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Tablet battery mod

Discussion in 'Power Electronics' started by jc0r, Jul 3, 2015.

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  1. jc0r

    jc0r

    3
    0
    Jul 3, 2015
    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
     
  2. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    Hello

    The problem is not balancing as you can balance cells like this that have protective circuits built in. The problem is the charge time. I am not sure on the battery charger IC use in these tablets maybe you can take a picture of it for us.

    Most of them are current limited to a certain amount. Say 1 Amp, so your 10 Amp hour battery is going to take more than 10 hours to charge. Maybe even longer if the charger has a built in timer, which most do. Say 6 hours so your battery will be less than 50% charged because the charging process is not 100% efficient.

    You may have to look into a more powerful charger that can charge these batteries and bypass the original battery charge IC.

    Adam
     
  3. jc0r

    jc0r

    3
    0
    Jul 3, 2015
    Hi Adam

    Many thanks for your response. Here are a couple of links to pictures of the 7" tablet i was planning on using.

    http://s10.postimg.org/eq4spkpsp/20150703_190137.jpg
    http://s10.postimg.org/4mg2tf921/20150703_190348.jpg
    http://s10.postimg.org/ytof1mhsp/20150703_190418.jpg

    However, have done some more reading and taking on board what you have said, i think i might be coming a little unstuck! As you can see from those pictures, there are 3 wires (red, black and yellow) from the battery protection board to the motherboard. I have since learnt having looked on the actual protection board, the yellow wire is marked T whereas the red and black are marked B+ and B-. I did get the multimeter on the yellow wire and it reads as ground along with the black. It would appear this T wire is for temperature protection? I did unsolder it and tested very briefly whether the unit would power and charge which it did, although i assume this wire is there for a reason and safety would be compromised.

    As most of the cells you can buy already have a protection board of their own but also only have red and black wires coming from them, i guess i would need to connect the existing protection board that came with the tablet to the new cells. How the existing protection board would handle mulitple cells, i have no idea.

    So, anyway, ive had another thought. I have another 10" tablet that i thought about using. The motherboard is similar in size to the 7" model, however, the 10" has two battery cells. paralled together and the capacity is not far off what i need anyway at 8000mAh. Unfortunately, the battery cells are very long so are not suitable for my project, however, i thought if i can get two other cells of similar capacity yet in more suitable dimensions, i could stack both those cells ontop of each other and wire them to the protection board that came with the 10" model? The other bonus with the 10" model is that it is charged with a seperate 5v DC style connector rather than using up the micro USB port.

    Heres some pictures of the 10" model.

    http://s10.postimg.org/p1h5i8hi1/20150703_191250.jpg
    http://s10.postimg.org/751ru3us9/20150703_191125.jpg


    Hopefully what i have said makes sense, once again, many thanks for your time.
     
  4. Arouse1973

    Arouse1973 Adam

    5,165
    1,087
    Dec 18, 2013
    I think its best not to mess with batteries, I could tell you a story about that :) and use equipment that was designed for the purpose. So if you have OEM equipment that might work, go for it.
    Adam
     
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