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.
Brad