ADC used for monitoring battery, drains battery when powered off. - Page 1
Forum section: electronics / projects

author: MrLikerBiker
author_rank: Newbie
timestamp: March 26, 2014, 04:03:02 pm
content: I thought his was going to be a simple addition to an existing project. I use a CR2450 Li Ion 3V 620mAh to maintain an RTC & SRAM on a Rabbit 3000 based module when the project is unpowered.http://www.digi.com/products/wireless-wired-embedded-solutions/solutions-on-module/rabbitcore/rcm3309recently, it seemed to be a good idea to use an unused input of an ADS7844 ADC to monitor the state of the battery. However, when the hardware is powered down, the ADC input appears to be acting as a forward diode to ground. This causes about 450mA to flow at about 3V which happens to be the voltage of the battery. Unsurprisingly, the battery doesn't last very long when the hardware is powered down.Any ideas?I could put a resistor in series in series, say, about 3M but then I suppose it all depends on how much current the ADC draws when sampling and whether it would cause a significant volt drop

author: dannyf
author_rank: Super Contributor
timestamp: March 26, 2014, 05:13:11 pm
content: It is called "ghost powering": the input pin + the clamping diode formed a power source.I would put a diode / mosfet on the adc's Vcc pin to prevent from being powered by the input pin.A series resistor to protect the input pin is always desirable.

author: redwire
author_rank: Newbie
timestamp: March 26, 2014, 06:58:34 pm
content: You need a several meg resistor and 0.1uF sample capacitor feeding your A/D, but anything you use to monitor the battery voltage will drain it. Some RTC's have a register flag for battery status.You can use software to determine if the battery is dead. Write a bit pattern like $A5A5 into RTC RAM and if you don't see that on power up, then the RTC is corrupt and battery probably dead.CR2450's are expensive!

author: MrLikerBiker
author_rank: Newbie
timestamp: March 27, 2014, 01:17:50 pm
content: redwire, sadly it's a custom RTC that is built into the processor. I can only read or set the date/time.Yeah, CR2450's are chunky cells. If the circuit is unpowered, 6uA is sourced from the battery. This gives a worst case lifetime of just over 11 years. That's 11 years of not being turned on. The battery maintains the RTC and SRAM. (The processor is on a module board which has about 512K of SRAM available). We have a very simple file system used in the design and the "header" information is contained in SRAM. Cycling the power without the battery in place results in the RTC being set back to 1980 and all files vanishing form the system. This why we went for overkill with the battery and why we thought it a good idea to add monitoring of the battery level to detect if it is still viable or has someone cheekily removed it.I think placing a resistor between the battery and ADC input of about 1M will be a simple solution. Hopefully the ADC will only draw a minuscule amount of current when sampling the input and not cause a significant voltage drop to the ADC input. Will have to experiment.

author: richard.cs
author_rank: Super Contributor
timestamp: March 27, 2014, 03:32:07 pm
content: Quote from: MrLikerBiker on March 27, 2014, 01:17:50 pmAs mentioned previously you will need a capacitor to ground on the ADC side (exact value will depend on the ADC but 10 nF is normally a good starting point). If you omit that capacitor you will get meaningless ADC values, although the input current is low the ADC shuffles charge in and out of the input pin during sampling so it needs to be low impedance to ac. Also remember to wait a few RC time constants after powering up the ADC before making a battery voltage measurement.

author: jrodgers
author_rank: Newbie
timestamp: March 27, 2014, 05:30:21 pm
content: Quote from: redwire on March 26, 2014, 06:58:34 pmhttp://www.ebay.co.uk/itm/5-x-CR2450-Lithium-Button-Cell-3v-Battery-24-5mm-Diameter-for-Watch-Car-Key-Fob-/330928128550?pt=UK_ConsumerElectronics_Batteries_SM&hash=item4d0cda8226I didn't think they were this expensive! And who has bought 5 of these packs at this price??

author: SeanB
author_rank: Super Contributor
timestamp: March 27, 2014, 06:32:33 pm
content: I will go tomorrow to Osmans and buy a box if i can shift them at that price. I might have to pay $2 per strip of 5.

author: Andreas
author_rank: Super Contributor
timestamp: March 27, 2014, 09:33:24 pm
content: Quote from: MrLikerBiker on March 27, 2014, 01:17:50 pmAnother solution could be placing a logic level FET between battery and ADC if you have a 5V supply which is switched off.The gate would be connected to +5V.Drain to batterySource to ADC input.Try a BSS138 with maximum gate threshold voltage of 1.5VSo unclamped measurement range is still up to 3.5V @ 5V gate voltage.With best regardsAndreas

author: jc101
author_rank: Frequent Contributor
timestamp: March 28, 2014, 10:42:06 am
content: Quote from: Andreas on March 27, 2014, 09:33:24 pmThat sounds familiar, I do much the same on a uC device using the BSS138, I connect the Gate to an output pin normally pulled low, and only turn on it on when I need to measure the battery level, when off it draws virtually nothing. In my case the supply is 3v max boosted to 5v so it's easy to do, the unit measures the battery and gives a battery low warning.