<%@LANGUAGE="JAVASCRIPT" CODEPAGE="65001"%> About RV Batteries
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Topic: RV Batteries

Presenter: Greg Holder, AM Solar, Inc

[Ed Note 10/2010: This presentation was first given in 2004. However much of the content would be the same were it given today.]

Deep Cycle Batteries: He defines 80% discharge and then recharging as one cycle. This would be around 11.8 voltes. 40% discharge and recharge is 1/2 cycle; and 20% discharge and recharge is 1/4 cycle. This type has fewer and thicker plates, versus "Starting Batteries", which have more, thinner plates for short bursts of high amperage. Starting batteries are NOT suitable for "house" batteries.

Most RVs come equipped not with a true deep cycle battery, but rather "RV Marine" type batteries -- a sort of hybrid between a true deep cycle and a starting battery (and less expensive for RV manufacturers to use.. These are definitely better house batteries than a starting battery would be -- so use them until they wear out and then replace them with a true deep cycle type.

Battery Types: Flooded lead acid batteries are either lead calcium "maintenance free" types, or lead antimony -- the more traditional type that has caps and to which you need to add water periodically. Most deep cycle batteries are lead antimony, since the "maintenance free" types (lead calcium) are sealed, but have low tolerance for "deep" discharge (below 40-50%). Lead antimony has higher tolerance for deep discharge, but they self-discharge faster. On balance, lead antimony is better suited for RVs, and true deep cycle batteries are of this type.

Gel Cell: Good for boats, where you're in rough seas, since the electrolyte won't leak out as it would with flooded batteries. But some problems, and in terms of utility are now totally replaced by AGM. If you charge gel cels at too high a charge, you'll actually lose some of the elctrolyte through gassing, and dry out the battery (shorter life).

AGM (absorbed glass mat) is a flooded lead acid battery, but instead of gel, it uses a fiberous mat which is 90% soaked in electrolyte. It is sealed, and the electrolyte is so immobilized that it can never come out.

Solar is nothing but a battery charger. Inverter is nothing without a battery. So need to understand batteries before you can understand either solar or inverter.

Only true way to know state of charge of a battery is to check its specific gravity. But very few RVers do this. Another fairly complicated way is to buy a meter for several hundred dollars (e.g., Link 10) that will measure that for you. The other way is to go by battery voltage. Preferred method is digital display, versus analog (needle). The idiot lights (red, amber & green) that come with most new rigs actually mean very little. The key to voltage checks is getting the battery "at rest". Yet that's virtually impossible unless you completely disconnect the battery, since there are always phantom loads (sensors, etc.). If you're plugged in to shore power, or use solar panels, they "charge" and make it impossible to know true voltage. Best time is first thing in the morning when you've not been plugged in (and before any solar influence). 12.65V is "full"; 12.47V is 75%; 12.24V is 50% 12.06V is 25%; 11.89V is just about zero.

How to know battery voltage: Only way to know for sure is to test the specific gravity of each cell. But his is so cumbersone that most RVers want another option. The built in systems of LEDs are at best an approximation. Best time to catch battery in the needed "at rest" condition is in the early morning -- unless you have solar, in which case you need to check before first light.

Primary causes of battery failure: Overcharging is one primary culprit. To charge, you need a source 14.1V ro 14.4V or more at room temperature. That's the gassing threshold for most lead acid batteries. You don't want to have higher voltage causing it to boil or "gas" from excess amperage. Gassing will occur at lower charging rates if the outside ambient temperature is hot -- such as parking on ashphalt during hot weather. Overcharging causes plate corrosion and/or water loss. In colder weather you may need to go above 14.1-14.4 volts to cause the needed gassing to stir the electrolyte. This is called reaching the edge of the gassing threshhold, which is needed to fully recharge the battery. Thus batteries will have longer life if there is a system for changing the charge rate depending on termperature. Lack of temperature compensation on inverter, charger, and solar is very important for RVs. Hot batteries require a lower voltage charge, while cold weather requires using higher voltage to "compensate" for the ambient temperatures. Unless the charging system can adjust the charging voltage either undercharging or overcharging is likely to occur. Temperature compensation is a crucial for RVs. If you don't have temperature compensation and the weather is hot, you'll have to add water more frequently, and keep batteries clean from the effects of gassing and spillage.

Undercharging is another problem, as it results in plate sulfation and electrolyte stratification. You'll know this when your battery used to last for 3 days, but now only lasts a day.

Vibration in an RV can cause some of the lead on the plate to fall off and piles up on the bottom of the battery. Eventually it builds up and will short out on the adjacent plate.

Shopping for a House Battery:

Cold cranking amps ("CCA") Available amperage at zero F for 30 seconds to 7.2V

Cranking amps ("CA") ia marine environment --same but at 32 degrees F

Hot cranking amps ("HCA") same at 80 degrees F

None of the above are useful measurements for RVs

Reserve capacity (usable minutes at 80 degrees F @ 25 amp draw to 10.5V)

Amp-hour capacity: Available amp-hours, at a fixed rate, over a given period of time (e.g. 20 hr rate, 5 hr rate., etc).

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 recommeds 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.