musskopf:
What's the 5V sensor specifically?
Its a prepackaged reflective laser sensor with built in gain and threshold adjustments.
Gahhhrrrlic:
The 9V form factor is all about convenience and space efficiency. It is inherently design-limited. The cells don't have the best aspect ratio and have too much internal resistance without enough electrode surface area. Good for a clock or smoke alarm as alluded to earlier but not much else. Since power banks are already regulated and rechargeable, they seem like a decent and affordable option, so long as the internal circuitry isn't too "smart" for the normal operation of the Arduino. If you don't go that route and your current demands are in the 10s of mA or higher, then custom rechargeable packs are the way to go and as far as they are concerned you are likely going to need some sort of regulating circuitry unless you happen to luck out on the pack voltage matching your needs within +/- 10%. That's my professional opinion.
The 9V "transistor" battery worked great in my first "transistor" radio back in 1968.
No doubt it did. Zinc based alkalines have a reasonable capacity and an excellent self-discharge rate compared to other chemistries. Therefore when you use them in low-drain applications, they last pretty much forever (well not really but it seems that way). I'm sure they'd also be the best choice for some ultra-low power arduino applications too but once you get into the 50+ mA regime (I'm talking about 9V specifically, AA tend to fare better), they suck donkey balls. As with most batteries, they have their niche.
I've been keeping an eye on supercaps and just plain big capacitors. I've seen one with a 6-pack of cans that can start a 6 cyl van engine 3 times before needing a full recharge, started the thing on 10V once.
One amp represents a rate of electron flow of 1 coulomb of electrons per second, so a 1-farad capacitor can hold 1 amp-second of electrons at 1 volt.
A 1-farad capacitor would typically be pretty big. It might be as big as a can of tuna or a 1-liter soda bottle, depending on the voltage it can handle.
Adafruit -- Super Capacitor - 2.5V 630 Farad - $19.95
When I was studying power supply electronics in school, the instructor always talked about adding BFC's to the circuit - Big Freakin' Capacitors. This here is UHFC - Unbelievably Huge Freaking Capacitor. If you have a super-cap project that needs up to 630 Farads of capacitance, check it out. Like most super-caps it has a 2.5V-max rated voltage, and remember that unlike a voltage-output battery the voltage drops immediately as it discharges.You can use this as a stand-in for a battery, its huge but can charge fast and doesn't degrade like NiMH or Li-Ion/LiPoly batteries. We like to refer to this MAXIM super-cap calculator to figure out how long we can run a project.
Still at 2V, 1260 Amp-seconds? With a boost converter you might get 30 minutes or more 100mA 5V out.
Capacitors are great for a host of applications. Funny you should mention them because I'm using them in 2 projects as massive power supplies (1 for my car).
The important characteristic to consider is energy rather than capacitance or voltage since both are factors of the capacitor's size and capacity. A capacitor bank which you'll find in a typical defibrillator kit holds at least 360J of energy but after a shock it's completely dead. If you used that 360J over a period of an hour you'd get exactly 0.1W of power because 0.1W (J/s) x 3600 seconds = 360J. Thus the capacitor bank would be rated at 0.1 Wh.
Take your typical AA alkaline battery. It's 1.5V, about 2000 mAh and to do an apples to apples comparison we need to discharge it in 1 hour as well, which means a current of 2A (unrealistic but ok for a theoretical calculation). 1.5V x 2A = 3W. 3W for a duration of 1hr is 3Wh. Already you can see that your average AA battery, modest and small as it may be, has 30x the energy in it as a bank of capacitors used in a defib kit. Simply put, the energy you can store in a dielectric in the form of an electric field is considerably less than the energy yielded from chemical potential. Of course the major disadvantage is the speed of energy release since no battery can drain itself in a fraction of a second.
Let's go back to actually providing something helpful to the OP.
adwsystems:
The 3 cell/3.7V options are not going to work. Thank you for the thoughts. There is not enough juice for the 5V sensor with a Vin min of 4V.
There are options. A 2S Lipo battery (2 cells in series) will bottom out at 6V. Or you can use a boosting circuit to feed the sensor from a lower voltage battery.
Right now you need to be an accountant. Take an inventory of your circuit, and catalog every place where power will be consumed; ICs, voltage dividers, biasing circuits, LEDs, actuators, everything. Measure, calculate, or estimate the amount of current used by each item. For devices with variable current demands or that will not be continuously operating, make an estimation of the duty cycle (the percentage of total operating time) of each particular current draw. If you're not sure how to do this for some things just do what you can, I'll help you as well as I can. It'll probably be hard to estimate the microcontroller's current usage since that depends on how you code things, so just focus on the other parts of the circuit for now and fill in the gaps later.
Do not, however, just make assumptions. Write this down and do it formally, because your intuition at this point is probably very bad. As an example, I helped someone here before with their scheduled solenoid project to control water into a garden irrigation system. The solenoid would be active only for about 30 seconds every 6 hours. Even derating the 250 mA current for the solenoid valve by its 0.14% average duty cycle, the solenoid was still the dominant power consumer in the circuit by a huge margin (estimated over 30 times overall), simply because the microcontroller and timing circuit were so far down in the microamp region. That is not something you would expect without doing the calculation.
Also consider how much weight and volume you are willing to have the battery take up, and what kind of battery life you are shooting for.
As far as helpful advice goes, you're right, let's get back on track. He should budget everything. That's good advice. I think it's just as important to minimize on system complexity and cost. One of those lithium backup power sticks or whatever you call them is probably the best blend of size, cost and simplicity in a portable power source. So long as there are no issues with the thing turning itself off at very low current draws, I think it's case closed on what type of battery to use. If for whatever reason it causes problems at some point in the program's execution where the current draw is very low, the substitution is clear: either NiMH or NiCd as they are the only battery chemistries that would run an unregulated arduino well, without additional circuitry. In that case I'd go for the NiCd and simply put 2 banks of 4 AA cells in parallel to get the needed run time but NiMh is good enough if you don't mind replacing them 4x earlier. However I'd probably just take my chances and buy the lithium power stick and try it out. It checks out for voltage, current and run time.
- Battery life is the requirement 16-18 hour run time is the target, 8-9 hours is the minimum.
- Interchangeable power pack is also a requirement.
- Rechargeable is also preferred, made easier by #2
The usage will be intermittent on a week basis but when used will be used hours on end. So again #2 will be of large benefit when you go to use it and find it is not charged.
The circuit has four major components:
- Sensor - 5V 60mA
- LCD - likely 16x2, hoping for no backlight, if I can get one with the correct colors and contrast
- Arduino - need something small, and run directly from 5-6VDC, so either Pro or Pro Mini as those are the only ones that can take a 5V input.
- Control buttons - not frequently pressed, so I would not include them
Do we know what the standard mode current draw is for the four Pro and Pro Mini models available? (let's not include anything I could do with power saving within the processor for the moment).
Others have reported successfully fooling the autoshutoff by periodically causing a burst of current draw. Maybe with a transistor/resistor load that is turned on/off just long enough?
CrossRoads:
Others have reported successfully fooling the autoshutoff by periodically causing a burst of current draw. Maybe with a transistor/resistor load that is turned on/off just long enough?
I will keep that in mind, but I already know the system will be pulling at least 60 mA. If I'm lucky, that in and of itself will be enough to stay above the minimum.
Is there any reason you're hesitant about using the lithium power banks?
Are you asking me? Because I have never done it before. I was wondering if anyone else had and what issues, if any, they had run into.