Originally Posted by PKI
The answer is yes. You will not be wasting your money with the addition of a second battery. However, it needs to match the existing battery or one will pull down the other. And that is a waste.
A wet cell or AGM battery can only be discharged to 50% (12.2 volts) without shortening it's recharge cycles life.
PKI makes a good point. For long life, fast charging, and maximum available capacity the batteries in a bank must be the same brand, type, and size.
To get more capacity cheaply, buy a duplicate of the one you have, a second battery box, and a pair of heavy connection cables.
There seems to be a widely believed myth that lead/acid batteries last twice as long if you only discharge them to 50% of capacity. The reality is they can be recharged twice as many times. Of course that means you only have half the capacity on each charge. Using 50% of your battery banks capacity is a good rule of thumb. It reduces the possibility damaging the battery by drawing it down flat. It provides reserve capacity for when things don't go as planned. But you will get the same number of amp hours of storage life out of either method.
Charging to 90%. Discharging to 10% is a typical test procedure. A good battery design may get 300 charge cycles before capacity drops to 80% of new.
Charging to 90%. Discharging to 50% is sometimes tested by manufactures as well. Usually the test yields about twice as many charge cycles. Of course you only get half as much storage capacity.
The biggest contributor to the deterioration is the distortion of the electrodes each time the battery is charged. It isn't the number of charges that matters, it is the amount of charge that distorts the plates.
Starting batteries are the most sensitive. The shape of the plates is intricate. The shape changes gradually loosing surface area with each charge and thus loosing cold cranking amps capacity. Drawing it down flat will pretty much destroy the intricate pattern on the electrodes.
Deep draw flooded cells are designed to be drawn down to 10%. Drawing them down flat damages the plates, but does not destroy them. Some are better than others. Having a reserve is an excellent strategy to avoid early failure.
Deep Draw AGM batteries are also designed to be drawn down to 10%. However, drawing them down flat does very little damage. The reason is the chemistry in the AGM battery runs out of charge in the electrolyte before it runs out of lead on the plates. An accidental full discharge makes very little difference to the life of the AGM.
On the other hand, AGM batteries are sensitive to overcharge or charging too fast. Both conditions will cause the AGM battery to vent. Loss to venting is permanent. It cannot be reversed. Most modern charger/converters in TT's and MH's will not charge too fast. They are voltage limited. Some can overcharge a little. They may not lower the fast charge voltage from 14.6 to 13.6 volts soon enough or switch from 13.6 to 13.2 maintenance charge soon enough. However, that loss is minimal and accumulates gradually. No big catastrophic failures here.
Deep draw AGM are optimized for applications like TT's. A good converter/charger and shore power or generator for charging are ideal for long life. Accidentally leaving the battery disconnect on for 3 months of storage will not kill it. There is less maintenance. AGM's won't spilled or spray acid on adjacent equipment. They do not vent explosive hydrogen gas as a part of their normal charge cycle.
Full timing with lots of boondocking or dry camping may benefit from gulf cart batteries. They generally are flooded cell, deep draw, and designed for fast charging. Buy them at discount prices, maintain them properly, and you will get good return on your money.