Using cell phone batteries in a laptop...

Preamble:
I have a Zbook 15 laptop that I'm building up into a credible gaming Rig where the dvd drive is sacrificed to fit in a TBD mxm-b card and additional 100-150w of cooling(by default there is about 100w of cooling)

The meat of this post:
Today I had something of an ephinany if I use four 3.7v cell phone batteries and hook them up in series it would give me 14.8v and could then be connected to the secondary battery connector of the laptop and allow me to hotswap the main battery when needed.

If small enough I could fit the battery pack into the 2.5 drive bay and simply use mSata drives for storage.

Questions for all of you:
-What are my oversights here?
-what are the highest capacity batteries that can be fit 4x in a longer and deeper than averge 2.5 drive bay?

 

I did some measurements and discovered the 2.5 drive bay can fit an item with 14.8/70/100.5mm HxWxL dimensions(with light dremeling) and with that in mind I set out to find the highest capacity batteries that could be fit into that space at x3(11.1v) and 4x(14.8v) and found a very cost effective 5 pack of 3.5/70/95 mm batteries that are 3.7v and 3000 mAh for $16

According to http://h20195.www2.hp.com/v2/GetPDF.aspx/4AA4-8356ENW my laptop's battery pack is 14.8v(8 cell) however the external battery that is compatible with my laptop is listed at 11.1v(9 cell) both use 8pin connectors and I'm not sure of the pinout for the external battery, Anyone got some insights? I tried looking up HP battery piunouts but I only found the 6pin battery pinout

 

I highly recommend you give some serious thought to safety before proceeding any further. Lithium batteries are great because they're energy dense, but that also makes them dangerous if anything goes wrong. Brand-name battery makers build multiple layers of safety in for this reason, but those batteries you're looking at aren't brand-name and you may find that you inadvertently bypass some of these. Even with all of the safety you still hear about battery incidences every few years.

The principle protections are for overcharge. Lithium batteries do not tolerate overcharge, and if they are overcharged they go into thermal runaway, potentially leading to what battery engineers call "venting with flame" but most normal people would call it a battery explosion. In addition to flame, it could also vent toxic gases as well.

Lithium batteries are also charged using complex algorithms. In theory you could charge one using a constant (safe) voltage, but this leads to reduced performance. Most chargers start out constant current, switch to constant voltage, and then cutoff at the end. If your cell characteristics aren't what your laptop expects, the charger may not end up cutting charge. In that case those cells you linked to have built in overcharge protection. However you are trusting that the OCP on your bargain chinese cells work, and that you don't inadvertently damage them.

Honestly I think the chance of overcharge problems are pretty low. That being said you'll likely have reduced battery life/longevity due to the mismatched charging curve. However, there are plenty of other ways for your battery to blow up.

Batteries can be damaged by mechanical stress. The damage can rupture insulating membranes, leading to an internal short, runaway thermal, and venting with flame. Here is video of a battery catching fire after being damaged.

Mechanical stress also includes thermal expansion, which all batteries undergo. In your case, if you stack those cells four high, you'll end up with a battery volume of 14x70x95mm, which gives you 5%, 0%, and 5% margin for thermal expansion. You are almost certainly going to run into thermal stress issues, which may or may not light your laptop on fire.

Finally even if you do everything right, there is a chance a manufacturing defect in the cell could cause a thermal runaway. I assume you won't be x-ray inspecting all of the cells you buy, in which case you have no way of knowing until it blows up on you.

None of what I've posted here is hypothetical. The last two issues, mechanical damage and manufacturing defect, are two separate independent causes of Samsung's Note 7 fires.

 

This was quite the in depth post. thank you for elucidating my oversights.

Preamble
In the past I had taken a Li-ion battery out of a deeply discounted iPod Mini charger and used it to replace 3 failing NiCd batteries in a Power driver I had. The transplant worked but the battery had no protection without the rest of the ipod charger product and ended up nearly doubling in size after I left the whole thing charging in hindsight I'm lucky I hadn't sealed the whole thing up yet and cause a lithium fire. other than preventing it with OCP I for one gave zero consideration to margin of error for thermal expansion.


TL;DR:
I was already thinking about OCP and using some kind of copper or aluminum sheet to dissipate heat from batteries into the Aluminum laptop chassis but what else can I do to make this project safer?(other than X-Raying cells) or should I just stop before I start?

 

I'm not a battery engineer, so I can't really tell you what are standard safety measures to take. I just know enough to know they can be dangerous. You could try asking around in more specialized forums like electrical engineering or battery engineering. Honestly though I think they're likely advise you against doing it.

BTW, that iPod mini battery that doubled in size wasn't from thermal expansion. It went into thermal runaway. Fortunately for you the battery contained it internally rather than venting flames and gas. The fact that it was a high quality name-brand battery probably contributed to the extra margin of safety.

And there is where I think your catch-22 is going to be. The high quality big name-brands generally won't sell you batteries because they don't want liability for safety issues, so you're left stuck buying cheap no-brands from Asia.

 

The battery and it's Mobile charger source were both generic but otherwise solid advice and I'll see if there is a more appropriate forum for this question.

In the meantime I think I'll just use an adapter to connect my 8 pin secondary port to spare brand name battery packs that I have

 

I wouldn't mess with it, other than for experimentation. To use it on your regularly used laptop, is only asking for trouble. My thought, just buy a small portable battery bank, fix it inside your laptop with the cord out the back somehow, and just go that route. Not as elegant, but you can just plug it in your laptop power input when you want to hotswap, and it will be an extra power cable to charge the battey bank, but likely can charge through your laptops USB port when it's plugged into the wall.

But honestly, just set up your laptop to hibernate, and swap battery that way a lot more clean execution and no hardware modifications needed. Will take no more than 60 seconds to hibernate and resume from hibernate.

Although you do bring up a good point. Ages ago, before external swappable laptop batteries became a small minority instead of a regularity, I thought manufacturers should include a small capacitor to give you a few minutes of battery life to hot swap a battery. But these days most laptops don't even have an easily swappable battery. They're all fixed internally.