AC and DC augmentation in BESS: the differences between the two approaches 

AC augmentation 

AC augmentation connects new batteries up to the AC bus of the BESS system, i.e. after the inverter. This means that any new batteries added to the BESS need to be installed alongside new inverters and control systems and leaves the rest of the system unchanged. AC augmentation is essentially the process of adding an independent battery system to the existing BESS. This approach enables incremental sizing and carries a low risk of technical complications.  

AC augmentation is a simpler approach from a technical standpoint that doesn’t require extensive modifications to the existing BESS and is a lot less disruptive to the everyday operation of the BESS compared to DC augmentation. Even though it is simpler installation from a technical standpoint, it requires a lot of extra permitting and regulatory approval that can make the installation process slow and complicated from a regulatory standpoint.  

It should be noted that these challenges are for grid-tied BESS only. BESS that are independent from the grid don’t need to undergo such lengthy regulatory processes, which makes off-grid BESS and islanded microgrids well-suited for AC augmentation. For grid-tied BESS, AC augmentation is used more for utility scale expansion and for long-term projects that have plans to be scalable over time. 

The following table looks at the advantages and disadvantages of AC augmentation: 

Advantages	Disadvantages
Lower integration risk	Can impact interconnection agreements
Has a minimal interference with existing BESS operations	Sometime new interconnection studies are required, including a reassessment of AC capacity, short circuit analysis and grid stability studies
Is highly scalable with greater flexibility if further augmentation is needed in the future	More expensive (higher capital costs due to the extra PCS equipment and transformers)
AC augmentation increases both the power (MW) and energy (MWh) of the system	More regulatory complexity which slows down the augmentation process for developers
AC augmentation is easier to finance and has simpler warranties	Requires more space than DC augmentation
Because the augmented batteries are decoupled at the AC-level, battery chemistries can be added to the installation that are different than the original installed batteries	Has a slightly lower efficiency than DC augmentation due to extra power conversion stages

While the above advantages and disadvantages cover AC augmentation in BESS, AC augmentation can be implemented (and capacity added) at either medium voltage level―at the medium voltage collection bus―or at the low voltage level, which is at the low voltage bus before the medium voltage transformer (MVT). 

Augmenting on the low voltage bus looks a bit like DC augmentation from a layout and design perspective because the new battery enclosures are added to the existing bus. Low voltage AC augmentation can help to reduce some the regulatory requirements and interconnection impacts but it requires the integration of an AC enclosure that is compatible with the current electrical layout and controls. AC medium voltage augmentation, on the other hand, is as stated above where full PCS and MVT equipment needs to be added alongside the battery enclosures. The only requirement is that there is the capability to integrate newer PCSs, MVTs and battery enclosures into the existing BESS. 

The following table looks at the advantages and disadvantages of low voltage AC augmentation and medium voltage augmentation:

Low voltage AC augmentation		Medium voltage AC augmentation
Advantages	Disadvantages	Advantages	Disadvantages
Lowest CAPEX option as no PCS or MVTs need to be added to the installation	Can tie the owner to using an AC enclosure, making it less flexible to use different battery chemistries	The most flexible option that allows different technologies, battery container supplier and battery chemistries to be different the initial installation without issue	Higher CAPEX option due to cost of PCS and MVTs
Lower risk of triggering a Large Generator Interconnection Agreement (LGIA) re-study because no PCS needs to be added	If any additional equipment is need but not accounted for during in the installation permit, re-permitting might have to take place		Will likely impact the LGIA because PCS are always added
	If enclosures are added that include string PCS, LGIAs may be impacted anyway due to an increased PCS across the whole project

DC augmentation 

The other main type of augmentation is DC augmentation, of which there are few routes to achieve DC augmentation. Often called an ‘in-rack’ solution, DC augmentation involves adding battery modules/racks directly into the DC bus and behind the existing PCS/inverters. This approach uses the existing inverters and control systems in the BESS installation, making it a more cost-effective and space-saving option, and allows energy capacity to be added without changing the grid interface. 

