Building custom rechargeable Li-ion batteries

Discussion in 'General Electronics Chat' started by jameselder232, Nov 7, 2010.

  1. jameselder232

    Thread Starter Member

    Oct 31, 2010
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    Hi, i wish to build a Li-ion battery from an old laptop battery pack. Now i have been researching and also have the battery pack opened and can see the 4 cells.

    Currently the pack has a rating of 7.4V 5200mAh capacity (2 cells in parallel, 2 parallel branches in series).

    Now I'm looking to add an extra couple of cells to increase the capacity of the unit, and so I'll add them in parallel to maintain the 7.4V voltage. I was wondering do the extra cells need to be the same capacity as the current cells? I know they have to be the same voltage, but I'm wondering about the capacity. And I'm planning to use the same PCB protection circuit that is present on the original pack, this should be okay right?

    I want to raise the capacity from 5200mAh to above 8000mAh if possible.

    Lastly if you have any advice as to where i can buy cells cheap in the UK please let me know.

    Thanks,
    James
     
  2. tom66

    Senior Member

    May 9, 2009
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    DON'T do this.

    The battery's electronics are specially designed to charge the cells. Temperature monitoring, current balancing, etc. all must be maintained, or your battery will either die quickly, or worse, it could burn (Li-ion and Li-po cells are very hazardous if abused.)
     
  3. Artikbot

    Member

    Nov 7, 2010
    45
    1
    The cheapest and best cells in price/performance/capacity in all the planet are found in Hobbycity.com

    Chinese, dealing BRUTALLY good batteries for less than you'd spend going out to have lunch at McDonald's. Shipping is VERY low aswell.

    I've got about 4 packs of em, from a tiny 7.4V 1Ah to a huge 14.8V 5Ah pack. All of them are super high discharge ratio batteries.

    Lithium Polymer chemistry, but as long as they're rated at 3.7V each cell, you won't have any problems with them. Li-ion and Li-Po are exchangeable with no compatibility issues. Just that lipos have higher discharge ratios and better weight/storage ratio :)

    If you're a radiocontrol hobbyist I'm sure you know em... I'm using them on a 530-Watt continuous BLDC motor, with more than 1.5kW peaks and they resist like bosses. One has two years on its back and still kicks butts like the first day. Awesomesauce ;)

    Edit: Of course, considering you don't exceed original pack's ratings, or even worse, remove charging circuit! As tom66 said, they're very very very dangerous (they tend to show up as huge fireballs of several feet in diameter) if used improperly.

    Before replacing the cells, make sure your charging circuitry isn't on the battery pack itself.
     
  4. tom66

    Senior Member

    May 9, 2009
    2,613
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    Bought a laptop battery from Hong Kong for my Inspiron 1300 (older laptop) for about £36, after 3 months it had packed in. Obviously this doesn't apply for all batteries from China, but buyer beware, you have little recourse if it goes badly.
     
  5. Artikbot

    Member

    Nov 7, 2010
    45
    1
    It's not a laptop specific battery pack itself, they're hobby hyper performance battery packs.

    Fast shippings, fair prices, and above all, EXTREME performances are what this batteries boast. And the best of it... Batteries don't meet the specifications... the EXCEED them xD

    With shops like this, I don't bother buying my training sessions packs from any other place in the world. Except for when it comes to racing-approved packs, then I'm forced to buy super expensive hard case approved packs.
     
  6. tom66

    Senior Member

    May 9, 2009
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    Oh yeah, those - sorry, too quick to post. They are very popular, very cheap and often meet their specifications, if not exceed as you say. I'm using one in my plane at the moment.
     
  7. jameselder232

    Thread Starter Member

    Oct 31, 2010
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    0
    The charging circuit is completely separate to the cells with 3 wires and 2 contacts connecting it to the cells (Obviously the contacts are temperature probes).

    I'm not a radio-control hobbyist, i'm actually planning to build my own rechargeable battery for a piece of equipment as to buy the manufacturer's one will set me back £70 at least.

    Sorry i just want to double check, as it wasn't clear from your post.

    Is it okay to use say 2x2600mAh & 1x2000mAh all in parallel to each other? Or should i stick to the original capacity of 2600mAh for all cells?

