How To Understand and Test Batteries Safely

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cubguy's dad
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How To Understand and Test Batteries Safely

Postby cubguy's dad » Sat Jan 24, 2009 6:32 pm

or: Battery Testing with Pictures

Battery Testing Basics:

Batteries are the heart of any tractor, auto or truck electrical system. How the battery functions and how to test a battery are usually not understood very well with many myths associated with them. I will try and provide a simple insight into this complex subject. Remember the goal here is to promote understanding not to teach chemical or electrical engineering.

Rule # 1: Safety:

Batteries contain Sulfuric Acid with is very caustic and will burn exposed skin, eyes and clothing. Handle batteries with care. Batteries when being charged or discharged give off hydrogen gas and oxygen; this is a HIGHLY EXPLOSIVE mixture. Personal protection equipment is a MUST. SAFETY GLASSES are a requirement when working on batteries. I personally have been up front and personal on two battery explosions, I was not hurt in either one but believe me when I tell you it is a BOMB. All it takes is a spark to ignite the gas, be careful when hooking chargers and jumpers. Even static electricity can ignite the gas, SO be careful.

Battery Fundamentals:

Batteries are a electrical storage devices. Batteries store electricity in a chemical state to be reversed at a later time back into electricity. A battery in its simplest of forms can be described as follows; two dissimilar metals, separated by an insulator, surrounded by a corrosive agent. For instance we can all recall the picture of a piece of copper and aluminum stuck into a lemon that results in enough power to run a small fan. Automotive batteries are a little more complex than that, but it is the same theory. Automotive batteries use a mixture of different materials with the bulk of materials being lead. To build this type of battery we will take several wafers of lead and an insulator and make a sandwich of sorts. Let’s put together about a dozen of these sandwiches and pack them into one compartment of the battery case, This will comprise a unit of the battery that we will call a CELL. The CELL is made up of enough sandwiches to give a chemical to electrical output of 2 volts. To build a 6 volt battery we would have a battery case that would be large enough for 3 cells, a 12 volt battery would require 6 cells. To make our cells function we need to have a corrosive agent which is called an electrolyte which is sulfuric acid or (H2SO4, remember this for later use) :idea: :!:

Typical Battery Construction:


The charging and discharging of the battery is simply a function of moving the Sulfur ( the S in (H2SO4[/i) in and out of the lead plates. A fully charged battery will have the majority of its SULFUR contained in solution as Sulfuric Acid. A discharged battery has the bulk of the Ssulfur contained in the lead plates, this in turn leaves the solution with NO sulfur component you then have ([i]H2O or water). The more charge a battery has the higher the acid concentration of the solution, the more discharged a battery is the higher the concentration of water is in the solution. The action of the electrical current applied to the battery moves the sulfur out of the lead plates and into the electrolyte and provides a storage or charged function. When providing an electrical current or discharging the sulfur leaves the electrolyte and moves into the lead plates. The more sulfur that leaves the electrolyte the weak the acid becomes until it is mostly water, that is why discharged batteries freeze solid in cold weather. However, not all of the sulfur leaves the plates during charging and discharging, a fraction of an amount stays. This process is called Sulphation. Sulphation leaves a coating that builds up on the plates that hinders the battery charging/discharging process. This process is the reason that batteries age and become ineffective over time. This leads us to our first test.

Hydrometer test:

A hydrometer is only used for testing a lead acid battery to tell the user the specific gravity of the solution. Specific gravity is a relative weight of density and measure corrected to a equal volume of water. The specific gravity of water has been established at 1.0000 as the first line on the float and is an indication of a measurement of pure water a fully discharged battery will be about 1.100 of specific gravity on the hydrometer.

On the other end, the specific gravity of a fully charged battery’s electrolyte is 1.265. as a reading on the hydrometer (means it has a high acid content or a lot of sulfur present in the solution. When the float floats and is read correctly, it is simply 1.265 heavier than a equal amount of water.

Image Image
On the el cheapo discount store styles, the float or balls may not have the same calibration of a professional style that costs more and accuracy will be affected. A much easier way of measuring the state of charge is with a DVOM on the voltage scale.

