Six 18650 cells become one 21 Ah battery, and that battery
reaches the rest of the probe through two wires and one plug. This is what joins
what, in what order, and which of those joints carries the whole probe's
current.
Companion to buoy-parts-book.html,
which carries the same power source from the plug onward. Topology from
ADR 0005 and architecture-power-and-wiring.md;
geometry from the 2026-09-03 integrated-platform note.
Before anything else
SoC-match the six cells to within ~50 mV of each other before you join
them. Nothing in this design limits the balancing inrush between mismatched
neighbours — the moment two cells at different states of charge are bridged,
the difference is dumped through whatever joins them, at whatever current the
copper allows. Measure all six, charge or rest them together until they agree,
and only then fit the bus.
Every joint below is soldered to a contact, never to a cell. The cells
are protected button-tops that press against spring and plate contacts — nothing
is soldered or welded to a battery anywhere in this build.
The pack, and where its two wires go
Six cells, twelve contacts, two bus bars, two wires, one plug. The
only current path out of the battery is the pair at the bottom right — which is
why the fuse sits in it, why it is 20 AWG, and why both leads are laced through
the platform before their joints.
Build order
Steps 1 and 2 cannot be swapped. Everything after the bus is
soldered is difficult to undo without heating a joint next to a cell.
Match the cells. Measure all six open-circuit. If any pair
differs by more than ~50 mV, rest or charge them together until they agree.
A mismatched pair balances through the bus the instant you close it, and
nothing in this design limits that current.
Fit the twelve contacts — one spring and one flat plate per
bay, uxcell 16.5 × 16 mm nickel-plated. They are fitted by hand after
printing; the end wall backs each one over 12.000 mm unbroken, and each
stands 1.801 mm proud of the wall into open air.
Lay each bus bar in its slot — bare solid 18–20 AWG copper
down the 1.100 × 111.2 mm channel in the end wall, and solder it to all
six contacts on that side. Do this with the bays empty: the cells go
in last, and a soldering iron does not belong next to a lithium cell.
Solder the two leads — 20 AWG silicone, red to the positive
bus, black to the negative — after lacing each one through its hole
in the platform base. Any later pull then lands on plastic instead of on
the joint.
Put the fuse in the positive leg, ~1.5 A, in series. Not the
negative leg: a fuse in the return leaves the positive rail live against
the hull and everything in it after the fuse has blown.
Crimp the JST-PH plug — red to the pin the gauge and charger
treat as positive. The shell does not key polarity for you; confirm it
against the gauge's own marking before the first power-up.
Drop the cells in, button-tops toward the plate contacts, and
fit the 3 mm foam pad over them.
Check before power: pack voltage at the plug (3.0–4.2 V,
never negative), continuity from each cell to its bus, and no continuity
between the two buses.
What each joint carries
Joint
Carries
Why it is what it is
cell → contact
up to ⅙ of the load
A pressure contact, not a joint. Nothing is soldered or welded to a cell anywhere in this build
contact → bus
up to ⅙ of the load
Soldered. The printed channel holds the bar straight and takes the mechanical load off the joint
bus → lead
everything
The single point where the whole pack's current leaves. Laced for strain relief before soldering
the two leads
everything
20 AWG silicone. ~0.6 A worst legitimate draw, the modem's ~500 mA burst included; all charge current flows back the same way
fuse, + leg
everything
~1.5 A, ~2.5× headroom over 0.6 A. Re-rated by ADR 0014 from 2.5–3 A — a 3 A fuse in front of a 0.6 A load lets a 2 A fault run indefinitely inside a sealed hull
JST-PH plug
everything
Not a choice — the charger board dictates the connector. The gauge sits inline behind it: pack → left port, charger → right
The parts, by name
What is settled, what is owned, and what nobody has picked yet.
Three of these five are still a decision.
Part
Spec
Stock
bus bar
bare solid 18–20 AWG copper, laid in the 1.100 × 111.2 mm channel in each end wall
not on any list — no product chosen
pack leads
20 AWG silicone — sized for the current each carries, silicone because it stays flexible cold
the BOM carries one ~$20 line for "JST leads, silicone wire" — no product chosen
fuse
~1.5 A inline — blade, glass or PTC, undecided. Re-rated from 2.5–3 A by ADR 0014: the old figure assumed a 1.3 A peak the supply never sees
3 needed, 0 owned, buy 4 (~$3) — PROC-08
fuse holder
follows from the fuse's form factor
not specified at all
the connector
JST-PH 2-pin, 100 mm pigtail — not a choice, the charger board dictates the family
4 owned, listed as "battery/panel leads"
Because the pigtails are pre-made, there is no crimping in this
build. You cut one to length and solder its two wires to the pack leads. No
crimp tool, no loose contacts, no housing to load.
⚠ The wire rule and the connector cannot both be obeyed
The wiring rules say 20 AWG silicone for the pack. The charger dictates
JST-PH. JST's own documentation gives the PH series a wire range of
AWG 32–24 and a rating of 2 A at AWG 24.