    If it safe to use different manufacturers in the one battery pack as long as they have the same rating?

    And lastly is it okay to increase the capacity from 5200mAh to 8000mAh and use the current PCB control circuit?

    And thanks for the advice tom66 but do you have any theoretical knowledge that suggests the control circuit will only work for the current 5200mAh capacity? As far as I'm aware, it is there to charge the cells to the voltage of 3.7V each, and then it controls the circuit to switch off when the current falls below a set percentage, and so the capacity should not matter.
     
  8. Artikbot

    Member

    Nov 7, 2010
    45
    1
    Stick to stock capacity. Don't do experiments with such a filmsy circuit (laptop's integrated circuits are VERY susceptible to changes of any type, you don't want to ruin it).

    Just make sure you follow the wiring of the pack in order to get balancing tabs for each cell, an unbalanced pack leads to poor performance, short lifespan and even cell failures.

    This might be useful :)

    http://scriptasylum.com/rc_speed/_lipo.html


    And remember to double check what does each charger pin do, and to which cell is it connected. You don't want to blow a cell up... And your desk, your laptop and possibly your hands with it ;)
     
  9. tom66

    Senior Member

    May 9, 2009
    2,613
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    Each cell is usually individually monitored for temperature, voltage and current. It's like a life support machine. Without it, you can get spectacular results.

    No - think about it, one will charge faster than the others leading to cell imbalance causing more problems.

    As mentioned before, it's not really safe to upgrade a pack. If you do upgrade it, the batteries should be matched - that is, you have to use batteries with similar parameters and not just capacity (terminal voltage, charge-discharge curves, charging temperature, etc.) Replacing some of the cells with a different type can cause problems.

    Well, I don't know the circuit. But I am talking from experience. The control circuits are designed for each battery pack to get the greatest charge/discharge efficiency. And, they have safety mechanisms built in, like temperature and current monitoring (usually for each cell.) If it doesn't have any expansion mechanism for adding more cells, I'd avoid it.
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    My electric model airplanes came with Li-Po batteries from China and I bought more. They quickly fail.
    The battery cells are 3rd generation.

    Now I can buy 4th generation cells that are less expensive, provide a lot of power for a longer time than the other cells and don't fail. Win, win and win. You don't lose anything if you buy them.
     
  11. jameselder232

    Thread Starter Member

    Oct 31, 2010
    40
    0
    Okay well i don't think i made it clear before. But basically i have a battery from a laptop which will not work due to motherboard issues. And i now need a rechargeable battery for a portable oscilloscope, so i figured i may as well recycle the laptop battery seeing as that still works fine.

    The laptop battery has a rating 7.2V 5200mAh.

    The stock rechargeable battery for the oscilloscope (if i were to buy it) has a rating 7.2V 8000mAh.

    So basically i plan to use that laptop battery to power the oscilloscope seeing as the voltage is the same. Its just the capacity is less, and i know this just means shorter battery life so should not affect the device in any other way, but if i could make the laptop battery perform like a stock battery it would be great that's all.

    Thanks,
    James
     
  12. tom66

    Senior Member

    May 9, 2009
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    Are you sure the interface is the same? The scope probably wants to communicate with the battery to determine important things like voltage, state of charge, etc... it will be a bugger interfacing it.
     
  13. Artikbot

    Member

    Nov 7, 2010
    45
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    Wait wait wait.

    7.2V... Li-ion chemistry? Ni-mh? Li-Po?

    If it's Li-ion, no problem. If it's Ni-Mh or NiCd, you're screwed...

    Since the pack you have there in the scope is bigger than the one in the laptop, this last one might fry during charge due to a higher C charge ratio (undesirable to go above battery's stock specs).

    I think you should consider making a fresh pack out of Hobbyking batteries, because they're very high current rated (above 25C SUSTAINED discharge ratios and 2C safe charge ratios).

    Good luck!
    Marc
     
  14. jameselder232

    Thread Starter Member

    Oct 31, 2010
    40
    0
    As for the website for LiPo batteries. I have opened the laptop battery pack already and confirmed that inside there is:

    4 cells arranged into two set of two parallel cells then connected together in series.
    The total rating of the pack is 7.4V and 5200mAh.
    There is a PCB inside, connected to the cells through 3 wires as the balancer hookup. With 2 probes connected which are stuck to the side of one of the cells.

