Using CR2032 cells for my application

[Ian.M]

No.  The load current will be non-linear with respect to voltage so your ohmmeter reading will be meaningless.

[uncle_bob]

It's early and my Swiss coffee machine is barely able to stoke me with caffeine fast enough.

Seems so, you didn't catch the 2nd one... you got the efficiency backwards (unless you're one of those "free energy" guys )

Hi

Ok, it's just plain early...

Bob

[sentry7]

Then approximate figures will have to do. The major parts are the MCU and display.

The MCU, as mentioned before, pulls 100mA, 3.3V, at maximum clock frequency (it's always running at this frequency, so I'm assuming constant pull).

The display has four backlights that sink, if I'm reading the datasheet correctly, 80mA, 3.3V, but I'm not sure if that is 80mA per LED (seems a bit much for one LED) or all together. Also, it doesn't give the current sink for the display itself without the backlights.

Datasheet for TFT - https://www.adafruit.com/datasheets/MI0283QT-11%20V1.1.PDF

In total, I'm looking at 180mA load current...or more if it's 80mA per LED.

[uncle_bob]

Then approximate figures will have to do. The major parts are the MCU and display.

The MCU, as mentioned before, pulls 100mA at maximum clock frequency (it's always running at this frequency, so I'm assuming constant pull).

The display has four backlights that sink, if I'm reading the datasheet correctly, 80mA, but I'm not sure if that is 80mA per LED (seems a bit much for one LED) or all together. Also, it doesn't give the current sink for the display itself without the backlights.

Datasheet for TFT - https://www.adafruit.com/datasheets/MI0283QT-11%20V1.1.PDF

In total, I'm looking at 180mA load current...or more if it's 80mA per LED.

Hi

Down on page 7 in the TFT spec they show that they are talking about all 4 LED's in parallel with 80 ma into the set of 4.

Bob

[dom0]

Consider also something like a 1/2AA lithium cell.  They typically have around 3.6V terminal voltage, and last very well in storage.

Those Lithium primary cells can supply a few MOS-RAMs for decades. They should also be able to deliver higher currents, but higher currents ~ low battery life <=> should use a rechargeable (secondary) cell.

The self-discharge of secondary Lithium cells is afaik _much_ higher than that of primary Li cells.

[sentry7]

Then approximate figures will have to do. The major parts are the MCU and display.

The MCU, as mentioned before, pulls 100mA at maximum clock frequency (it's always running at this frequency, so I'm assuming constant pull).

The display has four backlights that sink, if I'm reading the datasheet correctly, 80mA, but I'm not sure if that is 80mA per LED (seems a bit much for one LED) or all together. Also, it doesn't give the current sink for the display itself without the backlights.

Datasheet for TFT - https://www.adafruit.com/datasheets/MI0283QT-11%20V1.1.PDF

In total, I'm looking at 180mA load current...or more if it's 80mA per LED.

Hi

Down on page 7 in the TFT spec they show that they are talking about all 4 LED's in parallel with 80 ma into the set of 4.

Bob

Thanks for clearing that up.

So final approximate load current is 180mA at 3.3V.
84 hours x 180mA = 15.12 mAh

[Ian.M]

There's a module power consumption of 220mW listed on page 4 of the datasheet.  That doesn't match the backlight consumption calculated from If*Vf so must be the electronics.  I calculate the total including your MCU at 822mW.  Assuming a 90% efficient PSU and allowing 10% margin, that comes out at 1W, three times the load you were initially expecting.  Alkaline efficiency drops off rapidly with increasing current due to the losses in the cell's internal resistance, and my best estimate is you'd be lucky to get 20h from 4 D cells.

To get your required run time, you'd need 12 Alkaline D cells, at about $14 a set.  You'd do better to look at rechargeable LiPO packs. An 8000mAh 4s pack would handle it with enough reserve to avoid deep discharge.

[Kilrah]

Those Lithium primary cells can supply a few MOS-RAMs for decades.

Yep impressive, I have a training/experimentation computer on which the lithium battery stamped 10/87 is still going strong 



The (lack of) consumption of the RAM is jsut as impressive as the (lack of) battery self-discharge though. These things really only need to know that there's something to power them to maintain their contents.