However, DC augmentation requires the BESS to undergo some downtime because the installations are not a separate entity unlike batteries added by AC augmentation. While AC augmentation offers the chance to add the latest inverters to the newly installed parts of the BESS, BESS that undergo DC augmentation are limited by the existing inverters and there is less flexibility in the augmentation because the new batteries being installed must be compatible with the original installation. DC augmentation only increases the energy of the system, and not the power, as that is governed by the existing PCS, but the installation process is simpler, quicker, cheaper (no extra PCS required) and requires less regulatory approval than AC augmentation. DC augmentation also has fewer power conversion stages, so is slightly more efficient than AC augmentation. 

This means that that DC augmentation is best for sites that know they are going to need to augment to top up capacity in the future, and this future planning means that these compatibility issues are not as much of an issue. 

The following table looks at the advantages and disadvantages of DC augmentation:

Advantages	Disadvantages
No impact on interconnection agreements and unlikely to trigger new interconnection studies	Constrained by inverter limits meaning that only extra energy (MWh) is added and not power (MW) is added
Cheaper and faster than AC augmentation	Less flexible in designing and expanding because all new batteries must be compatible with the original installation
Lower cost than AC augmentation because only batteries are added to installation and not a lot of extra equipment	Causes downtime during the installation
Doesn’t change the grid interface, and avoids interconnection modifications, so is a much simpler installation that takes up less space than AC augmentation	Installation is more complex and many checks―such as voltage compatibility, BMS compatibility and PCS limits―need to be performed to ensure the system is suitable for DC augmentation
The sharing of a PCS leads to fewer power conversion stages, leading to a slightly higher efficiency than AC augmentation	PCS limitations in original installation can cause inverter saturation and clipping
Lower incremental cost per kWh	PCS limitations means that there is a limit to the number of augmentations that the system can undergo, leading to more restrictions over time
	The mixing of old and new batteries leads to more complex warranties and bankability concerns
	There’s the potential for safety concerns as newer cells have lower internal resistance and higher voltage profiles than aged cells, which can cause current imbalances, inefficient cycling, and accelerated wear on the new cells if they overcompensate for the lower performance of the older cells

There are few different ways of doing DC augmentation, that we cover below. These are: 

• Direct module replacement 
• DC direct 
• DC shuffling 

Direct module replacement 

Direct module replacement is the direct swapping of degraded modules with new (and compatible) battery modules that have a higher capacity, while using the same BMS, racks, and MVTs. Direct module replacement sometimes requires a DC-DC inverter/DC-DC converter to manage the modules as they can all have vastly different states of health (SOH). While replacing the degraded modules with new modules is the most common approach, modules can also be replaced with second life/recycled modules that are cheaper but don’t have as high capacity a brand-new module―so long as they are also compatible with the original installation. 

Here are the advantages and disadvantages of direct module replacement: 

Advantages	Disadvantages
No impact on LGIA or installation footprint as the original capacity and land are used	Still limited by the voltage level of the degraded modules (but this can be mitigated by DC-DC converters)
No need to relocate modules around the installation site	Labour intensive process
Doesn’t increase physical footprint of the installation	New modules need to have the same specifications and be compatible with the original modules

DC direct 

DC direct is the process of adding new battery racks or enclosures into the DC bus (alongside the existing racks and enclosures) instead of replacing degraded modules. This approach expands the total energy capacity and requires voltage control at the battery and enclosure level due to voltage imbalance on the DC bus, and it does expand the physical footprint of the installation compared to direct module replacement. Adding new battery racks/enclosures via DC direct augmentation requires there to be capacity available on both the DC bus bar and the PCS/MVT. If the new batteries being installed are different to the original batteries, then care needs to be taken to ensure that the new batteries are compatible with the existing systems’ controls. 

It can be challenging to add new battery racks and enclosures to the same DC bus a few years after the initial installation goes online as it is more difficult to synchronise the batteries for charging and discharging without the new batteries going offline. The overall operating voltage of all battery units can be reduced to protect new batteries from going offline but will slightly reduce the capacity of the new battery units. Within a few years of installation, these issues are not as prevalent.  

