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 $$ can be saved.
Step 1: The Schematic Diagram and Safety Considerations
I have include the detailed diagram of the charger. Have included the design of a Lithium battery pack. The diagram has detailed notes and labels for it to be self explanatory. Here is how to read the diagram:
On the left is the Constant Voltage Charger that has the following features:
1. Constant graphical battery monitoring - just like the one we have on cell phones
2. Battery temperature sensor (digital thermostat) - this makes the charger suitable for even the most sensitive batteries including NiCd and NiMH.
If your requirement is to construct a bare bone functional charger, you can eliminate the monitoring and thermostat modules and save $$.
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
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......