    I therefore worked out each cell must be 3.7V 2600mAh.
     
  15. Artikbot

    Member

    Nov 7, 2010
    45
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    So, your battery has built-in charging (or at least balancing) circuit.

    What's inside your scope's battery? If there are just cells, you should be able to do a fresh pack quite easily.

    If not, you should get the original replacement.


    Luck with the battery :)
     
  16. jameselder232

    Thread Starter Member

    Oct 31, 2010
    40
    0
    Thanks for the advice.

    With the interface i was just planning to wire the positive and negative wires from the battery pack direct to the pins on the scope, and neglect any of the data wires??

    They are Li-ion cells.

    If i make a fresh pack out of Hobby King cells i then need to create some kind of control circuit, and i figured this may be complex?

    One thing i did not consider though is how i would charge the laptop battery pack now, seeing as the motherboard is screwed, and so therefore i suspect the charging circuit on the motherboard is also screwed. If i used the scope to charge it i would probably encounter problems like you say with incompatible current flow.

    If i could get some advice with making my own battery pack i wouldn't mind giving it a go. Its just i thought i could recycle my current battery pack and save myself some money.
     
  17. jameselder232

    Thread Starter Member

    Oct 31, 2010
    40
    0
    Well the battery has wires connected as the link you showed me shows for a balancer hookup, so therefore i assume it does.

    Also i don't actually own a rechargeable battery pack for the scope as its an optional extra so i can't check inside and see the current setup. Its mains powered with the option of buying a rechargeable battery pack to make it portable. (3-4 Hour battery life with stock battery)
     
  18. Artikbot

    Member

    Nov 7, 2010
    45
    1
    If the scope has built in balancer, no problem.

    if it doesn't... Get an original replacement.

    No, you can't wire cells directly, that's gonna cause an unbalanced and low performance pack.

    You need to balance them, either externally (for my models I use a 20€ external discharge balancer with sensitivity up to the mV scale, just balance the packs right after every charge and job done) or internally, with the scope's built-in balancer.


    You'll need to check inside the scope's pack to see how it's wired up, and replicate that diagram but with fresh cells.


    Regarding laptop cells..
    I guess you lose nothing in trying. How much does the scope battery take to charge? If it takes 4 or 5 hours, you're fine. You're not gonna exceed the laptop pack's specs, they're usually rated for 1C ratios, although 0.5C recommended for mainstream Li-ion chemistries.

    It's late today... Will keep postin tomorrow morning :)


    Good nites, and good luck!
    Keep us informed!
     
  19. jameselder232

    Thread Starter Member

    Oct 31, 2010
    40
    0
    Okay thanks for all the help. In answer to a point you made, when i said directly, i meant from the ± terminals on the outside (plastic casing) of the laptop battery, thus i will still be using the pcb that is currently attached to the cells.
     
  20. jameselder232

    Thread Starter Member

    Oct 31, 2010
    40
    0
    I've had another idea.

    Firstly i checked out the official battery and it seems it doesn't hold its full charge capacity for very long, and has quite a low charge cycle count. So these are two big reasons why i would like to build my own battery, even if it ends up costing the same as the stock battery.

    So i found these batteries:

    http://www.bestofferbuy.com/trustfire-protected-18650-lithium-battery-2500mah-2pack-blue-p-6426.html

    They have built in protection. I was wondering if i connect a few of these together to give my 7.4V rating and also say 7500 or even 10000mAh, would it be safe seeing as the batteries have individual protection circuits, and so I'm assuming they will individually charge to the correct voltage and capacity?

    This way i could make a nice high capacity battery pack out of 8 batteries (4 packs). Connecting 4 in parallel (then repeating this), and then connecting these 2 sets of 4 in series, i would end up with 7.4V and 10000mAh capacity, and i wouldn't have to worry about over-charge.

    Just throwing some ideas about as the stock battery is awful from what i have read, and is costly for what you get.

    Thanks
     
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