Batteries 101 - Part 2: Battery State of Charge

How To Determine Battery State Of Charge

The next question that begs an answer is "How do I calculate my battery's state of charge?"  There are 3 main methods that you can use. 1. Voltmeter; 2. Charge Meter; 3. Hydrometer

1. Voltmeter

The most common method for keeping track of your batteries state of charge is a voltmeter. You usually place it in a prominent location in the home so it will be noticed periodically.  Monitoring the voltage can be an accurate method of measuring the battery's state of charge, except for one important factor.  It is only accurate when the battery has been sitting idle for a period of time. You'll want it to be at least several hours, but preferably an entire day.

Whenever a battery is being discharged (i.e. you are using electricity), the voltage is artificially lowered.  And when the battery is being charged (i.e. running the generator or if solar panels are charging), the voltage is artificially raised. This throws off the "true" voltage reading and can render it inaccurate for the purposes of finding the battery's state of charge.

But since this method is often the easiest, here is my tip for getting the best results.  Simply check the voltage first thing in the morning before you have started using any power.  The refrigerator may have been periodically running through the night, so the results won't be precisely accurate, but it should hopefully get you close. Once you determine the accurate voltage, how does that relate to battery state of charge?  Most battery manufacturers have a chart that spells out what the state of charge is when the resting voltage is a given figure.  A common 50% voltage for deep cycle 12 volt batteries would be 12.1 volts (24.2 volts for a 24 volt battery).  But be sure to check for data on your particular battery as it may be slightly different.

A voltmeter is built into many renewable energy system monitors, discussed in more detail below.

2. Battery State of Charge Meter

Outback FlexNet DC Battery State of Charge Monitor

  Commonly found in off-grid homes, these meters use simple math to determine your battery's state of charge and read the result to you as a percentage (i.e. 60% means that the battery has 60% of it's capacity left, or that you have used 40% of it's capacity).  Initially, the capacity of your entire battery bank is programmed into the meter.  Then wires are connected to strategic points in the power system so as to measure any input and output from the batteries.  The batteries are fully charged, the meter is set at 100%, and the rest is history.  It sounds easy and accurate, but I have found this method to be the most prone to problems and inaccuracies.

Unfortunately, electricity is not like math with hard facts that can easily be precisely computed.  There are many variables that we never even think of.  The size of copper wires for a particular application, the efficiency of the inverter, the efficiency of the batteries, the temperature of the batteries, and a host of other factors all impact the accuracy of this method.  And the longer the period of time since the batteries were fully charged, the more inaccuracies there are to add up and really throw off the reading. Every time the batteries are fully charged, the meter resets, once again yielding accurate results for a period of time.

How to Get Fairly Accurate Results

While it is prone to inaccuracy, I still believe there is a place for battery state of charge meters.  They can be a great way to double check the voltage method.  Also, it's the easiest of all methods to use.  And when experimented with and properly set, it can be fairly accurate, as long as you always read the meter with a suspicious mind.  It's great to let a computer assist and add efficiency to a task, but never let it think for you!

Here is how I have been able to get fairly accurate results with my meter.  Fully charge the batteries and reset the meter to 100%.  Then go about your regular life for several days, checking the state of charge using method #1 and #3 until the batteries approach the 50% mark (this may only be possible during a dark time for those of you on solar, and may not be possible for those of you with a hydro system).  At this point, get a very accurate reading using method #3.  Let's say it turns out to be 55%.  Now go to your state of charge meter and adjust the variable until the meter reads 55%.  The variable will be the capacity of your battery bank (amp-hour capacity).

Adjusting Battery Capacity

Why should you have to adjust the stated capacity of the battery bank when the manufacturer says is a certain capacity?  Once again, it's all of those inefficiencies we were talking about earlier.  And not all batteries actually hold the amount of power claimed by the manufacturer.  After making this adjustment, do some spot checks periodically before putting too much stock into what the meter says.  If done carefully, with a skeptical eye, your meter can be reading acceptable results and make it easy for even non-technical members of the family to keep tabs on the batteries.

One More Point

One additional point I must make here is the battery temperature factor.  Our batteries reside in a room attached to our home, outside of the heated space.  While this room never freezes, it does stay fairly chilly during the winter months.  The impact this makes on our batteries is substantial, because the capacity of a lead-acid battery is significantly reduced when cold.  So if your battery temperature fluctuates substantially, you may have to adjust your state of charge meter with the changing of the seasons to get the most accurate results.  But if your battery stays at a fairly constant temperature, there is no need to worry about this.  See the section below on battery temperature for more details.

Battery state of charge meters are built into many renewable energy system monitors, which are discussed in more details below.

3. Hydrometer

Hydrometer for checking specific gravity

This is a device that looks much like a turkey baster. But it checks the specific gravity of the electrolyte inside each cell of the battery bank.  Inside the glass tube is a small vial that floats when electrolyte from a battery cell is suctioned into the glass tube.  The floating vial is indexed with numbers and whatever number is intersected by the electrolyte level, that is the specific gravity reading.  This is hands-down the most accurate method of checking your battery's state of charge.  The only factor I am aware of that could lessen it's accuracy is if the battery temperature is significantly higher or lower than the standard 77°F (see below section on battery temperature for more details).

Sample battery state of charge voltage and specific gravity readings. Note that your battery may have slightly different stats.