[sentry7]

There's a module power consumption of 220mW listed on page 4 of the datasheet.  That doesn't match the backlight consumption calculated from If*Vf so must be the electronics.  I calculate the total including your MCU at 822mW.  Assuming a 90% efficient PSU and allowing 10% margin, that comes out at 1W, three times the load you were initially expecting.  Alkaline efficiency drops off rapidly with increasing current due to the losses in the cell's internal resistance, and my best estimate is you'd be lucky to get 20h from 4 D cells.

To get your required run time, you'd need 12 Alkaline D cells, at about $14 a set.  You'd do better to look at rechargeable LiPO packs. An 8000mAh 4s pack would handle it with enough reserve to avoid deep discharge.

I guess that rules out alkaline cells; I want to keep customer cost down. As far as LiPO packs are concerned, is there any compliance testing that needs to happen before I ship these?

[uncle_bob]

There's a module power consumption of 220mW listed on page 4 of the datasheet.  That doesn't match the backlight consumption calculated from If*Vf so must be the electronics.  I calculate the total including your MCU at 822mW.  Assuming a 90% efficient PSU and allowing 10% margin, that comes out at 1W, three times the load you were initially expecting.  Alkaline efficiency drops off rapidly with increasing current due to the losses in the cell's internal resistance, and my best estimate is you'd be lucky to get 20h from 4 D cells.

To get your required run time, you'd need 12 Alkaline D cells, at about $14 a set.  You'd do better to look at rechargeable LiPO packs. An 8000mAh 4s pack would handle it with enough reserve to avoid deep discharge.

Hi

Hey, back to a dozen D cells in no time flat.

====

Bottom line here is that the system is pulling a *lot* of power for a battery based device. The cost of the power system is indeed "non trivial". I would suggest carefully evaluating the power in each sub assembly and seeing if money spent there would make more sense than money spent on giant / expensive batteries. I would also take a look at things like the back light -- how bad is it at 1/2 power ? Is that "ok" or not? On battery powered system I have ever done, we spent a *lot* of time beating on the electronics (and firmware) to get the power down. There simply was no other way to do it that made sense cost or size wise.


Bob

[Ian.M]

While we are giving examples of ridiculously long batterry life, what about the world record?

Zamboni dry pile still in operation after 175 years.

https://en.wikipedia.org/wiki/Oxford_Electric_Bell

@sentry7,
Compliance depends on local regs, but one thing's for certain, you cant ship them Air-freight.

[sentry7]

Sounds like a dry pile is what I need.

[uncle_bob]

I guess that rules out alkaline cells; I want to keep customer cost down. As far as LiPO packs are concerned, is there any compliance testing that needs to happen before I ship these?

Hi

If you follow the news, the whole "lithium battery fire" scare is making headlines every few weeks (at least in the US). Whatever the rules are today, that hysteria is likely to make things even more complex over then next few months / years. If you will be shipping internationally, I'd bet the rules will vary by destination. Massive headache coming on ....

None of this in any way relates directly to the actual hazard involved with this or that battery design. We made some wonderful NiCad's back in the day that with their "rapid charger" could and would either melt the radio or set it on fire. We fixed things and nobody outlawed NiCad batteries as a result. There are Lithium designs that are a lot more prone to nasty things than others. The same is true to a lesser extent of lithium battery suppliers.

The issue will be regulations that are either written or interpreted in a broad fashion. There are an enormous number of examples of this and (unfortunately) few examples of it being done properly. Being able to dodge around what ever they write (in advance) is going to be tough.

=========

Is this beast going through UL or CE approval? If so that will cover a bit of this.

Bob

[sentry7]

...Bottom line here is that the system is pulling a *lot* of power for a battery based device. The cost of the power system is indeed "non trivial". I would suggest carefully evaluating the power in each sub assembly and seeing if money spent there would make more sense than money spent on giant / expensive batteries. I would also take a look at things like the back light -- how bad is it at 1/2 power ? Is that "ok" or not? On battery powered system I have ever done, we spent a *lot* of time beating on the electronics (and firmware) to get the power down. There simply was no other way to do it that made sense cost or size wise.


