Wire batteries in parallel
Computes current sharing as a real network analysis, not a rule of thumb. With four blocks and both take-offs at the same end, the first carries 77.9 A instead of 50 A — 93.5 % imbalance. A diagonal take-off makes that exactly 0 %. It also quantifies the years of life this costs the most loaded block.
The formulas behind the calculator
Every number above can be recomputed: the full calculation path, all assumptions and the data source with retrieval date — plus cross-validation against independent references. Disclosed, not claimed.
Safety-relevant calculation. This is an estimate based on the stated assumptions. The final design must be checked by a qualified electrician against the rules that apply where you are.
Data as of: 2026-06-15 · The technical review of this safety-relevant calculator is still outstanding.
| Step | Formula | Value | Provenance |
|---|---|---|---|
| Current imbalance | (I_max - I_min) / I_mean * 100 | 93.506 % | exact |
| Load factor of the busiest block | I_max / I_mean | 1.5584 | exact |
| Life of the busiest block | life_nom / f_load | 6.4167 a | assumed |
- Formula
I_i = I_total · (1/R_i) / Σ (1/R_j)- Valid for
- A real network analysis, not a rule of thumb: path resistances per wiring method (same-end / diagonal / busbar takeoff), current split by conductance, all three methods compared. Reference (4 blocks, 2 mΩ, 0.5 mΩ, 200 A): same-end 77.9/51.9/39.0/31.2 A = 93.5 % imbalance, diagonal exactly 0 %. Lifetime skew from the extra load (6.4 instead of 10 years for the first block). Assumptions: identical blocks, purely resistive, steady state. The Kirchhoff sum and diagonal symmetry are anchored as property tests.
- Not covered
- Resistance differences from ageing (they amplify the skew), dynamic behaviour on load steps, temperature drift of individual blocks, sizing of string fuses (DC fuse calculator) and cable cross-sections (battery cable calculator). The lifetime estimate is an order of magnitude, not a forecast.
- Data sources
- Manufacturer datasheets (Victron, Fronius, BYD) and IEC 61427-1, aggregated · retrieved 2026-06-15
Frequently asked questions
How do I wire several batteries in parallel so they share the load equally?
The take-off points decide it: with a diagonal take-off — positive at one end of the bank, negative at the other — current sharing is exactly symmetric, with 0 % imbalance. The calculator solves the sharing as a real network analysis, I_i = I_total · (1/R_i) / Σ(1/R_j), and compares all three wiring methods: same end, diagonal and busbar.
How bad is it to take both connections from the same end of the bank?
Far worse than the schematic suggests: with four blocks of 2 mOhm internal resistance and 0.5 mOhm per interconnect at 200 A, the first block carries 77.9 A instead of an even 50 A while the last carries only 31.2 A — 93.5 % imbalance. The most loaded block ages correspondingly faster and dictates when the whole bank must be replaced; the calculator quantifies the years of life lost.
Do I need a fuse on every parallel battery block?
Yes — for every parallel bank, from two blocks up. A short inside one block is fed by the rest of the bank, and battery fault current — unlike PV string current with its two-string exception — is not inherently limited; the marine standard ABYC E-11 requires overcurrent protection on every ungrounded conductor from the battery. The calculator therefore flags this at any block count; fuse sizing, including breaking capacity, lives in the DC fuse calculator.
Why does the first block get more current with same-end tapping?
Because its path to the load is shortest: current through block 1 passes fewer interconnect resistances than through block 4. Per the current divider, the shortest path carries the most — the first block delivers and charges hardest, runs warmer, ages faster. The bars show the distribution for your actual resistances.
What does diagonal wiring achieve concretely?
It makes all paths equal: tap plus at the first block, minus at the last — every current passes the same number of interconnects. Imbalance drops from double-digit percentages to near zero; the method bars show the difference for your case. Cost: one single longer cable. Few places in off-grid building buy so much lifetime for so little money.
From when is the central busbar the better choice?
From about four blocks or high currents: every block gets an equal-length cable to the central bar, the paths are identical by construction, and the bar distributes cleanly even if one block fails. Diagonal is the best compromise for two to four blocks; the busbar the optimum when space and budget allow — its sizing is the busbar calculator’s job.
How much faster does the most-loaded block age?
The calculator converts the current split into a lifetime skew: in the reference case (4 blocks, same-end takeoff, 2 mΩ block, 0.5 mΩ interconnect) the first block carries 39 % of the total current instead of 25 % — nominal 10 years become about 6.4 for it, i.e. 3.6 years lost. And since the weakest block dictates when the whole bank gets replaced, the entire bank pays that price. The nominal lifetime became an accessible input on 5 August 2026 (advanced options).
Where do I find my block's internal resistance?
On the datasheet — listed as “internal resistance” or “Ri”, typically 1–10 mΩ for LiFePO4 blocks, often stated at 50 % state of charge. The calculator deliberately guesses nothing here: this value directly controls how much the wiring matters relative to the block. Rule of thumb: the LOWER the internal resistance, the more unequal cable paths matter — modern LiFePO4 banks are more sensitive to same-end takeoff than old lead-acid banks.
Does the analysis also apply to charging?
Yes, with the sign reversed: charge current flows along the same unequal paths, so the first block also absorbs the most while charging and regularly sits fuller than the last. That amplifies the ageing skew, because the most-loaded block additionally dwells at a higher state of charge. The imbalance percentages are identical to discharging — the network analysis has no notion of current direction.
Why is the imbalance independent of total current?
Because it is a pure resistance ratio: each block takes the share its path resistance allows relative to the others — double the total current and all block currents double uniformly, the percentage stays. The total current decides whether the skew hurts in practice, though: 93.5 % imbalance at 20 A is cosmetic; at 200 A of inverter load it pushes the first block to its limit.
Does thicker cable fix the imbalance?
Only partially — it lowers the interconnect resistance and thus the imbalance, but not the root cause of unequal path lengths. The real lever is topology: a diagonal takeoff makes all paths exactly equal (0 % imbalance with identical blocks) and costs only moving one cable. A busbar with equal-length taps achieves the same and stays symmetric when more blocks are added later — thicker cable is the right knob only after that.