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Battery Storage Knowledge Bank

Understanding Batteries

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Key components of a PV battery system

There are two main components in a battery storage system: the battery inverter / charger, and the battery itself. These are often ‘packaged’ together in one cabinet.  The battery inverter is only required for AC coupled systems (see Inverter/Chargers for more information).

Most batteries come with a Battery Management System which provides protection against overcharge and deep discharge, and through which parameters such as Depth of Discharge can be set.

In addition the system will contain cabling and some switchgear. It may also come with a back up distribution board or emergency power supply, as well as some sort of monitoring system.

Various options are available, as follows.

Batteries: Lead Acid vs Lithium Ion vs Aqueous Hybrid

Most batteries marketed for PV systems are either Lithium Ion or Lead Acid. Aqueous Hybrid batterieshave also recently come onto the market from the US.

The table summarises the key attributes of each type of battery.

Key Attributes of Different Battery Types: Lead Acid vs Lithium Ion vs Aqueous Hybrid

 

  Lead Acid Lithium Ion Salt Water (Aqueous Hybrid)
Maximum Depth of Discharge (i.e. usable capacity)* 50% 80% 100%
Number of Life Cycles ~1,100 life cycles at 50% Depth of Discharge.Note that the number of life cycles is heavily dependent on depth of discharged charge profile and temperature. 5000-10,000 life cycles at 80% Depth of Discharge ~3000 life cycles at 100% Depth of Discharge, ~15,000 life cycles at 20% Depth of Discharge
Tolerence of Partial State of Charge (SoC) Low.The fundamental lead acid chemistry contains many side reactions, such as negative plate sulfation and positive plate corrosion. Sulfation occurs when lead acid batteries are deprived of a full state of charge. This causes the formation of large sulfate crystals within the negative plates of the battery that is irreversible and results in permanent capacity loss. Good.Lithium ion batteries can be held at a partial state of charge, or discharged to a 20% SoC. Good.Aqueous Hybrid batteries can be held at a partial state of charge or fully discharged without damage.
  Lead Acid Lithium Ion Salt Water (Aqueous Hybrid)
Nominal Capacity Typically Available on the Market for combination with Solar PV 2kWh to 24kWh Megawatt(s) Megawatt(s)
Expected Life at ~250 to 300 discharge cycles per year 4-6 years 20 years 20 years
“End of Life” Capacity compared to Orignal Capacity After the specified number of life cycles, the battery will typically be guaranteed to operate at about 60% of original capacity. After the specified number of life cycles, the battery will typically be guaranteed to operate at about 60% to 70% of original capacity. After the specified number of life cycles, the battery will typically be guaranteed to operate at about 70% of original capacity.
Charging Rate Full re-charge within an hour or so, subject to charger sizing. Full re-charge within an hour or so, subject to the charger sizing. Discharge rate can usually exceed charge rate if required. Slow charge / discharge rate (typically 4 hours to charge fully).
Temperature tolerance Recommended operating range 10 to 25C.Lead acid batteries are highly affected by temperature. The lifetime of lead acid batteries is cut in half for every 10C rise in operating temperature over 25C, due to rapid increases in the corrosion rate of the internal components of the battery.  Higher temperatures also reduce charge rates. Optimum operating range 15C to 30C (some can operate 0C to 45C). Optimum range     -5C to 40C. Can operate up to 40C without reducing the cycle life of the battery.
  Lead Acid Lithium Ion Salt Water (Aqueous Hybrid)
Other features Low-cost storage solution with the expectation of battery replacement in 5 years.There are two types:-Vented lead-acid, requiring regular topping up with distilled water as well as good ventilation. Must be kept upright.-Valve regulated including gel batteries, sealed batteries requiring less ventilation and little maintenance. Integrated Battery Management System ensuring safe operation and battery monitoring. The BMS provides deep discharge protection, voltage and temperature monitoring, as well as the charge balance between the cells.Originally developed for laptops, mobile phones, cameras as well as electric vehicles. Battery chemistry is optimal for long-duration, daily cycling application including residential and commercial solar.The only Cradle to Cradle Certified™ energy storage product on the market, verifying the product for safety to human and environmental health and sustainable manufacturing.System requires no costly thermal management or regular maintenance.
Safety Good.Suitable for residential use. Note that lead is toxic so batteries need care in the event of leak or damage. Install and operate in accordance with manufacturer instructions. Battery must be decommissioned by a qualified installer and disposed of via a battery supplier or dealer authorised to dispose of the battery. Good to very good, depending on system choice.Safety ensured by Battery Management System. Suitable for residential and commercial use. Some hazard in the event of fire / thermal runaway, although the best batteries have multiple safety mechanisms via the Battery Management System; must be in installed and operated in accordance with manufacturer instructions. Battery must be decommissioned by a qualified installer and disposed of via a battery supplier or dealer authorised to dispose of the battery. Very good.Batteries contain a safe and environmentally friendly water-based electrolyte. Non-toxic, non-flammable, and non-explosive, with no risk of corrosion to components in close proximity to battery. Chemistry is inherently safe and not capable of thermal runaway. The batteries are also entirely touch safe.
Approximate size and weight taken by battery or batteries with 5kWh of usable capacity 52cm x 104cm x 25cm.300kg 40cm x 16cm x 70cm.60kg 95cm x 100cm x 30cm.340kg
Brands Victron, Enersys Sony, LG Chem, Tesla, BMZ, NeeoQube, Intensium, Tesvolt, Aquion

