Step 6: Balance Charging

There are two ways to charge a battery like this. You can either apply 4.2 volts to individual cells or you can apply 16.8 volts to the whole battery. While the latter is more simple, in this case the former is better because we are using used cells. When new packs are made, they all use new batteries of the same amp hour capacity. This way they can be charged hundreds of times without going out of balance. However, balancing will extend the service life of any pack. With our pack, the batteries are in various states of wear and amp hour capacity, therefore balancing is a must. A balanced battery is one in which all the cells remain the same voltage.

Let me explain how packs can become unbalanced and why that is really bad. Batteries will inevitably wear our at slightly different rates. Say the voltage of each cell in your pack is 4.2 volts after charging when new.

Cell 1 is 4.2
Cell 2 is 4.2
Cell 3 is 4.2
Cell 4 is 4.2
Whole battery is 16.8 volts

Now say one of the batteries has started to wear out. They manifest their wear by sometimes refusing to charge all the way to 4.2 volts. Now cell 4 only charges to 3.8 volts. The charger doesn't know this but still charges to 16.8 volts and the other cells are charged higher to compensate.

Cell 1 is 4.3
Cell 2 is 4.3
Cell 3 is 4.3
Cell 4 is 3.8
Whole battery is 16.8 volts

You can see how one bad cell could destroy the good ones because the others are overcharged. Balance chargers never charge cells above 4.2 volts and they can tell you if one cell refuses to charge completely. My charger has four lights and shows blue for fully charged and red for not there yet. They make a volt meter here that is perfect for monitoring the health of cells.

