Introduction: My Protection Circuit
As many of you may know, this Lazy Old Geek loves Arduinos. Many of my projects are portable that need a battery source:
https://www.instructables.com/id/Arduino-Pedometer/
https://www.instructables.com/id/Arduino-Nokia-LCD-Sensors/
https://www.instructables.com/id/Ultrasonic-Tape-Measure/
So, I’ve been using Adafruit 6 AA Battery holder $5.00
https://www.adafruit.com/products/248
with 6 AA rechargeable batteries. (See Picture)
Another choice would be Adafruit 9V
https://www.adafruit.com/products/67
My Complaint:
Here’s why I don’t like 9V alkaline batteries
Expensive ~$3
Low capacity ~600 mAh
One time use
In comparison, I bought 16 AA 3000mAh NiMH rechargeable batteries for $9.38 on ebay.
6 AA would cost $3.50 but would have 5x the capacity and are rechargeable.
My Complaint about AAs:
Okay, so I’m a Geek and I’m not really satisfied with the 6 AA battery setup.
- It’s pretty heavy and bulky.
- My battery chargers will only take four batteries at a time, so I either have to charge them in two stages or use two different chargers. But I’m Lazy.
So, naturally, I’ve been looking for an alternative.
Hark! I find out about the 18650. These are Li-Ion batteries nominally 3.7V.
Li-Ion batteries are like those used in most laptops. Now, I did some research on Li-Ion batteries and like them:
Step 1: 18650 Battery
I bought 6 18650 3200mAh batteries for $10.85 on ebay. I don’t think this is still available but you can find pricing pretty similar.
The 18650 battery is nice but it does have its own problems.
Problem: It is a little bigger than a AA battery so it needs its own holder.
Solution: You can buy a two battery holder for $1.08 on ebay. (See Picture)
The reason I selected two batteries in series is so that it will provide enough voltage for the Arduino.
Problem: It is not a standard battery type so needs its own charger.
Solution: I bought a charger for $3.46 on ebay. (See Picture)
Problem: A bigger problem is that the batteries I bought are ‘unprotected.’ This is a little different from NiMH rechargeable batteries which can be discharged down to about ½ a volt. If Li-Ion batteries are discharged too much they will not be rechargeable. The 18650 is nominally 3.7 volts, will charge to about 4.25 volts but should not be discharged below about 2.7 volts. I believe there are some other requirements about not charging incorrectly but I am (assuming) my charger is taking care of this. And not discharging too fast but hopefully my circuits will prevent this.
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Warning: If appropriate charge and discharge methods are not used, there apparently is some danger of the 18650 batteries exploding.
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Solutions: See next step
Step 2: 18650 Protection Circuit
Solution1: You can buy ‘protected’ 18650 batteries with the circuitry built in. However, they seem to be about twice the cost.
Solution2: You can buy individual battery protection circuits and add them to your batteries. I just found a protection circuit for $1.27@ from Deal Extreme.
Arduino: Since I am designing a battery for the Arduino, I will need to use two 18650 batteries in series. This will provide a nominal 7.4 Vdc and up to 8.5 Vdc when fully charged.
Solution3: You can buy battery protection circuits designed for two batteries. Here’s one for $4.49:
http://www.batteryjunction.com/pcb-7-2.html
Solution4: Now I am a Geek so I decided to build my own.
Step 3: My Protection Circuit
Design: My main concern was not allowing the 18650 batteries to discharge below about 6 Vdc or 3 Vdc apiece.
*************WARNING****************************************************************************************
So I was trying this circuit out with an experimental audio amplifier and, as expected, I forgot and left it connected. Well, something happened and my amplifier chip burned out and shorted.
Good news is that the my circuit disconnected the batteries when they got down to about 6Vdc total.
Bad news is that the batteries discharged unevenly. One battery was about 3.7Vdc and the other was about 2.2Vdc. This is below the recommended discharge limit.
The not so bad news is that both batteries seemed to recharge correctly.
Now my opinion is that this uneven discharge was caused by the shorted amplifier drawing too much current (unevenly).
But this is a WARNING that this design may not protect your batteries correctly. I have redesigned this circuit and am waiting for some 1.8V zener diodes to test and finalize Design2. Once I get it working, I will add it to this Instructable.
*************WARNING****************************************************************************************
Simple Solution: I could just keep an eye on them and charge them often. Li-Ions are usually not hurt by charging too often. But I am Old and may forget and leave them connected.