This is the method of choice for determining state of charge when approaching dangerously low battery levels or when there are any abnormal or uncertain readings from method #1 or #2.  If in doubt, use method #3.  The downside is that it is labor intensive, especially for larger battery banks with many cells.  To thoroughly check, each cell must be individually sampled.  Ideally, it would be good to keep a log of what each cell reads when checked, but this is not a necessity.

How often do I methodically check each cell of my battery? 

Usually once every month or two in the winter, and almost never in the summer (since my battery bank is fully recharged virtually every day during the long sunny days of summer).  However, when I approach the 50% mark in the winter months and am deciding whether to start the generator or not, I may sample a couple of battery cells just to spot check and make sure my other readings are accurate.

Since this only takes a minute or less, I don't mind doing that every week or two.  But for a full fledged thorough check of every cell, I only do that once every month or two.  The purpose of that monthly or bimonthly check is to look for variations in readings between cells. Then I can determine if I need to perform an equalize charge (or overcharge) to bring all the cells back into equilibrium.

Real Life Example

In everyday practice,  here is my typical sequence for keeping up with the battery state of charge.  I use the voltmeter (located on my system monitor) most frequently. It lets me know when I am getting close to 50%.  Then I start comparing the voltmeter with the state of charge meter (also located on my system monitor) to get a second opinion.  Finally, once these two factors let me know we are getting close, or if I am getting a discrepancy between these two methods, I step out to the battery room and take a quick sample of two cells with the hydrometer.  Then I know for sure what is going on.

Effect of Temperature on Battery Readings

Effect of temperature on specific gravity

The standard temperature is 77°F.  Any significant deviation from this will impact the battery voltage, specific gravity reading, and especially the overall capacity of the battery.

For example, a battery temperature of 50°F will artificially increase the specific gravity reading from a hydrometer by roughly 0.009.  For example, if my battery is currently at 50°F and is reading 1.210 on the hydrometer, I need to subtract 0.009 from that reading to obtain the true reading of 1.201 (rounded to 1.200).  This is the reading that may now be compared to state of charge charts.  The reason for this correction is because virtually all of the charts are designed for use at the standard temperature of 77°F.  Some hydrometers come with a tiny thermometer and a built in index that is to be used for making this correction, but I have found the index to yield figures that are significantly different than what the manufacturer of my battery states.  So go with the stats you can get from your battery maker or a comparable one.

Effect  of Temperature on Battery Voltage?

For the effect of temperature on voltage, I have not been able to find a good chart or formula that even remotely corresponds to my personal experience, so I don't know what to tell you except some rough observations on my part.  I find the voltage for a 12 volt battery bank at 45-50°F to be 0.1-0.2 volts higher (0.2-0.4 volts for a 24 volt system) than it would be at the standard 77°F.  So if my batteries are at that temperature and are reading 12.1 volts, I automatically assume it is more like 11.9 or 12.0 volts.

Effect of temperature on battery capacity

If a 24 volt battery bank at that same temperature is reading 24.2, I would assume it is really somewhere around 23.8-24.0 volts.  This gives me a figure I can compare with the state of charge chart and find an estimate for my battery's state of charge.  Once again, this is not scientific and may be different for your battery. This is simply based on my own personal observations.

Battery capacity is probably the most significantly impacted factor of these three.  As the chart demonstrates, a battery at 30°F only has 81% of it's rated capacity.  So while my battery may hold 1,690 amp-hours at 77°F, it may only hold 1,370 amp-hours at 30°F!  Those are actual numbers given by the manufacturer of my battery.  This is why we adjust our state of charge meter in the winter to derate the battery's capacity and arrive at a more accurate figure for the state of charge.

System Monitors

Thankfully, we have monitors available that provide a great deal of information to us in one convenient box.  For years, the industry standard was the Trimetric Meter by Bogart Engineering.  It works well, is fairly simple to operate, and was the best (or perhaps only) option available for quite a while.  But in the last several years there have been more options emerging from the companies that produce inverters (i.e. Outback, Magnum, etc).  It is a logical step to tie in all the meters and all the controls for the entire system into one unit.  I have no experience with these new-comers, but they look very interesting.

Whichever route you choose to go, there are several pieces of information your meter needs to display:

  • Voltage
  • State of charge meter
  • Amps or watts in and out (current amount of charging or discharging). One nice feature on the newer Trimetric Meters is the option of having it display in watts. This makes it much easier for me to quantify than using amps.
  • Amp-hours in and out (this is the basis for the state of charge function).

Nice to have features include:

  • Days since full charge
  • Days since equalize
  • Some sort of alert feature that flashes or somehow grabs your attention when the batteries need to be charged or equalized.
  • History of power in and out for previous days

Meters/controllers that are tied in with the inverter will also come with a host of other functions that relate to controlling the inverter and charger. So this list is referring specifically to items I would want to see in a system meter.

For More Details

If you are looking for more details on batteries and how to set up a cost effective off grid power system, we have a lot of really helpful training for you.  I think the best place to start is a free video series that everyone interested in off grid stuff should watch.  The first video in this series is all about batteries!  To get free access to this thorough yet simple training series, visit this page.

Nick Meissner

Nick Meissner’s adventure with homesteading and off-grid living began in the late '90s with a less-than-bare-bones budget. Over the past 12 years, Nick has taught thousands of people about renewable energy, homesteading, water systems, and independence in general. He's deeply in love with his beautiful wife Lisa and thoroughly enjoys their two children.

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