Frequently asked questions.

Battery System

What are the charging and discharging rate of TerraBattery?


The nominal charge/discharge rate is C5 (0.2C) i.e. in 5 hours. The system specs are based on this rate.


The optimal charge/discharge rate is C10 (0.1C) i.e. in 10 hours. The system is expected to output up to 10% more than the rated capacity.


The fast discharge rate is C1 (1C) i.e. complete discharge in 1 hour. The system efficiency will be affected up to 30%.

Charging characteristics

Float charge: 1.59 – 1.60 V/cell
Boost charge: 1.65 – 1.75 V/cell
Buffer operation: 1.60 – 1.65 V/cell


What are the effects of temperature on the efficiency?

The expected output capacity at +50ºC is 95% while at -40ºC is 50% of the rated.


What is the self-discharge rate of the battery?

The expected self-discharge rate is about 0.5% a day.


What are the life cycles against depth-of-discharge (DoD)?

The following numbers are bare minimum and expected are twice as much with optimal operation and maintenance:

2000 cycles at 100% DoD (~6 years at daily cycling)

2500 cycles at 80% DoD (~7 years at daily cycling)

5000 cycles at 60% DoD (~14 years at daily cycling)

7500 cycles at 50% DoD (~21 years at daily cycling)

23000 cycles at 20% DoD (~63 years at daily cycling)


Is half capacity charge-discharge considered as one cycle?

No, it will be considered as one cycle but lower depth-of-discharge. The lifetime will not be affected in practice.


What is the expected degradation on an annual basis?

The TerraBattery system, unlike others, does not lose capacity permanently over time due to surface reactions. With an in-house developed technique, the capacity can be restored close to its original capacity. The expected degradation is 1% per annum under optimal operation and maintenance due to miscellaneous factors. The degradation rate will be higher at a deeper depth of discharge, as high as 3%.


What is boost capacity? When and how do I use it?

The system will be clocked at a certain discharge capacity to achieve optimal performance and lifetime. This clocking can be removed up to a maximum of 7 times a year resulting in 2x the output capacity. It can only be done by TerraBattery installers/engineers with authorization for safety and warranty purposes.


What are the energy density and specific power of the TerraBattery?

Energy density

30-45 Wh/kg (depending on cell size)

Specific power

10-30 W/kg (depending on the system)


What is the voltage window?

1.00 V/cell to 1.75 V/cell 


How are you coping with the general disadvantages of Ni-Fe batteries with the TerraBattery?

High price

The price of the installed capacity per kWh is in line with li-ion while the levelized cost of storage (LCOS) is much cheaper as compared to both li-ion and lead-acid systems. The LCOS takes into consideration the total cost of ownership and the expected lifetime of the system. The expected minimum lifetime of TerraBattery is 20 years with proper maintenance. Theoretically, it can go up to 40 years.

Low energy density and specific power

The focus is on stationary applications and not the ones where density is the main criterion, such as for EVs or mobile solutions.

Slow charge/discharge time

The nominal cycle time is 5 hours which is faster than lead-acid and fast enough for renewables integration. The fastest rate possible is 1 hour at the cost of efficiency.

Discharge rate of 0.5% per day

The self-discharge rate is non-linear and if daily cycling is performed, it does not affect the capacity output. The optimal usage is daily cycling and not long term backup storage.


Unless the application is restricted by space e.g. EV the size and weight are reasonable for applications like stationary storage. Sustainable and safer alternatives like flow-batteries are up to 100 times less dense. The efficiency is in line with lead-acid systems and not as high as li-ion. Unless the efficiency loss is unacceptable in said application, the LCOS is cheaper than both technologies.



What are the measures taken for hydrogen safety?

Hydrogen gas is produced in low quantities during the charging of the battery. The system is designed for outdoor placement and to passively ventilate the hydrogen. For special cases when the system is installed indoors, an active ventilation unit will be included.


Does the system require a battery management system (BMS) for operation and safety?

Nickel-iron battery is inherently robust and safe. No special BMS is required for operation and safety while the inbuilt control system can perform all the necessary functions.



Does the battery need maintenance? How often?

With the current state of technology, the battery requires water refilling every three months. Soon with a hardware upgrade that we are developing, the maintenance will be reduced to once a year. Along with water refilling, system rejuvenation is required every 5-10 years.


Do you offer an automated water refilling system?

Yes, the auto watering system comes preinstalled with the system.


What are the maintenance packages?

There are two maintenance packages, standard and premium:

It is a self-service package. The task of water refilling must be performed by the user via the auto-watering system that comes pre-installed. Materials, documentation, training, and online support will be provided. Cleaning and rejuvenation of the system every 5-10 years will be performed by trained company professionals.

It is a company-service package. The task of water refilling must be performed by the company. This is applicable only for The Netherlands. International support will be added soon. Materials, documentation, and online support will be provided. Cleaning and rejuvenation of the system every 5-10 years will be performed by trained company professionals.


What about maintenance and rejuvenation of the system in case the company isn’t operating anymore?

Since the systems are expected to have a life expectancy of 20+ years. It is important to consider customer support if the company no longer operates in 20 years. 

In the case that the company isn’t operating anymore, a detailed document for maintenance and rejuvenation will be provided. This document will explain the process in detail which can be performed by a locally certified engineer.


Customer Support & Services

What is the expected return on investment (ROI)?

The ROI for any storage system is largely dependent on the current energy price being paid and the number of applications the system is used for. In order to calculate the ROI, case-specific information is required. Kindly contact us for more information.


What is the residual value at the end of life?

The expected residual value of the system (metals) is up to 15% (depends on the price of metals at the given time).


What guarantees/warranties are offered with TerraBattery?

10 years limited warranty for the rated capacity output. 3 years complete replacement warranty.


Will TerraBattery facilitate the removal/recycling of the system at the end of life?

System removal/recycling will be facilitated by TerraBattery or a TerraBattery certified local installer.


What does your service level agreement look like and what are its costs?

The maintenance can be divided into two parts: water refilling and rejuvenation routine. For the water refilling, an auto-watering system is preinstalled. The operation of the watering system once every 3 months can be performed by a local engineer or handler with training from TerraBattery. For the rejuvenation routine, the expected time is once every 10 years. This can only be performed by our engineers. Costs can be estimated as per the case.


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