 

Nominal capacity, power, C-rate, depth of discharge and other parameters

There are several key parameters that need to be considered in comparing different batteries:

  • Nominal Capacity (Ah) and Discharge Current (A);
  • Nominal Capacity (kWh);
  • Power (W);
  • Depth of Discharge %;
  • C Rate (hours);
  • Usable Capacity (kWh);
  • Efficiency %;
  • Self-Discharge %.

Each of these is discussed below.

Nominal capacity (Ah) and discharge current (A)

Battery capacity shows how much energy the battery can “nominally” deliver from fully charged, under a certain set of discharge conditions.

The most relevant conditions are discharge current and operating temperature. Varying either of these can really impact performance, changing the capacity of the battery. See the example below.

Battery capacity is normally given in Ah (Amp-hours) at a certain discharge current (A). The higher the discharge current, the quicker the discharge and the lower the overall capacity (Ah).

Big Discharge Current = High Discharge Rate = Lower Overall Capacity

So for example, a lead acid battery might have a capacity of 600Ah at a discharge current of 6A.

With a higher discharge current, of say 40A, the capacity might fall to 400Ah.  In other words, by increasing the discharge current by a factor of about 7, the overall capacity of the battery has fallen by 33%.

It is very important to look at the capacity of the battery in Ah AND the discharge current in A.  As an alternative, look at the discharge rate or time (C Rate – see below).

Converting Ah to kWh

Electricity usage is billed in kWh. 1 kWh is the the electricity consumed by running a continuous load of 1000W for one hour.

The output of a solar system is also measured in kWh.

It is therefore helpful to know the capacity of a battery in kWh. This is worked out as follows:

Capacity in kWh = (Capacity in Ah x Operating Voltage (V)) / 1000

So if a battery has a nominal capacity of 500Ah and a nominal voltage of 12V, the overall nominal capacity in kWh is 500 * 12 = 6000Wh, or 6kWh.

Remember the battery only has this capacity when operating at the nominal discharge current 

Power (W)

The power output of the battery in Watts is given by Discharge current (A)* Voltage (V).

So if our 500Ah battery has an operating current of 20A and an operating voltage of 12V, then it has a power rating of 240W.

When sizing the system it is important to look at the likely power input (i.e. the excess solar power) and the required output of the battery, as well as the capacity of the battery in kWh.

The C-rate (hours)

Sometimes the battery specification may refer to the C-Rate or charge time (hours). The Nominal Capacity of the battery is given at this C Rate.

The discharge current can then be worked out from the C Rate and the Nominal Capacity.

For example if a battery has a C1 capacity of 400Ah, this means that when the battery is discharged in 1 hour, it has a capacity of 400Ah.  The discharge current would have to be 400A to discharge the battery in an hour.

