Technical Info

Technical Info- BMS

At a glance, a battery appears just to be a box delivering electric power. Thanks to technical advances in the battery industry; this is all most battery owners will ever need to know. If a battery is properly installed and maintained, you may truly ‘fit & forget’.

However, someone that has a more demanding application will discover the more complex side of the battery. Even with a grasp of electrical engineering, chemistry and metallurgy; one may still get confused by half-truths and whole falsehoods concerning this everyday item. These users will hugely benefit from a deeper insight into the workings of the storage battery – which we are more than happy to provide.

On these drop down headers we will address the most common problems a battery owner might encounter, so please click away.

No cranking capacity
ExpandThe end of a starter battery’s service life is usually announced by poor cranking, especially at low temperatures. That, of course, could very well be caused by normal wear. A battery is subject to two processes of natural wear: corrosion of the (positive) grid and loss of active mass. These processes are taking place gradually, never suddenly. Unfortunately that is not to say that the symptoms are not from one day to the next: a good night-frost can make painfully clear what at higher temperatures remained unnoticed!

If the problem does occur prematurely it has most likely been caused by overcharging and/or excessive temperature, in which case it is important to check the alternator to stop it from happening again!

A second cause can be found in loss of active mass by deep discharge. No surprise, if the battery has insufficient capacity to meet the energy requirements of the vehicle. The same applies, of course, for starter batteries that do not receive sufficient charge. Short trips during which the dynamo cannot fully charge the battery will, eventually bring the battery into a permanent state of deep discharge which severely cuts battery life.

Another cause of battery failure is often overlooked: acid stratification. The electrolyte on a stratified battery concentrates on the bottom, causing the upper half of the cell to be deprived of it. The upper half of the cell is deprived of acid, which will limit plate activity, promote corrosion and reduce performance. Whilst the acid concentration is light on top, it is heavy at the bottom. Such a high acid concentration artificially raises the open circuit voltage. The battery appears fully charged but has a low CCA.

Driving habits rather than battery defect are often the cause of battery failure. A manufacturer of luxury cars reveals that of 400 car batteries returned under warranty, 200 are working well and have no problem. Low charge and acid stratification are the most common causes of the apparent failure. The car manufacturer says that the problem is more common on large luxury cars offering power-hungry auxiliary options, rather than on the more basic models.

No reserve capacity
ExpandThe most obvious explanation is that the battery has reached the end of its service life by normal wear, so it has shedded most of its active mass.

The service life of deep cycle batteries largely depends on the depth of discharge. In fact, the service life of a deep cycle battery is often determined at the moment of purchase. It is at that point, when making the calculation of power consumption and charging facilities, that the number of cycles to expect are decided upon. So it must be ensured that the right choice is made by the customer.

Deep discharge due to lack of capacity of the battery, charger or alternator may also have accelerated loss of active mass and cause short cycle life.

Another possible cause of premature battery failure is sulphatation. In general, sulphatation results from two factors: state of charge and the time the battery discharged. Sulphatation not only hampers the charging process, but it can also form large crystals which bridge between two plates and cause a short circuit. In the event of such a short circuit the battery voltage will rapidly increase when connected to a charger. The charger will see this as an indication that the battery is fully charged and cut off, leaving the battery empty.

ExpandWhy do some VRLA batteries bulge? Why do some VRLA batteries appear “sucked in”?

To prevent the permanent loss of gases – thus recombination has time to take place, each cell can hold approx. 1.6 pounds per square inch (psi) of pressure without venting. Batteries with very large cells – such as the BCI 4D, BCI8D, DIN250, JIS150, JIS200, GC, electric vehicle and scrubber types – will bulge somewhat as this normal pressure builds. This is especially true in higher temperatures, because the polypropylene case is pliable. Therefore, a certain amount of bulge is normal. If a battery bulges severely on charge, this is not normal. It is an indication of a blocked valve or an overcharge situation. If the charger works properly, such a battery should be removed from service.

A sucked-in appearance can also be normal.