20 AWG does not fit a JST-PH contact. It is four gauges over the
largest wire the crimp is specified for. So one of two things has to be true, and
the tree has never said which:
The run steps down at the connector — 20 AWG from the bus, spliced
to the pigtail's own thin wire for the last 100 mm. That is what the owned
pigtails already are, and it adds one soldered joint per leg.
Or the whole run is 24 AWG and the 20 AWG rule is wrong for this
leg. Electrically that is fine: 0.6 A worst legitimate draw against a 2 A
contact rating, and about 20 mV of drop over 200 mm of both conductors. What
it gives up is mechanical robustness and cold flex, which is why the rule said
20 in the first place.
One thing does line up: the ~1.5 A fuse sits below the connector's 2 A
rating, so a fault the fuse tolerates is one the contacts can carry. That
ordering is worth keeping whichever way the gauge goes. Tracked as PROC-19.
Soldering the joints
Three kinds of joint, in the order you make them, and the rule
that governs all of them: the iron never comes near a cell. Every joint
below is made with the bays empty or on a lead that is not yet in the hull.
Contact to bus bar — the six joints down each end wall. Tin
the bar and the contact tab separately, then bring them together and heat
the joint until the solder flows, not the solder itself. The printed
channel holds the bar straight, so you are not fighting alignment while the
iron is on. Nickel-plated contacts take solder readily; the plating is there
for exactly this.
Bus bar to pack lead — one joint per leg, and the one that
carries everything. Lace the lead through its hole in the platform base
first. Strip about 5 mm, tin the strands so they cannot splay, hook the
wire over the bar, and flow it. Sleeve it in heatshrink afterwards: this
joint sits inside a sealed hull for months and nothing else insulates
it.
Pack lead to pigtail — if you step the gauge down at the
connector. Stagger the two joints along the cable so the red and black
splices do not sit side by side, tin both ends, and heatshrink each
separately before a common outer sleeve.
A cold joint here is the failure you will not find. It reads fine on a
meter at the bench, then goes high-resistance under the modem's 500 mA burst
months later, inside a hull nobody can open. A good joint is shiny and has
wetted the copper; a dull, grainy or blobbed one gets reheated with fresh flux,
not left alone.
The connector, and which way round it goes
A JST-PH 2-pin pigtail, 100 mm — you own four, listed for battery and
panel leads. It is not a choice: both candidate charger boards use this family,
so the pack side is fixed by the board.
Since PROC-18 closed, the pigtail does not go to the charger. It goes
to the fuel gauge's left JST-PH port; a second lead leaves the gauge's
right port for the charger's BAT pad. The gauge sits
inline so it measures the pack rather than the rail it runs on.
Pin
Wire
Goes to
1
red, +
the positive bus, through the ~1.5 A fuse
2
black, −
the negative bus, direct
⚠ The shell does not key polarity for you. A JST-PH housing mates only
one way round mechanically, but nothing stops the wires being soldered to
the wrong ends of it — and a pre-made pigtail's red and black are only a
convention, not a guarantee. Ring the pigtail out with a meter against the
gauge's own marking before the first power-up, and check the pack voltage at the
free end reads positive before it is plugged into anything.
The pack is one node, not a supply
The cells, the charger's BAT pad and the carrier's
J5 are all one electrical point. Nothing draws
through the charger — it only pushes current into that node when the sun
is up. So these two wires are not "the charging wires": they are the probe's
entire power path, in both directions.
The fuse in them is not the primary protection, and the record is
explicit about that since the pack went back to protected cells: each cell's own
PCB cuts at about 8 A, and a short pulls through all six at once and trips
them together. The fuse is redundancy — kept because it is cheap and already
on order, not because it is the thing standing between a 21 Ah bank and a harness
fault.
Open, and not to be guessed at the bench
HULL-23
The twelve contacts have no retention design. They
are fitted by hand after printing and there is no generator feature holding
them. How they stay put through a Drake Passage winter is undecided.
PROC-08
The fuse is not bought. Four × ~1.5 A, re-rated by
ADR 0014. No fuse holder is specified either — inline glass, a PTC, or a
soldered pigtail is an open choice.
HULL-27
Which end takes the spring. The note records one
spring and one plate per bay but not which goes where. The drawing above puts
the spring on the flat negative end, where it takes up cell-length tolerance
— that is the usual convention, not something this tree has decided.
HULL-27
The bus-bar channel is a reading, not a specification.
The 1.100 × 111.2 mm slot is unexplained by the generator; the 2026-09-03 note
infers its purpose from the fact that 18 AWG solid copper is 1.024 mm. The
spec models no bus bar at all.
HULL-24
Bay clearance is 0.200 mm a side — tight for ASA
before elephant's foot — and the bays leave 3.900 mm over a 3 mm foam pad.