UPS Battery Strings in Parallel: Current Sharing and Protection

Two identical battery strings are connected to one UPS. Does each string carry half the current when the mains fails?

That is the design aim, but it is not something to assume from the wiring diagram. Parallel strings share a DC bus voltage. Their currents depend on the batteries and on the resistance of each complete path to that bus. If one string carries more than its share, its cells work harder during discharge and the bank may deliver less runtime than the simple arithmetic suggests.

This article concerns battery strings connected in parallel to one UPS. It does not describe two UPS units operating in parallel; that arrangement has different controls and protection requirements.

• Equal string voltage does not prove equal discharge current.
• Matching cable size and length is a starting point; the complete branch path matters.
• Plan each string's protection and isolation under the approved design.
• Measure each string current during a loaded test, not just the bank total.

Within a string, battery blocks are connected in series: their nominal voltages add, while the string's Ah rating remains that of one block. When two matching strings are paralleled, nominal DC voltage stays the same and their nominal Ah ratings add. For example, two approved 100 Ah strings would be described as a nominal 200 Ah bank at the same nominal string voltage. Here, approved means compatible with the UPS and battery manufacturers' requirements and the project's electrical design.

The UPS maker specifies the permitted battery voltage range, number of blocks in series, charging settings and connection arrangement. The battery maker's data determines how a proposed bank performs at the required discharge rate. A 200 Ah label alone cannot promise a 20-minute runtime. Our UPS battery runtime sizing guide shows how to move from measured load to the battery discharge table.

Picture two strings feeding a 100 A battery-side load. In an idealised calculation, each supplies 50 A. If one path has higher resistance or one string has aged differently, the split could be 60 A and 40 A instead. Those numbers are an illustration, not a predicted split for any particular bank.

Check the whole path, not just the interconnecting cable. Battery condition, state of charge, temperature, terminal joints, protective devices, busbar connections and cable resistance can all change the current a string supplies. Equal-sized cables of approximately equal length to a common connection point are a useful starting point; EnerSys installation guidance calls for this to promote load sharing. Positive and negative routes both count. A neat drawing with equal cable lengths will not compensate for a loose lug or an older string.

The strings also share the charger when mains power returns. A large external bank may recharge slowly if the UPS charger is the limiting factor. The UPS battery charging guide covers charger current and the available recharge window.

Where the UPS design includes a common battery breaker, it can isolate the UPS battery input but does not by itself let a technician isolate just one parallel string. The approved design should identify the disconnect and overcurrent protection for each string, the common DC connection and the UPS battery input. One Vertiv UPS manual explicitly requires a separate disconnecting device for each parallel string so that a string can be taken out for maintenance. The exact device and arrangement remain model-specific.

Protection must be selected for DC voltage, prospective battery fault current and the required interrupting duty. The designer also needs to check conductor rating, device coordination and the possibility that healthy strings feed a fault in another branch. A device with a suitable AC rating is not automatically suitable on a high-voltage battery circuit. Follow the UPS and battery manufacturers' drawings and have a qualified electrical designer confirm the scheme.

Isolation changes the available runtime. If a bank needs both strings to meet its discharge duty, opening one string for service does not leave the full original autonomy in place. The remaining branch may have to carry more current than its design case. State the required operating condition during maintenance before treating multiple strings as redundancy.

Ask the installer to record the battery model and age, series-block count, cable sizes and routes, protective-device ratings, connection torque, string voltages and string currents. Before parallel connection, follow the UPS and battery manufacturers' commissioning instructions and the project procedure to confirm polarity and acceptable voltage or state-of-charge difference. Connecting a lower-voltage string to charged parallel strings can cause a substantial equalisation current from the existing bank, even with no UPS load connected. Once the strings are connected and floating at the same voltage, the operating check is how they share charging and discharge current.

Do not parallel strings of different chemistry, rated capacity, model or age unless the UPS and battery makers approve the arrangement. A replacement string with the same nominal voltage is not automatically a match.

During a controlled loaded battery test, measure each string current, along with total DC current, UPS load and DC voltage. Repeat the current check during recharge. A total-current reading can look normal while one string contributes much less than the other. An unusual split calls for investigation of the battery and its connection path; it is not a cue to add more strings blindly. Thermal checks of accessible terminations and periodic records make later deterioration easier to spot.

For a bank in a separate cabinet, cable length and voltage drop deserve their own check. Our battery cabinet distance guide explains why the cabinet location cannot be chosen from floor space alone.

For an online UPS with external battery, send the UPS model, actual load in kW, required minutes, battery make and model, proposed series count and number of parallel strings. Add the cabinet layout, one-way cable routes, site temperature and required runtime if one string is isolated. TAFENG can then check the proposed configuration against the UPS battery input and charging options before discussing protection, cable layout and battery discharge data.

More strings can add stored energy. Whether they deliver the required minutes, share current acceptably and remain safe to service is a separate engineering check.

• EnerSys NSB UPS Quick Start Guide — parallel-string cable sizing and length.
• Parallel Strings — Parallel Universes (Battcon paper) — how current sharing can vary between strings.
• Vertiv Liebert S600D User Manual — a model-specific example of individual string isolation.

This article is for general technical reference. Battery wiring and protection must follow the approved UPS and battery documentation, the project design and applicable local requirements, with installation and verification by qualified personnel.