If the battery state-of-charge is below 70% the battery needs to be recharged before you test it. The battery should not be tested if it has been charged or discharged in the last 6 hours and preferably 24 hours. This is called an open circuit voltage test. To measure the voltage you need a good digital voltmeter. Using the DC voltmeter check the voltage and compare it to this chart.
% State Of Charge | Specific Gravity Corrected 80 F | Open Circuit 12-Volt | Open Circuit 6-Volt |
100 | 1.277 | 12.73 | 6.37 |
90 | 1.258 | 12.62 | 6.31 |
80 | 1.238 | 12.50 | 6.25 |
70 | 1.217 | 12.37 | 6.19 |
60 | 1.195 | 12.24 | 6.12 |
50 | 1.172 | 12.10 | 6.05 |
40 | 1.148 | 11.96 | 5.98 |
30 | 1.124 | 11.81 | 5.91 |
Discharged | 1.120 | 11.80 | 5.90 |
Whenever possible you should avoid discharging a battery below 40%. Battery readings that are off of this chart indicate the battery was left discharged too long or the battery may have a bad cell.
The preferred method for testing the battery’s state of charge is to check the specific gravity reading of each cell. You can purchase a hydrometer at an auto parts store for about ten dollars. The electrolyte is a solution of acid and water so you need to wear goggles and gloves and avoid skin contact. Remove the vent caps and check the electrolyte levels. There has to be enough electrolyte in the cell for the hydrometer to pick it up. If you have to add any water you’ll have to charge the battery and let it sit for at least six hours before testing. Fill and drain the hydrometer at least twice in each cell before taking a sample. Take the reading and record it, then drain it back into the cell. Test all of the cells and replace the vent caps. If your hydrometer does not compensate for temperature you must correct the readings to 80 degrees F. Add .004 for every 10 degrees above 80 degrees F and subtract .004 for every 10 degrees below 80 degrees F. Compare the readings to the chart.
The battery might be bad:
■If there is a .050 or more difference in the specific gravity reading between the highest and lowest cell, you have a weak or dead cell(s)
■The battery will not recharge to a 75% or more state-of-charge level.
Remember, what you take out of your batteries must be put back in and if it’s not done in a timely manner the battery sulfates and can be permanently damaged. Single stage RV converter chargers have a fixed output voltage around 13.6 volts and over time will boil off the electrolyte in the battery. You need a three stage charger that can provide a bulk charge then an absorption charge and finally a float charge. There are RV converter chargers on the market that will do this.
If you purchase a multi-stage battery charger you need to know the charging current limitations of the battery being charged. For proper charging always follow the instructions that come with the battery charger. Make sure you match the charger to the battery manufacturer’s recommended charging voltages or match the batteries to the charger capability. Batteries should be charged as soon as possible after each period of use or whenever they reach a 70% state of charge or below. The batteries should only be charged in a well ventilated area and keep any sparks and open flames away from a battery being charged. Check the electrolyte levels before and after charging batteries.
If you put your RV in storage it is a good idea to remove the batteries and put them in storage too. This is quite simple to do. When you’re removing the battery always remember to remove the negative terminal first. Clean the batteries with a 50/50 mixture of baking soda and water if necessary. Check the electrolyte level and add distilled water if necessary. Test the battery state of charge and charge any batteries that are at or below 70%. A discharged or partially charged battery will freeze much faster than a charged battery. Store the batteries in a cool dry place but not where they could freeze. Batteries in storage will lose a percentage of its charge. Check the state of charge every month and charge batteries that are at or below 70% state of charge.
Happy RV Learning,
Mark Polk
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