Hi everyone.

I'm currently working on a simple PIC project that measures the CCA of a car when starting, but do not know where to begin.

Is there a simple shcematic which I can use to together with the formula found here to determine the CCA?

Thanks in advance.

Ngai Seng

 

Starting currents can be large, so you'll want to be careful in experimentation. I'd suggest considering using a Hall sensor to measure the magnetic field of the current flowing in the wire. Here's one reference that pops up in a google search: http://ece.wpi.edu/analog/mqps/currentmqp.pdf.

Within the past few years I came across a website that gave nice details about how to adapt a Hall sensor for measuring the starter current, but I didn't keep a link to it. But at least one person out there has done such a thing and, if you can find it, the information will probably save you some work. Also consider contacting the manufacturers of Hall effect devices, as they may have app notes or engineering experience doing exactly what you want to do.

You'll also want to worry about what happens in temperature extremes, as the behavior at e.g. the freezing point of water won't be the same as, say, 20 °C.

 

Here is a definition of the term "CCA" - http://www.bgsoflex.com/cca.html

This link gives different definitions, but a great deal more detail - http://www.batterystuff.com/tutorial_battery.html

By use of a heavy solenoid switch (like a starter solenoid - just made for the purpose), you can let the battery discharge through a resistance. The voltage across the resistance will let you determine the current, by Ohm's law.

Finding the resistance could be fun. Say the battery offers 350 CCA's. Taking the battery as a nominal 12 volt source (13.6 is more like it when properly charged), a resistor that will allow 350 amps is only 35 milliohms (34.286).

For a test, you will need to have the battery at some temperature and fully charged. You dump the battery through the resistor for 30 seconds and see what the voltage was at the end of that time. If you have an idea of the lower voltage limit, you will be able to test to see if the battery meets its spec for CCA.

Did I mention that being able to vary that resistance will be necessary? Different batteries are going to have different ratings. And that the resistance will have to dissipate the heat? 350 amps through .035 ohm is close to 4200 watts.

Your PIC can only handle voltages up to 5 volts. If it has a 10 bit converter, that is 1024 voltage levels that can be resolved. You can use a resistive divider to lower the battery voltage so it is always less than 5. Your resolution is going to be on the order of 4.8 millivolts, if that will be enough (that is a resolution of 13.06 millivolts scaled up).

 

beenthere's post made me realize that maybe the OP isn't interested in measuring the CCA capability of a battery, but rather is just interested in knowing what the starting current is. ngaiseng, please confirm what your objective is. If it is just to measure the current, the quickest solution is to use a clamp-on ammeter (it has to measure DC current of course). Such devices can be found on the web in the $100-$200 range; some of them require that you already have a DMM.

If you don't mind cruder measurements, years ago you could buy from numerous auto tool dealers a simple analog gauge that you put next to the starter cable and it measured the current. I don't know if they're still available and I only saw them in catalogs, so I don't know how useful they'd be if you could find one.

If you really want to measure the CCA rating, getting the load that beenthere mentioned is going to be the hard part. A year or two ago I had some project I was interested in that needed to dissipate lots of power, so I made some sketches and back of the envelope calculations for a water-cooled aluminum block that held a number of International Rectifier IRF540N MOSFETs that would be connected in parallel to get the requisite load resistance. I'm sure the envelope has been tossed , but I'd be happy to sketch out the ideas again if you want. You'd need access to a machine shop to make the thing though. Also, I'd ask the knowledgeable EEs here to suggest a way to connect all these FETs together in parallel in such a way that they're guaranteed to equally share the current load. You'd also want to worry about what happens if one of them fails -- you'd probably want to quickly shut the rest of them down so they don't overheat. This means a bit more effort on the control side.

Added: There are also some power FETs on the market that can take hundreds of amps -- a friend sent me one a number of years ago. They were for 12 V auto service, but required gate voltages around 18 V if I recall correctly. If you're really interested in knowing more about these, answer this post and I'll see if I can find the thing (it's around here somewhere, but I haven't seen it in years) and I'll get the manufacturer name and model number.

 

Hi everyone,

As someonesdad pointed out, I would like to build a simple device to just capture the CCA of cars and inform the driver if the battery is going to fail or if it is still good.

beenthere gave some very interesting tips, but what I really need is something captures the CCA rating when the car is being cranked.

What I did was use a PIC to determine the voltage and say if it falls below 11.5Volts, it starts capturing about 3 seconds of datas and average it out. If it falls below 9.5Volts, it means to say that the battery is weak. But I have had some smaller cars with very "light" starters and they never hit below 11.5 Volts when craking , making the PIC not reset.

Hope you can give me some ideas here.

Thanks in advance.

