Parallel 12V Power Supplies: Current Sharing in Tethered Drone with 15V 25A PSU and 15V 100A LiPo

User question

When you have a 12v dc power supply capable of 1a output and in parallel another 12v dc power supply but capable of 3a output and a load in series drawing 2 amps how many amps are being drawn from each power supply? Same? One more than other? I know that by putting them in parallel inceases amperage to 4a, but is the ratio 3:1 coorisponding to their maximuim amperage output?

In reality im building a tethered drone with a ground power supply(15v at 25a max) which is going in parallel to the drone with a small, lightweight, high discharge, emergeny, lipo battery(15v at 100a max) . This battery will act as a capasitor by handelling the high amerage usage of the drone that the tether cannot produce without damaging itself. My question is will the battery assist the ground power supply when drawing more than 25a?

Artificial Intelligence Response

Direct answer to the question

• The load current does not split in a 3 : 1 ratio that mirrors the supplies’ current-limit ratings.
• Each source delivers a current determined by
  I = (Vsource – Vload) / Rint,
  where Rint is its internal (output) impedance.
• For the drone: the 15 V/25 A ground supply will provide up to ≈25 A. When the drone demands more, the Li-Po—being a very low-impedance 15 V source—automatically furnishes the excess current.

Detailed problem analysis

1. Parallel supplies – theory
   • Real supplies never have identical open-circuit voltage \(V{oc}\) or internal resistance \(R{int}\).
   • The source with the slightly higher \(V_{oc}\) takes the lion’s share until its voltage sags (or it hits current-limit).
   • If two 12 V supplies have \(V{oc1}=12.04\;V,\;R{int1}=90 m\Omega\) (3 A model) and \(V{oc2}=11.98\;V,\;R{int2}=220 m\Omega\) (1 A model), a 2 A load settles near 11.91 V and the currents become
   \(I_1\approx1.52\;A,\;I_2\approx0.48\;A\).
   Ratio ≠ 3 : 1; it is set by the impedances, not the current ratings.

2. Drone hybrid supply
   • Tether losses: \(V{\text{drop}}=I\;R{\text{cable}}\). With 25 A and 0.03 Ω round-trip, \(V{\text{drop}}\approx0.75 V\).
   • Ground PSU set to ≈16.4–16.6 V (below 4 S Li-Po maximum 16.8 V) keeps the flight battery topped and supplies steady power.
   • At \(I{\text{load}}<25 A\): PSU supplies almost all current, battery trickle-charges.
   • At \(I{\text{load}}>25 A\): PSU reaches constant-current mode; its output voltage droops. When \(V{\text{node}}\) falls below battery voltage, the battery delivers \(I{\text{extra}}=I{\text{load}}-25 A\). Transient peaks (motor start, gust response) are therefore covered instantaneously by the Li-Po.

3. Dynamic behaviour
   • Li-Po response time ≪1 ms → excellent for surge absorption.
   • PSU recovery time dominates long bursts (>100 ms).

Current information and trends

• Commercial tether systems (Elistair Safe-T 2, Volarious V-Line Pro, Vicor UAV tether modules 2023-24 releases) all use a “battery-assisted tether” topology: 300–1500 W over the cable, Li-Po buffer on the drone.
• Modern ground boxes use bidirectional DC/DC converters so the flight battery re-charges through the tether during low-load periods, improving endurance.
• MOSFET “ideal-diode ORing” controllers (TI LM5050-1, LTC4412, MPQ5050) replace Schottky diodes, cutting conduction loss by >70 %.

Supporting explanations and details

• Equivalent circuit:

   +---- Rint1 ---- PSU1 (25 A) ----+
  V_gnd ---+ +--- V_node --- Load
  +---- Rint2 ---- Li-Po ---------+

  Solve \(I_1,I_2\) with KVL and Ohm’s law; whichever reaches its limit first clamps.

• Example: Drone pulls 60 A, tether R = 30 mΩ, PSU limited 25 A.
  PSU sees \(V_{node}=16.5-25·0.03=15.75\;V\).
  Charged Li-Po ≈16.2 V, so battery provides \(60-25=35\;A\).

Ethical and legal aspects

• Li-Po safety: thermal runaway, over-charge, over-discharge → mandatory BMS, flame-resistant enclosure.
• Aviation: tethered drones >400 ft or in controlled airspace require FAA waiver (US) or equivalent local authority permission.
• EMC & ground safety: high-current tether acts as radiating antenna; comply with FCC/CE emissions, provide strain relief and over-current protection.

Practical guidelines

1. Match voltages within ±50 mV at no load; add 50–100 mV programmable droop (“load-share droop”) to the PSU if available.
2. Insert a 40–50 A ideal-diode MOSFET on both sources to prevent back-feed.
3. Fuse both legs (PSU 30 A slow-blow, battery 80 A fast-acting).
4. Monitor in-flight \(I{\text{PSU}},I{\text{BAT}},V_{\text{node}}\) via smart shunts (INA226, MAX34417).
5. Use ≥12 AWG tether at 100 m (or coax with copper-clad aluminum + Kevlar) to keep losses <10 %.
6. Provide forced cooling for the PSU; 25 A at 16 V ≈ 400 W continuous.

Possible disclaimers or additional notes

• Unequal cell ageing in the Li-Po changes its voltage curve; periodic capacity check required.
• Cable damage or PSU failure could force full current into the tether; protection devices must withstand worst-case 100 A.

Suggestions for further research

• Bidirectional buck-boost tether converters (Vicor NBM2317, 2024) to allow lower-voltage, higher-current cable with active regulation.
• Solid-state circuit breakers (eFuse) for sub-µs disconnection.
• AI-based power-budget management for swarm tethers.

Brief summary

Current in parallel sources is set by voltage accuracy and output impedance, not by the name-plate current. In your drone, the 15 V/25 A ground unit will provide up to its limit; any excess above 25 A—whether transient or sustained—is instantaneously covered by the low-impedance Li-Po, exactly as intended. Proper voltage trimming, ORing protection, tether sizing and battery management are essential to make the hybrid supply reliable, efficient and safe.

Disclaimer: The responses provided by artificial intelligence (language model) may be inaccurate and misleading. Elektroda is not responsible for the accuracy, reliability, or completeness of the presented information. All responses should be verified by the user.