Bob

The MCU is a real beast mainly because it's a multicore parallel processor, so, there's no changing that. In order for it to do what I need it to do, it needs to be constantly running at maximum frequency, so the MCU is a constraint. The TFT is relatively small as it is, and the visibility at maximum viewing angle would just be sub-optimal at anything less than full power.

The battery capability is more of a leisure than a necessity. It would just be nice if the user weren't confined to wherever there is a outlet.

[uncle_bob]

...Bottom line here is that the system is pulling a *lot* of power for a battery based device. The cost of the power system is indeed "non trivial". I would suggest carefully evaluating the power in each sub assembly and seeing if money spent there would make more sense than money spent on giant / expensive batteries. I would also take a look at things like the back light -- how bad is it at 1/2 power ? Is that "ok" or not? On battery powered system I have ever done, we spent a *lot* of time beating on the electronics (and firmware) to get the power down. There simply was no other way to do it that made sense cost or size wise.


Bob

The MCU is a real beast mainly because it's a multicore parallel processor, so, there's no changing that. In order for it to do what I need it to do, it needs to be constantly running at maximum frequency, so the MCU is a constraint. The TFT is relatively small as it is, and the visibility at maximum viewing angle would just be sub-optimal at anything less than full power.

The battery capability is more of a leisure than a necessity. It would just be nice if the user weren't confined to wherever there is a outlet.

Hi

Ok, not directly related to your application (I don't know what that is):

In some cases, a "people in range" sensor is used. One example is the IR sensor on your cell phone. It shuts off the display (and likely other stuff) when the phone is at your ear. Other examples are seen in other areas. The idea is the same, if nobody is there to use the display turn it off.

If the battery is a "luxury", how much is it worth to the customer? Are you better off with a battery option than a "everybody gets a battery" solution. In some areas, the gizmo comes in one rack box and the battery takes up a whole other box. If they want the battery, they order it. All of the gizmo boxes have chargers etc in them. Those who don't want the hassle do not have to deal with it. Even lead acid batteries are a hassle to ship ...

If you go with a rechargeable battery, consider the whole issue of recharge time (and heat). I have no idea what the turn around at the end of the 84 hours needs to be. A 24 hour charge may be fine. If a 1 hour charge to 50% is not acceptable, you are back to either disposable batteries or swappable battery packs.

Lots of details.

Bob

[Ian.M]

Why not design it to run off 5V with a USB lead?  A cheap USB powerbank sourced locally by your customer could easily give 24H running time, and a high capacity one could meet your 84H target.

[uncle_bob]

Hi

.... ok so Ian.M beat me to the punch line.  Rats ... 

The only thing I'd add: If you have two USB connectors, you can do hot swapping of the power packs. That way you have zero turn around time on a unit that is back from it's 84 hour mission.

Bob

[sentry7]

So a more explicit description of this is an artwork display. People come in range of the module and it turns on, showing artwork.

The PIR sensor and the USB powerbank are great ideas. Doesn't make sense to leave it on when there is no one there to see it. When people leave the area, the entire unit can enter shutdown/sleep mode or just turn off altogether.

If the unit is placed in a hallway or living room, it is probably going to be off more often than on. But on the other hand, if it's in the office where the person is working all the time, then the device is probably going to be on more often than off.

[Fungus]

This product is not going to be sitting anywhere after assembly; it's going to be used regularly and the batteries will be replaced by the user. I would like at least 84 hours of continuous device operation @ 100mA minimum.

It's a simple calculation: you need an absolute minimum of 8400mAh. I'd add a bit extra, just in case, so 10000mAh.

(Which is quite a lot...)

It's really hard to give useful advice without knowing what it is though. eg. How big? Doe sit need to fit in a pocket? What exactly is it?

[uncle_bob]

Hi

Even in an office, most businesses run on sort of an 8 hour day. The other part of the day there is a lot less activity. It's a pretty good bet that the net active time will be < 12 hours. I'd bet on a lot less than that in most office locations.

Here at home, unless I put it in the main room, The on time would be a lot less. Here, I've got uuummm ... errrrr ... USB charge strips ... all over the place. If it's got a USB power connector on it, I know what to do.

Bob

[kolbep]

What about using a 12v, 7Ah Lead Acid Alarm / Gate Motor Battery (I think approx $12?), and a float charger
Or a 12v 14AH?