Balance chargers are more common in hobby batteries.  People use lithium polymer (LIPO for short) batteries to power model helicopters, airplanes, boats, etc.  LIPOs are cheaper and tougher, better for surviving crashes.  They require more balancing then lithium ion.  Some have C rates of 50!  That means a 5 amp hour battery could supply 250 amps and completely discharge in only a few minutes!  The chemistries are almost the same so the chargers are interchangeable.  I suggest looking through Hobbyking so you know what all is out there.  They have an especially dizzying array of chargers.
Would the board you use still work if I only connected 2 sets of four? Because I only need 5v and (please correct me if I'm wrong) if I have 4 of them then it would give out about 16.8v, and if I only had 2 of them it would only give 8.4v. Also, hoe would I connect a RCA cord to a USB/Micro USB? <br> <br>Thank you!
use a 3.7 to 5v boost board
<p>I love this project. I am a total novice when it comes to electronics but it is very interesting. I am trying to build a light weight battery to run my telescope. It will run on the 11.1 volt configuration. I have 21 good cells, so I was going to place them in a S3P7 configuration. My question to you is, can you recommend a PCB for that configuration? I the one you have is only for a P4? I do not understand that part of the build. Thank you so much for posting such a cool idea.</p>
just look on ebay
<p>Please correct me if i'm wrong here but the charge profile of a li-ion is constant current then constant voltage with capacity measured by current dropping off. if you are using a balance charger across 4 cells in parallel the current will be 4x higher than its expecting thus providing incorrect feedback to the balance charger.? you could over charge your batteries</p>
it just acts like a bigger battery
So where is the best place to get cheap or free batteries / cells?
<p>Get used laptop cells from your local recycling site, usually there only 2 or 1 damaged cells inside, that make the pack fail, so the consumer throws it away, although the other cells are in a good used condition.</p>
why 16.8volt you could just make a 7.2 volt battery pack use an 7805 to bring to 5 volts and give your self even more amp hours
<p>You would want a buck converter which can handle up to 2A draw. Its about 90% efficient, whereas the lm7805 dissipates the excess voltage as heat.</p><p>boost converters step up voltage and drop max current, which is not possible in dc-dc applications using a transformer. </p>
<p>use a car adapter to usb, they can take 5-24 volts in and 5 volts out</p>
What is a 7805?
l7805cv is a 5v regulator brings the 7.2-12v down to 5 volts but needs a heatsing for voltages higher then 7.2 or if it going to be under heavy load.<br>what i use with my diy mobile charger 2 18650 in series for 7.2 volt then reduced to 5 volt by the 7805 ok losses of heat but if you look at that compared to a dc -dc converter that can use 1.5amp the little loss in heat is worth it
7805 are extremely inefficient power-wise (hence the heat sinking). They burn of excess voltage with current as heat. Since the real rating of a battery pack is watt-hours, 1. rearranging the batteries for a different voltage wont change the energy capacity (watt-hours) and 2. a 7805 will drastically reduce the effective watt-hours.
i was compareing ah dc to dc boost converter like they minty boost to a 7805
the 7805 is self adjusting resistive voltage splitter, really. you're not gainging any amp-hours by using it, but you are reducing the voltage.
<p>Very nice!</p>
i think i just found 18650 at 5000mah (5amph) will know more when money comes to get them
<p>those dont exist.</p>
Be careful here.<br><br>I HAVE found some 4200mAh rated 18650 cells, but they were Ni-MH(like here http://www.all-battery.com/browseproducts/One--4-3-AF-%2818650-Size%29-4200-mAh-high-capacity-NiMH-battery.html).<br><br>Those are completely different animals from Li-Ion 18650s.<br><br><br>You COULD use them for the same style project, but you'd need a different charger/charging setup.<br><br>Not to mention, they are still 1.2 volt.<br>So to get equal capacity...<br>for every two 18650 Li-Ion cells(we'll use an &quot;average&quot; 2600mAh@3.7v) you would need 3 18650 Ni-MH cells.<br>2x2600mAh@3.7v = 5200mAh@3.7v<br>3x4200mAh@1.2v = 4200mAh@3.6v<br><br>Trade offs are, safer to use/charge, but more weight, more space, less capacity, and good luke finding cheap/free Ni-MH.
http://www.ebay.co.uk/itm/ws/eBayISAPI.dll?ViewItem&amp;item=270821277749&amp;clk_rvr_id=275620926262&amp;item=270821277749&amp;lgeo=1&amp;vectorid=229508<br><br>were the ones i was looking at to me it says 10x 18650 at 5000mah but it could also be 10 18650 5000mah total.<br>just depends id wording is right
I don't trust several brands of 18650s like trustfire and Ultrafire. They are poor quality batteries and they overestimate their capacity. There is no way a single 18650 could be 5 amp hours. A good quality cell could be no more than 3 amp hours. The cells on batteryspace are expensive but I am sure they are legit.
you are very right they turned out to be 1100mah
<p>Very informational instructable! I'm planning on using it to make a battery pack for a quadcopter. Originally, I was planning to use Panasonic NCR18650B's for it, but then I found <a href="http://www.ebay.com/itm/18650-3200mAh-3-7V-Rechargeable-Li-ion-Battery-for-UltraFire-LED-Flashlight-DB-/261530668000" rel="nofollow">these on eBay</a>. What do you think - can I use these? I'm going for ~ 2 C discharge rate. Thanks!</p>
<p>dont even buy those. they are so fake. buy a 12 cell laptop battery from amazon.</p>
