Make This Smart, Modular and Cheap Lithium Battery Charger System in Minutes!!

About: I am a professional electronics and computer systems designer so keep shooting guys!

There are countless ideas on the internet and also on Instructables on DIY Battery chargers and related systems. However in terms of cost and effort, most of them turn out to be on the higher side and in terms of the functionality on the lower side. Rarely are any of the idea fine tuned to the data sheets of the target batteries and therefore the majority appear to be rather amateurish in not out right dangerous for the batteries.

To offset the cost and effort and maximize the functionality, the idea here is to buy the low cost modules (power supply, battery monitor, digital thermostat etc.) from eBay and assemble a really sophisticated charger in minutes! I have fine-tuned the system for Lithium batteries. However with the level of built-in protection in terms of fixed voltage, battery monitoring and thermostat, this charger can be used to charge almost all kinds of batteries including NiCd, NiMH andLead Acid.

There are several variations in batteries - The Ampere Hour (AH) capacity, the voltage and most significantly the type. A little bit of high level digging into the manufacturer data sheets (just google the manufacturer with battery type) of the battery should be enough for you to be able to make a few quick adjustments for optimum charging of your battery using this charger.

You do not need too many discreet electronic components and should be able to assemble this charger within 20-30 mins. By reducing the use of discreet electronic components, soldering can be avoided, reliability can be increased and $$ can be saved.

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Step 1: The Schematic Diagram and Safety Considerations

I have include the detailed diagram of the charger. Have included the design of a Lithium battery pack. The diagram has detailed notes and labels for it to be self explanatory. Here is how to read the diagram:

On the left is the Constant Voltage Charger that has the following features:

1. Constant graphical battery monitoring - just like the one we have on cell phones

2. Battery temperature sensor (digital thermostat) - this makes the charger suitable for even the most sensitive batteries including NiCd and NiMH.

If your requirement is to construct a bare bone functional charger, you can eliminate the monitoring and thermostat modules and save $$.

On the right is the battery pack which includes the Lithium battery protection circuit - I included this because of the several stories about the cell phone battery packs exploding in the pocket. You may choose to remove the protection if your battery is self protected or if safety is not a big concern.

I do not accept any liabilities arising out of the implementation of the idea - however based on my knowledge and experience the system is robust and save.

Step 2: Understand What to Expect

You can assemble this charger for less than $15. Commercially available chargers of a similar rating and features cost about $75 or so.

I was able to fully charge my car batteries using this charger in an emergency situation. This speaks volumes on the versatility of this charger. Modules utilized here e.g. The power supply is short circuit and temperature protected. i.e. there are tons of cool hidden features. With time the components or their equivalents will become cheaper and the features will get richer

Everything except the power supply, the resistor are optional for the charger. The cheapest version of this charger can me built for as less as $4. $3 for a 1A power supply and $1 for the resistor

Step 3: A Quick Note on Lithium Batteries

The most popular size of a general purpose Lithium battery is the 18650. It looks like an oversize AA alkaline cell that we have all used. However the sad part is that eBay has several counterfeits. It is my general impression that no 18650 that costs below $5 would be even half worth as what it claims to be. Branded 18650s are generally superior than unbranded ones. Several branded 18650s come protected and if that is the case, you will not need the expensive protection circuit.

If you are just building the charger for a Lithium pack that you already have, you can ignore these instructions altogether.

When deciding on the voltage that you should set on the power supply, the thumb rule is to multiply the number of batteries by 4V. Once the voltage is set to this level, follow the instructions in the diagram to set the exact charging current.

For fast charging, charging current = AH capacity of the battery. If you have 2700mAH battery set the charging current to be 2700mA or 2.7A. For slow charging set the current to a quarter of this level. The thermostat is no longer required.

Step 4: How Did You House It? Post Your Pics...

I have just hidden mine inside a tool cabinet. Probably the power supply housing can be opened and some of the components can be hidden inside. I was tempted to hide the current limiting resistor in this manner but considering the fact that it can get really hot, I decided against it.

This intractable would have been more complete with a picture but sadly I do not have one I would be happy with so please help......

I welcome one and all to post ideas on a housing that is just as cool as this charger!!!...............

Step 5: Bill of Materials and Notes on Buying From EBay

I am assuming that the pics included within the schematic diagrams will be your ultimate guide to eBay purchase of components. Please do let me know if you need more info. I am not sure if instructables will allow me to post links from eBay but I can try.