 

You're using the wrong terminology, as CCA is what is used to measure the capability of a battery to supply current -- and is what beenthere's post addressed. CCA is not something consumers measure, as it's not easy to do.

It sounds like what you want is an ammeter to measure the starter current when the car is being started. This current will be a function of temperature, battery condition, wiring condition, etc.

If I had to attack this problem, I'd look at the feasibility of measuring the voltage drop along the cable from the battery to the starter -- it's a built-in shunt resistor. If you figure out a way to measure the drop along the actual cable and not include the connections, this might be a cheap, easy way to approach the problem (the connections can corrode, loosen, etc. and change their resistance over time). It's worth looking at. This shunt resistor will then have a DC voltage across it when the starter motor is cranking. You'd want to measure/condition it with e.g. an op amp as an amplifier or buffer. The cable resistance will change a bit with temperature, so if you're really picky, you'd want to take that into account. The temperature sensitivity of your measuring circuitry is probably more of a concern. This obviously depends on where you live -- where I am, it can get to -20 °C or lower in the winter and above 40 °C in the summer, so one would need to address temperature issues for a measurement accurate over those ranges -- and especially hotter, as the under-hood temperature on a hot day could be much more.

I make this sound simple because I haven't done it. A car's engine space is not a benign electrical environment and this measurement might have interactions with the magnetic field of the starting current or starter. But these shouldn't be too hard to iron out once you start experimenting. My biggest concern would be temperature effects -- a good designer would want to measure these and counteract them.

Again, if it were me, I wouldn't bother worrying about complications. I'd start off simple with an op amp buffer and run the output to an analog meter. I'd figure out some way to calibrate it (I'd get one calibration point with a clamp-on ammeter) and be done with it. It would be a useful tool once you got used to your car's behavior. Of course, if you're using a microprocessor, you can do fancier things. But the op amp/analog meter approach could probably be done in an hour or two. At least consider using it to get some knowledge of how things behave -- then you can get fancier later with the uP.

 

just capture the CCA of cars and inform the driver if the battery is going to fail or if it is still good.

That might not be the whole story. The specific gravity per cell is a good indicator of battery health, plus other indicators - http://autorepair.about.com/library/weekly/aa101604c.htm

 

CCA is a term that most definately is used to describe what a vehicle requires, as much as a batteries ability to fullfill that requirement.

You of course must establish some baselines, ie; oil viscosity, size of engine, required rpm, temperature, etc. You can then come up with a CCA for different engine groupings. I personally would use a shunt to measure the current. The question will be, do you measure current over time, or do you measure temperature rise and calc power consumption.

Now that you know the general CCA, rather than measuring current, you could monitor cranking rpm profile. Considering that the vast majority of battery problems in automotive result from poor charging via bad terminal connections, you may want to measure a battery's ability to recharge, rather than discharge.

Just some ideas.

 

I like the specific gravity measurement for battery health.

I have been using it for a little desulfation project Im working on.

I have yet to look into it, but an electronic specific gravity sensor would be great for that. If they exist.

You could plot charge over time with gravity and learn the changing health of batteries. Then you could adapt the data to a warning device.

 

and that would probably be the ideal approach. Driving a current through a small quantity of electrolyte?

 

Driving a current through a small quantity of electrolyte

Even if that turned out to be an excellent indicator, then you are faced with the problem of sampling all 6 cells. Some capillary-sized tubing might be necessary to get conductivity into a reasonable range.

The 1930's style SG indicator is just a calibrated float in the electrolyte. Some sensing of the float height in the fluid might be more easily arranged.

Some Googling found this - http://www.sbsbattery.com/subpage_index.php?_subp_=91 - That indicates something beyond the old float method is possible.

 

I still feel the best and most robust device for measuring the specific gravity of electrolytes like battery acid is the refractometer like the Misco hand-held ones. beenthere, the one you found measures the density by the "oscillating body" method -- looks like a nice instrument, but it will probably cost numerous k$, so it's out of the range of most of us.

The physical chemists have long known the relationship between specific gravity of the electrolyte and the electrochemical cell's voltage. Here's a screenshot of a spreadsheet I made up years ago from a formula I found somewhere (but the numbskull who made the spreadsheet didn't attribute the source ). If the battery is at thermal equilibrium and it's been a couple of hours since the last charge, then a DMM can measure the voltage and estimate the specific gravity of the electrolyte. Obviously, it'll be some kind of average over the battery's cells.

Oops, some of us are dragging the OP's topic off into the weeds...

 

Oops, some of us are dragging the OP's topic off into the weeds...

That seems to be the case, but predicting battery life is on topic. It may be that specific gravity or voltage across individual cells is a better predictor.

 

That seems to be the case, but predicting battery life is on topic. It may be that specific gravity or voltage across individual cells is a better predictor.