My Solution: So I decided to design a comparator that would shut off voltage to the Arduino when it drops to about 6 Vdc. So I use a comparator to disconnect power going to the Arduino.
For the Purists out there: Yes, I realize most Arduinos have 5 Volt regulators, many using the 7805 that suggest the input voltage is a minimum of 7 Vdc. Well, I’m pretty familiar with the Arduinos and most will work down to about 4 Vdc. One concern you may have is if you are measuring analog voltages and using 5 Vdc as the reference, then your measurements will be off. Well, for all of my analog circuits, I’ve been using 3.3Vdc as the Aref. One of the reasons is that when using the USB as a power source, most computer USB ports are not at 5 Vdc but can be around 4.7 Vdc anyway.
Alternative: My circuit can be altered so that it will switch off at 7 Vdc and 'guarantee the 5Vdc.
Theory of Operation: Originally, I wanted to design the circuit so that the comparator was also disconnected when the voltage was too low. Well, I couldn’t get that to work. So I left the comparator connected all the time.
See schematic. Z1 is a 5.1 Vdc zener diode. R1 supplies enough current to bias the diode. So pin 2 of the analog comparator, LM393 will have 5.1 Vdc on it. R2 and R3 form a voltage divider. When the battery voltage is above 6 Vdc, then pin 3 will be above 5.1 Vdc and the comparator output will be high. This will turn on the FET, Q1 a BS170 which will supply the ground for the output going to the Arduino. The positive voltage is passed directly from the batteries.
R5 and D1 are active when power is available so the LED will indicate power is going to the Arduino.
By the way, the high and low voltages on pins 6 and 5 are so that these inputs are not floating. I prefer not leaving unused inputs floating. Theoretically, the floating inputs could start an oscillation that would use up a lot of power and drain the batteries faster.
Alternatives: The LED doesn’t need to be in the circuit to reduce battery drain (~20mA)
For a different trip point, R2 and R3 can be changed.
For a different battery source, Z1, R2 and R3 can be changed.
For my circuit, I actually made some PCBs using toner transfer but will save details for another Instructable.
Protector Parts List
Prices:
http://www.taydaelectronics.com/
S&H not included
PCB
LM393$0.13
8 pin IC socket$0.02
1N4733$0.03
Resistors 1/4W metal film
1.2K
2.7K
1M
2.4K
13Kabout $0.02@
LED 3mm$0.02
FET BS170$0.26
2.1mm power plug$0.20
Total cost is less than $0.70. I realize shipping charges are not included. If ordered all together, it’s probably an additional $3.99. I order a lot of parts from Tayda on ebay, e.g., often like 50 resistors for $0.99 shipping included. For big mixed orders, it’s probably cheaper to buy direct from TaydaElectronics.com.
Step 4: Comparison
So I pretty much explained why I didn’t use a 9V battery source. Here is a comparison of this 18650 system with AA NiMH batteries that I have.
18650 Cost
2 18650$3.62
1 holder$0.54
1 Protector $1.00
Total$5.16
AA Cost
6 AA NiMH $3.87
1 Adafruit$5.00
Total$8.87
18650AAComments
Batteries26
Voltage8.5-68.4-6
Capacity3200mAh3000mAhDischarge time is going to vary with load
Cost$5.16$8.87
Weight~2.5oz~6ozI don’t have a scale but used my postal scale to see which was heavier (See Picture)
https://www.instructables.com/id/A-67-Cent-Postal-Scale/
Size1.7”x3.5”2.5”x4.5”
18650 wins for cost, weight and size.
Advantages and disadvantages: battery technology
AA NiMH: memory issue, higher self discharge rate
18650 Li-Ion: no memory issue, lower self discharge rate, aging issue
Complaint: Another of my complaints about AA NiMH batteries is if you don’t use them for a while, they will self discharge. The 18650s are supposed to have 10 times less self discharge rate.
Step 5: Conclusions
I haven’t done very much testing but this seems to work pretty good.
Technobabble: One test I did was measured the current with the protection circuit connected but no load. It was only 6.25mA even with the LED on. What this means is that when the protection circuit drops below about 6 Vdc and disconnects power to the Arduino, it will still be drawing current but it would take a long time for it to drop down to 2.7 Vdc, the danger voltage.
Concern: For the fellow Geeks, one of the things my circuit does not address is if the batteries discharge at different rates. Ideally, the battery protection circuit should be for each battery instead of for both at the same time. I am fairly certain the batteries will discharge fairly evenly since they are the same capacity and same age. But I will try to monitor this in use.
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