Post by thread:[EE] Handheld, batteries and more

Mauricio Jancic wrote: {Quote hidden}> Hi, > I need to develop an application that will run on 4 AA Ni-Me > batteries. It will have a PIC18F6xJ50 (3.3V) a RTC, external EEPROM and a > graphic lcd, capable of working at 3.3V and at 5V. > > The device is required to be able to charge the batteries from the > USB port. > > Assuming a battery voltage from 1.6V to 0.9V (the user would use > alkaline batteries) the voltage range of the device will be from 3.6V to > 6.4V. > > I can't find some elegant solution to solve both the supply to the > electronics and charging the batteries. Would it be correct to use some step > down switching to get 3.3V and a step-up to boot from the 5V on the USB to > the 6.4V needed to charge the batteries. > > Any comments will be really appreciated. > > Regards, > > Mauricio So you want something like USB => BOOST => BATTERIES => BUCK => GADGET No idea occurs to me that is elegant in terms of doing it all with one supply. Some questions arise though. The answers to which will point out the path to take. How much current does your gadget need? Does it need to run while recharging? Do you need to preclude customers from trying to charge alkalines? Is efficiency important? Size? Cost? If efficiency is not real important and overall current is low, a linear regulator might be the best choice for the buck job. Otherwise a buck supply is called for. Other things come to mind like diode or switch isolation of the batteries so they don't discharge back into the boost supply when it is unplugged from the USB. And finally, test your gadget with someone like me who is clumsy and will bounce the battery contacts several times giving brown out circuits a good test. Good luck!

Mauricio Jancic wrote: > Hi, > I need to develop an application that will run on 4 AA Ni-Me > batteries. It will have a PIC18F6xJ50 (3.3V) a RTC, external EEPROM and a > graphic lcd, capable of working at 3.3V and at 5V. > > You have located an LCD display that operates at 3.3V? > The device is required to be able to charge the batteries from the > USB port. > > Assuming a battery voltage from 1.6V to 0.9V (the user would use > alkaline batteries) the voltage range of the device will be from 3.6V to > 6.4V. > I don't understand this. Are you saying that the user MIGHT install 4 'AA' cells to get 6.4V? but they wouldn't be chargeable, would they?. > I can't find some elegant solution to solve both the supply to the > electronics and charging the batteries. Would it be correct to use some step > down switching to get 3.3V and a step-up to boot from the 5V on the USB to > the 6.4V needed to charge the batteries. > USB "seems" like a 5V source but is really only 4.3V in the big picture. I think you can charge directly from the USB source using a precision LDO regulator. The regulator might need to be 4.2V to supply enough charge current; not sure how to get that precisely. You COULD drive from the PIC using the PWM channel. --Bob > Any comments will be really appreciated. > > Regards, > > Mauricio > > > >

Yes. Optrex has a lousy LCD that works at 3V. It has an internal charge pump to produce the negative voltages, however that internal inverter does not work and you have to build an external inverter, so they say.. if interested this is it: http://www.apollodisplays.com/Products/PassiveGraphicDisplays/F-51852.html Since I dont want to use the external charge pump I just found this: HDG12864L-6 here http://www.hantronix.com/2_2.html I think the later is the best of both. The idea of the product is to use it with 4 nimh batteries, thus making 4.8V max. However, in the event that the batteries die, the user can install 4 "normal" batteries to have the device up and running. Thats why we have that broad battery voltage. The device should check if the batteries where changed and if so, logic beyond this is yet to be defined. To charge 4 nimh batteries I would need atleast 4.8 volts, ain't that right? Why did you say that 4.2 is enough? Regards, Mauricio On 11/21/07, Bob Axtell <spam_OUTengineerTakeThisOuTcotse.net> wrote: {Quote hidden}> > Mauricio Jancic wrote: > > Hi, > > I need to develop an application that will run on 4 AA Ni-Me > > batteries. It will have a PIC18F6xJ50 (3.3V) a RTC, external EEPROM and > a > > graphic lcd, capable of working at 3.3V and at 5V. > > > > > You have located an LCD display that operates at 3.3V? > > The device is required to be able to charge the batteries from the > > USB port. > > > > Assuming a battery voltage from 1.6V to 0.9V (the user would use > > alkaline batteries) the voltage range of the device will be from 3.6V to > > 6.4V. > > > I don't understand this. Are you saying that the user MIGHT install 4 > 'AA' cells to get 6.4V? but they > wouldn't be chargeable, would they?. > > I can't find some elegant solution to solve both the supply to the > > electronics and charging the batteries. Would it be correct to use some > step > > down switching to get 3.3V and a step-up to boot from the 5V on the USB > to > > the 6.4V needed to charge the batteries. > > > USB "seems" like a 5V source but is really only 4.3V in the big picture. > I think you can charge directly > from the USB source using a precision LDO regulator. The regulator might > need to be 4.2V to supply enough > charge current; not sure how to get that precisely. You COULD drive from > the PIC using the PWM channel. > > --Bob > > > Any comments will be really appreciated. > > > > Regards, > > > > Mauricio > > > > > > > > > > -

> > So you want something like USB => BOOST => BATTERIES => BUCK => GADGET Yes smething like that. No idea occurs to me that is elegant in terms of doing it all with one > supply. Some questions arise though. The answers to which will point > out the path to take. > > How much current does your gadget need? I estimate it will be below 150mA with 250mA peaks of no more than 1 min. Does it need to run while recharging? Yes Do you need to preclude customers from trying to charge alkalines? Ideally, yes. Is efficiency important? Is not the main concern. Size? Small :) Cost? Cheap :) If efficiency is not real important and overall current is low, a linear {Quote hidden}> regulator might be the best choice for the buck job. Otherwise a buck > supply is called for. > > Other things come to mind like diode or switch isolation of the > batteries so they don't discharge back into the boost supply when it is > unplugged from the USB. > > And finally, test your gadget with someone like me who is clumsy and > will bounce the battery contacts several times giving brown out circuits > a good test. > > Good luck! > -- Thanks, I'll look into your suggestions. Bye!

National or Linear (maybe TI) makes a SEPIC charger/supply that would work for this. It isn't an inexpensive part though. - Martin K Mauricio Jancic wrote: {Quote hidden}> Hi, > I need to develop an application that will run on 4 AA Ni-Me > batteries. It will have a PIC18F6xJ50 (3.3V) a RTC, external EEPROM and a > graphic lcd, capable of working at 3.3V and at 5V. > > The device is required to be able to charge the batteries from the > USB port. > > Assuming a battery voltage from 1.6V to 0.9V (the user would use > alkaline batteries) the voltage range of the device will be from 3.6V to > 6.4V. > > I can't find some elegant solution to solve both the supply to the > electronics and charging the batteries. Would it be correct to use some step > down switching to get 3.3V and a step-up to boot from the 5V on the USB to > the 6.4V needed to charge the batteries. > > Any comments will be really appreciated. > > Regards, > > Mauricio > > >

Listas de Correo wrote: {Quote hidden}> Yes. Optrex has a lousy LCD that works at 3V. It has an internal charge pump > to produce the negative voltages, however that internal inverter does not > work and you have to build an external inverter, so they say.. if interested > this is it: > > http://www.apollodisplays.com/Products/PassiveGraphicDisplays/F-51852.html > > Since I dont want to use the external charge pump I just found this: > > HDG12864L-6 > here http://www.hantronix.com/2_2.html > > I think the later is the best of both. > > > The idea of the product is to use it with 4 nimh batteries, thus > making 4.8V max. However, in the event that the batteries die, the > user can install 4 > "normal" batteries to have the device up and running. > Thats why we have that broad battery voltage. > > The device should check if the batteries where changed and if so, logic > beyond this is yet to be defined. > > To charge 4 nimh batteries I would need atleast 4.8 volts, ain't that right? > Why did you say that 4.2 is enough? > I must have been daydreaming; I was thinking of a LiIon cell. For NiMH, the max voltage to charge is 1.5V/cell, or 6V for 4 cells. I have found that pulse-charging (applying the max voltage but making the pulse narrower as the cell takes the charge) eliminates the "memory effect" problem. When I am done, I trickle-charge at 1/30C, which means that I am pulsing approx 1% of the time. --Bob A

Dave said: > Do the cells you're using allow any "trickle"? Most NIMH > don't. RECOMMENDATION: Wade through this if you have an interest in NimH trickle charging. What is allowed varies widely. Alas, its complex and varies by manufacturer and NimH subtype and capacity. - Some cells allow continuous charging at C/10 and even one year lifetimes at up to C/5 !!!! (or so the manufacturer's data sheets say). - Others allow C/30 to C/20 trickle either indefinitely or for a reasonable period. - Others say no trickle charging at all. - Higher capacity NimH cells seem to allow lower or no trickle charging while small capacity cells generally allow higher rates. - Lifetimes at higher trickle rates (where such data is given) decrease with ambient temperature. - Over temperature from trickle charging can be inferred from manufacturers comments as a significant limit to trickling but not necessarily the only one. The square-cubed relationship between battery area and volume may help explain why smaller capacity cells are generally more tolerant. (This 'law" also helps explain the differences between how an ant and an elephant are built). If you are interested in NimH trickle charging, and unfortunately I am, then you'll need to wade through a lot of material and may still be uncertain at the end. I'm going to have to learn more about this subject. The following will start you on the journey. Russell _________ For starters: NimH are generally reported as being less tolerant of trickle charge that NiCd. Reading what's said, and this may be wrong, the failure / degradation mode seems to be the ongoing heating effects. Looking at actual manufacturer's data sheets reveals a variable result. Varta allow C/30 to C20 trickle charging in their smaller AA cells (up to 1300 mAh) but not at all in their larger AA cells. They allow massive overcharge "trickling" in their button cells, even stating lifetimes at various rates. While this looks like a mistake in any one data sheet, it is repeated in many. Panasonic both allow trickle charging and suggest that long term it is inadvisable. ___________ Utterly superb collection of battery data sheets for a very wide range of batteries This is the best single source of battery data that I have ever seen. http://www.master-instruments.com.au/Data%20Sheets/ _________________________ VARTA 2700 mAh AA Trickle charge 0.03-0.05: NO! http://www.master-instruments.com.au/Data%20Sheets/NiMh%20Cyl%20&%20Prism/Varta%20VH2700A.PDF 1600 mAh AA - also NO trickle allowed http://www.master-instruments.com.au/Data%20Sheets/NiMh%20Cyl%20&%20Prism/Varta%20VH1600AA%20data%20sheet.PDF *** BUT *** 1200 mAh Varta AA NimH Trickle (0.03 - 0.05): Allowed (36 - 60 mA) http://www.master-instruments.com.au/Data%20Sheets/NiMh%20Cyl%20&%20Prism/Varta%20VH1200AA.PDF 1300 mAh AA - allowed _________________________ Here's an excellent Panasonic page on NimH charging, including trickle charging. They suggest in (9) a trickle charge rate of 0.033C to 0.05C. In (12) they seem to suggest trickle charging long term is bad, but by considering the overall gist of the article it seems that this is more precautionary than certainty. However, as always, YMMV. http://www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_NiMH_ChargeMethods.pdf _________ Authoritative *looking* page on Varta NimH Microbatteries. "Long life-expectancy at trickle charge" Up to 6 years at 20C Up to 3 years at 45C http://www.cellpacksolutions.co.uk/varta/rechargeable/Mempac/mempac-batteries.asp ____________________ Varta seem to rate their NimH button cells at All at 20 C C/30 trickle C/10 continuous overcharge <-- suspect C/5 1 year overcharge !!!!!!!!! <-- suspect Life expectancy: IEC cycle 1000 cycles Trickle charge [C/30] Up to 6 years at 20C Up to 3 years at 45C 380 mAh example http://www.master-instruments.com.au/Data%20Sheets/NiMh%20Button/V350H%20data%20sheet.pdf 15 mAh C/30 continuous trickle http://www.batterystore.com/Varta/V15H.htm Same cell - note strange and high long term overcharge specs http://www.batterystore.com/Varta/VartaPDF/V15H.pdf Range of Varta data sheets http://www.master-instruments.com.au/Data%20Sheets/NiMh%20Button/