Post by thread:Formula

At 01:35 PM 1/31/99 -0500, Darwin Reynoso wrote: >Hi >Can somebody tell me the formula to know how long >will a battery last supplying a 20 ma load. > >Thanks 'snot that simple. You'd think a 20mah battery would last one hour, but it will typically give you about half that. The Amp/Hour rating of a battery is called "C". You need a battery with a rating of at least 2C in order to get an hour's runtime. Depending on the chemistry, this can get a lot worse. Zinc-Air cells can give you a huge amp-hour rating for their size, but they only like to supply tiny currents. First, settle on a battery type, (Nicad, Gell, Alkaline..... ) then get with the manufacturer for current/runtime curves.

Okey Using a 9volts Duracell battery supplying a 20 ma load how long will it lasts Eduardo R. wrote: {Quote hidden}> >Can somebody tell me the formula to know how long > >will a battery last supplying a 20 ma load. > > > > Darwing, > It depends on the kind of batery you are going to drain that current (20mA) > from. > > For Rechargable type bateries: AA penlight = 500mA/h > for C type = 1500mA/h > > Then we have that AA will last 500mA/20mA = 25 hours when fully charged. > > for C type 1500mA/20mA = 75 hours > > You need to know the mA/h rate for the batery you are planning to use, or > calculate it from the above values in case you are using those or similar ones . > > Remember that n batteries in parallel supply n times the current, > increasing the time they last. > > Hope this helps > Best wishes > Eduardo R > > AC Power Control project based on PIC > http://www.geocities.com/SouthBeach/Cove/4535 > ICQ# 10909825 > spam_OUTeriveraTakeThisOuTumemphis.campus.mci.net > > CHILDHOOD CANCER > "Anyone whose family hasn't been touched by it > should get down on his knees every night and > thank the MAN upstairs"...........SAM COOPER

Even that's not fully answerable but it's closer. An ALKALINE 9v "transistor" battery (PP3 etc) has a capacity of around 500 mAH. This means that NOMINALLY it can provide about 1 mA for 500 hours or 10mA for 50 hours etc. For your 20 mA you would expect to get APPROXIMATELY 500/20 = 25 hours. This will depend on temperature, whether you do it in one burst or eg 10 minutes every hour (battery chemistry gets a chance to do things while its resting) and very importantly, how many hours you want to discharge it in (20 in this case) how you define the end point. An Alkaline battery starts off at about 1.5 volts per cell (there are 6 cells in a 9 volt battery) drops a few tenths of a volt fairly quickly then settles into a fairly flat decline until it gets to around 1 volt per cell and then plunges quite rapidly thereafter. Below 0.9 to 0.8 volts its fully gone and drops very rapidly to zero. Manufacturers publish curves of expected performance under various "standard" conditions. Rough guide. For discharge currents of 0.1 x amp-hour capacity or less: Time ~ capacity/time For more rapid discharges Time ~ capacity / (2 x time) For very fast discharges - much less than you would expect. Here are a few TYPICAL capacities for modernish batteries. All these are in milliamp hours. Alkaline AA 500 Alkaline C 7500 Alkaline D 15000 NiCd AA 500 NiCd C 1800 NiCd D 4000 Your mileage WILL vary. Tjaart noted a while ago (6/1998) that a Duracell D rated at 15000 mAH nominal will actually provide 12000 mAH at 300ma at 50 to 70 degrees Fahrenheit. (strange new fangled temperature units we don't know about over here). From: Darwin Reynoso <.....darwinKILLspam@spam@fuse.net> {Quote hidden}>Okey >Using a 9volts Duracell battery >supplying a 20 ma load how long will it lasts > >Eduardo R. wrote: > >> >Can somebody tell me the formula to know how long >> >will a battery last supplying a 20 ma load. >> > >> >> Darwing, >> It depends on the kind of batery you are going to drain that current (20mA) >> from. >> >> For Rechargable type bateries: AA penlight = 500mA/h >> for C type = 1500mA/h >> >> Then we have that AA will last 500mA/20mA = 25 hours when fully charged. >> >> for C type 1500mA/20mA = 75 hours >> >> You need to know the mA/h rate for the batery you are planning to use, or >> calculate it from the above values in case you are using those or similar ones. {Quote hidden}>> >> Remember that n batteries in parallel supply n times the current, >> increasing the time they last. >> >> Hope this helps >> Best wishes >> Eduardo R >> >> AC Power Control project based on PIC >> http://www.geocities.com/SouthBeach/Cove/4535 >> ICQ# 10909825 >> eriveraKILLspamumemphis.campus.mci.net >> >> CHILDHOOD CANCER >> "Anyone whose family hasn't been touched by it >> should get down on his knees every night and >> thank the MAN upstairs"...........SAM COOPER >

An ALKALINE 9v "transistor" battery (PP3 etc) has a capacity of around 500 mAH. : Here are a few TYPICAL capacities for modernish batteries. All these are in milliamp hours. Alkaline AA 500 NiCd AA 500 Those don't corrolate. A typical 9V alkaline battery contains 6 AAAA cells, so would of necessity have a much lower mAH rating than an AA. Also, a NiCd usually has significantly less energy than an alkaline (although a higher current capability) (as with the C/D cell numbers.) I therefore think that the quoted Alkaline AA capacity is incorrect. NiCd C 1800 NiCd D 4000 Another thing to watch out for is that many of the "consumer" D cells are actually C cells in a bigger package (keeps prices lower.) IIRC, the duracell web site includes a fair amount of technical information, including the discharge curves for their common batteries. BillW

>Here are a few TYPICAL capacities for modernish batteries. >All these are in milliamp hours. > >Alkaline AA 500 >Alkaline C 7500 >Alkaline D 15000 >NiCd AA 500 >NiCd C 1800 >NiCd D 4000 For comparison, from The Art of Electronics, I get these: Alkaline: D 10000 - 8000 C 4500 - 3200 AA 1400 - 1000 AAA 600 - 400 These are *measured* capacities at a continuous drain down to 1V/cell. The first number is at 10 mA, the second at 100mA. Note that the capacities go down significantly, especially with the smaller cells. So yes, a LOT depends on what kind of current you're drawing and whether it's continuous or intermittent. Dave Johnson