It is no surprise that COVID-19 has reduced the amount people drive. Additionally, new car sales have been down, causing vehicles to sit on lots for extended amounts of time. But what is happening to the batteries within those dormant vehicles?
Key Takeaways
- Batteries in vehicles left parked for weeks or months experience progressive discharge from parasitic drain, even when the car hasn’t moved.
- A partially discharged lead-acid battery (below 12.4V) begins the process of stratification — sulfate separates and concentrates at the bottom of the plates.
- Extended stratification causes corrosion and large sulfation crystal formation on the plates, which is irreversible and permanently reduces battery capacity.
- A discharged battery isn’t necessarily a bad battery — but one that won’t accept a charge, hold a reserve, or crank the engine after charging almost certainly is.
- Routine charging, cycling, and monitoring are the only reliable ways to prevent dormancy damage — waiting until the vehicle is in service is often too late.
Simply put, when a battery spends prolonged periods of time in a partially to fully discharged state (below 12.4V in many applications is considered partially discharged), it reduces the battery’s overall health and performance capabilities. And with parasitic draws in vehicles, even if the battery was fully charged when you parked it, that battery will become partially discharged over time.
Just because a battery is discharged, doesn’t mean it is bad. It just means it needs to be charged. If it doesn’t accept a charge (charge acceptance), can’t support small loads over time when charged (reserve capacity), or won’t start your car when charged (cranking health); it’s likely a bad battery.
As shown in Figure 1, when the battery is fully charged, it has a healthy mix of sulfuric acid (HS2SO4, or commonly referred to as battery acid) and water (H2O). As the battery gets discharged, the battery acid separates into hydrogen (H2), sulfate (SO4), and water (H2O).
Over time, the heavier sulfate mixture settles and concentrates toward the bottom of the plates. This condition is known as stratification. At this point, the battery will likely lose capacity and experience degraded performance. But with proper battery management, it may be preventable.
Irreversible damage occurs as the battery sits in this stratified state. When portions of the plates are exposed to a diluted electrolyte for long periods of time, the plates experience corrosion. In Figure 2 you can see the corrosion difference on the top section of the plate vs. the bottom section.
In addition to this, over prolonged periods of time large sulfation crystals form on the negative plate reducing the battery’s active material (Figure 3). Once crystals have formed you cannot desulfate a battery. If left unmaintained, the net impact causes a rapid acceleration of battery aging and performance failures. By charging and cycling you can reduce the creation of crystals and the impact they have on the performance of the battery.
Temperature can also impact the physical damage done to batteries at low state of charge. High temperature causes loss of functionality and crystal formation creating shorts. When low temperature occurs, batteries in low states of charge are at a higher risk of freezing causing physical damage.
Much of this can be prevented with routine battery maintenance. Depending on where your (or your customer’s) batteries are “sitting” that could mean: keeping the battery connected to a trickle charger while in the garage, taking your vehicle for a 20-30 minute drive on a more routine basis, monitoring and charging the batteries of vehicles sitting on the lot, checking the state of health and charge of a customer’s vehicle battery regardless of the reason for the visit, or even just connecting a customer’s battery to a power supply during service. In the end, not letting the batteries sit dormant and unmonitored will ensure it works when needed most.
If you would like to better understand how this may impact you or your customers, feel free to send an email to your Midtronics account manager, and we’d love to help in any way possible.
Frequently Asked Questions
What happens to a car battery when the vehicle sits unused for months?
Parasitic drain from vehicle electronics gradually discharges the battery even with the ignition off. Once the battery drops below roughly 12.4 volts, the electrolyte begins to stratify — sulfate concentrates at the bottom of the plates while the top becomes diluted. Left long enough, this leads to plate corrosion and sulfation crystal formation, both of which cause permanent capacity loss.
How do you tell if a battery that’s been sitting is bad versus just discharged?
Charge the battery fully first, then test it. A discharged battery that passes a conductance or load test after charging is likely still serviceable. A battery that won’t accept a charge, can’t maintain reserve capacity under a small load, or fails a cranking health test after a full charge is a bad battery that should be replaced — not recharged again.
What is battery stratification and why does it cause damage?
Stratification occurs when a lead-acid battery sits in a discharged or partially discharged state. The heavier sulfate mixture in the electrolyte settles toward the bottom of the plates, leaving the upper portions of the plates exposed to a diluted electrolyte. Over time, that diluted exposure causes corrosion on the upper plate sections. Simultaneously, large sulfation crystals form on the negative plates — and once those crystals are established, they can’t be removed through charging or reconditioning.
Does temperature make dormancy damage worse?
Yes, significantly. High ambient temperatures accelerate chemical reactions inside the battery, speeding up crystal formation and increasing the risk of internal shorts. Cold temperatures create a different risk: batteries in low states of charge have a higher electrolyte water content, making them susceptible to freezing — which can physically crack the case and destroy the cell structure.
What’s the best way to maintain batteries in vehicles that aren’t being driven regularly?
The most reliable approaches are: connecting the battery to a diagnostic charger or trickle charger during storage, taking the vehicle for a 20–30 minute drive on a regular basis to let the alternator fully recharge the battery, and — for dealership lots — monitoring and charging vehicle batteries on a scheduled basis rather than waiting for a complaint. The GR8 Series diagnostic chargers are built for exactly this kind of fleet and lot maintenance use case.
How do you test a battery that’s been sitting in storage or on a lot?
According to Battery Council International guidelines, batteries that have been in storage should be charged before testing. After a full charge, a conductance-based tester gives you an accurate read on state of health — charge acceptance, reserve capacity, and cranking performance. Test before assuming the battery is good just because it holds surface voltage.