Read my just posted answer to this question. While not identical it covers aspects which will answer some of your questions.
3 x 18650 LiIons (or any 3 LiIons) will have a fully charged voltage of 3 x 4.2V = 12.6V and a fully discharged voltage of ABOUT 3 x 3 = 9V. How low low goes is up to you. Too low and battery dies.
Read my answer above re balancing. It is not NECESSARY as long as you are CERTAIN that no cell is ever deep discharged AND if charging in series, as long as no cell is in constant voltage tail off mode while you are attempting to inject full constant current at 1C. 'Attempting to" period may be short.
IF you charge this off the bike and if all 3 cells are isolated from the world (but connected to each other) then my answers above re charging one at a time apply. You can charge 3 at a time with 3 chargers ** as long as** all charger outputs are truly isolated.
An easy way to get 12V is to use one of the many many available switch mode power supplies. You can get 1 or 2 or 3 cell LiIon to 12V capable supplies.
An 18650 LiIon cell is has a capacity of about 2000 mAH x 3.6V nominal =~~ 7 Watt hours. IF your flasher worked at 1 Watt average and was anything like serious it would blow following motorists off the road. Depends on design. 1 Watt at 10% duty cycle = 10 Watts when one. 1 Watt at 1% duty cycle = 100 Watts when on. Properly collimated a 1 att red LED willl do a very very very very good job. So a single 18650 cell with inverter of say 7% efficient (low) will run for 7 Wh/1 Watt x 70% = 5 hours. Ample for most people.
ADDED:
- OK, so some clarifying questions.
1) how can I be certain that no cell is ever "deep discharged"?
No cell ever under 3 Volt.
- Monitor voltage and prevent this happening
OR
- Never discharge beyond known capacity to ensure this is true.
Murphy says you will fail if you take the 2nd choice.
- 2) if I'm using a COTS charger (and charging each cell separately), what do you mean by the charger outputs must be truly isolated? If I'm using a physically separate charger for each cell,
If the cells are not connected in any way this is irrelevant.
This is an issue only if the cells are connected as in a battery holder.
Outputs are fully isolated from the charger input.
If you operate 2 chargers from mains simultaneously you must get no sensible voltage readings when measuring from eg V+ out of one to V+ out of other.
If you plav a resistor from V+ out of oneto Ground out of th eother no current flows.
Well under 1 mA would be acceptable.
I'd expect good isolation. What is to be avoided is having ground out hard connected to ground pin in.
3 truly isolated chargers will work happily on 3 cells in series if there is no closed current path apart from the cell interconnections.
- will it be safe if when they're done charging separately I put all three cells back into the same system together
Yes.
Using the relay to disconnect the neutral line doesn't really sound like you know what you're doing. You're exactly wrong in the safety department. In a perfect world, it's the hot that you want to switch. With only neutral switched, the heating coil is always energized, waiting to shock you.
Additionally, relying on a polarized electrical cord to try to make sure it's the hot you're switching has its own problems. You're better off using a double pole relay to fully disconnect the toaster from the electrical supply. Use a relay that is designed specifically to switch both legs of a power supply.
Also don't need to heat-shrink, glue, or electrical tape the wires, that's overkill and amateur hour. Hiding things in an attempt to increase safety is a sure way to decrease safety.
Use the project box, and mount your circuitry (including relay) on perf board. Simply solder the relay inline on the hot wire. Keep your low voltage relay control wiring (and arduino circuitry) physically separate from the relay and line voltage power wires. Do not let low voltage and high voltage come near each other, except at the relay, and even then, those come from different directions. Physically secure the power wires so they can't accidentally be pulled out. Be neat.
Your heating coils should be shielded from your pcb by a heat spreader, and your thermocouple should be next to the pcb, and not be touching anything. It's the air that heats up the pcb, and the air that heats up the thermocouple. You trying to bake the pcb, not broil it.
IMPORTANT SAFETY NOTE:
The electrical power that comes out of your outlets is extremely dangerous. It's not the fun-times easygoing stuff of 15 volts and below. It will surprise you, and given the opportunity, it will kill you.
From your question, you really don't seem to know what you're doing. I'm not trying to insult you, I just don't want you to die. Advice from the internet is not going to make this any safer. Sit down with someone in the real world who has worked PROFESSIONALLY with household electrical circuits and have them look over what you are doing.
Please be safe.
Best Answer
As others have said, this should not be sitting on your table or on the carpet. Exposed 110V metal carrying 110V is dangerous, and the pins on the back side of the PCB, the exposed screws, and any stripped wires could also cause shorts or electrocution. On the note of
A decent way to mains power to your device would be to add a panel-mount power connector like the IEC 60320-C14 receptacles commonly found on PCs, monitors, and TVs for power input:
You (and your potential customers) probably have a few cables for these receptacles lying around.
You can get these with integrated fuses and on/of switches; check the Connectors, Interconnects / Power Entry - Inlets, Outlets, Modules section of Digikey and filter for IEC 320-C14, receptacle - male pins, panel mounted, and the features you want.
The best output would probably be a standard electrical outlet. This would allow you to connect any appliances directly to your device. You can also get panel-mount receptacles for these plugs, for the US they're NEMA 5-15 receptacles:
For both the input and output, the wires can be soldered or screw-mounted to the receptacle, and connect (entirely within the enclosure) to the relay module. The project box can then completely protect the user.
And yes, if the box is metal, you must ground it in case one of the hot wires becomes disconnected and touches the box. With this ground wire, the circuit breaker or fuse will protect anyone (from electrocution) or anything (from fire) which comes in contact with the box. Without this wire, the size of the box amplifies the danger from a few small screws to an entire enclosure. Ground it, or use an insulating (plastic) enclosure.
As Brian already, said, the abbreviations stand for: