RTC backup power - Page 1
Forum section: electronics / projects

author: Alex
author_rank: Regular Contributor
timestamp: June 15, 2014, 12:41:11 pm
content: Hello folks,Working on an RTC-based design, the question of 'How much backup time do we need' pops up quickly.I realise this is application-specific; the application I am considering falls under the desktop/bedside clock category.I personally feel that backup power must be sufficient for electricity supplier planned and unplanned power cuts (a few hours?) and should not be expected to keep a clock going when e.g. in storage for a month.I wanted to kick-off a discussion to hear your views on what you feel is too little and too much backup time for this and other applications. 5 hours? 5 days? 5 weeks?Alex.

author: dannyf
author_rank: Super Contributor
timestamp: June 15, 2014, 01:07:26 pm
content: It is impossible to tell.It is not uncommon for PC motherboards to have RTCs running for 10 years or more.

author: theatrus
author_rank: Frequent Contributor
timestamp: June 15, 2014, 07:08:30 pm
content: A modern RTC can operate for longer than the normal 10 year shelf life of a lithium coin cell. Finding the smallest possible battery will shorten this, so if you are cost optimizing that's one significant variable.

author: calexanian
author_rank: Super Contributor
timestamp: June 16, 2014, 05:24:57 am
content: Typically a RTC will be powered from a 3 or 3.6 volt lithium cell and run at 32khz tuning fork crystal. Most micros support this in a low power mode. If you need a stand alone to query Microchip has their own little rolled 8 bit micro specifically made as a low power RTC. Dallas clock modules used to be a popular option but fell out of favor due to the battery being internal and not replaceable. In our designs a lithium 3.6 volt AA battery will keep the real time clock running for a minimum of 5 years plus actually running the micro for the rest of its activities. If it were just keeping time in sleep mode only waking up to handle the timer interrupt and incrementing the clock you could conceivably get the whole usable life out of the battery (10 year shelf life plus about 5 more in practice)

author: gardner
author_rank: Regular Contributor
timestamp: June 16, 2014, 06:59:14 pm
content: A coin-cell holder is a pretty cheap part. This and a lithium cell yields retention times longer than the useful life of most products.Do you have some specific requirement -- like to NOT have a lithium cell? Or to ENSURE data loss after some period?

author: Richard Crowley
author_rank: Super Contributor
timestamp: June 16, 2014, 07:40:27 pm
content: I've seen several "bedside clock-radio" type gadgets that use a 9V snap-terminal battery for power-mains backup.Of course, it runs only the timekeeping part, and not the radio or the display.It would appear that the major reason for using that kind of battery is that is so common to find by consumers.Your question omits several important facts that prevent more detailed response.The primary one being, how much standby current are you talking about?As others have mentioned, this problem was solved by mother-board designers a decade agowith a rather simple, small, and inexpensive solution: A small CR2032 coin cell in a 20-cent clip.

author: Jeroen3
author_rank: Super Contributor
timestamp: June 18, 2014, 07:06:59 am
content: supercap, how long can your run your nA RTC on 0.22 Farad? Few days?User replacable:cr2032 battery, socketed or not, standard design practice for years of timekeeping.Or two triple A batteries or maybe 9v battery. Gives you product lifetime RTC timekeeping. Until the batteries vomit. Has a few hundred mAh directly usable or with a little ldo regulator. You'd need a case for those though.

author: Psi
author_rank: Super Contributor
timestamp: June 18, 2014, 07:19:18 am
content: Quote from: Jeroen3 on June 18, 2014, 07:06:59 amAssuming RTC drawing 100nA100nA / 0.22F = 0.454 uV drop per second.Assuming 3V cap with 1V of usable charge (3v down to 2V)Seconds for a drop of 1V = (1V / 0.454 uV) = 2,202,643 seconds = 25.5 days

author: AndyC_772
author_rank: Super Contributor
timestamp: June 18, 2014, 07:30:49 am
content: I looked into supercaps a while ago; in practice, most (all?) of them have self-discharge currents which exceed the load that an RTC is likely to present. It's an essential part of the specification that many data sheets don't even include - and that's a clue IMHO!Moreover, that leakage scales roughly with capacitance, so using a larger value supercap doesn't actually help much.I concluded it was likely that a small-ish supercap should be able to run an RTC for a period of days, maybe a couple of weeks at most - but probably no longer, however big it is. Your best bet is to use an RTC which has as wide an input voltage spec as possible, so it can cope with a large drop in capacitor voltage.Something likethis oneis quite a good bet. Timekeeping voltage is specified from 1.3 to 5.5V, so use a 5.5V supercap and charge it up to as high a voltage as you're comfortable with.

author: Psi
author_rank: Super Contributor
timestamp: June 18, 2014, 08:31:40 am
content: I have some maxwell 2600F 2.5V boostcaps that are still at 1.2V after about a year of sitting around doing nothing (initial full 2.5v charge).Supercap self discharge isn't as much of an issue as people think.I have them in my car instead of a car battery.

author: AndyC_772
author_rank: Super Contributor
timestamp: June 18, 2014, 08:59:28 am
content: How electrically similar is a 2600F cap to the sort of 0.5F cap that might be used in an RTC backup situation?Do you have a data sheet for a small supercap that shows a low enough self-discharge to last more than, say, a week or so?

author: Psi
author_rank: Super Contributor
timestamp: June 18, 2014, 09:21:43 am
content: yeah, i dunno how well they compare.However i imagine there's quite a difference between cheap/good brands of supercaps.