Bill of Materials (see picture for specs)

1. Power supply

2. Power resistor

3. Battery monitoring system

4. Digital thermostat

5. LM7812

6. Battery protection circuit

7. 18650 Lithium batteries

7. Wires, Crocodile clips....

Step 6: Improvement Opportunities

No project is complete without a detailed comment on limitations and improvement ideas. Here are some that were obvious to me:

1. The temperature settings on the thermostat will disappear every time the power is disconnected. To me however, this is a small compromise compared to the protection and versatility that a thermostat offers. If this is an irritant, the following can be done:

1.1. Build yourself a simple comparator based analog thermostat for a few more $$. A variable resistor can then be used for temperature setting

1.2. Make sure that the charger is never plugged out or at least plugged out only when absolutely necessary

1.3 Use a small battery pack or a large capacitor as a backup for thermostat

1.4 Build the system without the thermostat - None of the choices appealed to me enough but I am dumping them here because everyone's application is a little bit different

2. If the charger is unplugged or if the power fails while the battery is still connected to the charger, the thermostat and the battery monitor will drain the battery at least a little bit. This issue can however be resolved by merely adding a 5A diode such that it prevents backward current flow from the batteries

3. The thermostat forces me to use a discreet electronic component 7812. It might be a better idea to add a buck-boost DC to DC converter that can take any voltage from 3 to 24V and delivers a constant 12V output. I did not add this for the sake of simplicity and low cost. I was able to remove the conventional 1MFD capacitor from the output of 7812 without the loss of any stability for the thermostat. If you are not so lucky, do consider adding a capacitor

4. The person assembling this system should have the ability to follow the instructions provided to set the charging voltage / current and be able to add / remove components as the need be. Do give the schematic a good read since most of the instructions are embedded within

5. Getting the temperature sensor to actually be in contact with a battery can be tricky. If you can place a copper / aluminum heat-sink very close to the batteries, the sensor can be probably slid inside the fins. Let me know if any of you have a better idea

6. I claim that this idea is modular but it still has discreet components. Sorry couldn't avoid them completely!

7. Feel free to add in the comments......

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    16 Discussions


    2 years ago

    I'm kinda new to the whole lithium-ion charging thing, so please bear with me.

    Looking at your diagram, I don't see where the actual charge circuitry is. I see a constant voltage power supply, a display, a thermostat, and a battery protection circuit. Where's the charging circuit? It's my understand - and I could be totally wrong here - that those balancing protection boards don't include any charging algorithm. In other words, they don't supply constant current until a certain point and then switch to constant voltage.

    On the diagram you point to the voltage adjustment pot on the power supply and say "turn this variable resistor to get safe yet fast charging current" - but that adjustment doesn't change the current. It changes the voltage.

    You specified a 2 ohm 10 watt resistor on the positive voltage feed from the power supply to prevent over-current. How did you determine the resistance value?

    For example, if I have a 12v 20A power supply, but want to limit it to 15A, what resistor would I use?


    3 years ago

    Let me get straight to the answers:

    1. I had to resort to the fixed supply dc-dc converter approach too after I noticed that any voltage lower than 20 v is not stable under high current conditions on the power supply model I purchased.

    2. I noticed that 19.5V which is typically the output of a laptop power adaptor is not enough of a drive to deliver 4A charging current into a Li battery pack with 5 or more batteries. If you must use a laptop adaptor either make sure that your battery pack has 4 or less batteries or use a buck-boost converter (these are available readily too at the same price as a buck converter)

    3. I observed that dc-dc converters do sink current so I am now forced to change my earlier stance (in the comment that I deleted) on paralleling them. I no longer recommend paralleling them since this may cause substantial damage.

    I do sometimes plan ahead and then wait endlessly to get the components from China. Sometimes by the time the components arrive, the design changes wasting time and money!


    3 years ago

    Lastly about the charging voltage: weigh the individual cells on a weighing scale. If the weight is 45g our more, feel free to go up to 4.15. V. If it weighs less than that do not exceed 4V.

    1 reply

    Reply 3 years ago

    Thank you very much for your answers! I had some luck and finally received an answer from the manufacturer of the bms - their engineer told me that the bms can handle up to 30V easily! I'm so glad. No worries about the buck converter anymore. yey!


