Hydrometers, ugh!

Hydrometers are the time honored means to measure wet cell battery state of charge. They compare density (weight per volume) to water by how things float. Battery acid in full strength has a specific gravity of 1.84 (times heavier than water) but only 1.277 in a fully charged battery (because it is diluted with water).

The specific gravity is useful because a battery converts acid to water in the process of making electricity and vice versa when being recharged. It is easy to make a hydrometer sensitive enough to measure the reduced density as the concentration of acid is reduced. This allows you to tell the state of charge of each cell in the battery.

The advent of digital voltmeters provides a better means for RVer’s to keep track of battery condition. A hydrometer a good way to contaminate cells, spill acid on things, and generally cause havok. Proper use of a hydrometer requires proper safety equipment (goggles, apron, glove, working space, ventilation) and appropriate emergency supplies (eye wash, neutralizing solution, observer) close to hand. Proper hydrometer use requires care, and temperature correcting. See the Trojan site for how to test your batteries.

For most small RV battery situations, a good voltage reading (to a tenth of a volt) will tell you all you really need to know, be safer to do, and be a lot more convenient. This is especially true when watching the voltage during your campout as you can see how fast your battery voltage drops.

The much more expensive amp-hour meters actually measure electricity going into and out of your battery. These will give you a much more accurate indicator of the state of your battery, even better than a hydrometer, but they are expensive and not really cost effective for most RV needs. The trimetric and link10 are popular models. Cruzpro is another.

With your voltmeter, you can check for the basics:

  • A float charging voltage over 13v and under 14v is OK. The closer to 14v the more care you to take to keep an eye on water levels, depending on how cold the battery is.
  • In use, the battery voltage should be over 12v or it needs charging.
  • In storage it should remain above 12.2 or so.
  • In measuring batteries, you’ve really got two issues to deal with. One is state of charge and the other is battery capacity. Hydrometers only tell you state of charge which can be determined close enough for most RV purposes by voltage under load. This is only a real concern when you have to stop using it because you don’t have a charge handy. Otherwise – put a maintainer and charger on the battery and keep it properly charged for best life.

    The other issue is that battery capacity slowly degrades as the battery ages and is best determined by watching how fast the voltage drops over time in typical use conditions. If it doesn’t last anywhere near as long as it used to, then it is time to replace the batteries in you bank.

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