Handling devices with high surge or startup current draw on off-grid sites

Handling motor/pump startup surge demand is an important consideration in all off-grid systems, regardless of the battery type used. 

There are some additional considerations that apply using Redflow ZBM2 / ZCell batteries in particular.

By way of background:

It is common for the startup surge demand of an electric motor to be several times the continuous energy rating. In this context, 'electric motor' includes power tools, fans, air conditioners, and pumps. Anything with an inductive or otherwise high startup current draw.

An electric pump rated for (say) 2kW can easily need more than 6-8kW of surge power for a short period of time when it first starts up.

This surge demand is often not specified on the device and there is merit, where possible, in measuring the real surge draw of such devices as part of the design process.

Generally for 'on grid' systems,  surge energy demands when high torque devices start up are not a problem, because a grid-interactive energy system can simply use the grid to supply any 'surge' energy requirement over and above the output of the on-site batteries and inverter that are being used.

For all off-grid systems, however, the surge/startup demand of devices on the site must always be considered and accommodated in a system design as there is no AC grid to act as a 'catch all' source of additional surge energy demand.

The AC inverters used on the site must have a sufficient output rating to handle these surges.

Often AC inverters can handle surge energy demands for short periods quite well - but this surge capacity should (also) be verified to be appropriate as part of the system design.

Designs using lead-acid batteries can often accommodate quite high surge energy demands, however the voltage drop that occurs can still drive the AC inverter below its cutoff voltage if the battery is undersized.  

Repeated surge loading above the rated output of the battery has the potential to damage the battery (often in a manner not immediately detectable) and hence may shorten its operating life.

Lithium batteries typically have quite a high high surge output capability but this will be subject to a limit that the Lithium Battery Management System (BMS) enforces - creating the potential for the off-grid system to drop out if this upper limit is exceeded such that the BMS disconnects the battery pack for self-protection. 

For designs using Redflow ZBM2 / ZCell batteries, as with Lithium batteries, the peak output rate supported is also subject to upper operating limits that are enforced by the battery controller in each ZBM2 device. If these limits are exceeded, the battery will temporarily disconnect as a self-protection mechanism.

Importantly, for a design using multiple ZCells, there are various circumstances (including ZCell maintenance cycle requirements) in which the rated maximum 125A output current is not instantly available from all batteries at once.

A safe and suggested rule of thumb for off-grid operations is to plan around the circa 3kW continuous / 5kw peak output capability of a ZCell, multiplied by "n-1" for a site with n batteries. E.g. for a 3 battery site, plan on 6kW continuous and 10kW peak (not 9kW and 15kW respectively). 

In the presence of a transient / surge demand peak that might be above the continuous output rating for the number of ZCells on the site ,there are a few ways that this can be accommodated (other than simply installing more batteries - which is the technically preferable approach but obviously costs more).

 

The traditional approach - and applicable just as well with Redflow batteries - to resolving high motor startup-loads is a 'soft-starter' or 'VSD' (Variable-Speed Drive) motor controller.

 

This is a commonly available device that is added in the power path to a device with a high startup surge current. It usually works by slowly ramping up the AC frequency of the energy being sent to the device, so that the electric motor concerned is able to slowly start moving and then build up gently to full operating speed. These are really effective, but you require one per surge-causing device. 

 

If there are a lot of surge-causing devices, or if soft-starters are not appropriate for other reasons, there is another potential solution to consider - the use of an Ultracapacitor (also known as a Supercapacitor).

 

An Ultracapacitor can deliver a very high surge output current flow alongside ZCells, although for a relatively short period (think in terms of seconds, not minutes). The use of a single Ultracapacitor of suitable rating can support substantial startup surge current demands on a whole-of-site basis - allowing the site to handle surge loads from both current and future electrical devices on the site.

 

Important considerations when using Ultracapacitors

• Ultracapacitors can discharge very high currents, higher than any other battery of the same energy size so extreme care must be taken when connecting it or disconnecting it, and appropriate circuit breaker protection is required
• Ultracapacitors can gather and retain electrostatic energy from the environment when in storage pending installation, and hence special care is required during their initial installation. They are usually shipped with a small shorting connection across their terminals for this reason.
• Ultracapacitors are also very fragile in terms of overvoltage so extreme care should be used when connecting them to a DC bus to avoid exposing them to surges of voltage above their rated upper limit.

Suggested Ultracapacitor Hardware

 

An Ultracapacitor product that Redflow has found to work with our batteries is this one from Maxwell:

 

http://www.maxwell.com/products/ultracapacitors/56v-modules

 

It is a 56V rated, 130 Farad device. It is available from a variety of hardware vendors such as Digikey and Mouser.

 

Another unit with similar specifications (not yet tested by Redflow) is available from Eaton:

 

https://www.eaton.com/content/dam/eaton/products/electronic-components/resources/data-sheet/eaton-xlm-62-supercapacitor-module-data-sheet.pdf

 

It is important to use a 56 volt rated unit (or higher), as the charging environment for '48 volt' battery systems involves charging voltages routinely being in the high 50's. 

 

Interfacing the Ultracapacitor

The interfacing approach when adding in an Ultracapacitor is remarkably easy - just wire it in to the DC bus in parallel to the batteries on the bus (using appropriate DC circuit breaker for safety purposes). 

 

There is no software configuration to do, and no ongoing adjustments to make - like all capacitors, an Ultracapacitor simply charges or discharges automatically in an effort to equalise itself with the observed voltage that exists on its terminals. In effect it moves energy into or out of itself in an effort to slow down the rate of change in bus voltage that occurs on the bus over time. As a result, it acts to absorb high voltage surges and it also acts to support (offset) low voltage sags - automatically.

 

Be very careful with Ultracapacitors during installation, disconnection, storage, and removal

 

Ultracapacitors can retain a very high energy output capability when disconnected - and they can even absorb static electricity from the local environment if left in storage without their terminals being shorted out.

 

In summary - handle with care!