Battery monitoring – the fuel gauge for the battery

Battery monitoring is like the fuel gauge for the auxiliary battery. Only more complicated because it is more difficult to elicit from a battery how much and how long it will continue to deliver energy.

Would you go on a journey and start an adventure without a fuel gauge or a way to measure the level? Probably not. Much depends on this simple information. The situation is similar with the additional battery. Depending on the trip and adventure, it is very helpful to know exactly the state of charge and the remaining capacity. In summer, we can do without the auxiliary heating, but not without the contents of the cool box. In winter, it must be ensured that the heating also runs the following night reliably.

Unlike the fuel gauge, determining the actual remaining capacity of a battery is much more difficult. Numerous factors play a role: battery type, state of charge, age, temperature, discharging and charging cycles, and the amount of current drawn at a certain time. We can’t just keep track of that with a piece of paper and a watch; it requires sophisticated measurement technology. There are differences in this technology, depending on which influencing factors are taken into account by the battery monitoring system.

This article will explain what information is important to you, how battery monitoring is structured, and how an ideal monitoring system works.

The point of battery monitoring

At meetings and forums, problems with the lithium RV battery can be heard and read repeatedly. Then people ask about a better battery, and many answers say that they haven’t had any problems with this or that type of battery for years, etc. The charging technology, especially with double battery systems, is always a top topic. And in recent years, solar power supply has been used more and more in addition to shore power. The supply system can sometimes be quite complex. Clamp together or more complex charging technology? You know that. It is often forgotten that many factors determine the long life of the battery. This starts with choosing the right type of battery for the individual purpose and sufficient capacity, continues with the charging technology, and ends with the maintenance and care of the battery.

Of course, a consumer battery system should be reasonably dimensioned right from the start, but static values are used. You know how much energy your loads use when running, but what’s left out is how often they will run. How often will the heating, the cool box, or the light come on, and the smartphone batteries be recharged? No calculation can take this into account beforehand, and it behaves differently from a trip to trip.

Where do you get the experience you can then draw on for sizing and customization? Exactly such information provides you with a capable battery monitor. And once you’re on the road, it will help you to start the charging times in good time, be it by driving, solar, or shore power.

The battery monitoring makes certain loading and unloading processes visible and understandable. The information about charging and discharging currents gives more than just the display of the remaining capacity. Monitoring can detect power leaks, defective consumers, or chargers. It can provide information about whether the charge controller is correctly adapted to the battery type and thus help to optimize your solar panels system.

If your monitoring masters the “State of Health” (SOH) indication, it will even show you the inevitable aging of the battery. On the one hand, you can see when a battery is better replaced before it fails in use, and on the other hand, you can also determine whether the battery is being treated optimally. If it ages far too early, something is wrong in the system.

With the information from a good battery monitor, you can make the most of the system, improve it and identify problems. Anyone who likes to travel longer and as self-sufficiently as possible will appreciate this.

Essential information about a battery

Numerous terms, abbreviations, and values accompany a battery. You can already read a lot from the data sheets and the labels on the battery. In addition, some terms describe the battery’s condition during operation, which only becomes relevant later. For example, battery types suffer greatly from long storage and insufficient charging. Other battery types are insensitive to this. But let’s go one by one. Let’s first clarify the most important terms relating to rechargeable batteries.

Technical terms and their meaning

Rated capacity

The rated capacity indicates the amount of energy that the battery can store when new. Depending on the type of battery, it can be removed completely or only partially. Lead-acid starter batteries, which are still often used as consumer batteries, are determined at 25 degrees Celsius according to standard 50342. The capacity can be drawn at this temperature for 20 hours until the 12V battery has reached the end-of-charge voltage of 10.5 volts.

Nominal voltage

This average voltage value can be measured between the battery poles. All consumers in the vehicle are tuned to this value. Typical are 12 volt battery in passenger cars and 24V lithium ion battery in commercial vehicles.

It is important in this context that the value applies to both discharging and charging. Although a little more current has to flow into the battery when charging than it can later release again, a specification of C20 means that the battery needs 20 hours at maximum charging current until it is full again. The different C rates for discharging and charging are specified depending on the battery and the application.

Full cycle

The full cycle is a complete discharge followed by a full charge.

Number of cycles

The number of cycles indicates how often the battery can be fully charged again. It is a direct statement from the manufacturer about the possible service life.

State of Charge—SOC

The SOC indicates the state of charge of the battery. But beware, there can be significant differences here, depending on what the battery monitor considers 100% or “fully charged.” If this is a static value (nominal capacity), the specification deviates from reality as the service life increases.

State of Health—SOH

This value is rather new in battery monitoring but important. It indicates the actual maximum capacity of the battery. This capacity decreases over time as the battery inevitably ages. The SOH value describes this aging, which can progress faster or slower. This value is logically the basis for the SOC, which is then close to reality.

