Portable power and batteries for field operation

Battery power changed portable operating twice. First when lithium iron phosphate made a usable capacity practical to carry, and again when the price fell far enough that a 20 Ah pack costs less than a decent antenna. A field station that used to mean a car battery in a milk crate now fits in a shoulder bag.

What has not changed is the physics. A 12 V pack can deliver hundreds of amps into a short circuit, lithium chemistries are unforgiving about charge profiles and temperature, and the fuse people fit is usually in the wrong place.

This page covers sizing, fusing, charging and solar, in that order.

Why LiFePO4 rather than anything else

The other advantage is the discharge curve. A LiFePO4 pack holds close to 13 V for most of its discharge and then falls quickly, so a radio sees a stable supply almost to the end. A lead-acid battery sags continuously, and a radio at 11.5 V produces distorted audio long before the battery is empty.

Sizing the pack

The mistake is to size on transmit current. In real operating, receive dominates because you spend most of the time listening.

1. Find your radio receive current and transmit current at the power you actually use. Both are in the manual, and both are lower than you expect at reduced power.
2. Estimate your transmit fraction. Casual operating is around 20 percent, a busy park activation with a queue is closer to 40, and a digital run is close to 50.
3. Average current equals receive current times the listening fraction plus transmit current times the transmitting fraction.
4. Divide the usable capacity by the average current for hours of operation. A 20 Ah LiFePO4 pack has about 19 Ah usable.
5. Work an example: a QRP radio drawing 0.5 A on receive and 5 A on transmit at 20 percent duty averages 1.4 A, which is roughly 13 hours from a 20 Ah pack.
6. Check the arithmetic for your own case on the battery runtime calculator.

Fusing, which is the part that matters

A LiFePO4 pack can deliver a very large current into a short circuit, limited by its internal resistance and its management system rather than by anything sensible. A shorted cable at the battery end, with the fuse at the radio end, is an unfused conductor carrying hundreds of amps.

• Fuse at the battery terminal, as close to the post as physically possible. That fuse protects the cable, which is what actually catches fire.
• Size the fuse to the wire, not to the load. The purpose is to open before the cable overheats. A 30 A fuse on 10 AWG cable is correct even if the radio only draws 5 A.
• Fuse both conductors on a radio power lead, which is why radios ship with fuses in both the positive and negative leads.
• Use a proper distribution point rather than splices. A Powerwerx PD-5F fused distribution block panel gives every device its own fuse.
• Never bypass a blown fuse. It opened for a reason and the reason is still there.
• Carry spares. A blown fuse ends an activation otherwise.

Charging correctly

Use a LiFePO4 charger

The charge profile is different from lead-acid: a constant current phase then a constant voltage phase at around 14.4 V, with no float and no equalisation stage. A lead-acid charger with a desulphation or equalisation mode applies pulses well above that and damages the cells.

Never charge below freezing

Charging any lithium chemistry below zero degrees Celsius plates metallic lithium onto the anode, which permanently reduces capacity and creates an internal short risk. Discharging in the cold is fine. Charging is not. Many packs include low-temperature charge cutoff, and you should not rely on it.

The battery management system is a safety net, not a controller

It disconnects on over-current, over-voltage, under-voltage and often temperature. Every one of those events is something you should have prevented. A pack that regularly hits its protection thresholds is a pack being misused.

Storage

LiFePO4 is happiest stored around half charge in a cool place, and it self-discharges very slowly. Unlike lead-acid it does not need to be kept topped up, and leaving it on a charger for months is worse than leaving it alone.

Adding solar

A folding panel turns a one-day battery into an indefinite one, which matters for multi-day events and for emergency preparedness. It is also a source of noise if you are careless.

• You need a charge controller between panel and battery, matched to LiFePO4. Connecting a panel directly to a battery is a way to destroy the battery.
• A FlexSolar 100 W foldable solar panel folding panel produces enough in reasonable sun to keep a QRP station running indefinitely while operating.
• Cheap MPPT controllers are switching converters and some of them are extremely noisy across HF. Test yours before you rely on it, and site the controller and its cabling away from the antenna.
• Panel output falls off dramatically with angle and shade. A panel flat on the ground produces a fraction of what the same panel produces aimed at the sun.
• Size the panel to the average draw, not the peak. A 100 W panel comfortably covers a station averaging 1.5 A.

What to buy

Field power

A pack sized to the day, a way to distribute it, and a panel if the day turns into a weekend.

GOLDENMATE

GOLDENMATE 12 V 20 Ah LiFePO4 battery

A 20 Ah LiFePO4 pack with a built-in management system. The standard size for a full day of QRP operating.

Check price$67.49

ERYY

ERYY 12 V 30 Ah LiFePO4 battery

Thirty amp hours for a station running higher power or a multi-day event, still at a weight you can carry.

Check price$75.99

DCDG

DCDG 12 V 6 Ah lithium battery pack

A small pack for a handheld, a QRP radio or a summit activation where every ounce is argued over.

Check price$34.99

FlexSolar

FlexSolar 100 W foldable solar panel

A folding panel that keeps a low-power station running indefinitely in reasonable sun.

Check price$80.99

Powerwerx

Powerwerx PD-5F fused distribution block

A fused distribution panel so each device has its own protection rather than sharing one fuse and one splice.

Check price$50.85

smseace

30 A quick-disconnect power connectors, 40-piece

Standard connectors. At a field site, everything plugging into everything else is worth more than it sounds.

Check price$9.99

The whole field station, priced with weights and a running total, is the portable field kit build.