Updated January 4, 2021!
Most true deep cycle batteries are at the 20 hour rate. A group 27 battery has about 100 amp hours (5 amps for 20 hours at 80 degrees F). The problem is no one uses exactly 5 amps. If you're pulling 10 amps, you may have a "70 amp hour battery"; if you only draw 2, you may have a 140 amp hour battery. So the amp-hour rating is not really more than a general way to compare like types of batteries.
Comparison Technologies -- How to compare different types of RV batteries.
Efficieny: Measures how much you have to put back versus what you took out. Flooded is 89% efficient; gel cell 95%; AGM is 99%.
Self discharge rate: Flooded 13%; gel cell 3%; AGM 1%.
Cycle life at 50% discharge limit: Flooded 1280; gel cell 400; AGM 1100.
Cycle life at 80% discharge: Flooded 850; gel cell 270; and AGM 550.
Charge set point: Flooded = 14.2-14.6; gel cell13.8-14.0; AGM 14.1-14.4
Float charge: Flooded = 13.2-13.7; Gell cell 13.2; AGM 13.2 -13.4
(Note from the last two measures the relative intolerance of gel cell types. You have several charging sources, such as alternator, shore power/coverter, and solar. It's very difficult to hit the tight tolerances of gel cell type batteries in RVs.)
Presenter highly recommends AGM batteries for RVs.
Input Charge: With flooded battery, charging amperage should not exceed 35% of the amp hours. Thus if you have a 100 amp hour flooded battery, you should not charge it at more than 35 amps. With gel cell, you can recharge at 50%; AGM you can recharge at 100%. Finally note that traditional lead acid batteries require maintenance, while gel cell and AGMs do not.
A note on adding water. Look down cylinder, and fill to the "slit". Always use distilled water. Fill only to the base of the cylinder. Don't fill above that point (water expands when it gets hot); and don't let water go down to where you can see the top of the plates. Reason for not filling above the bottom of the cylinder is that water will expand in volume when hotter, and will spill out the electrolyte, causing corrosion. To clean batteries keep caps on tight, use half teaspoon of baking soda in a cup of water and spray on. It will bubble and fizz -- then just hose it off.
Equalization: This pertains only to flooded lead acid deep cycle batteries. E.g. a Trojan golf cart battery. They will have six caps, representing the six cells within the batteries. Sometimes on recharge one cell will recharge less than the others. 95% of "dead batteries" really means just one dead cell. Equalization is a planned overcharge of the five good cells, while bringing the one bad one up to full voltage. It's better than the option of letting the battery sulphate and ruin that one cell, causing a "dead battery". Suggests equalizing not above 15V, and then adding water immediately afterward.
Best bang for the buck is the Trojan T-105 golf cart batteries -- if you have space for them. They are considerably taller, and won't fit in some compartments. 2 6s in series is better than 2 12V batteries. Longer life, more forgiving, will last twice as long. Cost in range of $65-$85.
Gel batteries cost twice as much, but require very exacting charging regimen. Stay clear of them in RVs.
AGM will cost about 2.5 times that of Trojans. But can be worth it for many RVers.
Dry camping typically takes 50-100 amp hours daily. 2-4 golf cart batteries usually enough for most rigs. 4 needed if you have an inverter.
There are date stamps on all batteries. They want to get rid of old batteries first. If you have different date stamps, you're likely to invite the "different ages" problem, which holds that you need to replace all batteries that are of the same age.
Note: Don't mix battery types, sizes or ages in the same battery bank. Typically if you need to replace on battery, it's time to replace all of them. by Greg Holder, AM Solar, Inc.
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