A partial vacuum can form within a sealed battery under various circumstances. Battery temperature and ambient pressure play a role, but predominantly the recombination and discharge reactions are responsible. After charging ends, the recombination reaction continues until most of the oxygen in the battery is consumed. The total volume within the battery decreases slightly during a discharge. Deeply discharged batteries often have a “sucked-in” appearance. Batteries with large cells may display this appearance even when fully charged.

A sucked-in-battery should be charged, but if it remains sucked-in after charging, the appearance can safely be ignored. However, if only a single cell displays or lacks this appearance a load test would be prudent.

No charge acceptance
ExpandBattery does not charge.

If a battery, in spite of a properly functioning and connected charger does not charge, the battery has most likely fallen victim to sulphation.

Sulphatation results from two factors: state of charge and the amount of time the battery discharged. Sulphatation not only hampers the charging process, but it can also form large crystals which bridge between two plates and cause a short circuit. In the event of such a short circuit the battery voltage will rapidly increase when connected to a charger. The charger will see this as an indication that the battery is fully charged and cut off, leaving the battery empty.

Sulphation always comes with a permanent loss of capacity. Mild sulphation in a flooded battery can sometimes be mended using a suitable charger, but doing so is time-consuming and full recovery may not be expected.

Acid leakage
ExpandBattery is leaking acid.

Battery acid is very corrosive and can cause serious damage to its surroundings.

Flooded batteries should always be kept in an upright position to prevent acid leaking through the filling ports or, in the case of a closed battery, through the vent. VRLA batteries do not contain free acid, so they can easily be installed in a tilted position. If a battery is leaking along the lid without signs of external damage, a manufacturing defect must be assumed in the sealing of lid and container.

Acid may also find its way through the terminal seal. The professional term for this phenomenon is seepage.

Finally if there is acid spilling through the filling caps. This is the result of a too high electrolyte level. It is often forgotten that batteries must first be charged and topped up afterwards!

ExpandIf a battery would be discharged in a cool room with a constant temperature of 0°C, the battery would give only around 80% of its rated capacity and would be completely discharged when the specific gravity dropped to 1.140. Electrolyte of this S.G. would freeze at around -15°C, giving a margin of 15°C between the electrolyte temperature and its freezing point.

Similarly, at an electrolyte temperature of -20°C a battery would give only 50% of its rated capacity and have a fully discharged S.G. of about 1.180. At this level of S.G. freezing occurs at around -26°C; the margin between the electrolyte and its freezing point is 6°C.

This acts as an inbuilt protection against electrolyte freezing because the capacity that can be taken from a battery is reduced at low temperatures and the fall in S.G. is consequently less. This means that for any given electrolyte temperature the capacity which the battery delivers is not sufficient to freeze. It follows that the only time the electrolyte would freeze is when the battery is fully discharged and then left in a cold room for a lengthy period of time.

ExpandExploded battery.

A charged battery is a stored energy device and appropriate risk management needs to be used in handling because if the stored energy is released all at once, for instance when a tool is dropped on the terminals, this can cause a full short circuit.

When reaching full state of charge, any lead-acid battery will produce explosive oxyhydrogen gas, dissipating from the battery vents. Under those conditions any spark can cause an explosion, that not only will damage the battery but also disperse acid into the surroundings. Anyone close to the battery may be injured.

Most battery explosions take place at the moment of connecting and disconnecting cables or terminals. In warehouse conditions a battery must not be foil-wrapped immediately after charging. A static spark from the foil might detonate the gas, which may be emitted during – at least one hour after – full state of charge.

If a battery explodes at the moment the power is switched on (starter, bow thruster) it usually is the result of a too low electrolyte level caused by excessive charging or poor maintenance. When the battery plates are not completely submerged in the electrolyte, the high current can cause a spark between the plates, detonating the oxyhydrogen contained within the battery.

Grid corrosion
ExpandThe electrolyte inside a battery is not only in contact with the active mass at the plate, but also with the uncovered parts of the grid itself.

Grid corrosion is a normal phenomenon in a battery where the lead of the positive grid is converted into lead dioxide. As a result of this conversion the electrical conductivity and mechanical strength of the battery gradually decline until the plates collapse. This is an inevitable process which has been taken into account in the design.

