Monitoring battery state of charge (SoC)

Cheap battery volt meters don’t give you good insight into how full or empty your battery really is. Without knowing the true state of charge you can damage your battery and seriously shorten its life.

If you’ve priced out proper battery monitors, you might have experienced sticker shock. Although you can buy a LED volt meter for under $10, true battery monitors cost a lot more – often around $200. Why? And what’s so important about using a proper monitor?

If you look on Amazon or eBay you can easily find battery monitors under $10. The problem is, an $8 battery voltage monitor like the one in the photo above – even if it’s showing an accurate voltage – isn’t actually much use in seeing how full or empty your battery is.

That’s because the relationship between battery voltage and battery state of charge (SoC) isn’t linear. It’s affected by several things like how much load is on the battery, the battery temperature, and even the battery type.

Battery voltage versus charge

Battery voltage goes down as the battery gets discharged. Unfortunately though, it’s not a linear relationship. The voltage decreases slowly until the battery is pretty far gone, then it starts to drop much faster.

Volts at different state of charge
Volts at different state of charge – notice how it’s pretty constant until SoC drops to around 30% (your battery will have a different curve to this example)

You can see that for this example, the battery voltage would read above 12v all the way down to 20% SoC. The difference between 70% SoC and 30% SoC is so minimal that it’s hard to tell how much capacity the battery really has left just from the voltage reading.

Batteries also drop their voltage when a heavy load is applied to them, and recover when that load is removed. For instance, running a large inverter from your battery bank can cause a sag of one volt or more.

Voltage sag under load
Voltage sag under load. Pulling a lot of power from the battery will make the voltage drop temporarily.

If you were reading the meter while a large load was pulling the battery voltage down, you’d think it was at a much lower state of charge than it really is.

Hot batteries read a higher voltage than cold batteries. If the battery you are monitoring is in the engine compartment or a storage locker close to hot asphalt, then on a hot day the voltage may make you think it’s much fuller than it really is.

Cables have resistance, which lowers the voltage. If you plug your voltage meter in at some distance from the battery you may not be measuring the true battery voltage. Instead, you could be measuring a lower voltage due to the resistance in the cable between the battery and the monitor.

Another issue is that different types of battery have different voltages when they are full, and different discharge curves. So, a voltage monitor with discharge percentages shown on it might be designed for a wet cell lead acid battery, but would be completely inaccurate for an AGM or lithium battery.

All of this is to say, just measuring your battery’s voltage isn’t enough to know what its actual state of charge is, and whether it’s OK to keep using it or not.

The danger of discharging your battery too far

Batteries work by storing electrical energy through a chemical reaction. When the energy is needed again, the chemical reaction is reversed and electricity is produced.

During this process of transferring electricity to chemical storage and back again the components in the battery suffer some wear and tear.

The deeper a lead-acid battery is discharged, the more its lead plates get damaged, and so the fewer total cycles it’ll support. Deeper discharging hurts the battery proportionately more than shallow discharges. That’s why most people recommend not taking a lead-acid battery below 50% SoC.

If the worst happens and your battery gets completely discharged, it can suffer other kinds of damage. Leaving some types of batteries completely discharged for even a couple of days will cause them to sulphate. The lead reacts with the sulphuric acid and forms a coating on the plates inside the battery. It’s difficult for sulphated batteries to ever fully recover their storage potential.

Lithium batteries are able to withstand much bigger discharges – down to 30% SoC on a regular basis, and even 90% on occasion. But, if a lithium battery gets completely discharged, that can be the end of it. Past a certain point, the battery just becomes incapable of accepting charge any more and turns into a very expensive door stop.

Good battery monitors will include an alarm, or even a relay that can be used to shut off the loads on the battery or start a generator to recharge the battery before it gets dangerously discharged.

What the cheap monitors are missing – a shunt

So, if voltage isn’t a sufficient measure, what is? Batteries store energy (power). If you know the battery’s capacity and how much power has gone in and out of the battery, you can determine its current state of charge. Power is measured in Watts. To calculate Watts you need to know both Volts and Amps.

A shunt lets you measure both the voltage of the battery and also the number of Amps of current that are flowing to it (when it’s charging) or from it (when it’s being used).

Blue Sea Systems 8255 Shunt
Blue Sea Systems 8255 Shunt

The shunt normally attaches to the negative terminal of your battery, then all your loads and charging devices attach to the shunt. That way, the shunt is able to measure all of the power that is flowing to and from the battery.

Two smaller secondary connectors from the shunt connect to your battery monitor. They let the monitor measure the resistance across the shunt, and because Resistance=Volts/Amps the monitor can work out the Amps of current and from that the power in Watts in and out of your battery.

You can obviously find cheaper devices out there, but if they aren’t using a shunt, then it’s unlikely that they are accurately measuring SoC.

Battery monitor options

Here’s a list of some popular battery monitors that use shunts. We haven’t put prices on the page because they vary from store to store, but expect to pay between $150 and $250 for one of these monitors. That seems like a lot of money, but it’s protecting your batteries. If the monitor lets you get even an extra year’s life from your batteries because you end up treating them better, it’s probably paid for itself.

Some models are pretty much plug-and-play, whereas others are not at all easy to set up or reprogram. Some come with all the parts you’ll need. Others require you to buy additional fuses, cables, connectors or displays.

