Battery Cooling and Ventilation [WIP]

https://pdm.getbild.com/0d9d31f0-3579-4fcd-82a8-b5347c436243/branch/main?dir=f26f8193-9b65-42b1-992e-b55a8219ed47&tab=project_files

DESIGN OBJECTIVE:

• Requirements

• Calculations

• Design

BATTERY OVERVIEW:

• https://www.molicel.com/inr-21700-p50b/

• Datasheet

• https://www.batemo.com/products/batemo-cell-explorer/molicel-inr21700-p50b/#get-report-popup

• Sigmaclad

• Nine cells in parallel per module, 32 modules in series per pack, 288 cells total

• About 120V total output, 40A expected current output for sustained speed

• 2.5V - 4.2V range, 3.6V nominal, 24W expected maximum per cell

• Single pole +/- via stripping casing, bottom poles attached to aluminum heat plate

• Max charge/discharge temperature 80C. Max ambient temperature 60C

• 12.8 mOhm internal impedance at 50% SOC

• INR designation → LiNiMnCoO₂

CALCULATIONS

Battery Heat Generation

The following section calculates the total heat generation of components within the battery enclosure. This includes the cells, bus bars, and contacts.

Source

Q [W] - heat generation

I_bat [A] - cell current → 6.6A

R_o [Ω] - ohmic resistance

R_p [Ω] - polarized resistance

R_i [Ω] - internal resistance → 13.5mΩ

T_bat [C] - cell surface temperature → 70C

U_OCV [V] - open circuit voltage

n - number of cells → 288

∆S [J/mol k] - entropy change

F [C/mol] - Faradays constant → 96,485 C/mol

The following equation splits the heat generation of the cells into two terms: irreversible (ohmic) heat caused by the internal resistance, and entropy change (∆S) caused by electrochemical reactions.

Q_bat = I_bat [ (U_wv − U_ocv ) + T_bat (∂U_OCV / ∂T_bat) ]

Q = I²_bat (R_o + R_p ) + I_batT_bat (∂U_OCV / ∂T_bat)

Q = I²_batR_i + I_batT_bat (∂U_OCV / ∂T_bat)

Reversible heat estimate (without physical testing):

∂U_OCV / ∂T_bat =−ΔS​/nF

Q = I²_batR_i + I_batT_bat (−ΔS​/nF)

Typical entropy values:

LiNiMnCoO₂ cells​ → ΔS ≈ -20-40 J/mol k

Since there are 288 cells in the battery:

Q_pack = 288 [I²_batR_i + I_batT_bat (−ΔS​/nF)]

Q_pack = 572W

Q_bat = Q_pack + Q_{contact_tot +} Q_{bus bar_tot}

Q_bat ≈ 600W

Bus Bar Heat Generation

ρ_sigma = 2.874 x 10^-8 Ωm (for sigmaclad 60)

ρ_{alum_20C} = 2.65 x 10^-8 Ωm (at room temp)

ρ_alum(T) = ρ₀​ ⋅ [1+α(T−T₀​)]

α=0.004 per C, T = 50C, T₀ = 20C

ρ_{alum_50C} = 2.97 x 10^-8 Ωm

Bus Bars:

Q_{Bus Bars} = I²R

Q_{Bus Bars} = I² (L ρ_sigma / A) (I = 6.666A for 60A pull)

Q_{Bus Bars} = 0.089W

Q_{Bus Bars}= 32(Q_contact)

Q_{Bus Bars_tot} = 2.86W

Collectors:

Q_Collectors = I²R

Q_Collectors = I² (L ρ_{alum_50C} / A) (I = 60A)

Q_Collectors = 0.12W

Q_Collectors = 10(Q_{bus bar})

Q_{Collectors_tot} = 12W

Natural Convection Calculations:

Q = h ⋅ A ⋅ (T_s − T_a​)

Q = h ⋅ A ⋅ (ΔT​)

Collectors:

h_alum = 10 W/m²⋅K (typical aluminum to air natural convection)

A = 2(0.0019) m² (top and bottom surface, sides negligible)

0.12 = 10 (0.0019 + 0.0019) ΔT

ΔT_Collectors = 3.14C

Bus Bars:

h_sigma = 10 W/m²⋅K (typical nickel to air natural convection)

A = 0.0023 m² (single surface)

0.089 = 10 (0.0023) ΔT

ΔT_{Bus Bars} = 3.74C

Cells:

h_steel = 10 to 25 W/m²⋅K (typical steel to air natural convection)

A = 288(0.00548) m²

600 = (10 to 25) (1.58) ΔT

ΔT_cells = 15.2 to 38 C

These values mean as long as the temperature difference between the surface and ambient air is ΔT, natural convection will dissipate all heat generated.

The ΔT_cells values mean that natural convection would most likely not be adequate to cool the battery given that the max ambient temperature allowed is 60C and the max surface temp is 80C. In other words ΔT_cells > 20C means that natural convection isn’t enough.

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Figure 1. Power v. Temperature testing by Batemo.com at 25C

The figure above is useful for context in calculating temperature. But the test was done in open air room temperature until 0% SOC or 80C, so we can’t really use the data. (Our power is 24W, 60/9A * 3.6V)

DESIGN

Cooling

The design will be a water cooling system consisting of a cold plate, pump/reservior enclosure, and radiator. The aluminum heat plate (bottom panel) underneath the battery pack will be attached to a water loop with parallel channels. This loop will connect to a water pump/reservoir combo input, and output to a radiator with two fans which has an exterior air intake and exhaust (ducts). The parts will be connected by tubes and quick disconnect fittings. The radiator/fans will be mounted to a 3D printed system attached to the frame. The reservoir/pump combo and control PCB will be mounted within an enclosure by the radiator. There might be an intake duct from the front cutouts to assist ventilation and cooling air temperature.

