How Alternator Charging Works
Many vans (including 2nd Generation Revels*) use an alternator to generate power that charges the house battery system. Alternators take mechanical energy from the engine and convert it to electrical energy for batteries.
Alternator output is controlled by a regulator to ensure that the batteries are not overcharged or undercharged. There are two types of regulators this article will reference: internal and external.
Internal regulators (for the purpose of this article) are simple on/off devices that live inside the alternator. They crudely control power output based on battery voltage.
Think of internal regulators like a light switch. They can either be ON or OFF. These are typically used in vehicles to charge the lead-based chassis battery and other electrical loads (the vehicle’s radio, headlights, etc.). These chassis batteries/systems are slow to respond to alternator power output, so there is ample notice (via voltage) when the battery is full. It is easy for an internal regulator with a simple on/off control to charge a lead-based battery.
Here is a metaphor for internal regulators. Imagine driving a dump truck on flat ground. You must drive as close to 60mph as possible using only full throttle or no throttle. You could easily keep the dump truck very close to 60mph because it is slow to respond to your changes in throttle.
Lithium batteries are like sports cars. In a sports car, if you have to drive exactly 60mph and are only able to use fill throttle or no throttle, then it will be almost impossible. You will either overshoot the speed or keep your average speed far below 60. The sports car is so responsive that it’s impossible to maintain a specific speed with a throttle that’s either fully on or off. This is why it is critical to have an external regulator with precise control for your lithium batteries.
External regulators (for the purposes of this article) have the ability to control the alternator’s voltage output very precisely. In the metaphor above, if you have the ability to feather the accelerator pedal to keep a sports car exactly at 60mph, you will be able to do so.
Charging Lithium Batteries
To charge lithium batteries (used to power the house system), you must maintain a specific voltage for enough time for the battery to be full. If we continue to use the car example, you must not only get to 60mph, but also maintain 60mph for a certain amount of time.
Revel alternator regulators for 2nd Generation Revels
If your Revel came from Winnebago with two Xantrex/Lithionics 125ah batteries, with a control panel that looks like the image above, it has an alternator that is internally regulated. The result is that the internal regulator can either be set to undercharge the batteries (keep speed well below 60mph) or overcharge the batteries (try to get to 60mph, with inevitable overshoot conditions). This internally-regulated charging is bad for the lithium house batteries.
Why is undercharging bad?
While undercharging will not damage your batteries, it leaves you with less available power than advertised (E.G. a 125ah battery may only have 100ah of energy in it). It can also make your battery monitor very inaccurate. Battery monitor percentage is estimated and must be corrected regularly with full charges (this is called drift, which you can read about here). If the batteries are never fully charged, then you cannot ever correct the battery monitor and trust it.
Why is overcharging bad?
Overcharging is bad for a number of reasons all related to how the batteries protect themselves from excess voltage. In simple terms, if the batteries are fully charged then they can shut a door to prevent overcharging. Every time voltage is too high, this door is shut. When the door is shut, the alternator stops producing power, voltage falls, and the batteries open the door again. When the door is open, the alternator begins to produce power again, and this vicious cycle repeats.
You may notice some seemingly-innocent results of this behavior. The indicator lights on your control panel may flash rapidly. This is a warning from the batteries. You may also notice your roof fan beeping. The roof fan beeps every time power to the fan is reset. The power is reset when the batteries shut down and then the alternator stops making power. The beeping is indicative of overcharging.
The bigger issue is that this cycle can damage the batteries. The device which shuts the door is called the BMS (battery management system). The doors are pieces of hardware on the BMS called MOSFETs. MOSFETs are special electrical switches controlled by the BMS to close the door and disconnect the batteries temporarily so that they are not over charged. This can cause interruption and unintended behavior, and potentially damage the BMS electronics (so, damaging your batteries).
How to Fully and Safely Charge Lithium Batteries from the Alternator
Upgrade to an external regulator that can precisely control voltage from the alternator to charge your house batteries. We recommend our external regulator conversion kit, which has been programmed specifically for charging your system, to fix this real problem.