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).
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.