State of Charge Voltage Test:

An easier test is the state of charge voltage test. Let me give a little background first.

Battery individual cell voltage ranges from 1.9999 discharged to a little more than 2 volts fully charged. A battery is made up of a group of cells that multiplied together comprise the Battery. Example a 6 volt battery has 3 cells to make up a 6 volt battery (3 x 2 volts) , a 12 volt battery will have 6 cells (6 x 12 volts) to make up a 12 volt battery.

Since a battery is a computation of it’s individual cells, it is important to remember that a discharged cell has a reading of just less than 2 volts whereas a fully charged cell will have a voltage of a little more than 2.1 volts, THAT IS NOT MUCH DIFFERENCE. However it is the multiplication of the cells together that determined the state of charge.

A fully charged (100% capacity) 6 volt battery will have 6.5 volts whereas a fully discharged (0% capacity) will have 6 volts. Example a battery with 6.3 volts will have a 50% charge. THAT is not much difference but it is all in the individual cell voltages.


Kyle’s battery shows about a 60% charge.

A fully charged 12 volt battery will show 12.6 volts fully charged to 12 volts fully discharged. Again a percentage of charge applies to voltages that fall in between 12.6 & 12 volts.


This 12 volt battery shows about a 25% charge.

Both batteries will need to be charged before we can go to the load or capacity test. The state of charge voltage test is the easiest way to test the batteries charge.

Battery Capacity Test:

Once we have determined that our battery has the correct charge we need to find out if it has any capacity (can it convert chemical into electrical energy).

To do this we have to pull a load on a battery to see if it has capacity or power to provide electrical energy. The battery needs to be above 40 degrees F temperature to be fairly accurate on the capacity test.

There are several ways to do this, but this is a low buck how to so I will stick to the tests with the best results for the dollar.

The first way to pull a load is with the starter on the tractor. Crank the tractor with ignition disabled for 15 seconds while you observe the voltage of the battery. The range will be 5.5 volts to no less than 4.5 volts. A good battery will fall into those ranges of voltages.


Kyle’s battery in his tractor does a good job on the battery in the vehicle load test.

The other way to test a battery is with a fixed load battery tester (weenie roaster). These are inexpensive the one pictured is about $41.00 and has a scale for 6 and 12 volts (I have other tests that I do with mine that I will share later).. These testers pull a fixed 125 amp load when you throw the switch. BE CAREFUL these testers get HOT. Hold the switch for about 15 seconds and read the voltage on the meter. The meter has a green through red scale to indicate the batteries condition under load.

Here is a picture of the tester:


This picture shows basic tester hook up. Red on positive, black on Negative.


This picture shows a load test on the battery. And the battery holds up well.


This picture shows Kyle’s 6 volt battery after being charged for ½ hour. It has a good charge as evidenced by the voltage reading.


Next we pulled our 125 amp load on this battery with the tester and it held up real good.


Kyle’s battery should hold up good as evidenced by the date code. This battery was made in June of 2008. The January of 2009 is when the battery should be removed from stock and sent back to the manufacturer. All batteries have this born on date and it is wise to check this when buying or testing to get an idea of the batteries health by how old it is.



The goal is to provide answers to common questions concerning electrical testing. Having the knowledge plus good basic inexpensive testing tools will make all the difference in solving your tractor problems. The testing equipment shown DVOM $28.00US, Battery tester $41.00US. Battery testing is not hard but like everything else it takes practice.

Remember batteries do get old and they do wear out. Buy a battery assuming a 3 year life expectancy, anything after that is a gift. Batteries are a PASS or FAIL component. It will either do the job or it will not. There is no in between. Batteries will lose ½ of there capacity from 32 degrees to 0 degrees. The chemical reaction slows down.

Batteries are cheap compared to the problems they can create. Buy the largest capacity battery that will physically fit in your application there is not much difference in price and it will do the job better and longer. Always keeping a good battery in your tractor or car will save your starter and your generator/alternator from premature failure.

Hope this helps someone out there.

Experiance is knowing what NOT to do the next time.......
1937 John Deere A (Big John)
1953 Farmall Cub (LiL Red)
Lots of projects.

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