Some have bluetooth or wifi connectivity for use with phone apps. Some have one or more relays that can be used to (for instance) turn off some loads if the SoC gets too low or start a generator. Some have temperature sensors that allow the monitor to show a compensated SoC or even prevent lithium batteries from charging in sub-freezing weather. Whether you want or could use any of these options is entirely up to you. But you should at least be aware of them before you spend your money.

  • Victron Energy’s BMV 700 is one of the cheaper options and it has everything you need in one package (shunt and display) plus you can order a separate temperature sensor to improve its accuracy (Amazon link for pricing). The unit has a built-in relay that can be used to trigger alarms or open or close a circuit (generator remote start, for instance) depending upon certain battery criteria. The unit logs historical data and can also “talk” to other Victron products via a cable or to a cell phone app using an optional bluetooth dongle. Their BMV 702 will also monitor voltage on a second battery, or mid-point voltage on 6v golf cart batteries wired in series. The units are configurable for lithium batteries.
  • Bogart Engineering make the Trimetric monitors. These are well respected units. They log data for the last five charge/discharge cycles as well as displaying current SoC. Designed to be a stand-alone unit, they also interface with the Bogart solar charger. One complaint: They look really ugly and they rely on a segmented LED display rather than a more thought through LCD interface. Does NOT have an option for temperature compensation, but will monitor voltage on a second battery. Shunt not included.
  • Blue Sea’s M2 DC SoC meter (Amazon pricing) has an OLED display which looks super-cool. If it follows Blue Sea’s typical build quality it’s probably worth the cost. It measures current on one battery and voltage on two others. It has an external relay. Includes a shunt.
  • Xantrex make the LinkLite which works with their inverters but also as a stand-alone option. Relay, secondary bank voltage monitoring, includes 500A shunt. (Amazon pricing).
  • Clipper make the BM-1 (100A shunt) and BM-2 (200A shunt) which are the same shape and size as their other marine instruments. They also make a compact version. These monitors have clear displays but no built-in logic to trigger relays and no temperature compensation.
  • Magnum’s ME-BMK monitor isn’t really a monitor at all – for that price (Amazon pricing), you’re buying a shunt and a control box. The box links to a Magnum inverter to help it do its job, not to any display unit. To actually see what’s going on, you also need their ME-ARC50 advanced remote control (Amazon pricing). Thats fine if you have a Magnum inverter that you needed the remote for (you can’t program the inverter settings without the remote) but it won’t work if you’re planning on using this as a standalone product.

Which monitor to choose?

Make sure that the monitor you choose contains all the pieces you need. For instance some just include the display module and not the shunt. Temperature sensors typically also aren’t included by default. Magnum’s ME-BMK is just the shunt and a control box, with no display unit.

It’s also worth downloading and reading the manual for these devices before you buy one. Some are easier to install and configure than others. The quality of the manual may also be a good indication of the type of support that you’ll receive from the company.

This is a device that you’re going to want to glance at regularly, so it’ll probably end up on display somewhere in your van. Some have an interface that only an engineer could love. Some make it easy to cycle between different display options, whereas others seem to need a degree in programming.

Aesthetics are another consideration. Surface-mount units are going to stick out visually more than recessed flush-mount versions will. LED displays will glow all night long whereas LCDs often have a controllable backlight. Even the color of the display surround might play a role in your decision.

Other options

If you have a flooded lead acid battery, you can get a good indication of the state of charge by measuring the specific gravity of the electrolyte (the sulphuric acid) in the battery cells. There’s a special tool for this called a hydrometer. You suck some of the battery acid up into the tool as if you were using a turkey baster. The specific gravity (density) of the battery acid causes a float to settle at a particular height against a scale. That then tells you the state of your battery cells.

This obviously won’t work for AGM, Gel, or lithium batteries because they don’t have the right kind of electrolyte. Using a hydrometer also involves playing with acid and properly interpreting the reading from the float inside the glass tube. It’s a much cheaper solution than the battery meter, but it’s not as convenient or necessarily as accurate if you aren’t experienced at reading hydrometer scales.

There’s also another product that doesn’t use a shunt but claims the same level of accuracy as shunt-based devices.

  • Balmar make the SmartGauge. This monitor does not use a shunt, so it’s not measuring amps. It claims to use “computer modeling” to make it accurate. It compensates for temperature, but measures temperature at the device rather than at the battery. It is not designed for lithium batteries. It has a relay for alarms or to trigger other events and will measure voltage on a second battery. It seems to be quite low-featured for its price. (Amazon link for pricing).

Our choice

We ended up using the Victron BMV 700 because it fits with the other Victron components we are using and also because it’s a relatively nice looking, flush mounted, easy to configure product that comes with all the pieces you need to make it work.

Victron BMV-700 battery monitor
Victron BMV-700 battery monitor

We wrote briefly about it already. The price was also good compared to some of the other options out there. It comes with the shunt, a long data cable, and the fused wiring to connect its power source. There’s a built-in relay that can trigger based on many different events. You can buy an optional temperature probe to attach to the battery. It also has an optional bluetooth dongle so you can read the settings remotely, or it will interface via a cable with the Victron Color Control GX unit. The BMV 700 and BMV 702 (which reads voltage from a second battery) have become quite popular with people on the Sprinter Forum based on their good value and ease of use.

Have we missed a battery monitor option? Do you have experience with any of these monitors? If so, let us know in the comments. 

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