You’re likely familiar with cold temperatures reducing battery performance. Winter is the stereotypical time for non-starts. But did you know that much of the damage to the battery may have been done in summer?
Although a warm battery performs better than a cold one, that warmth also speeds up ageing.
Batteries technically require a very specific temperature range for optimal efficiency, usually around 25°C. Any less – or any more – and problems start to emerge.
Realistically, there isn’t much you can do about the weather. There’s no use fretting about what nobody can change. However, a good understanding of how summer temperatures affect battery health, capacity and performance can help you maximise your fleet’s battery service life.
Ambient temperature vs operating temperature
Before we get started, a quick note. The air temperature in summer is only a contributing factor. The temperature that really matters is the battery’s actual operating temperature.
Under the bonnet, the battery can be much hotter than the outside air, even in winter. That’s especially true after long runs, stop-start traffic, heavy electrical loads or heat soak after shutdown.
Although this article focuses on summer conditions, the wider issue is high battery temperature, whether caused by warm weather, engine bay heat or charging load.
Heat can improve short-term battery performance
During discharge, the negative sponge lead plate and the positive lead dioxide plate both form lead sulphate. The reaction consumes sulphuric acid from the electrolyte and produces water, reducing the electrolyte’s acid concentration. Electrons released at the negative plate flow through the vehicle’s electrical circuits, providing electrical power.
The battery’s temperature affects how easily this happens. Of course, its temperature is largely affected by the environment immediately around it, the air temperature and the temperatures under the bonnet or engine cover.
When the battery is warm, the ions have greater mobility because heat gives them more kinetic energy and lowers electrolyte viscosity. Because they can move through the acid and plate pores more easily, the battery sees reduced internal resistance and can better deliver current.
However, better short-term current delivery doesn’t mean better long-term battery health.
Why summer heat shortens 12V battery life
The summer heat speeds up all those useful reactions. But it also speeds up the damaging side reactions. That means, although the battery might technically be under less strain at discharge, it actually ages faster.
The main reasons for damage are grid corrosion at the positive plates, water loss (gassing), self-discharge, sulphation and breakdown of the active materials.
Positive grid corrosion
Positive grid corrosion happens because the positive plate sits in a highly oxidising environment of sulphuric acid, oxygen, heat and high electrical potential. Over time, the lead-alloy grid forms corrosive materials that don’t conduct electricity well. The grid then carries current less effectively, so internal resistance rises and voltage drops harder under cranking load.
Corrosion also weakens the grid’s structure, so active material loosens or sheds. With less connected active material and higher resistance, the battery loses both cranking power and reserve capacity.
Gassing (water loss)
In flooded batteries, water can split into hydrogen and oxygen under hot conditions. These gases then escape the battery through pressure valves and caps. This reduces the water content of the electrolyte and lowers the electrolyte level. If electrolyte levels drop enough to expose the plates, it leads to sulphation and plate damage, meaning higher internal resistance and lower capacity.
In sealed AGM and VRLA batteries, gas is meant to recombine inside the battery. But high heat and overcharge can produce gas faster than the battery can recombine it. If the pressure rises enough, the valve vents gas and water will still be permanently lost, leading to the same problems as above.
Self-discharge
A parked vehicle battery keeps reacting even when the ignition is off. Add alarms, trackers, ECUs, cameras, telematics and other parasitic loads, and the state of charge (SoC) can fall quickly. And in warm weather, these reactions occur faster.
Low SoC then encourages sulphation. Soft lead sulphate becomes harder and less reactive. It blocks active plate area, reduces charge acceptance and weakens current delivery under load.
Why summer battery damage often appears in winter
The danger of the warm summer months is that it sometimes hides battery issues. In hot weather, the naturally lower internal resistance masks weaknesses. A battery on the verge of failing may still crank with seemingly no problems, and the driver has no reason to suspect it’s not perfectly fine.
Inside the case, however, heat may be ageing the plates, drying the electrolyte, corroding the grid, and increasing self-discharge.
And then winter arrives.
Cold weather has the opposite effect. It slows the chemical reactions and increases the electrolyte viscosity, leading to higher internal resistance. With extra demand from thicker engine oil, starter current demand also rises.
Now that same weak battery must work harder while delivering less usable capacity. It’ll fail before long. The best way to stay on top of this is with a scheduled battery maintenance programme in your fleet workshop.
How heat affects battery charging
Charging voltage is temperature sensitive. This applies whether the battery is being charged on-board by the alternator or in the workshop with a professional fleet battery charger.
A lead-acid battery needs lower charging voltage when it’s hot (because excess voltage causes overcharge, gassing and water loss) and higher charging voltage when it’s cold (because its reactions are slower, internal resistance is higher, and more voltage is needed to drive current into the plates).
With a proper battery charger, you shouldn’t have to worry about manually adjusting the charging voltage. However, for reference, above 25°C, the charging voltage should be reduced. Below 25°C, it should be increased. A common compensation figure for lead-acid batteries is about -3 mV per cell per °C from a 25°C baseline. A 12V battery has six cells, so that works out to about -18 mV per °C across the whole battery.
If the charging system or charger doesn’t compensate for the battery’s temperature, it will overcharge the battery. This causes gassing, which can lead to water loss which, in turn, leads to dry-out, high resistance and early failure thanks to sulphation.
How heat affects battery testing
The same principle also applies to testing. Whether you’re conducting a system test when the battery is connected to the vehicle or an isolated test with a Midtronics CPX 900 ROBIS, it must account for the temperature. Any good modern battery tester should do this automatically.
A basic open-circuit voltage test only shows the battery’s SoC. It doesn’t fully show its state of health (SoH), which is a more relevant reading for long-term battery capacity.
Your battery tester should also check the battery’s cranking performance, conductance, internal resistance, charge acceptance and reserve capacity. Store and log all these test results so you can compare current test results with the unit’s historical performance. That way, it’s much easier to spot trends, such as poor batteries on certain vehicles or routes or underperforming battery models across your fleet.
Although you could manually upload your data into a spreadsheet, we recommend ROBIS, our cloud-based battery management solution. You can connect compatible testers directly to your account, allowing for immediate upload, and view the results from an easy-to-use dashboard.
What workshops and fleets should watch for in summer
Carry out summer checks to keep an eye out for signs of degradation, even if your vehicles seem to be starting and running as normal. Here’s a checklist of potential warning signs:
- Low state of charge
- High self-discharge
- Poor charge acceptance
- High internal resistance
- Weak conductance
- Slow cranking
- Start-stop faults
- Swollen cases
- Hot battery cases
- Corrosion at terminals
- Repeat low-voltage codes
- Telematics drain
- Vehicles parked for long periods
Keep battery faults from becoming downtime
Here at Rotronics, our team supplies professional battery testers, chargers and diagnostic equipment for fleets and workshops. We stock only the most trusted models and walk you through how best to optimise their use in your specific context.
Instead of waiting for a costly wave of winter non-starts, get ahead today. Source your battery tester, system tester, or professional fleet battery charger from Rotronics. Get in touch for tailored advice and support choosing the right equipment.
