LiFePO4 (LFP) batteries for gyropup

Prices were checked on 2026-10-25. Every pack, BMS and charger named here is in catalog/lfp_batteries.json (historical file), with its sources and confidence. "Not published" means exactly that. Computed packs (ids custom-8s…) are labelled estimate: mass = cells × 1.15.

Target: 8S LFP: 25.6 V nominal (26.4 V with A123-type 3.3 V cells), 28.8–29.2 V full, ~20 V at cut-off. That sits inside the J501's 12–36 V range with 7 V of headroom; 10S (36.5 V full) would not.

Requirements: ≥ 250 Wh, ≥ 60 A continuous, short peaks of 60–150 A, as light as possible.

Baseline to beat: WREKD 7S2P Molicel P50B Li-ion: 1.05 kg, 252 Wh, 120 A, $294 (wrekd-nexus-7s2p-molicel-p50b-10000).

The short answer

LFP stores roughly 100–145 Wh/kg at cell level, against about 240 Wh/kg for the P50B pack. Every LFP option that meets 250 Wh is 2.6–3.4× heavier than the current Li-ion pack. On a 14 kg robot that is 12–18 % of the total mass. The leg actuators and the balance controller must carry it, and it should sit low in the body.

1. Ready-made 8S (or 2 × 4S) LFP packs

n.p. = not published. PCA = EarthX peak cranking amps (a few seconds). ¹ EarthX AN-1609 calls a battery below 11.5 V (2.8 V/cell) over-discharged; the BMS cut-off point is not published. ² Continuous rating from EarthX application note AN-1609, Table 1, for the series pair.

Series use of 12 V starter batteries. The note said to check this before using two 4S batteries in series.

• EarthX allows it. AN-1609 approves up to 2 in series and/or 2 in parallel with no extra electronics. Use the same model and age, and no cross-ties between strings. The series pair is de-rated: ETX680/ETX36 pairs get 100 A continuous, 54 A charge and 28.8 V max charge. ETX18 pairs get 60 A, 27 A and 28.8 V. So charge EarthX pairs at 3.6 V/cell (28.8 V), not with a 29.2 V LFP charger.
• Antigravity forbids it. Its powersports RE-START manual says: "Do not attempt to use the battery product … in series or parallel with each other; damage or fire may occur" (manual). Excluded.
• Battle Born quotes a 48 V maximum series voltage, but its 24 V packs are 14 kg. Excluded on mass.

Other ready-made options, rejected:

• K2 Energy 25.6 V 9.6 Ah (SuperDroid Robots). Search listing: $359, "Not Available". Under 25 A continuous, 40 A for < 2 s. The page is behind a bot wall and was not opened (listing).
• RC LiFe packs (Turnigy nano-tech LiFePO4) are only 2S transmitter/receiver packs of 1.45–2.2 Ah (HobbyKing). Four in series gives about 56 Wh. Far too small.
• Bioenno, DNK Power, National Battery Supply and similar 24 V packs are BMS-limited to 10–30 A. Examples: DNK's 8S3P 15 Ah tops out at 30 A and 3.5 kg; National's 25.6 V 12 Ah is 12 A.
• Robot-combat LFP today is almost all custom A123 builds; no current off-the-shelf 8S A123 robot pack was found. BatterySpace's two ANR26650M1B 8S packs are "not for general sales".

Only the EarthX ETX36D pair meets both ≥ 250 Wh and ≥ 60 A off the shelf.

2. High-rate LFP cells

³ From search-result snippets of NKON / eBay.de. The pages are behind bot walls and were not opened.

A123 availability and fakes (2026).

• Genuine cells are now an EU distributor item. NKON (NL) lists the ANR26650M1B (its "production year 2023" listing is marked discontinued), and akkuteile.de lists it too. No trusted US retail listing was found. Liion Wholesale and 18650BatteryStore do not stock it, and Lithium Werks' page gives only a contact form.
• Treat eBay/AliExpress "A123" with suspicion. Many are used pulls (often the older 2.3 Ah M1A) or relabels. An EEVblog seller of used modules warned that many A123 cells sold online are knock-offs.
• Buy only from NKON-class sellers. Ask for lot or date codes, check 76 g ± 1.5 g and about 6 mΩ AC IR, and capacity-test a sample at 10–20 A.
• US-stock substitute: Vapcell T25 (60 A, $4.99) at Liion Wholesale.

