Aux Battery
From VanagonWiki
A page devoted to the installation, care, and feeding of auxiliary batteries in the Westy. Also, subjects like deep-cycle vs. AGM vs standard vs. sealed batteries are appropriate. If adding a new section, try to keep it in alphabetical order. Avoid adding html code, such as "<br>", stick to plain-vanilla wiki markup.
Contents |
[edit] Charging The Aux Battery
[edit] From Household Current
See this entry on the solar page. (This section could use recommendations for specific "smart" chargers.)
[edit] From Solar Panels
See The Solar Page
[edit] From the Vehicle's Alternator
See this entry on the solar page.
[edit] Determining Battery State of Charge
(or, "How Much Power is Left In That Thing?")
[edit] Terminal Voltage
The state of charge of a battery can be determined by reading the voltage across its terminals with an accurate voltmeter, but the battery cannot be in use at the time: for accurate results it needs to be disconnected from any loads or charger for at least an hour to allow the battery voltage to stabilize. The longer a battery is allowed to "rest" before measuring its voltage, the more accurately the voltage is a true indication of its state of charge. If the battery is under charge, the voltage will whatever voltage the charger is putting across its terminals, and if the battery is powering something then the voltage reading will be dragged down by the load.
For a generic plain-vanilla flooded-cell lead acid 12 volt battery,[1] this chart shows the relationship between no-load voltage and state of charge at "room" temperature.
| No-load Voltage | Percent Charge |
| > 12.6 | 100% |
| 12.45-12.6 | 75-100% |
| 12.24-12.45 | 50-75% |
| 12.06-12.24 | 25-50% |
| 11.7-12.06 | 0-25% |
| < 11.7 | 0% |
For more information on the terminal voltage method, see State-of-charge reading based on terminal voltage, and "Lead-Acid Battery State of Charge vs. Voltage" (pdf).
[edit] Specific Gravity
| Measuring the specific gravity of the electrolyte with a hydrometer is a somewhat more accurate method of determining state of charge. Again, the battery has to have been at rest for at least an hour before an accurate reading can be obtained. With totally sealed batteries the hydrometer method is not possible and in any case, messing around with battery acid is probably not a good idea during a casual outing. Note that image has been lifted from batterystuff.com. |
More about using specific gravity to determine stage of charge can be found here.
A chart illustrating specific gravity and no-load voltage vs charge is here, and this FAQ gives more detailed information about the voltage and specific gravity measuring methods along with a mention of an industry method [2] which uses battery voltage and charging current to determine when a battery is fully charged.
[edit] Battery Monitor
The terminal voltage and specific gravity methods cannot be used to determine the battery's state of charge when the auxiliary battery is either being charged (by Solar, for example) or when being used as a source of power. Battery monitors, which use "coulomb counter" integrated circuits to monitor energy going into and coming out of the battery, can be used during charging and discharging and will display how much capacity is available, ranging from 0% to 100% -- like a fuel gauge. One such device, suitable for Vanagon and marine use is the Xantrex XBM. There may be others. (Other brands of coulomb-counter battery monitors should be added here.)
The disadvantages of battery monitors are 1. price and installation hassles (wiring, mounting), and 2. they are not 100% accurate because modeling the complex electro-chemical characteristics of a lead acid battery is not trivial. That said, with appropriate programming[3] and an understanding of their limitations battery monitors are accurate to within a few percentage points and, unlike the voltage-measurement or the specific gravity methods, they give constant "real time" indication of state of charge during actual charge/discharge usage.
[edit] Notes
- ↑ Contact your battery manufacturer for data specific to your battery -- if they can be bothered to provide you with that information.
- ↑ IEEE 450-2002 Annex B Recommended Practice
- ↑ Mr Squirrel has programmed his XBM to "know" that the battery is a 130Ah one, and that it is to be considered fully charged when the charge voltage can be held to 14.7 volts or more with less than 200mA of current for more than 1 minute. Once those conditions have been met, the monitor resets itself to display 100% charge. Before it resets, its displayed (calculated) state of charge displays as being no more than three amp-hours of full charge, which indicates that the discharge-charge modeling seems to be quite close. When the battery is powering stuff, the monitor measures discharge amps and time, curves the results and calculates amp-hours remaining.