Excessive grid corrosion is typically the result of structural overcharging or by means of too high charging voltage or too high a load factor. Also temperature is important, high temperatures will accelerate grid corrosion, while a moderate temperature will prolong service life.

Shedding of active material
ExpandThe active mass (paste) of the positive plate consists of lead sulphate. During charging this lead sulphate is converted into lead oxide, during discharge the lead oxide is converted back into lead sulphate.

Because lead sulphate has a larger molar volume than lead, the active material will shrink and swell with each cycle, weakening and shedding the active material and ultimately leading to loss of plate capacity.

Accelerated loss of active material, also known as a PCL or Premature Capacity Loss can be caused by deep discharge. Typical examples of this are often found in applications where starting or semi-traction batteries suffer from an inadequate loading regime. Because with each charge the battery will receive less energy than it delivered, it will at some stage operate on a discharge level that it was not designed for and end it’s service life much sooner than expected.

Typical of pre-mature mass loss by deep discharge is that, other than in a normal wear process, the active mass is disintegrated, whereas the grid is unaffected.

Terminal corrosion
ExpandCorrosion of the metal parts of a battery is the result of a chemical reaction between the terminal and the connections. There are three types of corrosion:

    1. Galvanic corrosion

Galvanic corrosion is caused by the potential difference between metals that come into contact with each other, in this case the material of the terminal and the connector. Corrosion generally comes as white lead or zinc crystals or, if the connections are made of aluminium, as aluminium sulphate. Bronze connectors will typically corrode with blue crystals. Very often, the corrosion shows a combination of white and blue crystals: white because of the lead in the connector clip and blue by the purchaser in the cable. This type of corrosion can be prevented by applying a terminal spray or Vaseline. If the corrosion has already set in, the terminals and connectors have to be cleaned first. Check for damage: a smooth, sleek surface is important for a good electrical conductivity.

    1. Electrolytic corrosion

If the battery contains too much electrolyte, because it has been topped up above the maximum level or in discharged condition. The battery’s acid may overflow and come into contact with the terminals and connectors, thus resulting in corrosion. This problem can be prevented by proper battery care.

Lesser known is that the electrolyte may also find its way to the outside through the terminal bushings. Batteries with a side terminal are more susceptible to this phenomenon called electrolyte creepage.

    1. Atmospheric corrosion

Another cause of terminal corrosion can be found in the acid vapor venting out of the battery when reaching full state of charge. This type of corrosion can be prevented by applying a terminal spray or a suitable grease such as Vaseline. If the corrosion has already set in, the terminals and connectors have to be cleaned first. Check for damage: a smooth, sleek surface is important for a good electrical conductivity.

ExpandWhen a battery discharges, lead and lead dioxide, which are the active materials on a battery’s plates, react with the sulphuric acid in the electrolyte to generate electrical current. A finely divided, amorphous form of lead sulphate is produced. During charging, the amorphous lead sulphate is easily converted back to lead, lead dioxide, and sulphuric acid, in essence returning the battery to its former state.

If a battery is not fully recharged soon after a deep discharge, the lead sulphate will crystallize. These large crystals clog the pores of the active mass and cover the plate surface so that charging becomes impossible. The result of sulphation is permanent loss of capacity. The sulphated active material of the positive plate is often light-coloured. A typical characteristic is the sulphate stripe at one-third of the height of the plate.

As the sulphation goes on, the active mass may be pressed out of the grid so forcefully, that the grids will bend! Charging a sulphated battery may cause dendrites to form at the negative plate. These sharp, needle shaped crystals can short circuit the positive and the negative plates.

ExpandIn a lead acid battery the electrolyte is a mixture of water and sulphuric acid. Stratification occurs when the water and acid separate, so that the heavy acid is concentrated on the bottom, causing the upper half of the cell to be acid poor. The upper part of the battery plate will sulphate because of depleted electrolyte, and the lower section will succumb to mass loss and grid corrosion due to overcharge!

