Battery Discharge

Thanks,

that is very informational, We may have to look into one of those test instuments that you have refered to,

Best regards'

RW

He said "car" battery. It can only be supposed that only CCA is shown on the battery...not reserve capacity...not uncommon for car battteries as that allows them to be value compared on the sales rack.

You would need to know the AH rating of the battery, and its Peukert's constant.

However, a good guess for your battery, if it is a standard starting battery for a car, would be that its AH rating is 55AH, at a 20 hour rate. A six hour rate for the same battery would be about 2/3 of 55 AH, or 37 AH. A 5 hr rate would be on the order of 30 AH. So you'd expect to be able to power your load for a little over 4 hours.

To get closer than that you'd need to know the actual AH rating of the battery and its Peukerts constant.

6hr vs 20hr rule of thumb:

http://gbindustrialbattery.com/Battery_Options/AH_Ratings.html

Hello everyone,

Not being a full fledge engineer (I only play one for 8 hour+ a day) Would one of you fine guru's please es'plane the 10 hr/20 hr rate you speak of?

I just did a project that runs on 4 AA bateries and I had a lot to learn when it came to "guessing" battery life. But don't remembering hearing anything on this "hr rate" thing.

Please a short sweet tutorial on this for us Daytime Engineers.

bill12780

Bill the battery meters are intended for lead acid technology and in larger sizes than what you get with AA size dry cells. Portable battery like AA usually give capacity in milliamp hours.

Oh Yes!

I understand that. I was just unfamilar with the 5-10-20 hour rate that everyone was refering to.

I did not understand that it was C10, C20 numbers that were in datasheets. It was just me doing my "driving the train" imitation.

I was mostly concerned that MY calculation for a 90 day battery life on my last project were wrong! hahaha!

But its all good. I do however like your inverter idea for real world testing. I have some work coming up that your idea will come in VERY handy...Half to buy you a beer! hahaha!

Thanks

bill12780

Bill when using an inverter remember that the inverter itself has a parasitic base load to power its own controller board. You can now get small inverters of as little as 75 watts. If you are doing dry cell powered projects this may be better. You do not want to load the bateries excessively. I used an old electric clock for a timer since these motors are immune to waveform distortion inherent with MSW type inverters. The MSW inverters can be 95% efficient when loaded to 90 - 100% of their rated output butefficiency drop off drastically as you lower the actual load compared to rated capacity. The parasitic load remains fixed and as your load decreases this now becomes a greater proportion of total load on battery. I liked the idea of the timer stopping when the inverter shuts off because I do not have to atend the test setup all the time. More multi day runs its easy to see if the clock has gone round once, twice, or whatever. Best of all such an inverter is cheap and so is an old electric clock. My total cost buying stuff at garage sale was $5

There is a way, but not from the CCA rating. You will need at least 2 different discharge ratings that are closer to 7 amps. The reserve capacity rating is how long it will discharge at 25 amps. You want to find maybe the 10hr rating and 20hr rating also known as C10 and C20. These will tell you how many amps the battery will supply for 10 and 20 hours without dropping below a certian voltage which if memory serves me correct is 10.5 volts. If you can get one of the two ratings then you can use it with the reserve capacity rating, but using those 2 will give you more accurate results. You plot these 2 points on a log log grid and draw a straight line through them. Follow this line to where 7A is and look over to the time and that will be your expected time for discharge at 7A.

Actually, even the 10 hour rating is too far off from the actual discharge rate. If he can find a 5 hour rate for the battery (or the Peukert's constant, from which he can calculate that rate) he'd get much closer. (Of course, simply doing some tests with the load, and a voltmeter to know when to stop the test, would give him very good figures.)

I would not call a $350 meter cheap. Xantrex Link 10 was the original meter invented to incorporate Peukerts calculation algorithm. Midtronics battery testers cost anywhere from $500 to about $1500 also not cheap. They use capacitance methods to determine state of battery health. The two types of meters will never agree totally but comes close.

To test batteries for actual capacity versus theoretical I connect a small inverter 150 - 300 watts to the battery and run a 100 watt bulb from the inverter.

The reason for the inverter is because they are designed to cut off when the battery voltage drops down to the 10.5V limit. I also run a timer in parallel with the light bulb load. Timer stops when the small inverter cuts off. Since I am more interested in relative capacity compared to actual capacity this works for me. I am using desulfating devices to recover badly sulfated batteries. These can be had for free in most marinas and some automotive supply stores.

My test procedure does not conform exactly to the Battery Institute test procedure but is a realistic test comparable to real world conditions.

OH!

The C10 rating...OK...I did not realize that is what that was for sure I guess...

My bad...

Nevermind on the tutorial request.

Bill12780

The absolute easiest way to do it is get some log log paper or better yet plot those two points in excel time vs amps. Use the reserve capacity rating RC and that rating is minutes at 25A. Use the C10 rating that is in Ah and figure out how many amps it is for 10 hours. Convert both to either minutes or hours and you have your 2 points. You can change the axis to log on both the time and amps axis (or whatever the plural is for axis). From there add a linear trendline and check the box that will display the equation on the graph. That equation is what you need. Plug back 7A into that equation (depends on which axis you chose for amps) and it will spit out you time (either in minutes or hours depending on which you chose in the beginning). I have used this method for specifying ratings to OEMs and I have also tested batteries to these predictions. This will give you a closer answer than any of those cheap battery testers will, besides batteries vary a good amount one to another and these ratings are minimum requirements.

This was all very helpful information, thanks all

Again, thank you for your generous offer.