Wagner, sorry for the late response. To shape our discussion:=20 What is better for battery-operated systems? 1. To feed them from say two or three 1.5V cells directly. 2. To feed them from step-up DC/DC regulator powered by _one_ 1.5V cell. You advocate first approach, I do second.=20 Factors to be considered: "Reliability" section: - loss of reliability due to extra contacts;=20 - loss of reliability due to using two cells instead of one;=20 - loss of reliability due to using extra components (DC/DC=20 regulator);=20 - some vagueness with reliability under floating voltage - =20 How do you like this from AN601: "...The number of devices that can fail before a particular endurance criteria is not met is also somewhat flexible. Even the most quality conscious manufacturer will occasionally have a failure, so a failure level is defined. The industry standard conditions for many types of reliability tests are set by JEDEC (the Joint Electronic Device Engineering Council). JEDEC defines that if 5% or less of a given sample fails at a given endurance goal, then that goal has been met... " "Cost" section: -extra cost due to extra contacts for extra cells;=20 -for a given "Watts*Hour" extra cost when using more then one cell; (one 1.5V 1A*H costs less then two 1.5V 0.5A*H and takes less room) - extra cost due to using extra components (DC/DC regulator, room cost); - extra cost of tests on reliability under floating cells voltage. "The Roads we Take" are on our own. And I am not sure your=20 road is cheaper: extra reliable contacts are not cheap;=20 for a given "Watts*Hour" extra cells cost more; tests on reliability under floating cells voltage are tricky=20 business and cost a lot; I agree with Dale: "All operated well within spec, and all still fail. Stuff happens. =20 The difference between an engineer and a top-notch engineer=20 is often the ability to design systems that take these things into=20 account, and balances reliability against cost." Mike. ---------------------------------------------------------- Wagner Lipnharski wrote: > Mike Singer wrote: > > Wagner Lipnharski wrote: > > . > >> Than, we just used the available space in the box, including > >> the one used by the step-up converter, to add a third AA cell. > >> Then, now we have power that would travel from 4.5Vdc > >> down to 3.7Vdc when cells would be almost discharged. > >> This lower VCC reduced the current consume from 62mA to > >> less than 55mA (4.5V) and 46mA (3.7V). Then we redesigned > >> the software and reduced external glue logic, replaced by few > >> CMOS components, the final current consume at 4.5V was > >> lower than 22mA. Now we expanded battery life to more than > >> 40 hours. It was almost 9 times better than the original promect. > >> We could keep going further, but the customer was very happy, > >> so we stopped there. > >> > > > > If you "could keep going further", do it! > > Keep on going further back to DC/DC converter! > > Look at 21372b.pdf TC125-126 PFM Step-up DC/DC > > regulator. (US$1.67 retail here in the Ukraine). (Attached .gif) > > You could use only one thick 1.5v battery. > > With TC125301 you get 3.0V _CONSTANT_ output voltage. > > Power consumption is less then at your 4.5V-3.7V. > > TC125301 eats less then 40 microA at 3V@30ma. > > I was told Seiko's are even better. > > You needn't carry out rather expensive tests on reliability > > under floating voltages (oscillator start-up or overdriving > > conditions for example). > > > > Mike. > > ------------------------ >=20 >=20 > I use to think that a battery is just a pack that supply Watts/Hour, the > same technology, lets say Alkaline, will deliver the same Watts/Hour no > matter what cell voltage or current, the same volume means the same > Watts/hour. >=20 > Lets see: >=20 > Suppose the circuit drains 15mA at 3V (full charge) and 11mA at 2.2V (end > life). The average consume will be around 13.5mA. Using two AA cells > (1A/h) you can have around 74 hours of operational battery life. >=20 > Now, suppose you use both AA cells in parallel, and use the TC125301 unit > to generate 3V fixed. > The conversion current will be 2x when batteries are fully charged (1.5V) > and 2.7x when at life end (1.1V). This gives you an average of 2.4x. > Consuming 15mA, the circuit will drain 36mA in average, from the cells. As > they are in parallel, now you have 2A/h, result in 55 hours of operational > battery life. Not even counting the fact that the 125301 efficiency is not > 100%. Suppose it is 85%, battery life will be down to 46 hours. >=20 > To have the same battery life (74 h), the battery drain should be around > 13.5mA constantly (per cell), it means the 125301 could not drain more than > 27mA (parallel batteries). Considering a worst case of 2.2V VCC, 11mA > consume, the 125301 will drain 1.46x current from batteries (fully charged > 1.5V), and 2x at the end of life (1.1V). Applying 85% efficiency, it goes > to 1.64x and 2.35x rate. Then, the 11mA circuit consume, will be draining > (11mA x 1.64) 18mA (fully charge) and (11mA x 2.35) 25.8mA (end life) from > the cells. The average here can go to around 17mA, what gives 2A/0.017 =3D > 117 hours. >=20 > This extra gain, even with the 15% loss of the efficiency of the dc/dc > conversion, is based on the higher current drain when battery is above 2.2V > (up to 3V), when the circuit is directly connected to 2xAA (no dc/dc). >=20 > One needs to consider if this extra operational hours (from 74 to 117) is > accepted by the customer, considering the final product price increase due > the dc/dc conversion (can not cost less than $2), final product can > increase $12 or more. For some products that customer uses no more than > one or two hours per month, he really doesn't care about the extra battery > time, but $12 more or less, can really makes a difference. >=20 > Want to do a comparison in the real world? >=20 > Cell phones. >=20 > Using top of the line dc/dc and battery technology, standby time of 80 > hours, talking time of 6 hours. > Ok, what about if it was 90 hours and 7 hours? what difference it really > does make? > I talk less than 1 hour/day, and ALWAYS will keep the cell in the charger > during the night. > Why I will pay more $$$ for a phone or a battery that can gives me 8 > talking hours? > Obviously some people requires that, but not me, and not a bunch of other > people too. >=20 > Other than that, who talks 8 hours a day in a cell phone, needs to do a > brain damage catscan, for two reasons, first, the RF damages the brain, > second, what a hell is he doing 8 hours/day in the phone? >=20 > Wagner -- http://www.piclist.com hint: The PICList is archived three different ways. 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