Saturday, January 29, 2011

Coach Battery Overview

The heart of your coach's 12-volt DC system is the two Trojan T-105 6-volt batteries that Lazy Daze installs in every vehicle. These batteries are what give you the freedom to head off into the boondocks and still enjoy almost all the comforts of home far from the grid. Trojans will generally last 5-7 years, and some basic care and careful use of the batteries can greatly extend their life. Those who have high power needs and/or boondock for lengthy periods often replace the Trojans with AGM batteries, which provide much more capacity. 

The three basic battery types
 Flooded lead-acid
The Lead Acid (wet cell) battery is made up of plates, lead, and lead oxide (various other elements are used to change density, hardness, porosity, etc.) with a 35% sulfuric acid and 65% water solution. This solution is called electrolyte, which causes a chemical reaction that produce electrons. When you test a battery with a hydrometer, you are measuring the amount of sulfuric acid in the electrolyte. If your reading is low, that means the chemistry that makes electrons is lacking. So where did the sulfur go? It is resting on the battery plates and when you recharge the battery, the sulfur returns to the electrolyte.

The Trojan batteries are of this type. Flooded batteries are by far the most common type of vehicle and marine battery. There are two basic types of these batteries: deep-cycle or marine type, such as the Trojans, and starting batteries as in up front under the hood. Starting batteries provide a strong burst of power for starting the engine, but do not take well to being deeply discharged.

Trojan manufactures three batteries - the T-105 with a 225 amp-hour capacity, the T-125 with a 240 amp-hour capacity and the T-145 with 260 amp-hours. The T-145 costs almost twice as much as the T-105. That's why some LD'ers have added addtional banks of T-105 batteries. For almost the same money, four T-105's will provide nearly double the capacity of two T-145's.

The Absorbed Glass Matt construction allows the electrolyte to be suspended in close proximity with the plates' active material. In theory, this enhances both the discharge and recharge efficiency. Common manufacturer applications include high performance engine starting, power sports, deep cycle, solar and storage batteries. The larger AGM batteries are typically good deep cycle batteries and they deliver their best life performance if recharged before they drop below the 50% discharge rate. When Deep Cycle AGM batteries are discharged to a rate of no less than 60% the cycle life will be 300 plus cycles.

Other than charging, these are truly no-maintenance batteries. A great advantage of AGM's is that they do not vent any gases and thus can be safely installed anywhere. AGM's are much more expensive than wet-cell batteries.

The Gel Cell is similar to the AGM style because the electrolyte is suspended, but different because technically the AGM battery is still considered to be a wet cell. The electrolyte in a Gel Cell has a silica additive that causes it to set up or stiffen. The recharge voltage on this type of cell is lower than the other styles of lead acid battery. This is probably the most sensitive cell in terms of adverse reactions to over-voltage charging. Gel Batteries are best used in VERY DEEP cycle application and may last a bit longer in hot weather applications.

Discharge of batteries
It is generally considered "not good" to discharge your wet-cell batteries more than 50% of their capacity. Doing so on a regular basis will greatly reduce the longevity of the batteries. So how do you know when you've used up half your amps? The simplest method is note the voltage of the batteries when there is no load on them.

Greg Holder from AM Solar gave these numbers at Life On Wheels in Moscow, ID: Flooded batteries (i.e. the Trojan battery almost all of us have) at 70 or 80 degrees will have the following charge vs. voltage relationship:

Rest Volts / State of Charge / Specific Gravity
12.65 / 100% / 1.265
12.47 /  75% / 1.225
12.34 /  50% / 1.190
12.24 /  25% / 1.120
11.89 /   0%  / 1.120

Greg pointed out that "crudely" rounding to 0.1 volts, 12.6 is fully charged, 12.5 is 75% and 12.4 is 50%. If you want your batteries to last for any length of time, do not discharge them below 50% of capacity. I do not have a good reference at hand but you can discharge a battery to 25% for something like 50 or 75 times before its worn out. You can discharge a battery to 50% of capacity for several hundred times and to 75% for 1000 or 1500 times. Deep discharge will sharply shorten life. Generally speaking, the difference between regularly discharging a lead-acid battery 10% and regularly discharging it 90% is a tenfold reduction in battery lifespan.

Again, this is RESTING voltage, hours after a charge, and with NO LOAD. And in reality, this is only slightly more accurate than the idiot lights on the LD Systems Status panel.

A much better method of monitoring your batteries is to install a battery monitor, which is a digital meter/computer that reports voltage, amp flow into and out of battery, amp hours remaining and operating time left. Here are two manufacturers:
Xantrex LinkLite and LinkPro

Charging batteries
OK - so now you have an idea of the state of your batteries. How can you charge them?
- If you access to an electric hookup, plug in and let your converter/charger take over. See the Converter/Charger FAQ for ways to improve charging.
- Run your generator to charge them.
- If you don't have a generator, start the engine and let the alternator recharge them.
- Install solar panels for recharging the batteries.
- When in storage, if you have an outlet nearby, you can hook up a battery charger on a timer (using the timer to prevent over-charging). 

Buying Batteries
When buying batteries, as with tires, it is good to know when they were manufactured. Here's how to read the date codes on Trojan batteries.
- Positive Terminal- Manufacturing Date. This code indicates the actual date when mechanical assembly of the battery was completed. At this point, electrolyte has not been added to the battery and formation charging has not taken place. LETTER stands for the month and could be anything from A to L (A=January, B=February, C=March, and so on); NUMBER stands for the date.
- Negative Terminal- Shipping Date. This code indicates the month and year when the battery was shipped out of the factory. LETTER stands for the month (see below); NUMBER is the last digit of the year.
- Example: Consider a battery with "I26" stamped on the positive terminal and "J2" on the negative. "I26" means that the battery was assembled, without electrolyte, on September 26th. "J2" means that it was shipped from the factory around October of 2002. 

Other Resources
Trojan Batteries Users Guide
Mark Nemeth's Battery Facts

Contributors: Gus Weber, Larry Wade, Linley Gumm, bumper, Andy Baird, WxToad
Return to FAQ Index