If the battery has a C20 capacity of 600Ah, it means that when the battery is discharged in 20 hours, it has a capacity of 600Ah. The discharge current would have to be 30A to discharge the battery in 20 hours (600Ah / 20h).

To work out the discharge time (the “C Rate”) from the Nominal Capacity and the Discharge current, divide the Nominal Capacity by the Discharge Current.  This will give you the C Rate.  So if the Nominal Capacity is given as 400Ah at a discharge current of 25A, this equates to a C Rate of 16 hours (400Ah divided by 25A). In other words all parameters are relevant to a charge cycle of 16 hours. They should not be assumed to hold with a charge and discharge cycle of 4 hours.

When comparing different batteries, it is important to compare the Nominal Capacity of each battery at the same-C Rate – in other words compare the performance of the batteries over a similar charge / discharge time period.

It is also important to look at the performance of the battery over the time period relevant to the application. For most solar applications, 8 hours is a relevant charge / discharge time period.  So look at the Nominal Capacity at the C8 rate.  This will give you the discharge current required to discharge the battery over 8 hours.  From this current and the operating voltage you can work out the continuous power output of the battery over 8 hours.

Example

The following data is given on the Specification Sheet of the S30 Aquion Energy battery (Aqueous hybrid):

Operation & Performance 
Cycle life 3,000 cycles (to 70% retained capacity)
Operating temperature -5-40°C ambient
Voltage range 40.0-57.6 V
Nominal voltage 48 V
Continuous power 680 W
Peak power 800 W
Continuous current 17 A
Usable depth of discharge 100%
Round trip efficiency >90%

 

Embodied energy (Wh)

Charge duration (hr)

4 8 10 12 20
Discharge duration (hr) 4 1,264 1,520 1,600 1,647 1,803
8 1,335 1,672 1,780 1,848 2,055
12 1,389 1,775 1,900 1,982 2,217
20 1,569 2,046 2,200 2,307 2,603

 

To make use of the information it is important to understand the charge / discharge profile of our application. Note that the Specification Sheet does not give one Nominal Capacity of the battery.  Instead, the left hand table gives the capacity of the battery in either Wh (top table) or Ah (bottom table) at different charge / discharge durations.

Typically solar charges and discharges the battery over a period of 8 hours. Reading the 8 hour performance from the top table on the Specification Sheet, we can therefore  assume the battery has a capacity of 1672 Wh = 1.672kWh.  From the bottom table, this is equivalent to 37Ah, implying an average operating voltage of 45.2V (since 37 * 45 2=  1672). Note that the operating voltage of 45.2V is within the expected range given under the Operating Performance table on the right of the Specification Sheet.

The discharge current required to discharge 37Ah over 8 hours is 4.6A.  The discharge power will therefore be 209W (45.2 V * 4.6A).

So if we want to be able to power a 1.2kW load for 8 hours from these batteries when fully charged, we will need six of them.

Reducing the charge / discharge cycle length

If instead our application charges and discharges the battery over a shorter period of 4 hours, the battery capacity will be reduced to 1.264kWh, equivalent to 28.7 Ah at an operating voltage of 44V.  Our discharge current will be 28.7Ah / 4h = 7.2A, and our continuous power will be 315W.

Increasing the charge / discharge cycle length

The capacity of the battery goes up to 2.6kWh if we increase the charge / discharge cycle length to 20 hours.

As you can also see, Aqueous hybrid batteries are ideal for slow charge / discharge applications, as indeed are most lithium ion batteries.

Note that on the Specification Sheet, the Continuous Power of the battery is given as 680W, at a Continuous Current of 17A (and by implication, an operating voltage of 40V).  But if we run the battery at 680W,  the charge / discharge cycle will be reduced to well under 2 hours…. So the 680W is a maximum for a high power, short duration use.

Conclusion: It is important to get behind the numbers! We can do this for you.

Depth of discharge %

The Depth of Discharge (DoD) refers to how much energy is cycled into and out of the battery on a given cycle, expressed as a percentage of the total capacity of the battery.