Depth of Discharge—DOD

This is the depth of discharge. DOD + SOC should result in 100% of the remaining capacity, which is still possible. A full discharge is 100%. There is a logarithmic relationship between DOD and battery life. The higher the DOD value, the shorter the number of cycles and thus the service life. In other words, the longer and more often a battery is fully charged and kept fully charged, the longer it will last.

Discharge voltage

A battery can be discharged down to this voltage. If the discharge continues and the voltage falls below this value, there is a risk of deep discharge and irreparable damage.

End-of-charge voltage / end-of-charge voltage

This value indicates the maximum voltage with which a battery can be charged. The main limiting factor is battery technology. The value should be specified or requested from the manufacturer.

What information is important

Now the question arises, what information should the battery monitor display? Everyone certainly has preferences. The simple user needs or would like less information than the inclined technology freak who wants to get the last little bit out of it.

Remaining capacity and remaining withdrawal time – SOC

The most important information is, of course, the remaining capacity and the time that consumers can still operate at a given load. The remaining capacity can refer to two different reference values. The nominal capacity is usually taken as the output capacity of 100%. If power is drawn, it is subtracted from it. The calculation also includes the consideration of the Peukert factor. This includes the level of the discharge current in the calculation. It is even more accurate if the State of Health (SOH) is used as the basis for the calculation. This value represents the aging and the decrease in the battery’s total capacity. This also makes the specification of the remaining capacity much more precise.

Battery Health Status – SOH

Battery ages treated properly or not. Knowing how large the maximum total capacity is is very helpful, as described, with the remaining capacity, but also to know when it is time to replace the battery.

If the battery monitor calculates the current drawn from the nominal value of the capacity, the indication of the state of charge or the remaining capacity is too high. Good systems also include the Peukert factor, which already makes the specification of the SOC more precise. Very good systems get to know the battery so well based on the initial discharge and charge cycles that they also include the aging (SOH). In other words, a very good system shows the SOC taking into account all accessible factors and the associated change in capacity.

Current draw

Not insignificant is the knowledge of the current power consumption. If you know how much electricity is currently being drawn, you can understand how much electricity you need in which situations and whether your consumer battery is correctly dimensioned. You can also see whether unwanted current is flowing (leakage current, defective device, etc.) and how high it is. Of course, you can see whether electricity is being stored or drawn when charging and discharging simultaneously, for example, with solar charger.


The voltage specification is also not entirely unimportant. When loading, it lets you know if it’s too low or too high. The former means a bad charge and probably a never fully charged battery. The latter can become dangerous if the battery starts to “boil” because the charging voltage is too high. Then it loses too much water, comes under pressure, and produces explosive hydrogen.

When discharging, you can track whether too high a load causes the voltage to collapse. When idle, i.e. (for some time) without a load, you can read the pole voltage (terminal voltage). For example, if it is below 10.5 volts in a lead-acid battery, it is defective.

Structure and components of a battery monitoring system

Since it is primarily about measuring discharging and charging currents, current measurement is a central part of any battery monitoring. With low currents of up to approx. 15 A, it would be sufficient to loop the measuring device into the circuit of all consumers. Incidentally, the right place for this is just before the ground pole of the battery or battery bank to be monitored. This is where all consumer and charging currents come together.

However, since the measuring range of normal ammeters is comparatively small, most monitoring systems have a special resistor, the shunt. It is usually connected to electronics that record all currents and their direction (charging/discharging). After a few discharge and charge cycles (synchronization), the battery’s current capacity can be determined. The nominal capacity is entered into the system statically so that conclusions can be drawn about the battery’s aging and the actual remaining capacity can be displayed.

In the picture, you can see a shunt, and across it, the electronics with the DEUTSCH connections of the bus system. Depending on the system, the displays, Bluetooth adapters, and alternators can be connected to this bus system.

How a shunt works

Current measuring devices usually only have a range of up to 10 to 15 amperes. If you want to measure larger currents, a trick is used. The voltage drop is measured at a resistor as small as possible to draw conclusions about the current. The resistance should be as low as possible so as not to reduce the current too much.

A resistor in an electrical circuit reduces the voltage depending on the current and the resistance. If the resistance and the voltage are known, the current can be determined using Ohm’s law: Current I = voltage U / resistance R. This principle is used with a shunt.

In this example, a voltage drop of 80 mV is measured at 12 volts and a shunt resistor of 0.5 mΩ. Using Ohm’s Law formula, this gives a current of 160 A.

Inaccuracies and drift

Battery monitoring can always be subject to deviations and inaccuracies. Mostly caused by currents below the detection limit and self-discharge. The detection limit is the minimum flow current to be recognized as a discharge current. As a rule of thumb, the maximum current allowed by the shunt can be used. The larger the maximum current that the shunt allows, the higher the detection limit. For example, if you have a small consumer, such as an LED on a USB charging connection, that allows 30 mA to flow, but the system only registers from 100 mA, there will be a deviation. However, these undetected discharges only lead to noticeable deviations the longer the vehicle is stationary, or the batteries are not charged.