Here are the advantages and disadvantages of DC direct augmentation: 

Advantages	Disadvantages
So long as the new battery units are compatible with the existing batteries and their control systems, new battery units from different suppliers can be installed without issue	Newer enclosures may need to be shunted to the voltage limits of aged enclosures
	Not always a viable option after the BESS has been in operation for a few years
	Re-permitting may be needed if it wasn’t accounted for in the original permit

DC shuffling 

DC shuffling is the process of analysing and reconfiguring and/or relocating existing battery modules within a BESS DC architecture to optimise the performance of the overall system and extend battery life. The modules are reorganised and grouped based on their current voltage range profile and similarly degraded modules are placed together. This mitigates the impacts of the degradation because the modules operate at the same voltage rather than having one module limiting other modules.  

The shuffling process on its own does not constitute an augmentation process as no capacity has been added, it’s just that the existing capacity has been optimised to deliver a better performance based on similar voltage/degradation profiles. However, the shuffling process also frees up capacity in the MVT/PCS that allows new capacity to be added. This is where the augmentation takes place, and involves adding a new string of enclosures behind the PCS (from where the batteries shuffled from) to ensure that the overall system retains its power capacity. 

This approach prioritises the distribution of the existing energy in the BESS to ensure a more balanced distribution of energy through the cells and modules during charging and discharging. For grid connected systems, DC shuffling is one of the most suitable augmentation strategies, but auxiliary load breaker and busbar limitations may present technical challenges for implementing DC shuffling augmentation. As there are no new interconnections being made, DC shuffling can also bypass regulatory approvals. 

Overcoming voltage imbalances with DC-DC converters 

Adding a DC-DC converter can overcome the voltage imbalance that exists at the rack and enclosure level when new modules are used alongside old modules. DC-DC converters do not provide any level of augmentation on their own as they don’t add any new battery capacity, but they help to make new battery capacity usable with the existing batteries within the installation. There are two main DC-DC converters that are used to manage voltage imbalances: 

Centralised DC-DC converters: used when capacity is added to the DC bus and when a voltage imbalance exists between new and original enclosures. 

Rack-level DC-DC converters: used between battery racks in an enclosure and is used when modules in battery racks are substituted for new modules, with the DC-DC converter harmonising the voltage at the DC bus level before the PCS conversion. These converters also isolate the voltage impacts of degraded modules at the rack level, so only the battery modules with the highest degradation need to be replaced. 

The main DC-DC converters added because of augmentation are rack level DC-DC converters. They enable the switching of voltage, current, and power control modes to manage the outputs of battery modules regardless of their SOH. These converters enable both the new and original battery strings to operate at their optimal voltages to deliver power to a common DC bus―which is set to a fixed voltage by the main inverter. 

If DC-DC converters are not included in the original installation, space is needed to add the converters, and the controls for the converters need to be compatible with the original system. Using DC-DC converters mitigates the impact of moderately degraded modules, so that they don’t have to be replaced, and doesn’t impact the LGIA as they are not changing any capacity. However, adding DC-DC converters will increase the CAPEX if converters were not included in the original enclosures and there is space to add them. 

Overbuild Vs augmentation 

While overbuilding is a viable process for ensuring capacity longevity that removes the need to revisit the installation down the line, augmentation is widely seen as the more cost-efficient approach to maintain capacity over time. Overbuilding locks in the CAPEX at the point of installation but if the degradation doesn’t progress at the expected rate (lower than expected), there is the risk that the extra upfront CAPEX just contributes to stranded capacity that never gets used.

On the other hand, while it requires more time to determine what augmentation is best, and when you should augment, it means that extra capacity is only added when it is needed. This ensures that no money is wasted on unused capacity.  

Additionally, battery chemistries continue to get cheaper and more efficient, so there’s the chance that locking CAPEX in at the start loses out on new technological developments, whereas augmenting can make the most of it, meaning that companies may get more capacity for less money. Though, if you think battery prices will go up, it might make more sense to overbuild now.   

There are more complexities that need to be navigated when augmenting compared to overbuilding, including ensuring cross-compatibility between old and new batteries, but from a CAPEX perspective, it is the safer option to ensure that money isn’t wasted. Whichever route is chosen, forward planning is key to ensuring that the BESS maintains a usable capacity throughout its installation lifetime.