<p>Dear experts,</p><p>Look like you can solve the problem that I am facing now.</p><p>I have about 20 x 5mw lasers and I need to design a system to drive it. At the moment I have 8 x 3.7V batteries (18650) connected in series and I found that even if I don't turn them on for a few days all the batteries became flat. Maybe because some of them are faulty and used up all the power?</p><p>The system that I am going to design include a Microchip PIC16F876A which needs 5V to drive it. So should I connect 2S4P (2 in series &amp; 4 in parallel) and use a voltage regulator to regulate it to 5V? I want to monitor it and charge them when required. Is this a good way to connect them? My concern is if one of them faulty then the whole thing will upset? How to avoid this? What is the best practice? I heard you guys mentioned about charging/discharging protection device - will this help? What sort of charger should I use because I am going to use this one from ebay to charge it:</p><p><a href="http://www.ebay.com/itm/5A-DC-to-DC-CC-CV-Lithium-Battery-Charger-Board-LED-Drive-Power-Converter-Module-/131065598006?pt=LH_DefaultDomain_0&hash=item1e841e5036" rel="nofollow">http://www.ebay.com/itm/5A-DC-to-DC-CC-CV-Lithium-...</a></p><p>If you have any suggestion please let me know.</p><p>Many thanks in advance</p>
<p>use a 12v car adapter to usb, they take 5-24 volts and regulate it down to 5v 1A</p>
seems everyone here is an expert,could you please help me?<br> I am using lifepo4 7 cells in series 2 in parallel,i dont know what kind of PCM I shoud use,my machine need 100A to boost first,then after started,the current only need around 30-50A.<br> <br> Please help to choose one good PCM from BesTech Power.
That seems like a lot of amps for just 2p
I found some trustfire batteries for $10 per 2 pack. <br> <br>With all the components for the summer project, my setup totals $200 plus unknown shipping for some things. <br> <br>If the batteries are found in laptop batteries, why not buy some of the mass-produced laptop batteries, take them apart, and save a few bucks over buying new individual cells? <br> <br>for example, buying 3 dell 11.1 volt, 4400 mah laptop batteries would cost $52 and gives 2 extra cells in case of a dead cell. <br>link: http://www.amazon.com/Dell-Inspiron-1545-Laptop-Battery/dp/B002MG6OO6/ref=sr_1_4?s=pc&amp;ie=UTF8&amp;qid=1366750261&amp;sr=1-4&amp;keywords=dell+laptop+battery <br> <br> <br>Individually, the best price/quality I could find was trustfire @ $5/cell with unknown shipping cost, which adds up to $80 plus shipping.
Buy 12 cell packs on amazon
If I want to make my own battery pack what consideration do I need to make for the wires? As the voltage and amperage increase do I need to use ever larger wires? If I just use the same large wire t/o the build won't the increased resistance keep any of the cells from contributing power? <br> <br>In short, if I want to build a battery that can power a 72 volt motor, what gauge wire should I use when wiring up my parallel/series pack?
I would use 14 gauge
<p>hello .. amazing project , well built </p><p>but i have some question here </p><p>the pcb has an over charge protection beside other functions . yet you used a charger to charge the batteries with ballance . im a little confused here !!</p><p>can the pcb be used to charge the batteries instead the charger ?</p><p>because when reading the specs of the pcb it says ( charge and discharge terminal on p+/p-)</p><p>plz do correct if i missed any thing .</p><p>(sry . old post but i need some answers .. thanks any way)</p>
You can but it won't balance charge
<p>You got it right. The PCB is a protection board which does all of the balancing on its own. Connect any power supply under 20 volts and it should work just fine. Buying a balanced charger defeats the whole purpose of trying to save money.</p>
<p>hello i need help i'm starting out making portable chargers and speaker systems powered by recycled li-ion batteries and this helped alot but couldn't i charge 2 li-ion batteries in parallel if it is being powered by a 5v charger </p>
Too much voltage, fire risk
<p>Just so you're aware, for less than $30 you can head on down to any battery shop and pick up an 8AH SLA that'll run all your equipment just as long, and doesn't require expensive or complicated equipment.</p>
A 14 ah sla can be replaced with 21 18650 cells, higher energy density and lighter, as well aas you can discharge below half without damage to the battery
<p>Hi there, stumbled upon your tutorial whilst searching for how to DIY a battery pack.<br><br>Here's my issue, I don't come from an electronics background so pardon me if this sounds a bit simple.<br><br>From what I gathered on your 2nd page and what I have in mind, I'm actually planning to make a single pack of 6 18650s, 7.4v, into a camera battery casing which is meant exactly for that, but I'm using higher capacity cells because the originals are quite low cap. <br><br>Anyway, from your explanation, it seems that I'm looking at a 2S3P configuration am I right? If I am, the problem now is the soldering. I need help wiring them to each other. Could you hit me up at ho.zhenjie@gmail.com? Appreciate it tons!</p>
Just make 2 packs of 3 cells, solder a wire across the top and the bottom of the two packs then join them together
I am making a small battery pack from a 12v usb charger and 3s2p18650 cells, using a drill battery as the housing. It will have the full capacity of an average laptop battery, as that is where I got the 6 cells. Once my balance charger ships to me and I have enough cells I am going to take an old broken goal zero battery pack and convert it to lithium.
<p>What a great write-up! Does anyone know the configuration I'd need for a 48V 20Ah battery pack?</p>
Assuming the cells are 2500mah then you would need 12s8c
<p>I am about to start a project but something in not clear to me about the PCB. Does this PCB only protect and balance the batteries or does it also have a smart charger function. In other words, can I just put a 12V 2A load on it or do I still need a seperate 12 volt Li-On/Li-Po charger??</p>
I also have a 18650 battery project, but spent much less for a 4S4P
<p>I have a friend that works for a cable company. They use back up battery packs in telephone modems and he had a bunch that were taken out of service and he gave them to me. They all had 2 or 4 18650 batteries in them. I ended up with 24 of them for free. Some were light blue and some of the better ones were purple.</p>
Thanks, these are the most detailed instructions ever about li-ion battery packs.

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