How do I buy the right laptop battery when all the sites tell me I need something different?
May 10, 2008 10:07 AM   Subscribe

Help my buy a new laptop battery!

I recognize that I should be able to figure this out myself. My laptop battery looks like its dying (battery light constantly flashing, and it won't charge), and I want to buy a replacement. There are many battery stores out there that will tell me what battery I need for my computer. The problem is, they all give me different answers! Voltage numbers are slightly off (some are 10.8, others 11.1), as are size ratios. Is that because they're all right, and all will work, or do none of them know what they're talking about?

My computer is an IBM Thinkpad T43 1871.

Lenovo tells me to get this one, but it doesn't explicitly say it works for the T43, just for the T40 series, and they don't have my particular T43 in there (they start at 1872).

Random site 1 tells me either this or this will work. Note the voltage differences between these and Lenovo.

Random site 2 tells me to get this. You get the idea.

Please help me.
posted by kingjoeshmoe to Computers & Internet (7 answers total) 5 users marked this as a favorite
I'd go with Random site 2. It's specific to your computer, in stock, and they have a 12 month replacement policy
posted by Mr_Chips at 10:31 AM on May 10, 2008

I have bought stuff from Random Site 2 without incident. So they're legit, anyway.
posted by rokusan at 10:33 AM on May 10, 2008

The computer will accept a range of voltages and amperages and these adaptors are (according to the vendors) within that range.

When I got a replacement for my Dell laptop, Dell sent me one that was 19.5 V and 4.62 A, when the old one had been 18 V and I think it was 3.5 A. Works fine.

If the vendor has an adequate guarantee, I'd buy based on price. Be sure that it doesn't say "power only" or something to that effect (that is, it will power the laptop but not charge the battery).
posted by winston at 10:35 AM on May 10, 2008

Also, the one at Random site 2 ( Laptop Battery Store) advertises the highest capicity (6600 mAH)
posted by Mr_Chips at 10:36 AM on May 10, 2008

I'm not the kind to usually mark all answers as best answers, but each of you helped me make my choice. Looks like its battery at random site 2, and I need not freak out. Thanks for your help everyone.
posted by kingjoeshmoe at 10:48 AM on May 10, 2008

The voltage differences are because batteries of all kinds have higher terminal voltage when fully charged than when partially discharged. Lithium ion batteries are nominally 3.7 or 3.8 volts per cell, depending on who you ask, but when fully charged they can easily be over four volts. Some companies seem to have decided that higher voltages make their batteries more marketable, or something, so they quote higher numbers.

Because LiI has such a high per-cell voltage (versus 1.2 volts per cell for NiCd and NiMH batteries and 2 volts per cell for lead acid, for instance), it's pretty much impossible to ever accidentally buy a LiI battery that's genuinely got the wrong voltage for your application. This isn't usually an issue anyway, since anything the same shape as the battery you've got should have the same cell configuration (the cells are standardised components), but it's still nice to know.

Note that many laptops these days give you the option of a low-capacity six-cell or a high-capacity nine-cell battery, which both come in the same casing. The way this works is that the cells inside the batteries are arranged in strings of three - 3 x 3.7V = 11.1V. There are three such strings wired in parallel with each other in the high-cap battery, and only two strings plus some empty space in the low-cap version.

I think the T40 series do all use the same battery, if the numerous eBay ads titled "Battery For IBM ThinkPad T40 T41 T42 T43..." are anything to go by. If I were you, I would definitely try a cheap eBay nine-cell pack from a well-feedbacked Hong Kong seller first. It'll probably be well under half the price, including delivery, of a genuine Lenovo battery, but is very unlikely to be only half as good.

Note also that LiI batteries have a definite lifespan, whether you use them or not. Things aren't now as bad as they were when I wrote this, but a new-in-box Lenovo battery that's been sitting on the shelf for a couple of years is still likely to be a poor performer compared with a cheap off-brand battery with new shiny cells in it. So, just like when you buy non-rechargeable batteries, it's a good idea to buy your laptop/cellphone/MP3-player batteries from dealers who have a lot of turnover, so you get fresh stock.

The extreme cheapskate option, for people who value their time fairly low (or actually ENJOY this sort of thing), is to crack open the dead battery and replace the cells. They're probably 18650-size (18mm diameter, 65mm length) or something similarly standardised, and bare LiI cells are very cheap on eBay. If the battery's built-in monitoring hardware won't believe that it's got new cells then this will be a problem, but I think you can almost always just cycle the battery once or twice to get the battery monitor to figure it out - and the battery monitor may be entirely built into the laptop, not the battery, anyway.

It takes a little practice to re-cell a battery, and opening and then re-sealing the casings is itself non-trivial. But if you're skint, and can yourself use a soldering iron or know someone who can, re-celling a battery is in my opinion a great way to spend an evening. Especially if you do it with fully-charged cells and short them out by accident.

(Here's a piece I wrote about re-celling an Apple eMate battery, which is just 1.2V AA cells but is still the same sort of thing. Cells with solder tabs are much easier for newbies to deal with, but the tabs may bulk the assembly out too much for it to fit into the casing.)
posted by dansdata at 10:57 AM on May 10, 2008 [3 favorites]

Piling onto what dansdata said, he's right.

I would only add that in my experience, an off-brand battery with an incorrect voltage *could* cause problems in the laptop. It can deal with not-quite-right voltages, but doing so might cause added stresses to the voltage regulation module (VRM) within the laptop. If it's too high, the VRM will run hot trying to burn off excess. If it's too low, the VRM will have to work too hard to get the correct voltages. And the charging circuits in laptop won't be charging the battery correctly if it wants different voltages than the charger wants to supply.

If it was me, I'd try to get a battery that is IBM/Lenovo branded. And as he said, from a dealer with high turnover. Maybe even directly from Lenovo. Or if you can, one of the companies that supplies their OEM parts.

You'd hate to save $40 on a battery, only to cause a system board to fail and create a $400 repair bill.
posted by gjc at 8:37 AM on May 11, 2008

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