Although this varies cycle to cycle, the maximum depth of discharge for lead acid batteries is typically at or below 50%. The cycle life of lead acid batteries is highly dependent on the State of Charge (SoC) that the battery is cycled at. Cycling that is done between 100% SoC and 50% SoC will last longer than the same depth of discharge cycling that is done between 70% SoC and 20% SoC. This is due to the crystalline structure changes that occur at the various states of charge.

In order to avoid the need to discharge lead acid batteries below 50% SoC, the nominal capacity of the batteries must be increased.  It is important to distinguish between the nominal capacity of the battery and the usable capacity of the battery, expressed as nominal capacity * maximum Depth of Discharge. Typically for lead acid batteries, the usable capacity = 50% of the nominal capacity.

Aqueous hybrid batteries can be cycled between any state of charge and to any depth of discharge without large impacts on the cycle life.  This reduces the need to over-size the system, it also adds flexibility to system operation. As you can see in the Aquion battery Specification Sheet above, the Depth of Discharge is given as 100%.

Lithium ion batteries can typically be discharged up to 80% before reaching a potentially harmful state of “deep discharge”.  They have a battery management system to prevent deep discharge (and over-charge).

Usable capacity and life cycles

By programming the system to limit the Depth of Discharge, the effective usable capacity of the system is limited.

Usable Capacity (Ah) = Depth of Discharge % * Nominal Capacity (Ah)

In general this will increase the number of available life cycles of the battery – the lower the programmed depth of discharge, the longer the battery will last.

System efficiency %

All batteries incur losses in the cycle of charge, storage and discharge.

The round trip efficiency of the combined charger and battery is usually in the order of 83%.

Self-discharge %

Batteries gradually lose charge over time. A typical lead acid battery will lose around 5% charge a month. Self-discharge rates are lower for lithium ion batteries, although the battery safety and control circuits incorporated into lithium ion battery banks do contribute to the standing losses.

Battery management system

Battery Management Systems are essential for Lithium Ion batteries. They perform several functions:

  • ensure safe system start up and shut down;
  • ensure the battery operates in optimum conditions, controlling voltage, current, and temperature;
  • indicate the available charge and discharge power and current;
  • balance the charge across cells;
  • provide information to the user about State of Charge (SoC) and State of Health (SoH) and operating parameters;
  • send alerts and if necessary put the battery into safe mode if certain critical thresholds (e.g. over-charge, deep discharge, operating temperature limits) are breached.

Cell monitoring and balancing

Cells in a cell assembly (stack) age differently depending on various factors. This ageing leads to capacity differences, variations in the internal resistance and different leakage currents, which in turn impacts the capacity of the cells and thus the voltages of each single cell. Improper charge states further reduce the life of the cells, with an ‘avalanche effect’ that has no remedy, unless cell monitoring and balancing is in place.

Cell monitoring and balancing systems can identify and correct continuously such differences in the stack. This process significantly increases the lifetime of the individual cells, but consumes power and thus increases the self-discharge in the stack.

There are three methods of balancing:

Passive Balancing:

Cells with an excessively high voltage are discharged by means of switchable or fixed bypass resistors.  The resulting electrical energy from the different states of charge is converted into heat and is lost.

Active uni-directional balancing (Power Pump or “charge pump” method):

The excess charge of a cell is “pumped” into the next with two transistor switches and a throttle. This process is also referred to as uni-directional active balancing, since it only allows controlled discharge of a cycle. The charge distribution of the cells is, however, only possible in one direction.

Active bi-directional balancing (TESVOLT):

In contrast to the uni-directional balancing each cell can be loaded from the other cells of a stack, or can be discharged in all other cells. This allows a fast and efficient balancing of the stack. This method is realized via bi-directional flyback converters, which are attached to each cell.

The bi-directional balancing system was developed and patented by Tesvolt. The BMS monitors the temperature, voltage and charge state of each cell and controls them in a cell assembly (stack). From the individually measured parameters the BMS determines the state of health (SoH), and the State of Charge (SoC) of each cell to allow the early detection of errors and to prevent damage. In addition it allows the charge of each cell to be defined, and seperates the charge of the affected cell from the other cells.

Bi-directional balancing is the best method, minimising self-discharge losses and reducing balancing time and power consumption.