Cmos battery test. - Page 1
Forum section: electronics / beginners

author: cyberwasp
author_rank: Contributor
timestamp: February 13, 2022, 03:07:23 am
content: I hope I'm in the right place. I haven't been building anything for a long time and my scope has been lonely and the old brain isn't recalling things to well. Recently I was having difficulty with my PC retaining its Cmos settings. Since being handicapped I waited for a friend to visit to assist me with changing it.Using a DVM the old 2032 shows 2.97v. I'd like to make a simple circuit that would simulate the cmos drain that I could monitor with the dvm or scope;) I know it sounds crazy and that's ok. Would just like to see what happens. I have a bunch of electronic parts including several arduinos if needed. the simpler the better.I was thinking just an led/resistor in series with the dvm or scope unless there's something better!! TIA

author: bob91343
author_rank: Super Contributor
timestamp: February 13, 2022, 03:38:01 am
content: There should be no drain from the battery. It's just a polarizing voltage and only miniscule leakage current is taken from the battery, which essentially should last for its shelf life.

author: cyberwasp
author_rank: Contributor
timestamp: February 13, 2022, 04:00:41 am
content: Quote from: bob91343 on February 13, 2022, 03:38:01 amThe old one was roughly 5 years, not including how long the motherboard was sitting in the warehouse. I was just thinking it would be something to get me back into experimenting!

author: m3vuv
author_rank: Super Contributor
timestamp: February 13, 2022, 04:10:01 am
content: seems like a waste of time regarding how much they cost,im suprised the cmos lost its marbles at that voltage,ive had them under 2v and still keeping the cmos data!

author: Vovk_Z
author_rank: Super Contributor
timestamp: February 13, 2022, 04:14:35 am
content: The capacity of 2032 battery is about 200 mA*h. It may work up to 5-10 years, let's take 10 years (with a good battery with small self-discharge). That is 87600 hours. So CMOS current is about 200 mA*h / 87600 h = 2 uA.

author: BrokenYugo
author_rank: Super Contributor
timestamp: February 13, 2022, 06:52:30 am
content: Quote from: m3vuv on February 13, 2022, 04:10:01 amI actually have one in a Super Nintendo cartridge that wouldn't hold at something like 2.9 or 3.0. A new battery fixed it and hasn't died yet, so I assume the SRAM is fine otherwise.

author: Shock
author_rank: Super Contributor
timestamp: February 13, 2022, 07:09:16 am
content: With infrequent use and no other parasitic load problem 5 years is about how long they last disconnected from the mains.

author: Per Hansson
author_rank: Unknown
timestamp: February 13, 2022, 03:29:24 pm
content: One thing not mentioned: when measuring battery voltage you should apply some load.Example: I have three CR2032 batteries that have been used for CMOS and RTC backup in mainboards till they failed.With a 10M ohm input resistance DMM they measure 2,9v, 2.7v and 2.2v respectively.With a 3K ohm input resistance meter (resulting in 1mA load) they measure 0.0v, 0.0v and 1.4v respectively.

author: jmelson
author_rank: Super Contributor
timestamp: February 13, 2022, 04:31:38 pm
content: Quote from: bob91343 on February 13, 2022, 03:38:01 amNo, on almost any PC, the CMOS battery also runs the real time clock. That has a 32768 Hz crystal oscillator and divider chain, running a time counter.Not a lot of drain, but not really zero, either.Jon

author: viperidae
author_rank: Frequent Contributor
timestamp: February 14, 2022, 03:16:23 am
content: Intel suggests a current of 5uA for the RTC of their PCH chipset. Which works out to be nearly 4 years for a 170mA cell.Power is also only drawn while the standby motherboard power is disconnected.