Can A Lithium Battery Damage Your Alternator?

We can answer our own premise for this story very easily. Yes, a lithium-ion battery can technically damage your alternator. However, "can" and "will" aren't the same thing, so let's unpack some other questions — like how and why this energy storage brick might fry your alternator in the first place. First, let's set the playing field. There are pros to lithium batteries, and some RVers and boaters want to have a lithium battery in addition to, or in place of, a lead-acid one because they are lighter, work great for power storage, and tend to last longer.

Many mild hybrids that combine gas engines with 48-volt hybrid systems already come with lithium batteries and starter-generators. Some of these run a fairly small motor between the engine and transmission off the lithium battery, and that motor can provide a little added oomph, in addition to the gas engine. Obviously, those cars are designed to use that lithium battery, and they get what's called a DC-to-DC converter that steps down the 48-volt system to run the car's 12-volt components. This is also why pure EVs have additional small low-voltage batteries.

But if all this has you thinking you might want to run a lithium battery under the hood of a car engineered to charge a lead-acid one, hit pause. A lithium unit has something called a battery management system (BMS), which is designed to prevent anomalous events from harming the battery — but if that BMS disconnects suddenly while the battery is charging, the resulting voltage spike could damage your alternator. Also, lithium bricks have unique charging capabilities and issues you should understand before considering the swap.

While a lead-acid battery acts like an electric buffer that absorbs voltage spikes, a lithium-ion battery's BMS can sometimes act like a kill switch, depending on its design. If there's a fault in the battery, the temperature gets too low, or the battery voltage spikes too high (known as overvoltage), the BMS can open its switches to disconnect the battery, causing the voltage in the alternator to briefly spike (load dump), even if the alternator is engineered to push just 14 volts.  That spike can cause the diodes in the alternator to fry.

That's bad news because diodes convert AC current from the alternator to DC current that your car uses for its electronics. Diodes are also single-direction; they prevent a battery from being drawn down when the engine is off. Therefore, a bad diode can be a parasitic drain that kills car batteries. If a BMS-triggered disconnect fries the diodes in the alternator, that's probably because the alternator wasn't able to regulate the sudden voltage spike in time. 

Luckily, some modern car alternators come equipped with the means of suppressing load dump spikes — most commonly avalanche diodes. The former work by passively clamping voltage past a pre-set voltage threshold, while the latter constantly monitors and increases and reduces the current going to the rotor's electromagnet. Because of this, the risk of fried diodes is much lower — but it's not zero, especially if the voltage spikes exceed what the system can reliably handle. How likely this is to happen will depend on many factors, including what kind of lithium-ion battery and alternator you're running, driving habits, and even outside temperatures.

There's another issue with going the lithium route: extreme temperatures. While lead-acid batteries are fine charging and delivering juice below zero, lithium batteries are less forgiving in freezing temperatures. In fact, the BMS will generally block charging if the internal temperature of the battery is below 32 to 25 degrees Fahrenheit. 

Low temperatures also drastically increase the internal resistance in a lithium-ion battery. For these reasons, lithium batteries often feature low-temperature cutoffs that prevent them from charging, which can also lead to a load dump. The good news is that some modern quality lithium-ion batteries are insulated, and premium ones even have internal heaters to safeguard against potential issues.  However, high temperatures can also be problematic for lithium batteries, as they speed up the chemical activity in the battery, increase discharging, and can lead to thermal runaway. More importantly, the BMS can once again step in to protect the battery. 

One YouTube channel put a secondary lithium-ion battery to the test, placing it under the hood of a Tacoma to power a fridge, camp lights, and camera charging equipment. The battery was insulated and connected to a DC-DC charger, but the findings raised concerns about whether it was safe to run in high temperatures. That said, how big of an issue lithium is will therefore likely depend on where you live and travel, and how long you keep your vehicle sitting in extreme temperatures.

The good news is, most late-model cars already have computerized voltage regulation through systems such as the powertrain control module. But is that enough?

For the sake of argument, let's say you go with a lithium-ion battery instead of a lead-acid one. In the case of a modern car, its computerized voltage regulation could be adequate to charge the lithium battery. Also, speaking of modern cars, most of the car's other electronics are going to be protected from a load dump in the event that a BMS disconnect in the battery leads to a voltage spike through the car's electrical system.

Further, Ford and GM provide documentation to suppliers that shows that they require circuitry that can handle such overvoltage. But, importantly, there's still no guarantee that your alternator won't be damaged if a lithium battery's BMS detects an unsafe condition and suddenly disconnects while charging. Is there any way to prevent it? Yes. According to owners posting on forums, one of the most popular solutions is hooking up a DC-DC charger.

The gist is that while a lithium brick might be the right replacement battery for your old Harley-Davidson, and maybe handy as a house battery for your boat or RV, it might not be ideal for your car designed for a lead-acid battery.

This article was edited to clarify how cold weather and battery management systems (BMS) can affect alternators connected to lithium batteries.