Please explain C rate to a beginner.

[Silver9]

I know Tom had mentioned discharging batteries at a 20C rate. Do I have this right? C rate = Amps x Hours. If I want to discharge a 12 volt battery in 8 hours, at a 20C rate, then I would divide 20 by the 8 to arrive at 2.5 amps.

So since Watts = Amps x Voltage. I would take 2.5 Amps x 12volts to equal 30 watts. So I would discharge my battery for 8 hours on a 30 watt load. Please help a grasshopper understand.


I don't think I would be able to do that with some of these batteries that I'm fooling around with trying to restore. I think an inverter would shut down way before 8 hours or the voltage would sink way low and shut the inverter down before you can pull a rate like that.

[Tom C]

the C20 rate of a battery is the amp hour rating divided by 20 hours of discharge, so a 20 amp HR rated battery divided by 20 would equal 1, which is the discharge current. you would then discharge your battery at 1 amp for 80% of that (80% Depth OfDischarge or D.O.D.) which would equal 18 hrs at 1 amp. C20 is the number of hours you discharge the battery.

Tom C

[Silver9]

Tom I'm not understanding what 80&#37 means. Is that a numerical equation? Can you explain please, sorry to be trouble.

Also I'm hearing that some folks on the forum are using the lawn tractor batteries. Now I know I haven't done the refinements to the energizer yet, when i begin with testing I will purchase two brand new batteries. The lawn tractor batteries are flooded type and are rated by cold cranking amps and not amp hours. I understand this to be so because starter batteries are just to start the batteries and then they are recharged immediately with the alternator but Deep Cycle batteries are rated by Amp Hour because they can deliver a current over a long period of time. So am I to purchase for my beginning test deep cycle batteries or just the flooded starter type?

[Tom C]

Sorry the forum screws up the percent sign it is 80 percent......

you will not find a small deep cycle lead acid battery. with that being said you are drawing low amounts of current from the battery you will be using. the average lawn tractor battery is between 10 and 20 amp hours, it just depends upon its size and plate thickness. you are correct there is no direct correlation between CCA and amp hours. as a general rule the higher the CCA the lower the amp hour rating, as the plates get thinner the higher you go.

you don't need new batts just good ones. do a baseline discharge to see where you are at.

here is a good battery info web page :

http://www.chem.hawaii.edu/uham/bat.html

Tom C

[Silver9]

Thanks again Tom for the info! More homework to do.

[aln]

silver,

rough estimate:
[cca / 20 = Ah]
Example: 160 CCA Lawn battery 160/20=8Ah.
[Ah/20 = c/20]
8Ah/20= .4 amps I have found this to work ok and its easy to remember. Aln

[Silver9]

Awesome aln, that's what I was looking for. Thank you so much! I didn't know really where to look for a conversion from CCA to AH. Paul

[Silver9]

Ok, just fooling with an old battery that I got from recycles, I have a 12volt motorcraft flooded cell battery which was holding at 11.25 volts. I went through a couple of cycles of charge and discharge on the SG. At that point I had no idea about the C rate as I was just using a inverter with a 6 watt CFL light bulb to discharge during the day. Right now the battery's resting voltage after one week is 12.80 volts. I have a SOLAR battery analyzer that I got from Amazon that tests batteries cranking amps and it said that the cranking amps was 580 instead of 850 so I know that even though the resting voltage is good the capacity of this battery is not good.

I have a 850CCA battery so a rough estimation of the Ah is 850/20 which gives me 42.5Ah. To discharge this battery at a C20 rate, it would be 42.5 / 20 = 2.125Amps. So since Watts = Amps x Voltage, Watts = 2.125Amps x 12Volts so I have 25.5 watts. So I would need to find lets say a light bulb or something comparable to discharge my battery at 25.5 watts. Does that sound about right?

Really it is quite exciting because from before there would have been no way this battery would even run an inverter to power a small wattage cfl. After a few charge and discharge cycles I was able to get the voltage up to the 12.8 and was able to use a 6 watts CFL and inverter for 12 hours and still had a voltage holding at 12.40. I think thats pretty good but I want to see if I can do better.

When I get my new circuit with matched transistors and resistors and pot, I will start my testing. Paul

[aln]

Paul, On an old sulfated battery that is rated "850 cca," notice, as you have measured, actual cca is 580 cca. So if we use our "rough estimate" formula we find 580/20=29ah. 29/20=1.45 amps. Your inverter will draw roughly a half amp just to run the fan. Figure that in when discharging. Get a 10 or 13 watt cfl and you are close enough to run it through cycles. Aln

[Silver9]

Thanks aln for all the help! So I'll use the 580 as the number at first to do my cycles for this interesting rejuvenation project. I believe the capacity will build. If so and lets say the capacity rises to say 700CCA from the tester then I suppose i would re-adjust my calculations to the new CCA number. If possible i would like to see if I can get it close back to the rated CCA of 850, I'll hope for the best.

Tom had mentioned above then that the discharge time should be 80 percent of the c20 time which is 18 hours. I understand this... But how far down do we discharge? I read the link that was provided above and they said that its not a good idea to go below I believe 11 volts under load. So what if the battery that your are trying to rejuvenate is dropping below 11 volts under load before the 18 hours are up, how to adjust for this?

Thats right the inverter itself is going to draw a slight bit of amperage, thanks for that bit of info too, that should also calculate in to the discharge amperage as well. Thanks for taking the time to post for this grasshopper. I appreciated your input!

[Yaro1776]

Paul

For fooling around you can visit the local NAPA store. The one here in Vermont stocks a dual filament auto turn/tail light assembly for which you can get a 6watt/25 watt 12 volt bulb. Works!

Don't be surprised if the old battery does not give you the amp hrs... For a regular car battery discharging down to about 12.1 to 12.2 volts while loaded seems to work OK. The deep cycle batteries are a different animal.

Yaro

Yaro

[Tom C]

with a starter battery (thin plates for hi current) do not go below 12 volts... actually some manufacturer's say 12.4 is fully discharged. look for your battery specs to determine. 11 volts on a starter battery is death....

Tom C

[Silver9]

Thanks Tom, I appreciate the info. Its hard enough to sort through all the different types and quarks that all these batteries have. I got it thanks for setting the record straight.
and Thanks Yaro I'll check into that too.

Paul

[Tom C]

you are welcome paul, half the battle is learning about batteries they are a science all by themselves.

Tom C