Your workshop phone rings. A driver reports that one of your trucks has been parked up for a few months. It was meant to be a spare unit, ready to roll when demand picked up again. But when the time comes to bring it back into rotation, the battery is, of course, flat. The driver wants to know if it’s salvageable or if a replacement is the only answer.
You might be familiar with this scenario if your fleet does a lot of seasonal or contract-based work. When the jobs aren’t coming in, your vehicles have nothing to do other than sit.
The main question is whether that dormant battery is worth your technicians’ time, or if it’s more cost-effective to write it off and fit a new one. This blog looks at what to consider.
What happens inside a dormant vehicle battery?
Batteries don’t like doing nothing. Lead-acid types are no exception. Even when a vehicle sits idle, chemical reactions continue inside its battery cells. Tiny currents flow between the plates, and small amounts of the active material slowly convert to lead sulphate. Water in the electrolyte can split into hydrogen and oxygen, and the plates can corrode, which gradually reduces the battery’s capacity.
Over time, these processes drain the battery, even though nothing is switched on. Flooded lead-acid batteries typically lose 3% to 5% of their charge per month at room temperature, while AGM or gel types lose slightly less.
Once the voltage falls below 12.4V for extended periods, sulphation starts to accelerate. Sulphate crystals harden on the plates, so fewer chemical reactions can take place, and you lose yet more capacity.
A dormant battery might still show some voltage on a multimeter. But that doesn’t mean it will crank your van or lorry engine. Available cold cranking amps (CCA) are usually much lower than expected, and that’s what counts for starting power.
Techniques to bring an old battery back
There are methods to recover capacity, though results vary depending on the equipment you have and how deteriorated the old battery is.
Slow, or ‘trickle’, charging with the correct profile is often the first step. Most high-quality modern chargers use staged charging to dissolve some of the sulphation on the plates and restore usable capacity. In some cases, technicians also use desulphation modes that send controlled pulses to break down crystal build-up.
Load testing after charging is the only reliable way to know if the effort has worked. A battery might look healthy on a screen but still fail when asked to supply real cranking current. That’s why reliable battery testers are essential. They give a clear picture of the state of charge (SoC) and state of health (SoH), so you don’t send a driver out with a unit that won’t start at the roadside.
When to restore and when to replace
Reactivation may buy you some time, but it isn’t always the right call. A three-year-old battery that’s been left dormant for a few weeks is a candidate for recovery. A six-year-old battery that’s been left over winter is usually going to be a waste of time and money. Of course, it all depends, so it’s always worth testing, just to be sure.
Best practice is to prevent your batteries from becoming dormant in the first place. In-vehicle charging is easy. Just connect your dormant vehicle’s battery to a charger once a month to top it up. You can even leave the charger connected when it’s not in use.
The best way to prevent dormancy problems is to assign a technician or a team to keep an eye on these vehicles. With semi-regular inspection and testing, they’ll be able to flag weakening batteries before they reach the point of no return. Preventing a battery from fully discharging takes more effort than letting it go, but it leads to far fewer complications and headaches when you need that battery back in service.
How can Rotronics help?
Rotronics works with fleets facing these exact questions every week. We supply a range of trusted professional battery testers to help you identify if a dormant unit is fit for reactivation. If that’s the case, our chargers apply controlled charging profiles to recover usable capacity where possible.
So, the next time a truck sits idle for longer than expected, you don’t need to guess whether the battery is worth saving. With the right tools and a structured routine, you’ll know when to restore and when to retire. Get in touch with Rotronics to see how our systems can help you reduce unnecessary battery replacements and keep your fleet on the road.