No... No, it's actually very interesting to see how you guru's think. I'm really a newbie and cannot offer much except to listen and try to understand what your thoughts are.

Please continue on....

 

Interesting as it may be, it may not get you to a useful point. Both my cars have sealed batteries, which may be pretty common of late. If that is the case, getting to the electrolyte or any individual cell is not at all practical.

Looking for the remaining battery voltage after some time on a load may be the only reasonable method you have to follow. Then the question may turn into how to determine the actual capacity of the battery in question so as to be able to come up with a reasonable resistance value. If a "quick-and-dirty" test seems more possible, one fixed resistor might be chosen, and the time the battery actually was connected to the resistance varied with the capacity - more time for larger batteries, for instance.

You might get away with a 120 milliohm resistance that would carry 100 amps at 12 volts. That is still a 1200 watt load, so there is the problem of heat plus verifying that a roughly 100 amp load can predict the battery's health. A bucket of clean water can soak up the heat (mineral oil might be better), but you might need a large sample of batteries to see if the test was anything like accurate.

 

Found something helpful on how to measure the CCA from a battery from another forum.

source

A number of comments on the message board have been about battery testing. Here is a response from Geoge Aumann:

Using a resisitive load to measure battery condition is a standard method. For each battery type a standard load is defined, and if the voltage under load drops below a certain level, the battery is bad or in need of recharging. For small batteries this load can be typical, like a 5 Ohm load for a AA Alkaline drops the voltage at end-of-life to 1.0 Volts. Using a load across the battery (for a few milliseconds) is used by laptop computers to assess the charge status of the battery.
For big batteries the "standard" load resistor may get to be very small. However, given the availbility of good and fairly cheap(under $40) 3 1/2 digit digital volt meters, it is not necessary (or safe) to draw a big current spike out of the battery to measure its internal impedance. For my Dynasty UPS12-310 High output battery I use a 1 Ohm 1% 20 watt resistor shunted with a bar directly to the battery terminals. The resulting drop across the 1 Ohm resistor is eazy to measure with the voltmeter set to the 200 mV scale.
A fully charged 12.6 volt lead-acid battery will have an internal resistance of about 0.01 ohms. My Dynasty UPS12-310 high output battery is spec'd at 0.0033 Ohm. Determine the internal resistance of the battery by measuring the terminal voltage with open circuit, V, and then the voltage drop across an accurately known resistive load R, voltage DV. The internal impedance of the battery, Ri, is then given by Ri= DV * R / V.
Example: V=12.60 volts and DV= 81 mV Volt using a 1 Ohm 1% 20 watt Ohmite resistor. Ri= 0.081* 1.0 / 12.6 = 0.0064 Ohm.
The power dissipated in the resisitor is V*V / R = 12.52 * 12.52 = 158 watt. The resistor will get warm very quicky. If this experiment is not finished quickly, the temperature increase will change the resistance. This will make the measurement inaccurate and will burn your fingers). Somebody on the email suggested pulling 200A, presumably using a 0.062 Ohm resistor. Pulling that much power (200 * 12.6 = 2.5KW!) has to be done fast indeed. Batteries of this size can be very dangerous.
​

Is there a way I can detect the load and disconnect the resistor from the battery terminals in a few milliseconds after the PIC has read the voltage to get the internal resistance?

Is there any other semiconductor that I can use to simulate the load instead of using the resistor?

Hope you can help.

Thanks

 

Someonesdad,
I wish you knew what the source of that formula was, because it's not correct.

The temperature correction on a lead-acid battery is a negative function; the colder the battery, the higher the voltage needs to be in order to be considered charged.

Have a look at this tech note: http://rep.mgeups.com/edg/edg/technote/battch.pdf
That's for a certain SLA type.

In most of the lead-acid or SLA datasheets I've reviewed lately, the nominal voltage is specified at a battery internal temperature of 25°C/77°F, and the temperature compensation is given as roughly -3mv per cell per 1°C deviation from 25°C.

 

You need a current transducer, like this:

http://uk.farnell.com/lem/hal-100-s/current-transducer-100a/dp/1617413

Run a battery lead though the middle. You could then use this to calibrate the voltage drop across a fixed length of the battery earth strap, feed it to an instrumentation amp and use that in place of the expensive, mechanically breakable transducer. Engine bays are unfriendly places for electronics.

 

Is there a way I can detect the load and disconnect the resistor from the battery terminals in a few milliseconds after the PIC has read the voltage to get the internal resistance?

If you want a heavy, momentary load, just figure out the resistance of the starter cable and that becomes your shunt and make your measurement when the car is being started. If you need to constantly monitor the battery condition, use the battery cable tied to chassis as your shunt.