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Author Topic:   Lead-Acid Storage Battery Compared to a Fuel Tank
jimh posted 04-29-2012 12:24 AM ET (US)   Profile for jimh   Send Email to jimh  
Everyone is familiar with a fuel tank as a way to store fuel or energy. We understand the concept that a fuel tank has a particular volume that it can hold. The volume of fuel in a fuel tank is a measure of how much stored energy is contained in the fuel tank. We easily say things like we have "a 70-gallon fuel tank."

Everyone is also familiar with the measurement of the fuel tank level or percentage of the total volume of the fuel tank that consists of fuel. We can get a fuel tank level measurement and convert this roughly to a volume, so we can say we have a "three-quarter-full" fuel tank. Using our 70-gallon tank, we'd mean we have a fuel tank with a fuel volume of about 52-gallons.

And everyone is familiar with the concept of the rate of fuel flow from the tank that is used by the associated engine. We can talk about a fuel flow rate in units like gallons-per-hour.

We also know that we store energy in the fuel tank by putting gasoline into it, and we use that energy as the gasoline flows out. We have fairly clear indicators of how much energy is left in our fuel tank at any time, and also the rate at which we are using the energy. If we have a 70-gallon tank, the tank is full, and the engine is drawing 7-GPH, we know we can run for ten hours.

Let's turn to a storage battery and use these same three concepts of the fuel tank. A storage battery, typically a lead-acid storage battery, is also a device that stores energy--in this case electrical energy. Just like a fuel tank, we would like to know information about the energy. How much can the battery hold? How much is in the battery now? And at what rate is the energy being consumed.

The capacity of a storage battery to store electrical energy is known as its Ampere-hour (A-h) rating. The more Ampere-hour capacity in a battery, the more energy it can store.

The amount of energy stored in the battery depends on its state of charge (SOC). Measuring the state of charge is not as simple as measuring a fuel tank level. The SOC can be inferred from several methods.

The most common means to infer the state of charge of a storage battery is to measure its terminal voltage. If we understand the chemistry of the lead-acid battery and we know the temperature, we can infer the SOC by careful measurement of the cell voltage. Terminal voltage works rather well with lead-acid batteries because they have a characteristic cell voltage change as a function of charge that is easy to measure.

Another method of judging SOC is to sample the electrolyte and measure its pH. This is often done by measuring the specific gravity of the electrolyte, which is proportional to its pH, with a hydrometer. This works well for flooded-cell batteries with open vents. With sealed batteries it is not a simple matter to get to the electrolyte, so this method is not very practical, nor is useful with with absorbed glass mat (AGM) batteries There are other more exotic methods.

An alternative method of judging battery state of charge is to carefully measure the current flow into and out of the battery, and count up the total electrical current flow. For example, if we pumped 35-Amperes into the battery, we should expect to be able to draw out 35-Amperes. In reality the ratio is not 1:1 because the charging current is not perfectly converted to stored energy, nor is the stored energy perfectly converted to electrical current. There is some loss in the process. But if we knew the proper ratio, we could track the current sent into the battery and have a measure of how much energy it holds. This is just like the fuel tank method of measuring the volume of gasoline added, except the fuel tank is more efficient at storing the energy--as long as it does not leak.

There are also other methods of evaluating the cell SOC. The SOC can be inferred from impedance measurement methods. Measurement of the AC impedance of the battery can be used to infer state of charge. There is even a method based on magnetic properties. Because lead and lead-oxide have different magnetic properties, the SOC can be inferred from a measurement of the battery's magnetism.

When electrical energy is drawn from the storage battery, we can typically measure the rate by measuring the Amperes of electrical current. Amperes are easy to measure; you just put an Ammeter in the circuit.

At first glance it looks like we have all the tools needed to measure the stored energy in a battery. If we have a lead-acid battery with a 70-A-h rating, its state of charge is 100-percent, and we are drawing 7-Amperes of current, we'd like to think that we can run for ten hours at that rate. But the problem is not that simple.

The primary problem in determining the amount of stored energy in a battery is to know its real storage capacity. When a battery is new it might be able to hold its rated capacity, e.g. 70-A-h, but as the battery ages and its cell chemistry goes through discharge and recharge cycles, it is typical that some of the storage capacity is lost. After a year a battery that started service as a 70-A-h battery may be diminished to only a 55-A-h battery. We can't really tell this from the state of charge.

Measuring the state of charge just tells us the state of the cell chemistry. If the cell is holding as much charge as its chemistry allows, we say it is at 100-percent charge. But without knowing exactly what the capacity of that cell has become with age, we don't know if we have 70-Ampere-hours of energy in the battery or perhaps much less.

Another problem is the rate of discharge. A lead-acid storage battery that is discharged at a high rate tends to lose its stored energy faster than if it were discharged at a more moderate rate. (This can be predicted by Peukert's law.) Typically a storage battery is rated by its manufacturer for a discharge of a steady current at certain rate for a certain duration, usually ten-hours or 20-hours. For example, a battery that can provide 7-Amperes for 10-hours would be rated as a 70-Ampere-hour battery. If you discharged the battery at a higher rate, say 21-Amperes, you would find that it would not last the proportional 3.3-hours.

jimh posted 04-29-2012 09:13 AM ET (US)     Profile for jimh  Send Email to jimh     
To continue the analogy of a battery as fuel tank, the electrical current corresponds to the rate of fuel flow. The size of the electrical conductor thus corresponds to the size of the fuel hose. In designing the fuel system we know the fuel hose must be sufficiently large to not restrict the flow of fuel to the engine and must be able to handle the flow rate needed at maximum demand. In the electrical system we must size the conductor to not restrict flow of electrical current during maximum demand.

In the case of a fuel tank and engine, the maximum demand occurs at maximum throttle, and is usually not more than about 20-GPH per engine, with each engine having its own fuel hose. A fuel hose with a 3/8-inch inside diameter can handle this flow rate. In the case of the battery, the maximum demand occurs during engine cranking, and the current demand is very high. The conductor must be sized so that it can handle several hundred Amperes of current. The variation in current flow is much greater than the variation in fuel flow, and the electrical conductors have to be much larger size than the equivalent fuel hose size that is used. Imagine the fuel system if your fuel tank had to deliver at a 250-GPH rate for brief periods. The fuel hose would need to be much larger--I don't know exactly how big, but certainly larger than 3/8-inch ID.

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