You do not need a Battery Monitor, but the “idot lights” will not let you know how full your battery is.
Would you drive your car without a fuel Gauge?
Knowing your battery bank’s State of Charge (SOC) is important so that you do not drain your battery too much and shorten its life. The standard battery gauge that most RV manufacturers equip their coaches and trailers with, the kind with three or four LED “idiot” lights, will never let you know the actual SOC of your battery. I’ve helped many RVers that did not include a battery monitor, and therefore damaged their batteries past usability within the first year! More info on How far to discharge your Lead Acid RV batteries?
The reason why it is hard to know the SOC of the battery is because when you discharge with heavy loads like a microwave or coffee pot the battery voltage will sag with the load. Lithium batteries do not sag nearly as much as lead acid batteries. When you charge with the Convertor or Solar the battery voltage will be artificially elevated making it harder to know how your battery is doing. You will need something better to get a measurement of your battery bank’s remaining capacity.
What is a battery Monitor?
A battery monitor that uses a shunt will be able to “count the electrons” that are coming and going from the negative side of the battery bank. It can take that information and display it in an easy to understand format. You can set it up to alert you when you reach 50% SOC or when a certain voltage is reached. The Victron BMV 712 will let you use an app to help you setup and monitor your battery. It will use algorithms to correctly monitor capacity with Peurkert Law challenges! This will set you back about $200.
What is a Voltage Monitor?
Now you might be thinking there has to be an alternative, there is, a Voltage Monitor. Some people will be well served with a cigarette lighter voltmeter and a cheat sheet to guesstimate the battery SOC. Weekend warriors who only use their RVs for a few days at a time or if you view a pair of cheap 6 volt flooded cell batteries as disposable every few years could find this a good fit. You will need a cigarette lighter port or you can always add one to help you keep electronics charged. This voltage monitor setup will cost about $12.
If you are still reading you will benefit from a Battery Monitor.
You maybe full time, taking extended trips, using an inverter that can run heavy loads or invested $1,000.00 or more in high quality batteries. A Battery Monitor will help extend the life of your battery and gauge your solar harvesting vs useage. When the sun goes down my battery bank is keeping my food cold, my furnace heating and my Maxxair fans cooling my home.