, because I am therefore open to a whole new world of possibilities, because the smaller the mAh rating, the smaller the battery capacity, hence the smaller the dimensions!

 

[hankhsiao]

Thanks guys, I ended up sourcing this battery cell and it looks like all the dimensions are right. Now all I need to do is wait for it to go back in stock, place an order, receive it and solder them back into place and I'll be good to go. Thanks for all your responses, it was very educational and informative and made me feel like this is a great place to be!

http/www.all-battery.com/37volt-2100mah15cli-polycell.aspx