Tablet battery mod

[jc0r]

Hi all. I am looking for some advice regarding lithium batteries. I am in the process of making my own portable mini pc. The pc itself is a small motherboard out of a 7" tablet. This is powered by a standard single cell 3.7v lithium battery, however the capacity is only 3500mah. I am looking for significantly more capacity, approximately 10000mah.

Having a look on eBay I've stumbled across this

http://pages.ebay.com/link/?nav=item.view&id=111286960678&alt=web

The reason I am looking at this specifically is because of the dimensions. Most 10000mah batteries are in excess of 140mm long and around 100mm wide however they are thin. Thickness is not so much as an issue for me, but the enclosure I need to use dictates I cant really look at length our width more than 80mm.

My concern with the battery I have linked is, although it fits my criteria for dimesions, I am unsure what effect 3 smaller cells paralleled together will have with regards to charging and balancing. I plan to charge the cells in the same manner that the existing 3500mah cell is, using the tablets micro usb port. Is this something I need to be concerned about?

Many thanks

[Mephitus]

So long as they are the same rating/size, they should balance out charging without any worry. Heres some information you might find useful for setting up the charger circuit: http://www.atbatt.com/chargers/sla-battery-charger/how-to-charge-sla-batteries

[amyk]

Lion cells can be wired in parallel and they will act like one big cell. Make sure you get their voltages exactly the same before connecting them together, otherwise huge currents will flow between them.

Charging will obviously take longer, but shouldn't be a problem as long as the charge/monitor circuitry is not "overly smart" (unlike the ones in some laptop packs).

[jc0r]

Many thanks for you help. I'll give it a shot then 

[michaeliv]

The battery that you link to has built-in circuitry to deal with the cells separately so basically you can think about it as a single cell for the purposes you're looking at.
You can also put 4x18650 batteries in parallel - you will get a 65mmX70mmx18mm battery with 10000mah for about $10 (get the cells from a laptop battery) or 13500mah for about $30 - use NCR18650B  3400mah cells.
Also, by using LI-ION you avoid all the issues with LI-PO battery such as easy to damage / lower endurance / higher cut-off voltage / etc.

[jc0r]

Thanks Michaeliv. I did look into using 18650 cells for the reasons you had stayed. My concern though was, when the existing LiPo battery in the tablet runs flat, my multimeter reports a voltage of 3.7, I assume fully charged it's around 4.3v. I see that 18650 cells report flat around the 3v mark, if that's the case, I doubt the motherboard would be able to operate at such a low voltage?

[michaeliv]

The reason for that is that the built-in circuitry doesn't allow the lipo to discharge below 3.7v because lipos get damaged easily. Fully charged it should be around 4.2v.
Your tablet has it's own cutoff voltage. If It can't opperate down to 3.0v then it will know that and shut down at 3.5v or whatever. Which is something else to keep in mind, if the tablet was made to discharge down to 3.5v - when you reach 3.7v the tablet will display that you have 5-10% battery life but then suddenly shut down unexpectedly because the LIPO circuitry decided that the battery is fully discharged.

[jc0r]

So if I was to use 18650 cells, from what I can see, I would need to roughly double the capacity for whatever I desire as they tend to hit 3.7v at 50% capacity as this chart would have me believe?

http://www.all-battery.com/productimages/li-ion/18650-2200mAh-discharge-curve.jpg

[michaeliv]

What do you mean double the capacity ? How is 3.7v relevant here ?
The chart you give is for discharging at 0.5C.
If you have a high enough load the cell will hit 3.7v after 1 second of usage - even if fully charged.
Check out

- at 5.0A the cell reaches 3.7v almost instantly.
Then when you disconnect the load the voltage will go back up ( this is probably what you measured .. ? )
From what I can see there would be no obvious advantages to using LIPO for your scenario other than the slightly smaller size. LiPo are generally used when you need funky battery shapes / high drain currents.

EDIT: Oh I think I get it about the 3.7v. If you think that The LiPo will give you 10k mAh when discharging to 3.7v but a simmilar Li-Ion will give you 10k mAh when discharging to 3.0v - I don't think that that is the case. I haven't seen this stated anywhere explicitly but the mAh rating is likely the total charge stored in the actual battery chemistry, regardless if you can use it or not - so if anything the 10kmAh LiPo will give you less mAh because of the high cut-off voltage. The reason why I think this is that LiPo and LiIon have similar energy density( high-end Li-Ion a bit more ).

[jc0r]

Yeah, before I go ordering what could be a potentially dodgy LiPo pack of eBay from China, I'm gonna make a battery pack out of the 18650s and see how they fair.

The Panasonic NCR18650B you mentioned seemed to be one of the best there is, just need to find a place to buy them here in the UK

Many thanks for your help and patience

[mikerj]

The battery that you link to has built-in circuitry to deal with the cells separately

I really doubt there is any circuitry in the pack.  Why would you need any for cells in parallel?

[michaeliv]

The battery that you link to has built-in circuitry to deal with the cells separately

I really doubt there is any circuitry in the pack.  Why would you need any for cells in parallel?

I was saying that you don't deal with multiple individual cells, you only deal with the overall battery - and you can let the designers worry about how they avoid issues with cells in parallel (If any, not implying any would exist) - so you can treat the battery as a closed box system, not a interconnection of cells.
The 'circuitry' could be a direct parallel connection between individual protected cells.
Or it may be a parallel connection between unprotected cells with protection circuitry on top of them.
Or It might be that the unprotected cells are individually connected to the smart protection circuitry which ensures that if one cell gets damaged and drops to a very low voltage, the other 2 cells won't try to charge it therefore dropping voltage and getting damaged as well. Also possibly completely isolating badly damaged cells.

[onlooker]

It will be easier to just buy a USB power bank case like this,

http://www.ebay.com/itm/USB-LED-Mobile-Power-Bank-Charger-Pack-4pcs-18650-Li-ion-Battery-Case-Holder-/331553298945?pt=LH_DefaultDomain_0&hash=item4d321dda01
(Yes, I bought 2 from the seller recently. I do not yet know if they can work to the spec).

The case is a little too long for your use, but you can use the board. Then you need to provide 4 (or more) pieces of 18650 of your own.

It can supposedly supply max  3A @ 5V.  It has all the circuitry/"protection" needed to charge and discharge the batteries.

It is also better to use the 5V output (your tablet MB should have a usb charge port), instead of, directly connecting the PC power input to the battery terminals, since you can further avoid the limitation of the 3.7V cutoff you mentioned (If the 5V output is too high for your PC input, you can add a step-down converter from 5V to 4V for a few dollars).