Stratification occurs if a battery is kept at a state of charge of less than 80% and never has the opportunity to receive a full charge. That may be the result of self-discharge in combination with long-term storage, but also in situations like short distance driving while running windshield wipers and electric heater.

Stratification can be remedied by an equalization charge: the gassing induced by the high charging voltage will mix the electrolyte (not in VRLA).

Valve Regulated batteries contain immobilized electrolyte and as a result, this phenomenon will not occur in gel batteries. In AGM batteries stratification will only occur in very tall stationary batteries. These batteries are usually fitted in tilted position.

Thermal Runaway
ExpandThermal Runaway can best be described as a battery meltdown. It’s a phenomenon that in lead acid batteries only occurs in VRLA batteries.

The chemical reaction of the recombination process in a Valve Regulated battery is an exothermic process: it generates heat. When a battery is overcharged in high ambient temperature, the exothermic process will increase the temperature within the battery faster than can be dissipated. The increasing temperature will decrease the charging voltage and at the same time increase charging current. This will increase the battery temperature again and start a self-feeding heat/current cycle that will cause the battery to bulge and eventually melt. There is a risk of explosion by internal short circuit and the presence of large amount of oxyhydrogen.

Thermal Runaway is a problem that is first and foremost caused by the charger, not by the battery.

Where to find the serial number for my Victron Energy unit
ExpandFor smaller items such as the Bluepower chargers, DC/DC converters, MPPT charge controllers the serial number can be found labelled on the reverse of the unit. For larger units such as MultiPus & Quattro units, the serial number is on the inside of the unit behind the front panel. The front panel will need to be removed to access this.

How to update firmware on Victron Energy
ExpandFor SMART enabled items firmware can be updated through the VictronConnect app. For VE.Bus products updates can be done by downloading the VE Configure Tools software The free download from can be accessed via the Victron Energy website

The difference between SMART and Non-SMART Victron Energy products
ExpandWhen you see SMART related to a Victron Energy product, this indicates the item is Bluetooth enabled and can be monitored and configured through the VictronConnect app.

The difference between the Victron Energy BMV units
ExpandBMV700 – This is the base model for batteries in the 6.5 – 95 voltage range and capacities from 1 – 9999 Ah.

BMV702 – The 702 has the same features as the 700 with the addition of an auxiliary input. This can be used to measure the midpoint voltage of a second battery or temperature.

BMV712 – Like the 702, the 712 also has an auxiliary input but with the addition of Victron’s smart technology which implements Bluetooth connectivity.

Choosing the correct battery charger for your battery bank
ExpandWe suggest the battery charger should be sized between 10 & 20% of the battery capacity.  Example: For a 12V 100ah battery bank a suitable sized charger would be between 10A & 20A.

How to choose the correct size inverter capacity for your application
ExpandWhen sizing an inverter, you need to consider the peak power and as well as the continuous power. Peak power is the maximum power the inverter can supply which is usually no longer than a second. Appliances such as compressors, AC units & pumps have a greater peak start up requirement than running power. Continuous power is what the inverter supplies on a continuous basis. This could be the power required to run a microwave or all loads totalled up.

The difference between Isolated and Non-Isolated DC-DC converters
ExpandIsolated units have a separate, galvanically isolated input and output whilst the non-isolated units use a common negative return.  The isolated units are also adjustable in output voltage whereas the non-isolated units are fixed.

Which Victron Energy BMS would suite a lithium battery
ExpandFor use with a VE.Bus MultiPlus and Quattro in the system you would require the VE.Bus BMS as this has the VE.Bus interface. The miniBMS can replace the VE.BUS BMS in certain applications and similar to the VE.Bus BMS has two outputs.

How to choose the correct DC cable
ExpandUsing the correct size cable is important and can only be determined once you know the currents in the system. To size the correct size cable for your system you can:

– Check the Victron Energy product manual, they recommend cable sizes and the fuse size for their products in the manual/datasheet.

– Victron ToolKit App – This helps to calculate cable size and voltage drop. You can select Voltage, cable length, current, and cable cross section.