The State of Health (SOH) of a battery is more or less an arbitrary measure of the condition of a battery compared to the manufacturing standards. The measurement is calculated in a percentage value.

When we buy new batteries we expect them to have a full SOH, but that is rarely the case. In general, a completely new battery does not start with a 100% SOH as it takes some time for the acid solution to permeate the plates to achieve the full potential of the battery. On the other hand, batteries deteriorate simply by being stored unused. Batteries will slowly but constantly discharge during storage, causing them to deteriorate over time. This discharge produces accumulating lead sulfate, which turns into crystals that do not go away with a normal recharge. Over time battery plates become completely covered in sulfate crystals causing them to die a premature death.

Batteries should come with a stamped manufacturing date. It is not recommended buying batteries that have been sitting on the shelf for more than 6 months.

There are external and internal indicators of the SOH.

External factors include rusted terminals, bad connections, cracks and bends.

A mechanical inspection is simple and informative.

1. Check if the connectors are clean and free of rust. Those are easy to clean up or replace.

2. Cracks: Any cracks on the battery body are a reliable indicator of the need to replace a battery, especially on flooded batteries or batteries than have been dropped or left without charging in regions that experience heavy winters. Letting a battery discharge during the winter will cause the solution to become mostly water which can freeze and cause the batteries to crack.

3. Bends: batteries that have been boiling tend to develop “pot bellies”. The positive plate on those batteries has a limited viability as they can crumble and will basically come apart. For a golf cart owner is better to replace those as soon as possible. On Solar /wind banks you can get more out of them. In any case make sure to check internal factors to make an informed decision.

Internal Factors are tied to the State of Charge (SOC), the internal resistance, the number of charge-discharge cycles and ability to accept a charge.

The State of Charge (SOC): it tells you how much “fuel” you have left in your battery. The SOC of a battery is measured in percentage units where 0% is empty and 100% is full, with a whole range in between. It is an indirect measurement of how much energy you have in your battery.

There are two main types of methods of measurement the SOC: The chemical type and Voltage.

1. The Chemical method can only be used with flooded batteries. You will need access to the electrolyte to measure the specific gravity of the electrolyte (sulfuric acid).

Specific Gravity or better called Relative Density is how the density of a substance relates to another one that is being used as reference. The compared substance can be more or less dense than the established unit. In our case the reference unit is water that has a density of 1.0 and our analyzed substance (electrolyte) is going o be the sulfuric acid in the battery. (See Chart 1 for details).

2. The Voltage method can be used with any battery type flooded and sealed. It measures how much electrical energy is flowing inside a circuit from one point to another point. The electrical potential that is measured in volts. It measures the voltage from the positive pole in the battery to the negative one.(More information here)

The voltage is compared to the SOC of the given battery. Different battery companies have their own charts of discharge curve to calculate the SOC but in general they are similar. (See Chart 1 for details)

Chart 1. Each battery manufacturer has a State of Charge Chart that you can reference. Below is an example of a State of Charge Chart from Trojan Batteries:

Percentage of Charge	Specific Gravity Corrected to 80o F	Open	Circuit	Voltage
		6V	8V	12V	24V	36V	48V
100%	1.277	6.37	8.49	12.73	25.46	38.20	50.93
90%	1.258	6.31	8.41	12.62	25.24	37.85	50.47
80%	1.238	6.25	8.33	12.50	25.00	37.49	49.99
70%	1.217	6.19	8.25	12.37	24.74	37.12	49.49
60%	1.195	6.12	8.16	12.24	24.48	36.72	48.96
50%	1.172	6.05	8.07	12.10	24.20	36.31	48.41
40%	1.148	5.98	7.97	11.96	23.92	35.87	47.83
30%	1.124	5.91	7.88	11.81	23.63	35.44	47.26
20%	1.098	5.83	7.77	11.66	23.32	34.97	46.63
10%	1.073	5.75	7.67	11.51	23.02	34.52	46.03

The Internal Resistance/Impedance: As far as the lead acid battery circuit concerns the values of internal resistance and Impedance are the same. It is the measure of the opposition that a circuit presents to the passage of a current when a voltage is applied. When a battery is new, the resistance tends to be 1 Ohm per cell, over time batteries accumulate lead sulfate crystals increasing the resistance. The higher the value the worst condition is the battery.

Every battery type has its own resistance value. Please check the datasheet for the batteries you are using to get the exact number. The resistance value is very important when recovering batteries as it can show the improvement in the battery.

Number of charge-discharge cycles: Different manufacturers have an expected number of charge-discharge cycles that they expect a battery will last. Based on this information they offer the limited warranty that, after a certain period, becomes prorated. Manufacturers will cover the warranty 100% for a short period, but as the product ages, they’ll assume less and less responsibility for the quality of the battery. Again sulfation and electrolyte lost will reduce the number of cycles a battery should last. The higher the number of cycles and the warranty a company offers the better the chances are that the battery has a better SOH.

Ability to accept a charge: This point was left to last because ultimately the real test for a battery is if it can accept charge and release it. There is a huge list of things that can prevent a battery from accepting charge. A few of them can be reversed, others not. We will go over this point in more detail in the next article.