    3 years ago

    i know a far cheaper one quite simples laptop power supply cc/cv step down converter set to 16.8 volt as that is max voltage of a 4s pack and a volt meter display thes is best to have on battery along with the 4s pcb for over discharge protection and done this would cost about 10 bucks

    6 replies

    Reply 3 years ago

    If you recycle a laptop power supply, you can cut down the cost by $6 approximately because you still got to pay for the converter. If you would like a simplistic solution, everything including the DVM that you mention is optional more do if the batteries are self protected. Even for unprotected batteries, after the charger output is set to around 17v for a 4 pack, there is not much to protect from. My design is scalable in terms of type and number of batteries. I would still think that unless an effort is made to oversimplify In the interest of your hobby, there is no real need to knock off too many parts on this design.


    Reply 3 years ago

    17 volt is to much you would over charge the cells at current i have solar panels that run into a dc step down converter set to 12.6 volt i have a 3s protection circuit for stopping over discharge


    Reply 3 years ago

    You are pretty much at a liberty to set a voltage of your choice but the data sheets of 18650 for samsung, Sony and a hand full of other manufacturers clearly indicate a max. saturation voltage of 4.25V.

    I don't think that using a DC converter is such a bad idea at all but it just sounds amateurish to me that I first convert 110V to 20V DC and then convert 20V to the exact charging voltage, The power pack I have suggested merely costs $10, can pump out 5A of juice at what ever voltage you need right up to 24V. So while I appreciate your comments, I wouldn't design the system based on your suggestion just to save $6.


    Reply 3 years ago

    but you are also using a power supply to drop from 110-230 down to x volts then dropping that for each different pack formation advantage of mine is not only will it charge mutliple different packs but also can be a bench power supply to power lots of diferent things and can be used to charge any form of battery


    Reply 3 years ago

    I think you are going a bit overboard without even reading the schematic. I am saying so because if you really paid attention you would see that the recommended power supply also has a built-in voltage control. This is actually more suitable for a test bench because it is a single component vs. your idea of two. I have so far not seen too much of merit in your comments and will spare myself from responding to them any more. Thank you!


    Reply 3 years ago

    Hey...I had to resort to the fixed supply- dc converter approach too after I noticed that any voltage lower than 20 v is not stable under high current conditions on the power supply model I purchased. The under cabinet accent lighting 27a already designed the way you describe above so it was easy for me to switch to the design. Thanks!


    3 years ago

    I'm really digging your setup man. Simple and cheap! But the thing is, I don't have all these parts at home nor is it possible for me to get them fast and I want to get this thing working now! You know what I mean right? ;)

    So I'm trying to an slightly different approach with the parts I already have laying around and as you seem to be an experienced person on this topic I may ask you for some small advice?

    In general it is the same approach that dasimpson1981 mentioned. I have an spare laptop power supply with 19v and 4.5A and I tried to step down the 16.8v that i need for my 4s3p 18650 pack which is coupled with a bms that inlcudes balancing. So far so good, but I have to major problems with this setup and may you can help me with that.

    1. Current limit of the buck converter. The one I have can only deliver 2A safely but to charge 4s 18650's I'd prefer 4A charging. As i have two of them I ask myself if it is possible to use buck converters in parallel to "double" the possible current. Do you know that? Or do you know a different way to decrease these 2.2v in general?

    2. Charging voltage. I'm quiet unsure with which amount of Volts I can feed my BMS to charge the pack. Because the BMS I use is a china product (pretty similar to the one you have used) it is hard to get my hands on a reliable datasheet. I already tried to feed it with exact 16.8V but the pack was not getting charged so I need a little over voltage I guess. But I'm afraid to raise the volts and risking to damage the bms. Best solution would be that I can feed it with my 19V power supply without the need of the buck converter. That would solve everything.

    Sorry for my extensive post in your commands but maybe it also helps other people with similar problems.

    Thank you


    3 years ago

    Will this charger charge a 20 volt cordless drill lithium battery

    1 reply

    Reply 3 years ago

    Yes it's 24V and that's enough fur your application.

    Unfortunately the current ones of my tool cabinet will add no value! I will keep updating this post as and when I have a better housing. I am planning to get a nice plastic one for around $10. For a start I thought that it was important to create and upload the blueprint for the comments and improvement ideas to start coming in.