Advice on: DIY laptop cell replacemnets

[InHisName]

If you have been there, done that with DIY cell replacements of laptop battery, then read on and advise. Below is link that got me to making this message that I have been thinking of doing for last few weeks.

http://www.candlepowerforums.com/vb...50-3100-mah-vs-2400-mah&p=3945669#post3945669

I have4 BM T21 batteries, 2 dead, and 2 nearly dead. I've been planning to put some of those new NCR18650's into it. After much reading it seems that that could be a big waste of $$$$. I saw a chart on panasonic's site that showed the MAH curve vs voltage. Seems the 2200, 2600, 2900, and the 3100 have about the same MAH from full to as low as 3.4v. The bigger ones run to lower voltages for the greater capacity.

I installed a battery monitor in my laptop and when it got to less than 4%, it had just dropped to 3.47 which inicates that I'm not sure which cells would really do the longest hours/minutes. For a battery's cells to go from 4.2 to 3.45v isn't near the full capacity but is all that comes from the battery as it sits. So 3600mah or 4400mah are neither very real.

So the multi-faceted question I have, if you've been there, done that is: Should I even bother with DIY cell replacement ? Or just blindly order from lowest price on eBay from some chinese builder ? If I were to do it myself, which cell size gives the most REAL minutes run time ?
---- Is there some way to change the laptop battery pack 4% point from 3.47 to 2.8 ? Then maybe the 2900 or 3100 might be more useful.

 

[czAtlantis]

I was also thinking about battery pack rebuild (not now, my battery is still good but it won't harm to know about it)- you could see my posts in the topic you linked to.
Well when your laptop shows 4% at 3.47V it doesn't mean anything - it has just saved from previous discharges battery discahrge curve and it knows that this battery has at 3.47V only 4% capacity left. Maybe when you replace batteries and run full charge and discharge cycles it will remember new settings. Anyway- laptops should terminate at 3.0V/cell no matter what % does it show (but it will cause situations like you run on battery, have only 3% left and now the battery metter will be stuck at 3% for another 30minutes until finally reaches 0% and shuts down). This works in every laptop differently so it is hard to tell without testing battery pack on power supply or simply measuring how it behaves.

In Acer I used to own there were 2 "catch points" when discharging- at some voltage no metter how many % left it jumped to 7%. Now it waited for some other voltage setting (it could be easily 30 minutes stuck at 7%) to again relase counting down to 6% 5% 4% 3%. Now again at 3% there was some catch point where it waited to another voltage threshold for few minutes and finally counted down to 2% 1% 0% and at 3.0V/cell it died.
When the pack was old (higher internal resistance) and I ran some cower consuiming operation, voltage dropped and power meter dropped easily from 30% to 7%. When I used the laptop again in idle with low power drain, it stuck easily 30minutes at these 7%.
So it is really quite complicated and requires testing.

I was looking at cycle life of 2600mAh Sanyo and 3100mAh Panasonic and the panasonic has much better capacity after 500cycles.Another advantage could be that Panasonic cells are rated to 2.5V discharge and when discharged to 3.0V it could have even better lifespan. And I also read discussion saying battery packs with Panasonic cells last much longer than Sanyo ones.
http://forum.thinkpads.com/viewtopic.php?p=614680
But it is quite old and concern only classic Cells - not the new technology used in 3100 and 2900mah ones

EDIT: 3party battery - I wouldn't buy it. There will be low quality cells inside - so you will get low runtime and low lifespan. + you can also have problems with electronics showing nonsense % readings etc.. I would buy it only if I have guarantee it has branded cells inside (preferably panasonic but also LG, Sony, Sanyo..) but at this moment it will be much more expensive.