The assumed Peukert factor also inevitably leads to small deviations since, in most cases, this is based on long experience and measured values and only forms an average value for each battery type.

Each full charge of the battery also resets the deviations. That is why the monitoring systems offer manual or automatic synchronization. When all parameters are reached, which means a full charge, the deviations are reset.

Connection types

In many cases, the onboard electronics in off-road and touring vehicles have been modified to a greater or lesser extent. In the simplest case, only the starter battery has to handle all the loads. Or there are one or more consumer batteries fed by the alternator, shore power, and solar. Battery monitoring must therefore be inserted at the right places in the installation.

The principle must be observed to record all currents, charging and discharging currents. This is the only way to make statements about residual capacities and aging as accurately as possible. The best place for this is usually in the ground return line of at least the batteries to be monitored. If other power sources can be used, for example, by connecting the starter battery, their ground must be routed via the measuring shunt.

The following shows some common constellations and how battery monitoring can be used there.

A starter battery

The simplest installation consists only of the starter battery. Here the measurement shunt is used just before the ground pole of the battery. So it can measure all incoming and outgoing currents.

Starter and consumer battery

One of the most common uses involves a consumer battery. In that case, there are separate ground lines, one back to the starter battery and one to the consumer battery. Since the currents of the consumer battery are of interest, the shunt belongs in their ground line. If the battery monitoring also allows for monitoring the voltage of the starter battery, the masses of the load and the starter battery must be combined on the load side of the shunt. The other side of the shunt then goes to the ground of the consumer battery.

Starter and consumer battery with charge controller or battery-to-battery charger (B2B loader)

Suppose the consumer battery is charged via a charge controller or similar, for example, a B2B loader. In that case, care must also be taken here that its mass runs through the shunt so that the charging currents can also be considered.

Consumer battery with solar controller

The installation with the solar controller is similar to that with the B2B loader. The difference is that the consumer circuit is routed via the solar controller, not the consumer battery. This means that the ground connection on the charging side of the solar controller must run through the shunt.

A system with ground shutdown

In our vehicles, we have switched off the circuits via the ground. This means that the vehicle’s electrical system, consumer network, or both can be switched off with one turn. Therefore, for battery monitoring, there is a good collection point for all currents behind the collection connection.

Battery type

The type of battery, for example, wet lead-acid (flooded lead-acid), AGM (with various sub-types), carbon foam, or LIFEPO (lithium iron phosphate). On the one hand, this setting sets some key parameters, such as end-of-charge voltage, charging current, or Peukert factor, which the monitor uses to recognize when the battery is fully charged and how the remaining capacity can be correctly calculated.

Rated capacity

This value tells the battery monitor how high the maximum capacity is. In the case of lead-acid batteries, this usually means the C20 value. This is the 100% value of a healthy battery and the starting value for the battery age calculation.

Charging Voltage and Taper Current

In particular, when several consumer batteries have been connected as a battery bank, it isn’t easy to determine the full charge. Slightly different states of charge between the individual batteries in a battery bank, as is always the case, lead to compensating currents so that the charge does not end in a manageable manner. In practical terms, however, the battery bank can be regarded as fully charged. These two values can be adjusted so that this is displayed accordingly via battery monitoring. They also help to reduce the deviations mentioned above.

These two parameters are populated with appropriate default values by selecting the battery type. If a full charge is never displayed, although the charging current has already dropped to the minimum value (usually 2-4% of the nominal capacity) or the charge controller is working in trickle charge mode, these values can be adjusted so that the monitoring then displays a full charge.

“Charge Voltage” should be slightly (please refer to the manual) below the measured voltage when the current “Taper Current” has been reached. “Taper Current” is the small current that still flows when the battery is fully charged. It is usually 2 to 4% of the nominal capacity. The battery (bank) is considered fully charged if both states apply.

This is usually not necessary with a single consumer battery.

The meaning of this value has already been discussed. If the battery manufacturer specifies a Peukert factor for its battery that deviates from the standard value for this battery type, it should be set accordingly. This calculates the remaining capacity much more accurately.


There is no longer just the one wet lead-acid battery that has accompanied us for decades. Today there are numerous battery types with different characteristics and requirements. This also leads to more complex battery technology. Modern, high-quality, and powerful batteries are many times more expensive than the classic starter battery. This investment should be protected as best it can.

In addition, the number of electrical consumers in the vehicle and on the road has increased. In addition to the lighting, there are coolers, heaters, smartphones and tablets, WLAN routers, camera batteries, and maybe the battery-powered milk frother. That’s why it’s good to know how long you can stand in a nice place before the batteries need to be recharged or whether your solar installation will provide the necessary energy to be able to heat the next night.

Battery monitoring will help you with all these topics. It should therefore show you at least the incoming and outgoing current, the remaining capacity, as an optional extra, the battery aging. A sensible investment for touring vehicles and long-term travelers.

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