Battery bank capacity and multi-bank wiring

Two questions come up constantly once someone moves past a single drop-in battery: how much capacity do I actually have, and how do I wire more than one pack together without creating a hazard.

Both have clean answers. The capacity one is arithmetic that people routinely get wrong by using the headline number instead of the usable one. The wiring one has a specific failure mode — parallel strings — that a surprising number of otherwise careful installations get wrong.

→ Size your pack against your own boat and runtime →

Part 1 — Computing real capacity

The four numbers you need

Everything below comes off the battery's datasheet. Read them once and the rest is arithmetic:

A note on the first two, because this is where most errors start. Nominal voltage is not charged voltage. A 16-cell LiFePO₄ pack is 51.2 V nominal and about 57 V fully charged. Energy computed at the charged figure overstates the pack by roughly 11%. Use nominal.

Rated capacity is quoted at a specific discharge rate, usually something gentle like 0.2C or 0.5C. Propulsion draws harder than that, and real capacity falls as the rate rises. The datasheet will normally give a small table or a derating curve; if it does not, ask, because at propulsion currents the difference is not a rounding error.

The arithmetic

Energy in kilowatt-hours is the product of nominal voltage and amp-hours:

kWh = (V_nom × Ah) ÷ 1000

So a 51.2 V, 200 Ah pack is 10.24 kWh nominal. That is not what you can use. Apply depth of discharge:

Usable kWh = kWh_nominal × DoD

At 80% DoD that same pack gives 8.19 kWh usable. Going the other way — you know the energy you need and want the amp-hours — rearrange:

Ah = (kWh_required × 1000) ÷ V_nom

The direction of that last one matters. Amp-hours are meaningless without a voltage attached. "I need 400 Ah" is not a specification; "400 Ah at 51.2 V" is. The same energy is 800 Ah at 25.6 V or 200 Ah at 102.4 V, and the cable and fusing consequences of those three choices are wildly different.

Where the vendor-specific numbers live

Integrated battery systems — Victron's NG line, MG Energy Systems' modular packs, and the others that compete with them — change nothing about the arithmetic above. What they change is where you read the numbers and what enforces them.

Whichever system you are specifying, pull these four from the current datasheets rather than from any guide, this one included:

1. Per-battery or per-module nominal voltage and rated Ah — these give you kWh per unit via the formula above.
2. Maximum number in series, and maximum in parallel. Both are hard limits set by the manufacturer, they differ between models, and they sometimes differ between firmware revisions of the same model. Exceeding either voids support and can exceed the BMS's measurement range.
3. BMS continuous and peak current rating. This is the one that bites on propulsion. A bank can hold plenty of energy and still be unable to deliver your continuous DC current, because the BMS current path is rated well below what the cells could supply.
4. Whether the BMS is per-battery or per-bank. This determines whether you are allowed to parallel at all, and how balancing behaves across the bank.

Point four is the real divide, and it separates the two architectures you will actually be choosing between.

Pattern A — bank-level integrated BMS

A drop-in battery with its own internal BMS, or a busbar-integrated BMS supervising a set of matched batteries. Victron's NG line is the familiar example.

The vendor has done the combination engineering for you within its stated limits, which is the attraction. The consequences:

• Series is often restricted or forbidden. Where each battery has its own internal contactor, one unit opening leaves the full bank voltage across it. Many manufacturers prohibit series for exactly this reason. Check before you buy, not after.
• Parallel is usually the supported growth path, up to a stated maximum count.
• The current ceiling is the vendor's, fixed by the integrated contactor and shunt. You cannot specify around it — if your continuous DC current exceeds it, you need a different product or a higher bus voltage.

Pattern B — modules plus a separate master BMS

A stack of battery modules supervised by one external BMS that sees every cell in the string and drives contactors you specify. MG Energy Systems' packs with their master BMS units are a common example on propulsion builds, and the pattern is also what you get from prismatic cells with a REC, Orion or Batrium BMS.

• Series is the normal way to build voltage. One BMS supervises the whole string, so there is no question of one module's contactor opening against the others. This is why the architecture turns up on 96 V and higher propulsion systems where Pattern A often cannot reach.
• Capacity follows the module arithmetic directly: modules in series multiply voltage and leave Ah alone; a second parallel string doubles Ah. Whether the master BMS supports more than one string, and how it expects them to be fused and balanced, is a datasheet question with a real answer — ask it.
• The current ceiling is partly yours to choose, because the contactor and fuse are external components you specify. That is more freedom and more responsibility: the BMS will command a contactor you selected, and the fusing rules in Part 2 are yours to get right.

