Concept #2 Evaluation / Design tradeoffs of using two batteries

After putting together our through-hole alphanumeric concept we're slightly concerned with the brightness.

We kinda assumed that since we were going to use put a resistor for each segment that we would be able to suppy enough current to the display to make it bright.  However, it turns out we are limited by the amount of current the PICs source.

If we raise the incoming voltage to around 4.5V then the PIC is able to supply enough current that the segments look bright enough (as you raise the Vdd the pic is able to supply more current... we think). 

At this point in the development of the parking spot recaller 2000 our research and development time is a major consideration.  If we don't finish this soon it's never going to be sold.  Therefore, we're willing to take some product part cost increase so that we actually do get our product on the market.

One idea we considered is adding a second battery.  If we could add another battery that was smaller.  We would be increasing brightness and potentially decrease the overall size of the unit.  Both of which would improve the concepts design criteria (see pugh matrix). 

Costs of Adding an additional battery:

3V 12.5MM 35mAH Cell = ~0.85
3V 12.5MM w/ Legs = ~$1.50
3V 16MM Cell = ~$0.83
3V 16MM w/ Legs = ~$1.68
3V 20MM 220mAH Cell = ~$0.33
6V 165mAH 23mm w/ Legs = ~$3.10

CR2032 Coin Cell =
Coin Cell Dual Battery Holder = $0.90
Coin Cell 20mm Dual Battery Holder = $0.55 (BH800S-ND)
Retainer Clip which require SMD Pad = ~$0.30

Best Deal is to go with the CR2032 Coin cell which is cheeper then other cells by a factor of 3 since it's the most commonly used coin cell. 

Cost = 2*0.33 + 0.55 = $1.21

So, the only question is will two batteries stacked on top of each other be too tall.  From the datasheet the profile is 8.89mm which is about a mm taller then the LED Display which is 7.80mm tall.  That's a little bigger then i'd like it to be.  We are planning on putting a red gel over the display to block any light from going through other then the red light.  This makes the display easier to read. 

If we can deal with the added 1mm height for adding about $0.70 in direct costs its worth not having to reinvest any more man hours in R&D.  If we sell about 50 of these units, the additional cost is 50*$0.70 = $35 dollars which is worth it.  Also, we have a number of CR2032 batteries from previous projects we could use and if we have a Vdd of 6V we can get away with using the non-low power PIC chips which cost a little bit more and are harder to find.