Primary Batteries¶

Slocum gliders run on one of several primary (non-rechargeable) pack types: alkaline, 3S lithium primary, and the newer 4S lithium primary — each available in standard and extended (energy-bay) configurations. Choosing a type is a trade-off between cost, energy, and handling/shipping burden.

Source

Paraphrased and consolidated from the Slocum Glider Operators Manual, the Teledyne Webb Research (TWR) user forum, and the UG2 community Slack. Capacity and derating figures evolved over the years and depend on configuration — treat the authoritative values as those in your glider's autoexec.mi and the current TWR specifications, and contact Glider Support before changing abort settings. See also the capacities figure on the Batteries page.

Pack types & chemistry¶

Type	Configuration	Nominal voltage	Notes
Alkaline	C-cells	~13 V	Lowest energy and cost; needs a Teflon slide sheet and aft bracket (see below)
3S lithium primary	3 DD cells in series (78 DD + 3 C emergency, standard G2)	~10.8 V	Lithium thionyl chloride (Electrochem). Stop diving and drift at the first shelf
4S lithium primary	4 DD cells in series	~15 V	Newer; can fly on the first (11.5 V) shelf toward recovery
Emergency battery	3 C cells	—	Lithium: a standalone pack in the forward section. Alkaline: included in the aft pack

Lithium content (for paperwork)

Each DD cell ≈ 10.2 g lithium; each C cell ≈ 2.2 g. A standard pitch pack is 36 DD cells, the aft pack 42 DD, plus 3 C cells in the emergency pack.

Capacity & undervolts settings¶

Set f_coulomb_battery_capacity and the mission undervolts abort to match the installed pack — uncomment the matching line in autoexec.mi when you change type. Representative values:

Pack	`f_coulomb_battery_capacity` (Ah)	Undervolts abort (V)
Alkaline (nominal)	120 (≈153 typical G2 estimate)	10
4S lithium, standard	~498–550	12
4S lithium, extended energy bay	~800	12
TWR rechargeable, standard	215	12.5
TWR rechargeable, extended	300	12.5

Zero the coulomb counter on every battery change

Each time new batteries are installed (or rechargeables are charged), zero the counter or you risk spurious aborts:

put m_coulomb_amphr_total 0
exit reset

Confirm the value starts climbing from 0. With capacity and undervolts set correctly, m_lithium_battery_relative_charge then tracks how much of the pack has been used, and the glider aborts on MS_ABORT_CHARGE_MIN when remaining_charge_min (default 10 %) is reached.

Lithium + GliderDOS

Avoid running the glider on lithium batteries while sitting in GliderDOS — the coulomb meter does not record amp-hours consumed below that software level. Use AC/DC wall power for extended bench work.

Derating & end-of-life behaviour¶

Storage age: TWR recommends derating capacity by ~3 % per year of storage. (Some operators have seen larger-than-expected losses on partly-used packs left in storage — derate conservatively and budget margin.)
Temperature/variability: an additional ~10 % derate is commonly applied to cover temperature swings.

Voltage shelves. Lithium primary packs hold a fairly flat voltage, then drop onto a shelf near end of life before a final steep decline to shutoff (~10 V):

3S: stop diving and put the glider into a drift once the energy reaches the first shelf.
4S: you can keep flying on the first (~11.5 V) shelf toward a recovery, but the appearance of a second shelf drop may be the last time the glider communicates. Never deplete either type to the second shelf operationally.
Practical undervolts strategy for 4S: set the abort to 12 V as an early alert that the glider is approaching the 11.5 V shelf; ~5 % or more of the pack's energy may remain on the shelf, so a lower abort (e.g. 9.5 V) can be used deliberately to fly home.

Passivation after storage¶

Lithium primary (thionyl-chloride) cells form a passivation layer on the anode when they sit unused. The layer is normal and self-healing, but it limits how much current the pack can deliver until it is "burned off." A glider that has been in storage for months can power on and run light loads fine, then fail to source the high current the buoyancy pump needs to pump out at depth — giving a stalled or very short first mission.

Wake the pack with step-down dives

After extended storage, don't send the glider straight to full depth. Fly a series of progressively deeper step-down dives so the rising pump load burns off the passivation layer gradually. This also gives you frequent early comms and a look at the data before committing to long deep dives. Rechargeable Li-ion packs are not believed to passivate the same way, but step-down dives are good practice regardless.

Switching between alkaline and lithium¶

Swapping chemistry is more than a battery change — several hardware and software items must move together:

Mainboard connector / enable circuit. TWR lithium packs have an enable circuit; the mainboard has separate connectors for each type, so switching types means moving the power connector to the matching one. (Non-TWR lithium packs, e.g. custom builds, generally wire to the alkaline connector.)
battpos calibration. Uncomment the matching f_battpos_safety_max, f_battpos_cal_m, f_battpos_cal_b set in autoexec.mi for your pump and chemistry — the lithium pitch pack sits further forward, so the travel limits differ. (Field note: on a G2 with a 200 m / 800 cc pump, alkaline f_battpos_safety_max should be ≤ ~1.4 in so the forward battery doesn't hit the pump.)
Mechanical. Alkaline needs a Teflon sheet on the forward hull for the pitch battery to slide against, and an aft bracket (with the two battery-securing pin holes). With lithium, roll adjustment is done with pie weights / added mass rather than the bracket.
Emergency battery. The lithium emergency pack is a standalone forward pack and must not be used with alkaline (whose emergency cells live in the aft pack).

Re-check the compass after any battery change

Batteries carry a magnetic field that can affect compass calibration. At minimum, do a four-point compass check before each deployment after installing new batteries.

Shipping & transport¶

Lithium primary packs are Class 9 dangerous goods. Ship/declare using the correct UN number and have the Safety Data Sheet (available from TWR / Electrochem) on hand.

Item	Standard pack	Extended (energy bay)
UN number — batteries in boxes	UN 3090	UN 3090
UN number — batteries installed in glider	UN 3091	UN 3091
Cell count	78 DD + 3 C	114 DD + 3 C
Total lithium mass	~803 g	~1170 g
Total capacity	~702 Ah	~1026 Ah
Max battery weight	~20 kg	~30 kg

Ground transport exemptions vary by region

Some operators classify a glider-with-batteries as a research vehicle, which can fall under the same exemptions as e-scooters/hoverboards (e.g. "Marine Research Vehicles … Special Case 67" under certain national rules). This is region-specific — confirm with your dangerous-goods authority before relying on it, and note that vessel (at-sea) rules differ from road rules.

Lithium safety on vessels¶

Operators increasingly need written procedures for carrying and charging lithium batteries on ships. There is no single standard yet (a community/UG2 best-practice effort is underway), but field-tested measures include:

Charge outside the vehicle's accommodation spaces, and (on G3s that allow it) charge while the glider is powered on so you can monitor internal temperature; the rechargeable BMS reports per-sub-assembly thermistor data.
Thermal imaging camera during charging and as a pre-deployment check.
Lithium-rated fire blankets / bags sized for a full glider (spec them properly — consumer e-bike sizes are too small), and consider PyroBubbles in shipping containers.
A practical containment / ejection plan: rather than an (impractical) explosion-proof cabinet, keep the glider on the back deck with a way to put it over the side if a pack goes into thermal runaway — e.g. a steel cable and crane, or a release ramp.

These hazards apply to rechargeable lithium-ion packs too

The same handling, charging, and shipping caution applies to the rechargeable lithium-ion packs — they are a different chemistry/transport class but pose comparable fire risk.