Neither pattern is better in the abstract. Pattern A is less work and less rope; Pattern B scales to higher voltages and larger packs and expects you to act as the system integrator.

Check your BMS current rating against the configurator's DC continuous current output before you commit to a bank layout. That figure, not the kWh, is what disqualifies most otherwise sensible plans.

Part 2 — Connecting multiple banks

Series: more volts, same amp-hours

Wiring packs in series adds their voltages and leaves capacity unchanged. Two 51.2 V 200 Ah packs in series give 102.4 V at 200 Ah — the same energy as the parallel arrangement, delivered at half the current.

That halved current is the entire argument for series. Every cable, fuse, busbar and contactor downstream gets easier. Against that:

• Every pack carries the full bank current, so the weakest unit limits the whole string.
• A series string of packs each with its own internal BMS is risky unless the manufacturer explicitly supports it. When one BMS opens its contactor, the others are still connected, and the full bank voltage lands across the opened device. Some units tolerate this; many do not. This is a question to answer from the datasheet before you buy, not after.
• Voltage class jumps have regulatory and practical consequences: above 50–60 V DC, installation, labelling, isolation-monitoring and disconnect requirements tighten.

Parallel: more amp-hours, same volts

Parallel adds capacity at constant voltage. It is the natural way to grow a bank you already own, and it has the failure mode worth knowing about.

Each string needs its own fuse. This is the rule in the diagram above and it is the one most often missed. The reasoning:

A single fuse at the busbar protects the load side from the bank. It does nothing about a fault inside one of the strings. If pack B develops an internal short, pack A sees it as a near short circuit and delivers its full prospective fault current into it — and that current never passes through the busbar fuse, because the busbar fuse is downstream of both. The only device that can interrupt it is a fuse in pack A's own string. With two strings you need two; with four, four.

Beyond fusing, three things decide whether a parallel bank behaves:

• Match the packs. Same chemistry, same capacity, same age, and ideally the same batch. Paralleling a new pack onto a three-year-old one means the new one carries a disproportionate share of every load and ages faster.
• Bring them to the same state of charge before connecting. Two packs at different SoC will equalise through whatever cable you just connected, at whatever current the voltage difference and the cable resistance allow. That can be thousands of amps for a moment. Charge both to the same point first, then connect.
• Make the connections symmetrical. If pack A is connected with 0.5 m of cable and pack B with 2 m, pack A carries noticeably more current and runs hotter for the life of the installation. Take the positive from one end of the bank and the negative from the other — diagonal, often called Kelvin connection — or run every string to a common busbar with identical cable lengths. Not approximately identical. Identical.

Isolated: separate banks, controlled transfer

The third option is to not combine them at all. A propulsion bank and a house bank stay electrically separate, with a DC-DC converter moving energy between them in one direction under control.

This costs you the ability to pool capacity, and buys:

• No shared fault path. A fault in one bank cannot propagate into the other.
• Different voltages and chemistries become possible, because nothing has to match.
• Independent BMS behaviour. The house bank going into protection does not take propulsion with it, which matters more than it sounds at two in the morning.

For most electric boats this is the right answer for propulsion-versus-house, with series or parallel used within each bank. Pooling everything into one giant bank looks efficient on a spreadsheet and concentrates every risk in one place.

Choosing between them

The mistakes that cost money

1. Sizing on nominal kWh and forgetting DoD. The pack is 20% smaller than the sticker before you have left the dock.
2. Quoting Ah without a voltage. It is not a specification until a voltage is attached to it.
3. Sizing the bank on energy and ignoring the BMS current limit. Plenty of capacity, unable to deliver your continuous draw.
4. One fuse for a parallel bank. Covered above; it is the one with fire at the end of it.
5. Connecting packs at different states of charge. A large, brief, entirely avoidable current.
6. Asymmetric cabling to a common busbar. Quietly unequal current sharing for the life of the installation.

Related reading

• Battery sizing for electric boat range — how to arrive at the kWh figure in the first place
• LiFePO₄ batteries for electric boats — chemistry, cell voltages, and cycle life
• Electric boat DC-side safety — fuses, contactors, pre-charge and isolation
• BMS selection for marine propulsion — what the BMS has to do and how it is wired