Cycle Charging: What is it?
We’re so glad you asked. Cycle charging is a term used in the microgrid space, most commonly for off-grid systems with storage and generators. It’s a management strategy in which excess power produced by a generator is used to charge the energy storage system.
How can cycle charging help the performance of my microgrid?
There are many benefits that can be found in a cycle charging approach. It allows for a generator to operate in the optimal loading range, minimize the number of run hours and lessens the likelihood of the generator needing to start multiple times a day. All of which saves on yearly maintenance costs and extends the life of the generator. Not to mention it can eliminate hours of noisy generator sounds for a community.
Rather than the generator “chasing” the system load, it can be run at its maximum efficiency setpoint. Over time, the saved fuel burn can provide significant economic returns.
Is cycle charging always done in the same way?
There are actually a couple of ways it can be approached. A generator can either run at full or near full capacity, using all excess power for charging. The result is a constant setpoint for the generators.
Alternatively, the generator can “follow” the load producing a certain amount of above the actual consumption. The result is a constant charge rate for the batteries.
Examples in the field
Compare the two photos below. The below picture comes from Ageto’s web-hosted dashboard for our minigrid project in Totota, Liberia. Here, load-following cycle charging is used to optimize the system.
These trend lines show two days of cycle charging. The solar production curve (green) is spiky, indicating a cloudy day of moderate production. While the energy storage state of charge (pink line) still climbs during the day, it is not enough to carry the load (orange) through the night and must be supplemented by the generators (red).
To contrast, this picture shows an off-grid resort in Colorado. The generators run at their maximum efficiency set point to recharge the batteries.
Is Cycle charging the right approach for my system?
A lot of systems can benefit from cycle charging. Some systems however are not sized optimally for the approach. If the storage is not large enough to carry the load for more than a couple hours, this is likely not the best approach. To avoid wear, generators are ideally not started more than once a day. An alternative approach may be to turn off the generators only when solar and storage can meet the load during the day.
Conclusion
For any off-grid system, it is ideal to be able to operate on solar power and energy storage all the time. However, with budget and size constraints (and with generators historically being present already) it does not make enough sense to operate an off-grid system without dispatchable generation source like generators. Using a cycle charging method can often provide the best of both worlds, allowing the system to operate with generators off as much as possible, without running the risk of power capacity shortages.
