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There are many reasons why your car battery could be dead. In this short article we explore several things to consider when asking the question “Why is my car battery dead?”
For an extensive article about extending your car battery life you should read our article How to extend your car battery life.
The first and most obvious reason is your battery could be old. Five or more years for a car battery is a good life. Car batteries are primarily only used for cranking the car engine so they should last long since the depth of discharge is typically low which reduces the amount of cycles they experience. However as they age they experience a more rapid falloff of capacity and eventually go flat faster which is why older car batteries will start to die more often as they age.
No two car batteries perform alike. Why? Well on paper they do, however the real reason no two batteries perform the same is because they are subjected to different environments. Cars have different starting requirements and accessories that may need powering when the engine is off. Ambient temperatures have significant effects on car batteries and temperatures vary around the globe. The biggest reason batteries perform different is due to the users. Assuming the car battery is used for a typical car with a combustion engine, the user will have their own unique patterns such as frequency of starting the car, driving distance, non-driving durations and accessory use when the car is not running. This is why there is a large fuzzy area in the real life performance of car batteries.
Parasitic draw can be difficult to assess. If your battery is relatively new and believed to be in good condition but still goes dead overnight or after a few days of not driving it, then you should definitely have an experienced car battery specialist or trusted mechanic test it for parasitic draw. Parasitic draw is a load put on a battery when the car is not in use. Meaning everything is off, key is out of the ignition all doors are closed including glove boxes, trunks and anything that would have a light associated with it when it is open.
Hood lights need to be disabled when testing for parasitic draw. Some luxury cars maintain a load on the battery for a certain amount of time after it is parked and its system shut down, etc. Read the manual for how long or let your car sit for an hour prior to testing it. Key Fobs can also activate systems in a car so leave them away from the car when you do testing.
Parasitic draw troubleshooting can be a tedious time consuming task. Correcting the problem can be tedious as well when electrical diagrams need to be analyzed. The most common method for testing for parasitic draw is to use a voltmeter and pull out fuses one by one until the amperage drops to an acceptable load of
The biggest culprit of car battery failure is due to sulfation. This is when lead sulfate crystals form on the lead plates eventually reducing the current within the electrolyte. As batteries sit without being charged, they start to sulfate. The longer a battery sits with increasing depth of discharge, the sulfation gets worse. In some cases it may not be recoverable. The way to reverse this is by either pulse charging the battery or by slow trickle charging the car battery. Sulfation may eventually cause the active material on the lead plates to break off and fall into the bottom of the cells which eventually will cause a short circuit as the material builds up in the bottom. There is no way to reverse this. Deeper cell basins is not a solution either since batteries rely on the balance of chemistry and deeper basins would mean larger casings, more electrolyte and thicker plates, etc.
Water plays a critical role in the electrochemical equation of a car battery. Although most car batteries are sealed and maintenance free, they do usually have a water level indicator that will allow you to observe if the water level is good. Low water level means the electrolyte will be out of balance and the battery cells will become too acidic resulting in sulfation on the battery plates. The non-conductive lead sulfate crystals will hinder flow of current through the electrolyte.
In most cases your car is being charged by the alternator and cars are sophisticated enough to not overcharge your battery. Water is typically lost during overcharging of a car battery. This may occur if you use a car battery charger that is not sophisticated with a built in float mode and ambient temperature sensor. Battery capacity changes according to the ambient heat it is subjected to. Colder temperatures decrease battery capacity while hotter temperatures increase battery capacity. As a battery is heated up from overcharging, it will allow more current to flow into the battery. If the charger does not take ambient temperature into consideration it could generate too high of a charge voltage causing excessive current to flow into a battery once it is fully charged. This excessive current will cause decomposition of water in the electrolyte resulting in premature aging of the car battery.
Furthermore, excessive overcharging could lead to thermal runaway where the battery continues to heat up and capacity continues to increase so the charger is able to push more current into the battery. This could destroy a lead-acid car battery in a few hours.
Remember if you use a car battery charger, be sure it has a float mode and an ambient temperature control sensor so it will not over charge the battery.
Most car batteries have caps that can be removed (this may require a flat screwdriver to pop them open) so you can refill them with distilled water if necessary. Car batteries also known as SLI (starting, lighting, ignition) batteries don’t require water very often if ever. To get the most out of your car battery life you should understand how to check if the water is low. Usually there is a clear “eye” on the top of the battery that displays a green light if the water level is good and a dark light if the battery needs water. Be sure to use distilled water. Do not use tap water as the minerals could react negatively with the battery chemistry.
Believe it or not, most batteries stop working due to an interruption of the flow of electrons between the cathode and anode. Battery terminals are a typical suspect to a dead battery. You should regularly inspect both positive (+) and negative (-) terminals. Corrosion on the terminals is commonly due to electrolysis occurring from the different metals on the terminals and battery cables. It also occurs when there is an imperfect seal between terminal posts and plastic casing allowing sulfuric acid to with lead terminal posts. White powdery buildup is often lead sulfate which is toxic. Do not touch or inhale this. See our article on how to clean corroded motorcycle battery terminals which would be the same method used on car batteries.
Car batteries will self discharge when not in use. If it is connected to your car it could also have a small load from accessories or car computers that will draw current from the battery which also aid in draining the battery when the engine is not running. In general an AGM, or lead-acid car battery self discharges at a rate of three percent per month. This rate increases when ambient temperature exceeds 20 degrees Celsius (68 degrees Fahrenheit). The more cycles a lead acid car battery has experienced will also start to reduce the capacity of the battery and eventually even slow discharging rates or small loads could cause the battery to go flat.
Bad cables causing short circuits and bad alternators which don’t effectively recharge the battery are sometimes the culprit as well. If you suspect a bad alternator you should take your car to a car battery specialist or a mechanic that can test the battery and check for parasitic drain.
If your car battery keeps going dead (flat), replacing sometimes may solve the problem but in some circumstances it could just be patching the problem. If the battery is less than a few years old you should have your alternator tested and have a parasitic draw test performed by a professional. You don’t want to keep throwing money at your battery when the real problem may be another component of your car.