A 4G or LTE modem is a deceptively difficult load. Its average consumption is modest — often 1-5 Wh per day for a modem that wakes, uploads and sleeps — but during transmission it draws a current spike many times higher. Systems sized only on daily watt-hours fail here, because the battery and cabling cannot deliver the peak.
The load profile
| State | Typical current at 12 V | Duration |
|---|---|---|
| Sleep / idle registered | 10-40 mA | Most of the day |
| Receive / idle connected | 100-250 mA | Seconds to minutes |
| Transmit burst | 0.5-2 A | Milliseconds to seconds |
| Weak signal transmit | up to 2.5 A and longer bursts | Repeated |
Note the last row: in a marginal coverage area the modem transmits longer and more often, sometimes at higher power. A site that works well in the city can drain a battery in the field purely because the signal is worse.
Design rules for cellular sites
- Size the battery on discharge current, not only on Wh. Choose a pack rated comfortably above the transmit spike.
- Add a capacitor bank near the modem so the burst is supplied locally rather than through long thin cable.
- Keep DC cable runs short and thick. Voltage drop during transmit is the most common cause of modem resets.
- Use a modem with a proper sleep mode and configure reporting intervals to match the actual need, not the default.
- Check coverage before design. Signal strength at the antenna position changes the whole energy budget.
Antenna, mast and surge protection
An external antenna on a mast improves signal but also raises the lightning risk. Fit a coax surge arrester at the entry point, earth the mast, and keep the antenna cable route as short and direct as possible. If the site is in a high-lightning area, an external directional antenna at lower height often beats a tall mast for both signal and survivability.
Energy budget example
A logger polling every 15 minutes and uploading hourly, with a modem transmitting 40 times a day at 1.5 A for 3 seconds:
Transmit energy = 40 × 1.5 A × 3 s / 3600 = 0.05 Ah — negligible. The real consumption comes from idle-connected time and the logger itself. This is why measurement beats estimation: the datasheet average rarely matches the deployed reporting profile.
When to add a second battery
Two separate packs — one small for the modem and one larger for the rest — used to be common practice. With modern LiFePO4 packs that have high continuous discharge ratings, a single well-sized pack plus local capacitance is simpler and more reliable. Split only when the modem is far from the main battery or when the loads have very different voltage requirements.
FAQ
How much solar power does a 4G telemetry site need?
Usually 60-150 Wp with a 3-7 day autonomy battery. The array is rarely the limiting factor; battery discharge capability and cable sizing usually are.
Why does my modem keep rebooting?
Almost always voltage sag during transmit. Check battery discharge rating, cable cross-section and connector quality before replacing the modem.
Can I use a small solar panel just for the modem?
Yes, if the modem is the only load and the panel plus battery are sized for the worst month. Separate small systems are common for standalone gateways.
We design telemetry power systems around the real reporting profile. Send us your modem and logger details or see small solar kits for sensors.