Apologies this has probably been asked before - I have been looking but I think I don't know the terminology to answer my question.

I have a micro-controller based project and I want to power it with three LiPo 2000 mAh cells in parallel. I think I understand how to do that part the issue is with charging the batteries - which I know how to do separately but the design is such that the batteries cannot be removed.

Ideally, I want the batteries to charge quickish (couple hours). I thought about using USB but 500mA would be a slow charge. I would like to use a standard UK wall plug which I think is 12v which would be too powerful for the components in my projects (4.7V).

So I need to kind of turn the current off (is this called a circuit breaker?) just when the batteries are charging but want this to work automatically and then when they are charged I can remove the charger and turn the project on manually with a push button.

Another thing is I need to keep the charging components really small too like under 20 mm width and 60 mm length. I would like the wall plus to be mini USB ideally.

Also if the battery charging is kinda on a separate circuit (or part of the circuit is turned off - including the micro-controller) could I use AC rather than AC-DC

I hope that makes sense. Any help is hugely appreciated

  • \$\begingroup\$ Suggestions: The following pedantic observations are offered with the intention of making your questions less likely to be "beat up on" and closed by the beat-up-&-close brigade.| This is a general micro-controller based question - mentioning specific brands can be counterproductive. Try to use slightly more formal language and make sentences "scan" in English. Use mA and mAh correctly - they are NOT interchangeable. | like -> eg, kinda, i \$\endgroup\$ – Russell McMahon Oct 29 '15 at 3:27
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    \$\begingroup\$ With your level of knowledge I strongly advise that you obtain a prebuilt LiPo / LiIon charger or at a minimum use a purpose designed IC or module. | A LiPo cell will usually allow charging at up to 1000 mA per 1000 mAh's of capacity (= C/1 rate) BUT see data sheets. So 3 x 2000 mAh cells in parallel would allow up to 6000 mAh max charge current or 3A for 2 hour charging to about 80% of capacity. 500 mA charge will take over 6000/500 = 12 hours. | Using a simple supply to charge will destroy the cells. \$\endgroup\$ – Russell McMahon Oct 29 '15 at 3:31
  • \$\begingroup\$ Thanks for the tips regarding how to ask a question - I will take this on board. \$\endgroup\$ – 1debstep Oct 30 '15 at 18:52
  • \$\begingroup\$ The information about the batteries is useful however, I don't think I made my question clear enough. what I was wondering about is how I would stop the current flow circulating through the rest of my project whilst the batteries are charging. \$\endgroup\$ – 1debstep Oct 30 '15 at 18:57
  • \$\begingroup\$ If that's the main question it is easily addressed. Having the battery charging or not makes no real difference to the connected load. At all times during charging a LiPo Vbattery is within the range you might see when the charger is disconnected. The charger will NOT "make current flow" in the load any differently than the battery alone would. If the load can deal with the battery voltage as required the charger adds nothing to the problem. \$\endgroup\$ – Russell McMahon Nov 3 '15 at 13:29

If you want to deliver a full charge to LiPos, then you need something a little more sophisticated than 'switch it off when charged'.

LiPOs use a 2 stage charge.

In the first stage, the terminal voltage stays below 4.2v, and the charger limits the current to within the manufacturer's specifications for max charge rate. Depending on the quality of the cell, this phase need only take minutes, or with lower current you can drag it out as long as you like.

The first stage finishes when the cell voltage reaches 4.2v. At this stage, the cell only has around 70% full charge. Note that some commercial 'fast chargers' call 'charged' at this point.

In the second stage the charger limits the voltage to 4.2v, and the cell takes an ever decreasing current. Do not exceed 4.2v, or you will destroy your cell. If you stop the first stage at 4.1v, you will improve the life of your cell somewhat. The second stage finishes when the current drawn by the cell has dropped to 0.03C. This stage may take an hour, and cannot be rushed without damage to cell.

You can obtain LiPo charger ICs that will do the control and threshholding for you.

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    \$\begingroup\$ Not a big deal, but most of the current chargers that I have seen terminate the second stage of charging when the charge current reaches 10% of the current used in the first stage. \$\endgroup\$ – Filek Oct 29 '15 at 6:57
  • \$\begingroup\$ Thanks for the information LiPOs it is useful. I am not sure my question was particularly clear. \$\endgroup\$ – 1debstep Oct 30 '15 at 19:12
  • \$\begingroup\$ The crux of my question is how I would stop the current flow circulating through the rest of my project whilst the batteries are charging because they won't be removeable. Saying that if 4.2V is the max the rest of my project could handle the current. Is this bad for the components though? \$\endgroup\$ – 1debstep Oct 30 '15 at 19:20
  • \$\begingroup\$ It's easy enoguh to put a FET switch between the batts and th rest of the circuit, or disable a regulator. I don't understand your sentence about the rest of the project handling the current. \$\endgroup\$ – Neil_UK Oct 30 '15 at 21:00

This will not anwser your question, but I'll post here because I can't comment still. Obviously, i think this may help in certain way.

There is no such thing as "standard wall plug", and you can find many types on the internet or in most eletronics store. One exemple is this one that I've found on ebay, which has 5V output that may fit your needs: Link

You can see that its rated for 2A max current. As you are using 3 batteries with 2Ah, that will take 1 hour per battery, or 3 hour total to charge then. I think may be hard to find a wall plug rated above 2A.

Other thing is that USB 3.0 has a dedicated charge channel that can handle 1.5A, instead of the 500mA you mentioned. This will make the charge time go up to 4 hours, or 1 hour and 20 minutes per batery. You can read more about USB here: Link

I know it dosen't anwser your implementation problems but hope this make some things clear.

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    \$\begingroup\$ good point about USB 3.0 higher charge current. With respect to charge times, the lithium chemistries use a constant current charge until the batteries reach 4.2v (at this point somewhere around 70-80% full), then hold the voltage consant at 4.2v while the current decreases to a preset value (usually 10% of the current in the first stage) filling up the rest of the battery. Because of the second stage, charging times are longer than expected. For example, 6000 mAh battery, start charge at 2000mA, will take: 6000\2000 * 1.4 = charging time. This works for most Lion batts. \$\endgroup\$ – Filek Oct 29 '15 at 7:05
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    \$\begingroup\$ This is really helpful indeed and 3 or 4 hours is the kind of charge time that is acceptable for the project - Thank you \$\endgroup\$ – 1debstep Oct 30 '15 at 19:02
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    \$\begingroup\$ And I didn't realise USB 3.0 had a higher charge rate - thats perfect \$\endgroup\$ – 1debstep Oct 30 '15 at 19:43

I just realised I am over complicating my thoughts and got in a tangle haha

I realise how to do this now looking at the sketches here made something 'click'

Here is where i found my answer http://www.electronics-tutorials.ws/transistor/tran_7.html

Just in case anyone else is having a moment ;)


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