Post by thread:NiCd Charge State

I have a project where I need to determine the state of charge of a 7.2 volt NiCd battery with a PIC. Is it possible? How can you measure the nearly flat discharge curve? I was planning on using an 8 pin PIC with A/D. As usual, space is tight. The load is resistive and relatively constant. I considered trying to keep track of charging time and discharging time, but I do not think that would work in this application. I would like to use any battery and immediately know the state of charge. Paul A. Brown Product Engineer Elliott Company Jeannette, PA

Take a look at the datasheets for IC's that the manufacturers call 'Gas-Gauge' IC's as this may give you an idea what it is/not measuring to estimate the life left in the cells. This is tricky!! SW. ____________________Reply Separator____________________ Subject: NiCd Charge State Author: pic microcontroller discussion list <spam_OUTPICLISTTakeThisOuTMITVMA.MIT.EDU> Date: 17/02/00 08:34 I have a project where I need to determine the state of charge of a 7.2 volt NiCd battery with a PIC. Is it possible? How can you measure the nearly flat discharge curve? I was planning on using an 8 pin PIC with A/D. As usual, space is tight. The load is resistive and relatively constant. I considered trying to keep track of charging time and discharging time, but I do not think that would work in this application. I would like to use any battery and immediately know the state of charge. Paul A. Brown Product Engineer Elliott Company Jeannette, PA

Paul, I don't believe there is a way to instantly determine the state of charge, as cell voltage varies just as much with cell temperature, manufacturer, memory effects, age, and even manufacturing batches. If there is a way - I'd love to know - (It would have saved me months of gas gauge code writing) My current favorite gas gauge is the MAX 1660 - It gives you the raw counts (charge / discharge) and is reasonably priced - you have to do the rest in software. Dallas has one with built in temperature histograming (don't know the price). If you want it all done for you - try the Benchmarq (now TI) range - expensive, but ideal if you are only designing for a small production run of high value kit, and haven't got the time to code up for a simple charge count / discharge count gas gauge. A suggestion - Don't give users a percentage charge remaining, Give them 4 or 5 bars to indicate charge. (I have no end of people complaining that I'm upto 5% out when the battery goes flat) If your unit is near enough constant current, then if you use the time the unit is on, and calculate in the battery's self discharge (calculate time from last charged ), and you only used a 4 or 5 bar indicator, I think you could have a pretty good stab at a time till flat battery indicator. It falls to bits totally, if the unit is not constant current, the user is switching off and on a lot, if the user only partially charges the unit, or (to a lesser extent) if there are large temperature changes. Sadly, to know the state of charge, you have to fully charge to have a starting point. If you need anymore help just shout, Steve Lawther > {Original Message removed}

If you find a way to do that with reliable results, please let me know... Paul Brown wrote: {Quote hidden}> I have a project where I need to determine the state of charge of a 7.2 > volt NiCd battery with a PIC. Is it possible? How can you measure the > nearly flat discharge curve? I was planning on using an 8 pin PIC with > A/D. As usual, space is tight. > > The load is resistive and relatively constant. I considered trying to keep > track of charging time and discharging time, but I do not think that would > work in this application. I would like to use any battery and immediately > know the state of charge. > > Paul A. Brown > Product Engineer > Elliott Company > Jeannette, PA

Judging from the replies, the only way to predict charge state is to use a "gas gage" type of chip to measure and characterize the charge/discharge cycles of specific battery. I will be discharging using a PWM signal with a nominal frequency of 1kHz. Will the gas gage devices work with a pulsed discharge, or do they need a mostly continuous discharge? Paul A. Brown Product Engineer Elliott Company Jeannette, PA

The best way to do this is probably as follows: assuming unknown state of battery, e.g. instrument gets turned off and on several times between recharging 1. measure the battery voltage at regular intervals e.g. every minute or two. 2. compare voltage against previous reading, allowing say a two bit noise value. If a reducing voltage trend is indicated then possibly measure more often to see if you are going around the discharge end point knee. 3. Apply an extra load to the battery (will require one output bit) and measure voltage while load is on. Immediately after measurement switch load off. As this will take minimal time the effect on battery life will be minimal except in the most low power situations. 4. If the load can reduce the voltage by a count of say 10 (will be dependant on the size of load used), then the battery has reached the end of life discharge point. you may find that the gas gauge IC's use this kind of measurement technique anyway.