Computed 8S packs (estimates; catalog ids custom-8s…)

Mass = cells × 1.15 (estimate; the BMS adds 50–200 g on top). Dims = bare cell block.

⁴ Uses the store's 92 g shipping weight per cell; the true cell mass is not published.

3. BMS for 8S LFP, 60–150 A

For the robot, CAN plus a 0 °C charge block matter more than 300 A.

• The Battery Hookup unit fits. Its 100 A continuous and 200 A for 10 s cover peaks of 60–150 A, and the robot can read SOC and faults on the CAN bus it already has.
• Choose a JK for more current headroom. Pick the 200 A or 300 A JK if 150 A peaks last longer than about 10 s. Note that a JK reports over Bluetooth only.

4. Chargers for 8S LFP (28.8–29.2 V)

• EarthX pair or A123 cells: set 3.6 V/cell (28.8 V).
• Generic 3.65 V/cell LFP: 29.2 V is allowed, but 28.8 V costs little capacity and leaves room for regen.

5. Recommendation

• Best ready-made: two EarthX ETX36D in series. It gives 327 Wh at 26.4 V. EarthX's own series rating is 100 A continuous and 28.8 V / 54 A charge. It weighs 3.6 kg, 2.55 kg heavier than the 1.05 kg WREKD 7S2P P50B, and costs $738.
  • It is the only off-the-shelf option found that meets ≥ 250 Wh and ≥ 60 A.
  • The downsides are two boxes, no data link and no published low-temperature behaviour.
• Best custom: an 8S4P pack of genuine Lithium Werks/A123 ANR26650M1-B cells. It gives 264 Wh, a 208 A continuous cell rating and 480 A for 10 s. It weighs about 2.8 kg (estimate), about 1.75 kg heavier than the Li-ion pack.
  • BMS: the Battery Hookup 100 A CAN BMS with a 0 °C charge cut-off ($65), or a JK-B2A8S20P/30P for more current.
  • Cost: about €254 ex VAT in cells from NKON, plus about $65 for the BMS, plus spot-welding and nickel.
  • US-stock alternative: Vapcell T25 cells in the same 8S4P layout, $160 in cells, about 3.4 kg on the store's shipping weight.
  • For maximum energy per kg, an 8S2P Vapcell G65 32700 pack gives 333 Wh at about 2.7 kg. It has no current margin (60 A continuous, 110 A for 5 s), so it is second choice.

Every LFP option makes the robot about 1.7–2.6 kg heavier for the same energy. Keep the Li-ion pack unless charging safety outweighs about 12–18 % more robot mass.

6. Design notes for an LFP gyropup

• Regen at full charge can open the BMS. A full pack (28.8–29.2 V) has no room to absorb wheel or leg regen. The BMS opens on over-voltage, and the J501 and motors lose power. Stop charging at about 95 %, keep a brake resistor or clamp on the motor bus, and use a BMS whose charge limit is above the worst regen current (Battery Hookup: 60 A).
• The flat LFP curve makes voltage a poor fuel gauge. Use the BMS's coulomb counter over CAN. Suggested UVLO thresholds: warn at about 3.0 V/cell (24 V), crouch and stop at about 2.8 V/cell (22.4 V), BMS cut at 2.5 V/cell (20 V).
• Charging below 0 °C: A123 cells tolerate small currents. Headway allows only 0.1C below 0 °C. Pick a BMS with a hardware 0 °C charge block.
• The J501's 36 V limit is not at risk. An 8S LFP pack peaks at 29.2 V, so it has more headroom than 7S Li-ion (29.4 V).

Sources

Every row above links its source inline. The main documents:

• EarthX: AN-1609 series/parallel note, ETX36D, ETX18F, ETX24C, Aircraft Spruce EarthX page.
• Antigravity: RS user manual (series use prohibited).
• Lithium Werks: ANR26650M1B page; A123 2011 datasheet.
• Headway: HW 38120HP spec, short sheet.
• Vapcell / lygte-info: G65 32700 test, IFR26650 test.
• BatterySpace: 25.6 V LFP pack list.
• BMS: Battery Hookup, Fogstar JK 8S30P, Daly M-series.
• Chargers: iCharger X8, ToolkitRC M8AC, Victron IP65 24/5.