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Need help with adding batteries and a charging circuit/PCB to my project? Answered

Hello all willing to help. I am building a laptop/dock thing for my shield tablet. I need to add a charging circuit or PCB to make my project portable. I am working with this board for the LCD from an old laptop as well as a few lithium ion batteries. what I need is a circuit that can charge the batteries as well as deliver 12v to the LCD & 5v to the tablet. The board for the LCD came with power brick that outputs 12v 4a max( I also have the power brick for the laptop that I pulled the screen from). If more information is needed leave a comment I will reply as fast as I can. Thanks in advance for all the help

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-max-

Best Answer 2 years ago

5V and 12V switching regulators are very common these days and you can find chips to use and the datasheets for them will tell you how to use them and all the performance data. Making one would be very cheap especially if you reuse chips from old junk. But making a good (low noise) switching regulator on a protoboard by hand is difficult, just make sure to keep connections close together, short and thick, with ground plane. Just make sure the chip you use is able to accept the voltage range of the lithium batteries, and of course can output the voltages and currents you need. for 4A, you may need an external switching transistor driven by the special IC.

http://www.mouser.com/Semiconductors/Power-Managem...

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You can also buy a module. That has a little less design flexibility, less options but more likely will work as expected. Probably the better choice for those not familiar with electronics. These are pretty common, can be found on eBay, amazon, DIY places, etc. like the other case, try to find one that suits your needs well.

https://smile.amazon.com/s/ref=nb_sb_noss_2?url=se...

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As for charging circuit for lithium ion, it is pretty simple. Constant current at less than 1C until 4.20V is reached, then constant voltage until the current falls to 1/10th C. (and if the battery is lower than 3V per cell, either refuse to charge or at 1/10th C until the cell voltage is brought back up to 3V.) You can even use a lab bench power supply setup with 4.2v @ 2A (maximum) or so for a good 18650 cell. Similarly, there are plenty of charger chips on the market, and a few different types and stuff. For this, the extra complication of switch mode charger is unnecessary for lower current charging. And of course you can also use a prebuilt module as well if you are not up to the task of soldering up and testing a chip based on the application note.

If you get the monitor powered with 12V, there is a very high likelihood it already has a 5V and 3.3V regulator in it for logic, and the 12V may only be for just the backlight. If you do not need much current, you can probably tap off those voltage rails to save space.

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kleinish-max-

Answer 2 years ago

I picked out this protection module( http://www.all-battery.com/protectioncircuitmodulepcbfor111vli-ionbatterypack3cellswithalimit-32006.aspx ) for ease. I was wondering if you could check my wiring diagram to ensure that my circuit is correct and safe. thank you

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-max-kleinish

Answer 2 years ago

Actually, unless the cells are very carefully matched, (they should be from the same batch from production, which is very hard to guarantee unless you order them straight from the factory.) do not connect any in parallel at all. It is just too many things to consider and worry about, because charging them in parallel can be a problem, as charging requires a constant current process, and due to differences in ESR and stuff, Even if the entire parallel arrangement is receiving a fixed current as a whole, each battery might draw a completely different amount of current based on the specific voltage across the parallel arrangement. So I would recommend to just not do it. If the cells have protection PCBs it might be more OK, but not recommended.

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kleinish-max-

Answer 2 years ago

All the batteries that I will be using came from the same laptop. connecting them in parallel should give me more amp hours correct? I will have them all charged to as close to the same voltage as I possibly can as you mentioned in your previous comment. I don't see the circuit pulling more then 1 A max. I will do some more extensive testing before I connect all the batteries. I did read this article on the characteristics of rechargeable batteries (http://www.ti.com/lit/an/snva533/snva533.pdf). I will continue doing research before continuing with the construction of the circuit if you have any suggestions on articles that would help me please link them. thanks for all the help.

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-max-kleinish

Answer 2 years ago

That is not a bad article, but it is only the very bare bone basics, and doesn't discuss all the problems that can occur in (especially older) batteries in parallel. Realistically, you will probably be fine if you take care of the pack and treat it with caution, but there is a greater chance something can go wrong. How were the batteries originally connected? Were any of them in parallel? Keep them in the same configuration if they were would be the safest. They were carefully matched in factory to have as close to equal capacity and ESR so that they would share current equally during charging and discharging.


So long as each individual cell does not discharge more current than it can provide. (which means that the maximum you should pull from any 2 cells in parallel is the maximum current a single cell can provide just in case the partner cell is a dud and not working.) This is especially true for charging. You'll need to charge the parallel pack more slowly so as to not exceed the maximum current flow through any individual cell, for the same reason as before: If one of the cells in parallel acts like a dud, then the other will be taking the brunt of the current, Which means that the charging current needs to be calculated as if you were only charging 3 of those cells in series rather than a 3S2P pack. In other words, based on calculated capacity, the current needs to be chosen such that it would take 2 hours to charge in theory. (If one of the cells was in fact a dude, however, then it might not take 2 hours anyway due to the lost capcity)

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-max-kleinish

Answer 2 years ago

Thanks for best answer! When you connect batteries in parallel, make absolutely sure that they are charged to exactly the same voltage with a multimeter. 0.1V difference could lead to damaging currents through the cells, where the only limiting factor to current is ESR and resistance of the wires connecting them. Also, it is best to make sure that the capacity of each bank of 3 parallel cells works out to be exactly the same capacity (AH).

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Also, double check the tabs on the cells are connected to the correct pads on the PCB. I think a few of them are mixed up, which might lead to either an error light or magical blue/grey smoke. And connect the voltage regulator PCB you plan to use directly across the battery pack.

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If you have not bought batteries yet, I recommend picking up several of those plain green panasonic 18650 batteries. Just beware there are many fakes on the market and any battery that is xxxFire is garbage.

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kleinish

2 years ago

I am planning on using mains. Maybe a split powered mod in the future

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iceng

2 years ago

Solar charger or mains for power ??