POWER GOAL: <25W

Ventilation

Regulations require a negative pressure fan assisted ventilation duct out the exterior of the vehicle away from any airstreams toward the driver. This is in the event that the battery explodes and there are dangerous fumes. There will be a 4in. duct intake to interior of the battery box and fan assisted exhausts.

Parts Selection

Radiator

• https://koolance.com/hx-240xc-radiator-2-fan-120mm-30-fpi-copper?srsltid=AfmBOoq4sMlnqOSSvEyK-fRTDcupgF3z5_n3zp0-cPsaEae84yp77AHd

• Estimate 1250-2000W heat transfer depending on intake duct speed

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Reservoir/Pump

• https://www.amazon.com/dp/B0CWTPP6X7/ref=sspa_dk_detail_0?pd_rd_i=B0CWTPP6X7&pd_rd_w=n4yhb&content-id=amzn1.sym.f2f1cf8f-cab4-44dc-82ba-0ca811fb90cc&pf_rd_p=f2f1cf8f-cab4-44dc-82ba-0ca811fb90cc&pf_rd_r=AFW6RRAX5M455HNTE4V8&pd_rd_wg=dDiG2&pd_rd_r=34b5a2c1-a288-49b7-a7d8-84a4cf83ac4b&s=electronics&sp_csd=d2lkZ2V0TmFtZT1zcF9kZXRhaWxfdGhlbWF0aWM&th=1 (order here)

• https://freezemod.com/product/freezemod-computer-water-cooler-magnetic-levitation-domestic-ddc-water-pump-pub-gcdcb-pm/ (more details)

• Highest flow rate (800 L/H) for <15W, cheap and lightweight

Fans

• https://www.digikey.com/en/products/detail/delta-electronics/AFB1212SH-F00/2560460

• <5W goal and high static pressure, 120mm

Tubing/Fittings

• https://koolance.com/tubing-clear-uv-reactive-pvc-13mm-x-16mm-1-2in-x-5-8in-3m?srsltid=AfmBOooJ2Rww4TUZ8V3FsRl22lMOn6h4hL1bdEdWH-Aw6INYNRU5qa2c (tubing, 13mm ID 16 OD)

• https://koolance.com/fitting-single-compression-for-13mm-x-16mm-1-2in-x-5-8in-black-fit-v13x16-bk (tube to pump/radiator fittings)

• https://koolance.com/fitting-single-swivel-angled-for-13mm-x-16mm-1-2in-x-5-8in-fit-l13x16 (90 degree)

•  https://koolance.com/quick-disconnect-no-spill-coupling-male-panel-barb-for-id-13mm-1-2in-qdt4-m13-p (cold plate QD fittings male)

• https://koolance.com/quick-disconnect-no-spill-coupling-female-panel-barb-for-id-13mm-1-2in-qdt4-f13-p (QD female)

• https://www.digikey.com/en/products/detail/festo-corporation/NPHS-S6-M-G14/13977124?gclsrc=aw.ds&&utm_adgroup=&utm_source=google&utm_medium=cpc&utm_campaign=PMax%20Shopping_Product_Low%20ROAS%20Categories&utm_term=&utm_content=&utm_id=go_cmp-20243063506_adg-_ad-__dev-c_ext-_prd-13977124_sig-CjwKCAiAiOa9BhBqEiwABCdG83GYjUwYOaXHrRCshNrJvYzM-OyM5_uF4XxXJBAXDW4EMnihEF0qFxoCdR8QAvD_BwE&gad_source=1&gclid=CjwKCAiAiOa9BhBqEiwABCdG83GYjUwYOaXHrRCshNrJvYzM-OyM5_uF4XxXJBAXDW4EMnihEF0qFxoCdR8QAvD_BwE&gclsrc=aw.ds (alternative)

Cold Plate

• Carbon fiber composite rectangular tubes to act as water channels, manufactured with 3D print mold

• Six tubes in parallel attached to 3D printed intake/exhaust manifolds → water loop

• Water loop is attached to bottom BB panel with thermally conductive epoxy

Intake/Exhaust Air Duct

• Intake duct from front intake of aeroshell to the radiator

• Exhaust duct from radiator to back exhaust of car

• 3D printed radiator mounting and front intake/exhaust designs

Pump Control PCB

• Inputs: temp and flow sensors, manual controls, dashboard controls

• Outputs: pump motor, LCD display with temp, flow, set speed, radiator fan output

• Connected to CAN with PSOM

Temperature Sensor

• https://koolance.com/sen-ap008b-coolant-temperature-sensor-brass-10k-ohm

• Thermistor, inline.

Flowrate Sensor

• https://koolance.com/ins-fm17h-coolant-flow-meter

• Hall Effect → vibration proof, more accurate. Inline.

• Requires ADC to read values

Internal Ventilation Parts

• Duct from front intake to interior battery, two exhaust ducts to exterior of car (fan assisted)

• 3D printed parts

• 2 fans

CAD

Mockup

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Mockup with Placement

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Alternative View to Show Air Ventilation Holes

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Radiator Mount

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Radiator Mount with Exhaust Funnel

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PumpRes + Pump PCB Enclsoure

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Intake Manifold

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Composite Water Channel

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Cold Plate

Mounting

Radiator

• Slide-on mount that is attached to frame tabs

• Mounted onto the interior side of frame, above the battery

Pump Enclosure

• Frame tabs on side of frame

Cold Plate

• Permanently epoxied to bottom panel of battery box (aluminum heat plate)

Simulations

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Radiator Funnel Velocity

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Radiator Funnel Pressure

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Intake Manifold Velocity

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Intake Manifold Pressure