Batteries: the 2S LiPo

Strido runs from one 2S lithium-polymer (LiPo) pack: 7.4 V nominal, 5000 mAh, about 37 Wh. LiPo stores a lot of energy for its weight and can deliver large currents, but it is also the one part of the robot that can start a fire. This page explains the numbers on the label and the habits that keep the pack healthy and safe.

In Strido

• Part: LiPo 2S 5000 mAh hardcase with XT60, max 138 × 47 × 25 mm (Gens ace G-Tech class), BOM line lipo_2s_5000.
• Accessories: balance charger with AC input (B6AC class, charger), LiPo safety bag (lipo_bag), LiPo cell checker / low-voltage alarm (lipo_checker).
• Data: hardware/electronics.yaml → power.pack (7.4 V nominal, 8.4 V full, 6.4 V empty, 5000 mAh) and rails.pack (6.0–8.4 V).
• Firmware: low-battery warning at 6.8 V, servo cut-off at 6.4 V (firmware/strido_esp32s3/lib/strido_core/include/strido/safety.h), measured by the INA260.
• ROS 2: /battery_state (pack voltage and logic-branch current).

Cells and the "2S" label

A LiPo cell is one lithium-ion pouch. Its voltage depends on its state of charge:

Cell state	Voltage per cell	2S pack
Full (end of charge)	4.20 V	8.4 V
Nominal (label value)	3.70 V	7.4 V
Strido warning	3.40 V	6.8 V
Strido servo cut-off	3.20 V	6.4 V
Lowest rail value the logic branch is checked for	3.00 V	6.0 V

2S means two cells in series: the voltages add, and the capacity stays that of one cell. (A "2S2P" pack would have two parallel pairs, doubling capacity.) Two cells were chosen because:

• 2S (6.4–8.4 V in use) sits just above the 6.0 V servo rail, so the UBECs only need to step down a little.
• 2S is the common RC-car pack, so packs, chargers and XT60 leads are cheap and easy to find.

Capacity and energy

Capacity (mAh) is charge $Q$: 5000 mAh means the pack can deliver 5 A for one hour, or 1 A for five hours, before it is empty. Charge is current integrated over time, $Q=\int I\mathrm{d}t$, so at constant current $Q=It$. Energy (Wh) also accounts for voltage. Electrical power is $P=VI$; integrating it over the discharge gives the energy, and if the voltage stays close to its nominal value this reduces to

$$E=\int VI\mathrm{d}t\approx V_{\mathrm{nom}}Q=7.4\mathrm{V}\times 5.0\mathrm{Ah}=37\mathrm{Wh}$$

(1 Wh = 3600 J, so this is about 133 kJ.) The nominal voltage is used because a LiPo's voltage falls gently from 4.2 V to about 3.2 V per cell over most of the discharge; 3.7 V per cell is a good average.

Energy is the number that predicts runtime. docs/electronics.md estimates about 60 W drawn from the pack while trotting (12 servos ≈ 50 W, plus Pi, LiDAR and the rest, through 90 %-efficient converters). With a usable fraction ${\eta }_{\mathrm{use}}=0.8$ of the stored energy:

$$E_{\mathrm{usable}}\approx 37\mathrm{Wh}\times 0.8=29.6\mathrm{Wh}$$$$t_{\mathrm{run}}=\frac{E_{\mathrm{usable}}}{P}\approx \frac{29.6\mathrm{Wh}}{60\mathrm{W}}\approx 0.5\mathrm{h}\approx 30\min \text{trotting}$$

Standing draws less, so the doc estimates about 45 min. The 80 % factor is there because the firmware stops the servos well before the cells are truly empty. Both runtimes are marked [ESTIMATE]: measure your own with /battery_state and a clamp meter.

37 Wh is also below the 100 Wh limit above which shipping a LiPo needs a surcharge (docs/bom/sources.md).

C-rating: how fast the pack can deliver

The C-rating expresses current as a multiple of capacity. With $Q$ in ampere-hours, a rate of $C$ means the current

$$I=C\cdot \frac{Q}{1\mathrm{h}}$$

so 1C on a 5000 mAh pack is $5\mathrm{A}$, and 60C is $60\times 5\mathrm{A}=300\mathrm{A}$ (the Gens ace pack's continuous rating; 100C gives $500\mathrm{A}$ burst). Equivalently, a pack discharged at $n$C empties in $1/n$ hours.

hardware/electronics.yaml records a max_discharge_a of 300 A for this pack (60C continuous, Gens ace GEA502S60X6HGT). The robot's worst-case pack current is about 23.6 A (servo and logic branches, docs/verification/electronics.md), so the pack works at under a tenth of its rating.

