Managing Your Batteries: Page 2 of 3

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

Counting Coulombs 

Using specific gravity to determine SOC is messy and time-consuming, and can’t easily be automated. Voltage readings are woefully inaccurate. So what’s left? Coulomb counting.

A coulomb is the amount of electric charge that is transported in 1 second by 1 amp. Devices that count and total coulombs are called amp-hour meters or watt-hour meters. A shunt—a high-power, precision resistor that is not affected by temperature—is used. The meter measures the voltage drop across the shunt and, using Ohm’s law, calculates the amps and/or watts going into or out of the battery bank. The meter also tallies how long this current is moving in either direction, giving you amp-hours or watt-hours.

More sophisticated amp-hour meters may use multiple shunts, so you can separately monitor your RE energy inputs—like from a PV array and a wind turbine. Many include a battery temperature sensor, which improves accuracy.

Coulomb counting is not infallible, as charging efficiency and self-discharge will both change as a battery ages. But the technique is very convenient and gives you a darned good estimation of battery SOC. Reading an amp-hour meter is so easy that anyone can learn to do it, and then start the backup generator before the SOC is low enough to cause battery damage.

Amp-hour meters are generally set up to display a simple “percent of full” reading, but they store much more data. When set to show amp-hours, the meter reads “0” when the batteries are full. As energy is used from the battery bank, the meter counts down in negative numbers; as the bank is charged, it counts up. A positive number indicates incoming energy that was not stored because the batteries were already full. Even the most basic of these meters stores some historical data, like the maximum depth of discharge since the last reset and the number of hours the battery bank was under a certain set critical voltage. This data can be used to troubleshoot a system or watch for problems.

Meter Choices

There are a variety of options available with amp-hour meters, including their ability to read multiple shunts, and their capability for having remote displays, data storage, computer interfaces, and even Internet monitoring via smartphones or websites.

Internal meters. If your battery-backup grid-tied PV system is simple, your battery monitor will be simple, too. Stored energy from your batteries is used only during a utility outage, and grid energy is usually used to quickly charge them again after power is restored. Some newer battery-based grid-tied inverters already have their own metering for the battery bank, which you can monitor through the system status display and built-in computer interface.

System-integrated meters. Many newer inverters and charge controllers can be networked together using special hubs and routers for monitoring. Battery monitors can be easily added to that network, so that all of the data from every device can be read on the system status display. This puts the entire system’s performance at your fingertips. Even wireless links to the monitor are possible.

In some cases, you’ll have to purchase extra equipment, like hubs and displays, for this networking capability. Also, the communications protocols used by equipment manufacturers are proprietary, so if your inverter, charge controller, and battery monitor are made by different companies, you’ll need  a laptop computer to integrate the data.

Stand-alone meters. These include the original amp-hour meters, and are the most versatile. No matter what sort of system you have, what resources provide your energy, or who designed your power system how long ago, you can monitor your battery bank with a stand-alone meter, and there are models that can monitor multiple shunts for more detailed data.

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.

Show or Hide All Comments

Advertisement

X
You may login with either your assigned username or your e-mail address.
The password field is case sensitive.
Loading