The plates and the grid alloy get all the engineering attention; the strip of metal joining one cell to the next rarely gets any. Yet an intercell connector is just as capable of costing you the string as a weak plate is — it can throttle the current a healthy bank is trying to deliver, waste real energy as heat at the exact moment you need every watt, or become the single point that fails under a fault current the cells themselves would have survived. A German stationary-battery maker’s own installation handbook treats connector sizing as a first-class engineering calculation, not an afterthought, and the arithmetic behind it is short enough to actually use.
Two families of connector
Flexible connectors, made from the same stranded, high-conductivity cable used for welding leads, carry more current at a given cross-section — and a lower voltage drop at the same current — than an equivalent rigid piece, because stranding packs more usable copper into the bend. They also do a mechanical job a rigid bar cannot: where cells sit at slightly different heights, or a rack crosshead flexes, or the whole row expands and contracts with temperature, a flexible connector absorbs the movement instead of transmitting it into the cell’s post seal. That is why inter-row and inter-tier connectors — the longer runs, more exposed to rack tolerance — are almost always the flexible type. Rigid connectors, ordinarily machined or bent copper bar, cost less and are easier to make in a non-standard cross-section, which is why they dominate the short, fixed, cell-to-cell joints within a single row. Either type is fitted last and tightened once during installation, which is exactly the moment our guide to battery commissioning asks you to record torque and take the first resistance reading — the only point in the bank’s life when a loose connector is cheap to catch.
The voltage-drop arithmetic
The voltage lost across any connector follows one relationship: drop equals length times current, divided by the product of the conductor’s conductivity and its cross-sectional area. For copper, that conductivity constant is a fixed physical number (about 56 metres per ohm-millimetre-squared) — which means the only variables an installer actually controls are the connector’s length and its cross-section. Make the run as short as the rack geometry allows, and size the cross-section to the discharge current you actually expect, not the smallest bar that happens to fit. The German handbook’s own worked reference — built from the equivalent German cabling standard, DIN VDE 0298-4 — gives continuous current limits for single-core, heat-resistant cable at thirty degrees Celsius ambient: about 98 A for a 16 mm² cross-section, 158 A at 35 mm², 198 A at 50 mm², 245 A at 70 mm², 292 A at 95 mm², and 391 A at 150 mm². Those figures are rated for continuous thirty-year duty, and the same source makes a genuinely useful point most buyers never hear: because a stationary battery typically discharges only four hundred to twelve hundred times across a ten-to-twenty-year service life, an occasional discharge can legitimately load the same connector four to five times harder than the continuous rating without over-stressing it. A connector sized purely off the continuous table, in other words, can be needlessly oversized for an installation that spends nearly all its life on float.
Heat is the practical limit
Ambient temperature changes everything in the table above — the same handbook lists a correction factor that drops a connector’s safe current by roughly a third between twenty and fifty-five degrees Celsius ambient — which matters directly for a battery room in a hot climate or an unventilated cabinet — one more reason ambient temperature belongs in the brief for our guide to battery room design, not just in the connector calculation. The connector’s own temperature rise during a discharge depends on the same variables again: its length, the discharge current, its cross-section and how long the discharge lasts, with the heating effect levelling off for discharges beyond about three hours because the connector reaches a rough thermal equilibrium with its surroundings. A connector that runs noticeably hot to the touch after a routine discharge is undersized for the duty it is actually being asked to do, whatever the catalogue said it was rated for — and it is worth remembering, from our guide to why one weak cell can drag down a whole string, that a hot or high-resistance connector can make a perfectly healthy cell look weak on a routine reading.
Copper or aluminium, and the joint between them
Large installations sometimes move from copper to aluminium busbar for the row-to-row or row-to-charger runs, purely on cost and weight — aluminium is lighter and cheaper per amp of capacity, at the price of a larger cross-section for the same current, because its conductivity is lower than copper’s. The complication is not the aluminium itself but the joint where it meets copper: aluminium forms an oxide skin within seconds of a fresh surface being exposed, that oxide is a poor conductor, and a copper-to-aluminium joint left unprotected is a textbook case of galvanic corrosion in the presence of any moisture — the same underlying mechanism our guide to battery terminal corrosion covers at the terminal post itself. The international compression-connector standard, IEC 61238, exists largely to make this joint properly — the right jointing compound, the right plating, the right compression tooling — rather than leaving two dissimilar metals bolted together and hoping. If a design uses aluminium anywhere in the run, ask specifically how the copper-aluminium transition is made and protected; it is the one detail in an otherwise copper-dominated bank worth double-checking.
Ask for the connector calculation, not just the connector
A battery bank’s cells and grid alloy usually get specified properly; the connectors joining them often do not. Before your next stationary or traction installation is finalised, ask us for the connector cross-section, material and expected voltage drop we have calculated for your discharge duty — a five-minute calculation now is cheaper than a hot connector, a voltage shortfall, or a corroding joint discovered on the day the bank is actually needed.