Why buy far more C than you need? Every cell behaves roughly like an ideal voltage source $V_{\mathrm{oc}}$ (the open-circuit voltage) in series with an internal resistance $R_{\mathrm{int}}$. By Ohm's law the terminal voltage under load is

$$V_{\mathrm{term}}=V_{\mathrm{oc}}-I{R}_{\mathrm{int}}$$

and the pack also turns $P_{\mathrm{loss}}=I^2{R}_{\mathrm{int}}$ into heat. A pack with a higher genuine C-rating usually has a lower internal resistance, so it sags less when twelve servos pull a current peak together. As an illustration (not a measured value), a pack with $R_{\mathrm{int}}=10\mathrm{m}\mathrm{\Omega }$ at the robot's worst-case $23.6\mathrm{A}$ would sag by $0.24\mathrm{V}$ and dissipate $5.6\mathrm{W}$. Less sag means fewer false low-battery readings and more voltage headroom for the UBECs. Cheap packs often overstate their C-rating, which is why docs/bom/sources.md lists the LiPo among the parts where the brand matters (hardcase, real C rating).

Load sag and the cut-off filter

Sag is also why the firmware does not cut off on a single low reading. The INA260 is read every 100 ms. The servos only relax after 5 consecutive readings below 6.4 V (0.5 s), so a short current spike in mid-step does not drop the robot. A pack that really is empty stays low and trips the cut-off.

Charging: always a balance charger

LiPo cells charge with constant current, then constant voltage (CC/CV): the charger pushes a set current until the cell reaches 4.20 V, then holds 4.20 V while the current tapers. Going above 4.20 V per cell damages the cell and can make it unstable.

In a series pack the charger only sees the total voltage, and the two cells are never exactly equal. One cell could reach 4.30 V while the other is still at 4.10 V, and the total would still read 8.4 V. The balance lead, the small extra plug with one wire per cell junction, lets a balance charger measure each cell and bleed off the higher one. Strido's B6AC-class charger does this.

Rules of thumb (see Battery University in Further reading):

• Charge at 1C or less: 5 A for this pack.
• Always plug in the balance lead and select the LiPo program with the right cell count (2S).
• Charge inside the LiPo bag, on a non-flammable surface, and never leave it unattended (docs/electronics.md, safety rules).
• Let a warm pack cool down before charging it.

Storage voltage

A LiPo ages fastest when it sits full (4.20 V per cell), and it is damaged if it sits empty. For storage longer than a few days, bring each cell to about 3.80–3.85 V (about 7.6–7.7 V for the pack). Most balance chargers, including the B6AC class, have a storage program that charges or discharges to this level automatically.

The LiPo checker

The cell checker plugs into the balance lead and shows each cell's voltage and the total. Use it:

• Before a run: both cells near 4.2 V and within a few hundredths of a volt of each other.
• After a run: cells still at or above about 3.2 V and still balanced.
• Before storage: cells at storage voltage.

A cell that drifts away from its partner run after run is the first sign of a failing pack. In alarm mode the checker beeps when any cell drops below a threshold. That adds a second, independent warning on top of the firmware's.

Safe handling

LiPo fire risk

A damaged, over-charged or over-discharged LiPo can vent and burn violently. A LiPo fire cannot be put out by smothering it.

• Inspect the pack. A hardcase helps, but check for swelling ("puffing"), dents, cut wires or a hot pack after use. Retire any puffed pack.
• Never short the leads. The pack can deliver hundreds of amps. That is also why the arming loop key must never fit the battery's own XT60. See Power for the contact rule.
• Fuse close to the pack. The 25 A main fuse protects the 14 AWG trunk right after the battery input.
• Unplug after use. Even with the logic switched off, the robot stays connected to the pack through its wiring. Disconnect the battery when you finish.
• Transport and store the pack in the LiPo bag, at storage voltage, away from heat.
• Dispose properly: discharge fully with a charger's discharge program, then take it to a battery collection point.

Check yourself

1. What is the pack voltage of a 2S LiPo at 3.4 V per cell, and what does the firmware do there?
2. How much current is 2C for this pack?
3. Why does a balance charger need the balance lead if it already measures 8.4 V at the main leads?
4. Why does the firmware wait for five consecutive low readings before it cuts off?
5. You will not use the robot for a month. What should you do with the pack?

Answers

1. 6.8 V; the firmware raises the low-battery warning.
2. $2\times 5\mathrm{A}=10\mathrm{A}$.
3. The total can read 8.4 V while one cell is over-charged and the other under-charged; only per-cell measurement shows and corrects that.
4. Current peaks make the voltage sag briefly. Requiring 0.5 s below 6.4 V ignores spikes and reacts only to a truly empty pack.
5. Bring it to storage voltage (about 3.80–3.85 V per cell) with the charger's storage program and keep it in the LiPo bag.

Further reading

• BU-409: Charging Lithium-ion (Battery University)
• BU-808: How to prolong lithium-based batteries (Battery University)
• TI INA260 datasheet: the sensor that measures the pack voltage
• Related pages: Power, Safety, Electronics, wiring and power