Portable Power

Supplying Power to Your Equipment in the Field

What kind of battery do I chose?

What are you going to use it for?

Battery Chemistry

• Lead-Acid (Car battery)
• Sealed lead-acid (SLA)
• Absorbent glass mat (AGM)
• Lithium Iron Phosphate �(LiFePO4)
• Lithium-Ion (USB Power Banks)

��We’re not going to go into the weeds on how the different chemistries work, this is more of an introduction on their strengths and weaknesses and how to choose the right one.

Battery Chemistry: Lead-Acid

• Commonly used for vehicles
• Unless labeled “Deep Cycle”, built to start an engine
• Initial high amperage output to “cold start” vehicles
• Widely available and affordable
• Heavy, hard to transport
• Voltage drops off significantly during use
• Needs to be maintained
• Off-gasses during charge!
• Can leak if not stored properly
• Operates well in a wide temperature range

Battery Chemistry: Sealed lead-acid

• Referred to as SLA
• Commonly used for alarm systems, RC toy�cars, golf carts, UPS
• Generally cheap
• Small and hefty, easy to transport
• Voltage drops off during use
• Needs to be float charged
• Safer than lead-acid
• Doesn’t off-gas during charge
• Can be stored on its side
• Fairly wide temperature range (-4 - 122 F)

Battery Chemistry: Absorbent Glass Mat

• Referred to as AGM
• Commonly used where a lot of power storage is needed, in a climate-controlled environment.
• Sold by automotive battery stores, not cheap
• Super heavy, hard to move
• Voltage drops off during use
• Needs to be float charged
• Safer than lead-acid
• Doesn’t off-gas during charge
• Keep above freezing�(32 - 122 F)

Battery Chemistry: Lithium Iron Phosphate

• Referred to as LiFePO4
• Starting to replace SLA due to power density and weight
• Availability improved bringing prices down
• Very light, easy to carry
• Voltage remains steady until none
• Holds charge for long periods
• Self-aware: BMS
• Keep above freezing�(32 - 122 F)

Battery Chemistry: Lithium-Ion

• Referred to as Li-ion
• Used in small devices like cell phones, handheld radios, power drills
• Widely available, cheap
• Super light, portable
• Voltage remains steady
• Holds charge for long periods
• No maintenance
• Keep above freezing,�don’t operate when hot�(59 - 95 F)

20,000 mAh at 5 V USB-A and USB-C PD

Use cases

• Outdoor or vehicle: �Lead-Acid

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• Indoor, uninsulated �(RV exterior or shed): �Lead-Acid or Sealed Lead-Acid

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• Indoor, climate controlled: �Absorbent Glass Mat (AGM), LiFePO4 or Li-ion

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• Portable (Car to picnic table): �LiFePO4 or Li-ion

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• Portable (Hiking): �Small LiFePO4 or Li-ion

How long will it last?

Understanding capacity, duty cycle and runtime

Understanding capacity

Most batteries list their Ah or mAh, some list Watt Hours

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Convert with Watt’s Law

• Amp-Hours (Ah):
• Measures current over time.  
• Watt-Hours (Wh):
• Measures total energy capacity.  
• Conversion: �Wh = Ah x Voltage; Ah = Wh / Voltage

Converting capacity with Watt’s Law

Converting the 20,000 mAh, 5 V battery to its aH at 12 V:

1. Calculate Watt-Hours (Wh):
• First, convert mAh to Ah: 20000 mAh / 1000 = 20 Ah
• Then, calculate Wh: 20 Ah * 5 V = 100 Wh
2. Calculate Amp-Hours (Ah) at 12V:
• Now, divide the Wh by the new voltage: 100 Wh / 12 V = 8.33 Ah�

Therefore, a battery that supplies 20000 mAh at 5 volts would supply approximately 8.33 amp-hours at 12 volts.�

Why? Watt-hours ideally remain constant regardless of voltage.

Radio Voltages

• Mobile and desktop radios like 12 V or better
• Modern batteries, car alternators, and AC adapters may be 13-15 V
• Radios with internal batteries often come in at lower voltage
• Use external batteries that supply 12+ volts for higher watt-output from the radio�
• Yaesu FT-817nd rating:
• 9.6 V, 3 watts
• 12 V, 5 watts

Runtime and Duty Cycle

Duty Cycle should be considered when calculating how long a battery will last.

• How much time is spent�TX vs RX over time
• FM: Continuous draw �at TX, each person �may talk 90 seconds
• SSB: Short bursts
• FT8/JS8: 15-second�cycles
• Radio at idle still draws�some power

Image borrowed from https://www.geeksforgeeks.org/duty-cycle/

Calculating Duty Cycle

Duty Cycle (%) = (Transmit Time / Total Cycle Time) x 100%

• FM is continuous steady draw at transmit (TX)
• Talk time: 90 seconds per person, in a conversation
• With two people, 90 seconds each = 180 seconds or 3 minutes
• Duty cycle is 50% = (90 / 180) x 100%

How long will it last?

• Yaesu FT-817nd:
• 2 amps at 12 V when TX
• .5 amps at RX
• Lithium-ion battery: 8 aH
• If duty cycle is 100%: 4 hours
• Duty cycle: 50%

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Average Current: �(Transmit Current * Duty Cycle) + (Receive Current * (1 - Duty Cycle))

(2 amps * 0.5) + (0.5 amps * 0.5) = 1 amp + 0.25 amp = 1.25 amps

�Runtime = 8 Ah / 1.25 A = 6.4 hours – longer, if you turn the TX power down!

Battery maintenance & Safety

Charging, storage, care, avoiding fire

Battery Management System (BMS)

Remember where we said some batteries are “self aware?”

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Noting that commonly, “batteries” are made up of smaller cells:

A BMS is an electronic system that monitors and manages a rechargeable battery or battery pack. It's like a smart controller that ensures the battery operates safely and efficiently.

...BMS Continued

• Monitors temperature
• Shuts off if too cold / warm
• Prevents charging if too cold

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• Monitors voltage
• Cuts off if too low
• Prevents overcharging

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• Monitors amperage
• Cuts off if draw is too high
• Radio draws too much
• Protects itself from shorting

Chargers

Select a charger that states that it...

1. Supports your battery’s chemistry
• This will ensure it provides the right voltage
2. Provides the amperage the battery supports
• Smaller batteries expect a slower charge rate than larger
3. Power source matches charger’s amperage
• AC Adapter will overheat and could catch fire, if it supplies �5 amps and the charger draws 10+

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Solar panels should only be plugged into chargers that support their higher voltage and variable amperage!

Fuses

• Take into account the �amperage capability of �your battery�Check label or manual
• How much will your radio �draw at transmit?�Radio specs
• Pick the inline fuse accordingly

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Battery Safety

• Don’t use when above 104 F
• Charge above freezing
• Use correct charger
• Use fuses!
• Avoid shorting
• Don’t store in devices
• Best to store ~50%

Wrapping up...

Closing thoughts and use case

Summary

Now we should be able to:�

• Choose the right battery chemistry for the right use case�
• Select a capacity to get us through our activation without hauling more than we need into the field�
• Use and take care of our batteries safely

Show and Tell

Photos: https://photos.app.goo.gl/in3U85UbU5FRUD3m9