And yes there is confusion about battery voltage numbers. Double speak you ask? It would seam so. This column is in response to a comment on last weeks column where I indicated that 9.6 volts was a low cut off for a battery that was either dead or needed a charge. Rob R. wrote in a comment, “9.6 Volts…you need help…Please read something like http://www.phrannie.org/battery.html“. Well I read the indicated article and we are saying the same thing but using a different route to get there.
First consider this. It is a nice summer warm afternoon and you are working in your yard. You notice your garden hose lying on the ground in the sun. Now it is not one of those expensive re-enforced nylon hoses but like mine, a bargain brand plastic hose that will carry water. It is puffed up twice it’s size. It is connected to the water faucet and the faucet is on. The nozzle on the other end of the hose is off. You pick up the nozzle and turn it on and immediately the hose reduces in size to normal. What happened is when the nozzle is off the pressure in the hose is the same as the pressure in the water system of the house. As soon as you open the nozzle water flows and the pressure in the hose drops pushing the water out of the end of the hose. So we have what we call static and dynamic pressures.
In an electrical system we have the same conditions only instead of pressure we have voltage, the push, and amperage or amps, the flow. When a battery is sitting and not being asked to provide current for something the voltage seen at the terminals with a voltmeter is static voltage and can be used as some measure of a batteries state of charge.
Immediately after charging a battery the voltage will be higher than normal as we are reading a surface charge on the battery. Normally, before taking a voltage reading to determine the state of charge of a car battery we would turn the headlights on for a couple of minutes to remove the surface charge and get to the real numbers. All batteries when coming off a charger or as the engine is shut down and the alternator stops charging will have a surface charge. The surface charge is just what the name implies, electrons sitting on the surface of the plates and not really adding to the capacity of the battery or it ability to produce current for a load.
Then if the voltmeter read 12.7 volts we would consider the battery fully charged to its capacity and if the voltage was 12.0 to 12.2 it would need more charge as it is close to dead. Now these numbers are with no current flowing or resting voltage. And the capacity of the battery may not be what the capacity was when the battery was new.
If the battery passed that test then it would need a load test to determine the capacity, the ability to produce current under load. The accepted standard for testing batteries for the past 100 years is to use a load that is three times the amp hour capacity rating of the battery or one half the cold cranking rating. Either or both of these rating should be marked somewhere on the battery.
This load is applied for 15 seconds and while the load is applied the voltage should not drop below 9.6 volts.
The same voltage numbers can be used to check the starter and battery in your rig in one operation. With a voltmeter connected to the battery engage the starter. If the voltmeter stays above 9.6 volts both the starter and the battery is acceptable. If the voltage drops below 9.6, check the battery as described above. If the battery tests OK then the problem is the starter. A battery can read good in a static test but may have deterioration on the plates and not have capacity, two separate things.
These are basic battery test and can be found in any automotive text book or Society Of Automotive Engineers literature. The auto industry is the biggest user of 12 volt direct current batteries and systems. The procedures and numbers used are for the electrical systems and do not vary because the battery is in an RV or a compact Chevrolet.
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Hi Brad & Lucy,
I just found your Blog after Lucy gave me the www. address. Hope you are doing well .Will you be in Seattle for Thanksgiving, as planned ? Bet you are having a blast ! Good luck !
Brad most excelten article. we have to agreee with the others though about the different capacty batts having different load rate times but the low voltage cutoff should be the determining bottom factor. However, one then assumes that the battery was fully charged and let set about a hour before the test was begun to get a true picture of its condition.
Load bank testing , as I call it, is the best way to insure that you have a great trip with the RV without any electrical problems. One should be careful not to demand too much current suddenly from any battery (checking manufacture data) or one could find a exploded view of it all over the floor. Oh and always practice safety… protection of the eyes is very important working around batteries so we found. You may want to expand on that too. Again good article, thanks
some of the other techinical batteries that should disucssed are the gel cell, flooded and the sprial wound optima ones.
Now if we can just figure out what the volt meter is. How to hook it up to the terminals etc… if not part of the load test equipment . Most of the car voltmeters are to say the lest in- accurate so we have found. Thus, using a good quality volt meter is a good idea along with the load bank.
Also a lot of MH and RV’s have dual batteries with rotary switches…. one needs to assume that the battery being tested is removed from the system also.
Would also be nice to see a article about battery sulfation and how the voltage current plays with that in the RV after the battery sits.
and of course the proper way to charge it back up after the load bank test. (constant current or constant voltage?)
Looking to read more about your work.
Larry, a lot depends on the type, brand and model of the battery. For the lowest voltage allowed and still be okay, some brands are better than others.
Also the maximum “at rest” voltage after charging depends on the brand. The Optima brand I am using has an “at rest” voltage of 12.92 volts. That would be the voltage reading at 100% charged.
Optima AGM deep cycle batteries are engineered to be effectively discharged down to 10.7 volts. They then need to be fully charged using a charge voltage between 13.5 and 15. 2 volts. That of course is one cycle. I believe they can be cycled 1000 times.
If you want to really know what is going on with your batteries, an Amp Hour meter is the best way. There is nothing like having numbers and measurements to make a record be of any use.
Using the battery manufacturer’s specs, you program the AH meter with the high and low voltage and current limits. From that point on you will get a discharge-charge cycle reading for each time. You will also get an accumlative AH reading over the life of the battery. And yes, you got to spend the money.
By the way folks, Brad Sears is a retired automotive technology teacher. So, he has a professional’s knowledge of automotive “things”. His way of testing is quite valid for the scenario he presents. Some tests will not work with some types of batteries and some tests will work with several different type of batteries.
One test that will not work with a sealed AGM battery is of course the specific gravity test. But that test is nearly foolproof with a standard starter battery.
I believe the test you specify. Using a load 3 times the capacity of the battery to test is a test to determine if the battery is “good”. i.e. will it accept and hold a charge. It is not a test to determine when to charge the battery. The voltage on a deep cycle RV or golf cart battery should not be allowed to fall below about 12 volts. To do so repeatably will drastically shorten the life of the battery.
Fess up you made a mistake.
Good post, but for me, determining the state of charge of a partially discharged battery was always confusing because under load, the measured voltage depends on the state of charge and the current draw. For instance, and using your example, a freshly charged battery without the surface charge and with a discharge of, say, 0.1 amp, the battery voltage might read something like ~12.69 volts, whereas if the same battery were discharged at three times the amp hour capacity rating, the measured voltage might be 9.6 volts. So, sitting warm and snug boondocking in my RV and avidly watching the volt meter as the furnace kicks on under battery power (I get my jollies in strange ways sometimes), how do I know when to go start the generator because my batteries are reaching that critical 50% discharged level? I’ve read a lot of articles, blogs, etc. on batteries and RV electrics, but the answer to this question has been a bit vague. I stumbled on this as the best information I’ve found: http://www.rollsbattery.com/?q=node/51. The right answer seems to be to buy one of those monitor panels that track battery current in and out, or, lacking the funds to do that, use the SWAG (Scientific Wild-A** Guess, with emphasis on the “scientific”) method .
I love your posts, Brad, keep them coming!
Brad, you “done good” with your water hose and water pressure and flow example. This is a very accurate comparison. Do not be swayed by naysayers like Rob.
Besides Phrannies link detailed about all RVers should read and make a copy of the following
Brad just buy a $30 Battery Load tester and for God’s sake quit comparing water flow with electricity. The comparison has long been called a poor if not detrimental comparison.
Electricity has always confounded me. However, with your description using the hose, I finally, finally get it! Thanks.