Measuring and Tracking The Specific Gravity of Lead-Acid Batteries
Keeping track of the health of your batteries is really important. One way you can help predict their eventual failure is by measuring and tracking the Specific Gravity (SG) of the electrolyte (acid) in your batteries.
For this job, you’ll need to have a couple tools. You want to have some distilled water, baking soda (in case there is an acid spill) and a hydrometer.
A hydrometer is a device that measures the density of a liquid relative to that of pure water. In the context of batteries, it measures the strength of the acid in them. As your batteries charge, the density of the electrolyte will increase.
Another factor that affects the density of the electrolyte in your batteries is temperature. As the temperature of the electrolyte increases, its density will decrease. So, when measuring the Specific Gravity (SG) of your batteries, you have to compensate for the temperature of the electrolyte by either using a hydrometer that has temperature compensation, or by using math (UGH! MATH!) to compensate for the readings you take.
I have two hydrometers. So should you. Having more than one gives you the ability to check your readings to make sure that your hydrometer is still working properly.
My preferred hydrometer is made by Midnite Solar. It’s the Hydrovolt. It’s a temperature compensating hydrometer, so I get accurate readings without the dreaded math. It costs more to purchase than a standard hydrometer. Hey, I’ll pay good cash monies to avoid math. Won’t you?
My other hydrometer is a non-compensating unit. I picked it up at an auto parts store. I keep it around just so I can occasionally double-check my readings when I don’t get the numbers I’m expecting.
On Your Marks…
This part of the process is really easy. First, make sure that your batteries are already full. Both full of water and full of charge. Ideally, once your batteries are fully charged, you should disconnect them from your system and allow them to rest for about an hour so that the electrolyte levels can balance out. However, in a real-world situation, that’s just not practical. So, just make sure the batteries are full of water and full of charge before you take your measurements.
Keep a log. Either in a notebook or in a spreadsheet. You want to keep track of the SG in every cell of every battery over time. I number my batteries 1-16, and each cell gets a letter designation. So the first cell of the first battery is 1A. The second is 1B, etc.
In my log, I record the date, time, ambient temperature, electrolyte temperature, SG and whether I am using a temperature compensated hydrometer or not. I also keep track of the voltage of each battery.
You can use this data to track the life cycle of your batteries and even catch when one battery or cell is underperforming and take preventative steps to either correct it or replace it.
This is the easy part. Take the cap off the battery and draw up an entire bulb of electrolyte from the cell you want to measure. Hold it up parallel to your eyes and level. On the Hydrovolt, the two wheels will turn and point directly to the SG reading.
On the standard type, you will look to see where the liquid level crosses the scale on the small bulb inside. This can be difficult to read. Keep in mind that it is not temperature compensated, either.
Record all these values in your log. Return the electrolyte you just drew up back to the cell that you got it from. You don’t want to mix the electrolyte between cells. It will throw off your future SG readings.
A fully charged cell with proper temperature compensation should read between 1.265 and 1.275. Lower values can indicate an insufficient charge or damage to your batteries. Higher values are also not great. That can indicate that you don’t have enough water in your batteries or that there is some other issue that needs to be addressed.
Note: Every manufacturer uses a slightly different formula for their electrolyte. So, make sure you consult your battery manufacturer to see what the proper SG reading for their batteries is when they are fully charged.
Clean Up Your Mess
When you’re done taking the measurements, you need to clean out your hydrometer. There will be a little bit of acid left in there. It can corrode the components of your hydrometer, and even cause incorrect readings in the future. Luckily, cleaning is really easy. Just draw up an entire bulb of clean, distilled water and gently swish it around in the hydrometer. Then squirt it out. I just squirt it on the ground.
That tiny amount of acid won’t hurt anything. However, if you’re concerned with the environmental impact, place it in a sealed glass container and contact your local government for proper disposal instructions.
If you’re not using a temperature compensating hydrometer (why?!), check out this page. It goes into the math of how to calculate the proper SG based on your readings and the temperature. This should be enough to convince you to buy a Hydrovolt. However, if you’re a math geek and want to save some money, go for it!
What to do Now?
If your SG readings are low across your entire battery bank, I’d suggest running an equalize (EQ) cycle on the entire bank. This is a controlled overcharging of the batteries and can often help boost low SG levels. If you have just one battery that is drastically low, it may be advisable to remove that single battery from the system and run an EQ on just that one battery. Of course, that would mean taking your battery bank offline for several hours.
If you still have one battery that is consistently underperforming, it may need to be replaced. If your battery bank is old (more than 1-2 years) It’s better to plan the replacement of the entire bank if you have a bad battery or two. Because of the way lead-acid batteries work, putting a new battery in with several old batteries will cause the new one to “age” quickly to match the others. So, if your bank is getting old, just do what you can to keep it running until you can replace the entire bank.
Also, consult the internet! I rely heavily on the knowledge base at the Northern Arizona Wind and Sun forum. Those people really know their stuff. While it can get a bit technical at times, most of the folks there are willing to slow it down and explain. After all, their goal is to educate you, right?
Wrapping it Up
Stay tuned for part three of this series, in which I go over the importance of keeping your battery terminals and cables clean. It can make a huge difference in the performance of your battery bank!