Managing Your Batteries

Intermediate

Inside this Article

Battery Bank
Battery Bank
A Hydrometer
A hydrometer with temperature correction is the most accurate way to determine battery cell state of charge.
Samlex Meter
A simple “gas gauge”-style meter, which converts voltage to reflect a “full-to-empty” reading.
A Shunt
A shunt is a precision resistor that provides accurate voltage drop measurements by which amperage is calculated.
Shunt Wiring Schematic
Always wire your meter’s shunt into the negative side of your battery bank.
OutBack Mate 3
The OutBack Mate 3, a system-integrated monitor, keeps tabs on the whole system, including battery SOC from the FlEXnetDC (next image).
Outback FlEXnetDC Meter
This system-integrated meter feeds the battery SOC data to the network monitor
PentaMetric Meter
A multifeatured stand-alone meter.
Battery Bank
A Hydrometer
Samlex Meter
A Shunt
Shunt Wiring Schematic
OutBack Mate 3
Outback FlEXnetDC Meter
PentaMetric Meter

Heavy and expensive batteries can be the weakest link in a renewable energy system. And if you abuse them, they can wear out fast. Here’s how to get the most out of your investment.

Take the initial cost of your battery bank, and divide by the number of years until it needs replacement. That’s your annual “battery bill.” If you can stretch battery life to eight or 10 years, the bill is minimal. If you ruin them in a year, that’s a big bill, and you probably were not paying much attention to them. Overcharging, undercharging, and high and low temperatures can all count as “abuse.”

Determining if your batteries are being used—or abused—is where battery monitoring systems come in. Professional RE installers and troubleshooters, and system owners alike, can analyze the stored data for clues as to what went wrong—or right—with a system. And battery-destroying problems, like loose connections or chronic undercharging, can be detected and nipped in the bud.

Accurately determining how full your batteries are (state of charge; SOC) is a complex problem. When you fill up your car with gasoline, all the fuel you pump into the tank stays there until you are ready to use it. Filling batteries is different—more akin to a having a small hole in the bottom of the gas tank. Batteries lose a little electrical “fuel” each day, even if unused—called “self-discharge.” Plus, a portion of the energy that’s pumped in never even gets stored, because battery-charging efficiency is never 100%. To further complicate matters, the faster the rate at which that stored energy is used, the smaller your tank becomes. Finally, temperature also affects battery storage capacity and longevity.

Specific Gravity

The most accurate way to determine SOC is to measure the electrolyte’s specific gravity for each battery cell. In a fully charged lead-acid battery, the electrolyte is a strong mix of sulfuric acid and water;  in a fully discharged battery, the mix is mostly water. A specific gravity measurement compares the electrolyte’s density to that of water at the same temperature.

A hydrometer is the standard tool used for measuring specific gravity and typically costs about $30. The denser the liquid (and thus the higher the SOC), the higher the float rides. Choose a hydrometer that includes a thermometer. Colder electrolyte is denser—without temperature compensation, a hydrometer will show an inaccurately high SOC. To use the device, first put on your protective eyewear, rubber gloves, and old clothes. Then, using the bulb, fill the hydrometer full of electrolyte. Record both the number showing at the liquid level and the temperature reading. Do this for every cell of every battery in the bank, then compute the SOC for each cell.

This is a time-consuming and potentially back-breaking task, with the prospect of a mess at any moment. Thankfully, there are simpler (and more automated) solutions available for battery monitoring. But make no mistake—a good hydrometer is an essential item in any battery tool kit. It provides the bottom line on determining battery SOC.

Monitoring by Voltage

The simplest and cheapest way to monitor a battery bank is by simply reading its voltage with an accurate voltmeter. A variety of products are available—some show the voltage reading directly; others using a “gas gauge” format, showing voltage on a scale from “full” to “empty.” Most cost less than $50.

However, this technique works only under certain conditions, and the range from empty to full covers only a short range of voltages, so accuracy is compromised. For voltage monitoring to accurately assess SOC, the battery bank must be “at rest” for at least two hours, with no energy moving in or out. A voltage reading during charging can confirm that a battery bank is full, but offers no other information. When the household is using energy from the battery, the voltage reading will be artificially low; when the battery is charging, it will be artificially high. For a PV system, checking battery voltage is best done during early morning—before loads are in use and before PV modules start sending energy to the batteries.

Comments (2)

dolph's picture

WHY, is there no mention of Nickel Iron batteries?. You would think that after the over whelming articles posted of the joy of using nickel iron batteries versus lead acid, you would have a multitude of nickel iron articles regarding them. But i don,t see this. I know they are expensive, I say we need a voltage step up system, from the 1.2v of the single ni/fe battery, up to 12v and 24v or 48v so small off grid systems can use just (one) or a couple ni/fe batteries to get the needed voltage, And at a high enough current as well. A modular device, with several step up phases.Then a few 500 or 1000 Amp hour batteries, can be used instead of the 20 to 40 needed for a large system. just step up the voltage to the inverter voltage. Is there any manufacturer, who makes such a devise already?.

Michael Welch's picture

Hi Dolph. While it is true that one of the reasons for multiple batteries is to attain the required voltage, multiple batteries are also needed to increase the total amount of energy available.

For example, your 1.2 V battery at 1,000 AH equals 1.2 kwh. That is not very much energy made available for loads, and not very much storage for solar or wind input.

Also, inverters can use huge amounts of power for surges, like when the fridge comes on or you first turn on a motor. A step-up device like you mention would have to handle hundreds or even thousands of amps at that low of a voltage; and that makes the device cost a lot more.

And don't forget that the lower the voltage, the greater the wiring losses, which means low-voltage systems need to use much more expensive cabling. This is the main reason why PV system designers use higher voltage systems. These days it is even rare to see them design off-grid systems at 12 V -- 24 V and and 48 V systems are much more common.

While I personally still hold out some hope for nickel iron batteries, most people will not use them. The initial cost is high, as you point out, but that is offset by their longevity. Another problem with NiFe batteries is that they have a very high self-discharge rate, which means it takes more energy to keep them topped off. On the good side, they can be discharged a lot more deeply than lead-acid batteries.

Unfortunately, most of the NiFe batteries that have been available in recent years were poorly made, with a lot of failures. With the manufacturers in China, failed batteries are hard to deal with.

I have heard that there are better NiFe batteries coming, and we shall see if the sellers and installers of off-grid systems like them enough to spec them in these systems.

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