Batteries for an OSBB: how to assess capacity and avoid mistakes

Usable capacity is not nominal capacity

Nominal capacity is stated by the manufacturer. Usable capacity is lower: the system does not discharge the pack fully, the inverter has losses, available energy drops in the cold, and over the years the pack loses capacity. So the calculation for OSBB (condominium association) batteries goes from the required energy to the nominal capacity, not the other way around.

The platform engine divides the required energy, with a reserve, by the inverter efficiency, the permitted depth of discharge, a temperature correction and an aging margin. The typical values of these parameters in the settings are assumptions without a confirmed source, so they should be replaced with data from the datasheet of the chosen equipment. The battery capacity calculator shows every step and a list of missing data.

Battery chemistry: what to compare

Several types are considered for stationary systems: lead-acid, including gel and AGM, and lithium, notably LiFePO4. Specific cycle counts, weight and safety figures depend on the manufacturer, so they are taken from documentation. Here only a qualitative comparison of the criteria worth asking about is given.

A more detailed overview of LiFePO4 and a comparison of form factors is on the LiFePO4 page.

Elevators and pumps: why they are special

An elevator consumes differently depending on car load and direction of travel; there are regeneration modes. Pump groups run in cycles, and for them the duty factor and the number of starts per day matter. Inrush currents depend on the drive type: direct start, soft start or frequency converter, see the datasheet. A benchmark from market research: inrush currents can be one and a half to three times the running ones, but this does not replace the data of your equipment.

For the elevator, the mode of returning the car to a landing floor is considered separately: it needs less energy than regular operation, and the requirements for it are set by the manufacturer or maintenance organization. Without datasheet data for the drive, the calculation result carries the label "engineering calculation required".

Room, ventilation, safety

The control system, protections, ventilation and fire safety fall within the responsibility of the designer and the installation company. This material contains no installation instructions. For an OSBB the practical questions are: where the battery room will be, whether there is safe access to it, how it is separated from residential areas, and who holds the keys and the inspection log.

• Clarify in writing the manufacturer's requirements for temperature, ventilation and clearances.
• Ask which certificates and test protocols the supplier hands over.
• Agree with the insurer whether installing batteries affects the building's insurance terms.
• Decide who in the OSBB is appointed responsible for inspections and contact with service.

Questions for the supplier and acceptance

• Usable capacity and permitted depth of discharge: under what conditions are they stated?
• Communication protocol with the inverter: confirmed by the manufacturer or only promised?
• Warranty: in years, in cycles, in capacity loss, and who bears responsibility in a dispute?
• Possibility of expanding the system, restrictions on mixing packs of different ages.
• The procedure for disposal and return of spent packs.
• Service terms: call-out, response time, spare parts.

Life cycle: from calculation to replacement

The battery part lives longer than the warranty letter if there is a plan behind it. Plan at least three checkpoints: acceptance, the first annual inspection, and a decision on expansion or replacement when the available capacity approaches the threshold adopted in the calculation. The threshold is worth writing down in advance so that it is not decided under the pressure of an outage.

Think separately about financing: saving money for replacing packs is better built into the estimate and reporting than left until the moment of failure. The procedure for forming the estimate and designated contributions is described in the OSBB section.

Typical mistakes when planning capacity

• Calculating for the whole building instead of a list of critical consumers, and then being surprised by the volume.
• Using advertised values for depth of discharge instead of datasheet ones.
• Not accounting for winter conditions in the room, where the pack charges worse.
• Believing that a twenty percent margin covers inrush peaks: an energy margin and a power margin are different things.
• Not leaving a fixed reserve charge level if the system also saves on peak draws.
• Forgetting that residents expect light in all stairwells, although backup is planned for only some of them.

Documents and roles for the OSBB

For the battery part it is useful to have a separate package that passes from one board to the next. It should contain the pack datasheets, the acceptance act with the test discharge result, the room diagram, instructions for on-duty staff actions and service contacts. Without this, in a few years nobody will remember why the system is configured this way.

• Responsible person from the OSBB: receives reports, keeps the inspection log, organizes test switchovers.
• Service organization: performs inspections, software updates and pack replacement.
• Design engineer: refines the calculation if the composition of loads changes.
• Lawyer: monitors warranty terms and the responsibility of the parties.

Organize public reporting on funds spent by the rules of the fundraising section, and attach the technical acceptance results to the report.

Seasonality and operating mode

In winter, outages often last longer, and packs are colder and less efficient. In summer it is the opposite: room temperature may be high, which also affects life. So the same calculation is worth checking in two states: in the coldest and the warmest season, using the temperature data from the datasheet of the chosen equipment.

How to compare proposals for the battery part

Two proposals with the same nominal capacity can differ significantly in usable capacity, warranty terms and scope of work. So create a comparison table with the same rows for all bidders and require it to be filled in writing. Add the price columns last, so the technical differences are not lost behind the total.

What to do next

Make a list of critical consumers with datasheet data, calculate the capacity in the calculator, check the inverter, and only then compare proposals. For long outages, consider combining with other sources: this topic is covered in the backup power section. When the initiative is ready, register the organization.

In this section

• LiFePO4 batteries for an apartment building

  An overview of lithium iron phosphate batteries for an OSBB: BMS, voltage, temperature, cycles, room safety, supplier questions and an acceptance list.

• Energy storage systems for a building

  What an energy storage system for an OSBB is: components, modes, integration with panels, loads, limits and questions for the designer.

Frequently asked questions

Why is a battery's usable capacity lower than its nominal capacity?

Because the pack is not discharged fully, the inverter and low temperature add losses, and capacity declines over the years. The specific limits are stated by the manufacturer in the documentation.

Can I calculate elevator capacity from the motor power alone?

No. You need the drive datasheet data, the starting power and the real operating mode. Without them the result requires an engineering calculation.

Does the platform guarantee the number of hours of standalone operation?

No. The calculations are approximate, depend on the entered data and assumptions, and the final conclusions are given by the design engineer.

Next steps

Battery capacity calculatorBackup power calculatorRegister an OSBB