Both EVs and internal combustion engine (ICE) vehicles use 12V batteries. In an ICE vehicle, the 12-volt battery provides high current to the starter motor. It also supports lighting, control modules, infotainment, security systems and other low-voltage loads such as the central locking and alarm. But its main job is starting the engine.
In an EV, there is no starter motor. Instead, the 12V auxiliary battery powers the low-voltage systems that allow the vehicle to wake up, initialise, close high-voltage contactors, communicate between modules, run safety systems and enable charging.
So, the battery is no longer responsible for cranking an engine. But it’s still responsible for making the vehicle usable. And when problems develop, the car or van still won’t work.
That’s why, although EV 12V maintenance is still critical, it requires a different approach.
The 12V battery has a different job in an EV
In an ICE vehicle, we often primarily judge a battery by its starting performance (cranking amps or cold cranking amps, CCA). If the battery can’t deliver enough power to crank the engine, the vehicle won’t start. Simple.
In an EV, the question is different, and a little more technical. You need the auxiliary battery to reliably support the vehicle’s low-voltage electrical architecture. The load profile is based around control units, relays, sensors, telematics, ADAS, body control functions, battery management systems, security systems and gateway modules.
A weak EV auxiliary battery might not produce an obvious starting symptom. Instead, it can cause module dropouts, failed wake-up, charging faults, contactor control issues, warning messages or loss of communication with the vehicle’s on-board diagnostics.
A charged traction battery doesn’t prevent 12V failure
An EV can have plenty of charge in the high-voltage battery and still fail to enter ready mode. That’s because the high-voltage battery and low-voltage system are completely separate. If the 12V battery can’t power the systems needed to initialise the vehicle, the EV might not close its high-voltage contactors or enter ready mode.
This can lead to misleading symptoms. The vehicle may not unlock normally. It may not communicate with diagnostic equipment. It may not open the charge flap. It may refuse to charge. It may display unrelated warning messages.
Without proper 12V testing, your technicians might waste their time chasing high-voltage faults, software issues, charging problems, or module failures when the root cause is simply a low-voltage auxiliary battery.
The traction battery also needs testing, charging and module balancing. This requires completely different, specialist units. Check out the Midtronics xMB-9640 Aftermarket for module balancing and the Midtronics GRX 5100 for more in-depth traction battery servicing.
EV auxiliary batteries are charged differently
In an ICE vehicle, the alternator supplies the electrical system while the engine is running and tops the 12V battery back up. The same principle applies to an EV, but the 12V battery is usually charged from the high-voltage battery through a DC-DC converter. The converter steps high voltage down to the low-voltage bus and maintains the auxiliary battery according to the vehicle’s control strategy.
The DC-DC converter is very different from an alternator. Unlike an alternator, the DC-DC converter is governed by software-controlled charging events, sleep and wake cycles, and OEM-specific battery maintenance strategies. This, of course, requires specialist training.
Now, the EV might maintain its 12-volt battery’s condition itself. However, this depends on the manufacturer and the software. For instance, the vehicle might wake periodically to charge it. It could also fail to maintain it if the battery is worn, damaged, deeply discharged or affected by parasitic loads.
Long story short? A charged traction battery doesn’t mean the 12V battery is healthy.
Testing and charging EV auxiliary batteries
As discussed above, there is no cranking load in an EV. The auxiliary battery needs to support the low-voltage control architecture that allows the vehicle to wake up, communicate, charge and enter ready mode.
That makes your tests very different. EV 12V testing needs to look beyond open-circuit voltage. It needs to confirm whether the battery can support low-voltage system demand, accept charge, retain charge and remain stable under electronic load.
As such, you need equipment designed for this very specific role. The Midtronics xLVS-9000 is designed for EV 12V auxiliary battery testing and charging. It assesses auxiliary battery condition and includes a built-in 70 Amp charger, so you can charge the battery while the system monitors its response.
EVs still need 12V maintenance
The main point is that if the 12V battery fails, the EV will still break down.
Here at Rotronics, our team supplies professional battery management solutions for modern vehicle environments, including the Midtronics xLVS-9000 EV 12V auxiliary battery tester and charger.
If your workshop, dealership, fleet, or service operation needs a better way to test and charge EV auxiliary batteries, speak with Rotronics about the right 12V battery management solution for your operation.
