Brian:

Howard is probably the best reference I know of as a user as to battery life.  His AGM’s (Lifeline – the same company that makes aircraft batteries) lasted about 5-6 years as I recall.  Like all batteries their capacity just started going downhill.  You will have to search his Blog to find it.

For most users I would not recommend the Lithium Batteries.  AGM Batteries, properly cared for, don’t just quit.  They just star running down and get to a point where you have less and less useable amps.  And yes, never replace one battery.  Doing so will almost guarantee the other batteries will go bad much sooner.

Remember, AGM batteries, like lead acid, should not be discharged more than 50% of their ratings as a normal event.  That means if you have 300AH worth of batteries you should use only 150AH.  This means having a good battery monitor that will give a good “gas gauge” reading of how much you have used and what the charge percentage is.  Voltage is not a good indication of charge for this purpose.

The TriMetric battery monitor is a very good choice IMO.  It’s simple to use and very easy to install.  This link is for two of the models.  They make a TM2025-RV as well.  They all work the same, just different form factors.

http://www.bogartengineering.com/products/trimetric/from_tm2020_to_tm2025

Although expensive, if the budget will allow, Lifeline AGM’s are well worth the money because they are truly maintenance free.  If you should forget to properly service lead-acid (wet cell) batteries they can be damaged.  So, IMO for the long term you will more likely get better life out of the AGMs just do to the practical nature of servicing the batteries.  Others may have different opinions as they never forget to service their batteries. But most forget and I know I did in earlier days.

As you may know, AGM’s can be placed anywhere in the rig, without ventilation, and in any direction.  I.e. there is no “up” side other than access to the terminal.  This can be a huge advantage if you need to place them in the basement due to space limitations.  They do not vent any gases.  I do recommend 6 volt batteries wired for 12 volts.  Lifeline makes all different sizes and voltage configurations.

http://www.lifelinebatteries.com/rvdeepcyclebatteries.php

Finally, you need a good charger.  A three stage “smart” charger for the AGM’s.  While very rugged, for long life they require a charger that will charge them properly and moreover not over charge them.

Trust this was helpful as a first overview.

Here is the link to Jack’s Battery and Solar page.  Get a cup of coffee, there is a lot of info here including about battery chargers.

http://www.jackdanmayer.com/rv_electrical_and_solar.htm

We've been running LFP for 3 years now.. here's our page on them with costs and such:

www.technomadia.com/lithium

We did a 2.5 year update earlier this year in a video that is also on the page (excuse the poor quality.. bad internet day).

We've not used them long enough to give a long term projection, and still consider them a bit in the non-mainstream play land. Unless you're looking to be on the cutting edge and take the risks that go along with that - we don't recommend them for most folks. But happy to share our journey with them.

By the math, the LFP *should* be cheaper in the long run than AGM. But that math is all theory right now. But, we are incredibly happy with their performance and all the other benefits they give us.

- Cherie