How to Monitor Your Batteries Voltage

Just because your voltmeter tells you the battery is full, should you trust it?

Many of us have a house battery in our van or RV that we are charging either off our engines alternator or starting battery, or you have installed solar panels which are charging our  house battery. Hopefully you have a charge controller with a digital display that give you the volts of your battery or you have installed you own voltmeter  to monitor your batteries State of Charge.

Monitoring your State of Charge is critically important to the long term health of your battery bank, because they want to be as close to full as they can be all the time. Every time you draw power out of your battery it is being discharged somewhat and that is called cycling it. If you take your battery down to 50%, or half of it’s capacity, you want to bring the battery back up to 100% full as soon as possible, preferably the next day!  That’s considered one full deep cycle.

The difference between a starting battery and a true Deep Cycle battery is how many cycles they are designed to handle. A starting battery is never supposed to be deep cycled and after 3-7 accidental deep cycles, it will be destroyed and need to be replaced. On the other hand, a quality Deep Cycle battery is designed to intentionally be discharged to half throughout their life and are purpose-built to be able to handle it. Going to 50% is not damaging to them. It’s common for a quality Deep Cycle battery to be able to be deep cycled 500-1000 times.

But, if you are abusive to your batteries, and cycle them down below 50%, you are actually damaging them each time and reducing the number of deep cycles they have. You really want to avoid that!!!

On the other hand, if you only take it down to a quarter, 25% discharge, that’s only a partial cycle and a quality Deep Cycle battery might could do that 1000-3600 times.  A high quality Deep Cycle battery could last 10 years if it’s lightly cycled (25% or less) every day!

A Marine Battery is a hybrid battery, it’s designed to both start engines and to be cycled. However while it’s okay at both jobs, it’s not great at either. I wouldn’t expect to get much more than 100-200 Deep Cycles out of one, and maybe less. Going below 50% will cut that even much more.

As you can see from this, knowing how deeply you are cycling your house battery is very important to getting as long a life as possible out of it. If you lightly cycle (25% discharge) it every day  it will last much longer than if you deeply cycle (50% discharge) it every day. Because Deep Cycle batteries are expensive (from $100-$350 each), and we often have 2-4 of them (totaling $200-$1200) you want to get the longest life possible out of them!

Below is a chart that lists battery voltage and what it means to the State of Charge of your battery:


As this chart makes clear, it’s very important we know at any given moment exactly what the charge of our battery is but that’s not as easy as it seems.  If you’ve ever watched your voltmeter you know it can read as high as 14.4 during the day and and as low as 11.6 when a big load is put on it. Worse, it can seem to change very suddenly by as much as two volts or more. With it jumping around like that, how are we supposed to know exactly what the voltage of the battery really is?

How to Get “Good-Enough” Accuracy

The truth is that total accuracy is complicated, but ball park is pretty easy and close enough to monitor it so we get maximum life out of our battery. Here’s what you have to always remember about the voltage readouts on your batteries:

  1. A battery that’s being charged–power going in–will read falsely high. The bigger the charge going in, or, fuller the battery is, the further the voltage readout is from accurate.
  2. A battery that’s being drawn from–power going out–will read falsely low. And, the bigger the draw going out the more inaccurate the readout is.
  3. A well rested battery–which means hours without any activity  (nothing going in or out) is pretty darn accurate. The longer it’s rested, the more accurate it is. When rested overnight it will be basically correct, after a week it will be dead accurate.

Two extreme examples:

  1. A battery properly being charged at absorption voltage can read 14.8, but stop charging it and it will fairly quickly return to reality which is still about 12.8
  2. If my battery reads 13.2 when I turn on my microwave, the huge draw makes it very wrong, it might read 11.6-11.9. As soon as I turn it off it jumps right back to 12.7 or 12.8.
This is a friends battery bank of 8 golf carts. You better believe he watches the voltage closely so he doesn't have to spend $1000 any more often than he has to!

This is a friends battery bank of 8 golf carts. You better believe he watches the voltage closely so he doesn’t have to spend $1000 any more often than he has to!

Check the Batteries When they are Rested

Assuming you sleep at night with everything turned off, in the morning before the sun hits its it, the voltmeter is very accurate. But you might think to yourself, “I need to know more than once a day what the accurate voltage of my battery is.” And of course you’re right, once a day is no where near enough. But by being aware of when it’s wrong and when it’s right, you can learn to compare the two and learn to make a mental correction for it’s inaccuracies.

You can do that by watching it and learning how it works. Over time you can get a pretty good feel for what it really is by knowing what the volt meter says and what’s been going on with it. For most of us, that will be accurate enough.

For example, it’s most important to know your state of charge in the evening after you’ve been using it because that is when you are most at risk of drawing the battery down below the all-important 1/2 full mark at 12.2 volts. If you are close to 12.2, you want to stop using all electricity you possibly can to save your batteries.  Fortunately, there is a fairly easy way to do that. Here’s how:

  1. In the evening just before bedtime, write down the date and the voltage of the battery.
  2. Next, turn everything off so the absolute minimum amount of power is going in or out from the battery, preferably nothing.
  3. In the morning, before the sun hits the panel, write down the voltage.

By comparing the two voltages, you’ll know how far off it is on a typical night and by watching it over a period of time you’ll know what is safe at night and what is not. For example, if your voltmeter reads 12.1 at night before bed, and you check it the next morning when it is rested and it reads 12.3, you know that you can take the battery down to 12.1 at night because that is a false reading, it’s really 12.3. From then on, never let your battery go below 12.1 at night and it will be a happy battery.

Battery Monitors

It is possible to know exactly what your state of charge is by using a battery monitor like a  Bogart Trimetric but you will also have to set up a shunt for it to work. I’ve got to be honest with you and tell you I know nothing about either, because that level of work and information about my batteries simply does not interest me. Rather than spend $200 and a lot of time learning all that, I’d rather just buy another solar panel with that money so I wasn’t always right on the edge of running out–just the opposite, I want to have a system large enough I never have to monitor it closely because it never starts to run down.

My suggestion is you plan your system for the shortest day of the year on a rainy day. If your battery is okay then, it will be ecstatically happy the rest of the time!

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I've been a full-time VanDweller for 12 years and I love it. I hope to never live in a house again!

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