AGM and lithium iron phosphate store, charge, and report energy differently, so the same numbers on two spec sheets can describe very different batteries. Here’s how to translate those numbers and match the chemistry to the mission.
Underneath the cameras, every mobile surveillance platform is an energy system. Its battery bank determines runtime when charging dips, recovery speed, and whether operators see a problem before coverage fails. Yet the two numbers most often used for comparison — amp-hours of storage and watts of charging — do not translate directly between AGM and lithium iron phosphate.
Two chemistries in plain terms
AGM (Absorbed Glass Mat) is a sealed lead-acid battery with the electrolyte held in fiberglass mats. That makes it spill-proof, position-tolerant, and maintenance-free. AGM has decades of field history, needs no electronics to operate, accepts a charge in freezing weather without a heater, and feeds one of the world’s most established recycling streams.
Lithium iron phosphate (LiFePO₄) is the most thermally stable of the common lithium chemistries used in off-grid power systems. Each battery ships with a battery management system (BMS) that supervises the cells, balances the pack, cuts off unsafe conditions, and keeps records.
Same numbers, different meaning
Put a 100Ah AGM battery next to a 100Ah lithium battery and the spec sheets look interchangeable. They aren’t, not because either label is wrong, but because the two chemistries deliver and absorb energy on different terms. Comparing them honestly means translating three numbers.
1. Nameplate storage vs. usable storage
A 12-volt, 100Ah AGM battery holds about 1,200 watt-hours on paper. Lead-acid pays for depth of discharge with lifespan: cycled to about 50%, it delivers its rated 300–500 cycles; drained deeper on a regular basis, cycle life falls off quickly. Operated the way its manufacturer intends, that battery offers about 600 usable watt-hours.
A 100Ah LiFePO₄ battery (12.8V nominal) holds about 1,280 watt-hours, and 80–100% of it is usable without a meaningful life penalty — while still delivering roughly 3,000–5,000 cycles.
The translation: compare usable watt-hours, not amp-hours
- 100Ah AGM: ≈1,200 Wh on the label → ≈780 Wh usable at the typically engineered % depth of discharge.
- 100Ah LiFePO₄ : ≈1,280 Wh on the label → ≈1,020–1,280 Wh usable, largely independent of load.
2. Rated capacity vs. capacity at your load
Lead-acid capacity is commonly measured at the 20-hour, or C20, discharge rate. Under that test, a 100Ah battery supplies approximately 5 amps for 20 hours. It may not deliver the same 100Ah under a heavier load.
What is the Peukert effect?
As current draw increases, a lead-acid battery’s effective capacity decreases. Internal resistance and electrochemical limitations make voltage fall faster, so the battery reaches its cutoff sooner. Doubling the load can reduce runtime by more than half: the nameplate capacity has not changed, but less of it is available at the higher discharge rate. This is the Peukert effect, and it is much more pronounced in AGM and other lead-acid batteries than in LiFePO₄.
When cameras, infrared illuminators, communications, and heaters draw more than the C20 test current, AGM may deliver less than its rating. Voltage sag can also trigger a low-voltage cutoff while energy remains in the battery. LiFePO₄ loses far less capacity at higher currents and maintains relatively stable voltage until nearly empty. At light, steady loads the difference may be modest; as continuous demand rises, lithium’s advantage grows or the AGM bank must grow with it.
3. Charging watts vs. recovered energy
Capacity tells you how much energy a bank can store, not how much charging energy it needs or how quickly it can accept it. Three characteristics matter.
Round-trip efficiency. Using representative design values of 80–85% for AGM and 95–98% for LiFePO₄, every 1,000 watt-hours supplied produces approximately 800–850 recoverable watt-hours from AGM and 950–980 from LiFePO₄ — a lithium advantage of roughly 100–180 watt-hours. Actual efficiency varies with the battery, temperature, charge rate, state of charge, and surrounding equipment.
Charge acceptance. Per 100Ah of capacity, AGM is commonly charged at approximately 10–25 amps; a comparable LiFePO₄ battery may accept 50–100 amps, depending on its specifications and BMS. Lithium can therefore make better use of a high-output charger or short window of strong solar production. The charger must still supply that current: on a 20-amp charger, neither battery initially receives more than 20 amps.
The absorption tail. AGM charging is front-loaded. In instrumented testing, batteries charged at their maximum recommended rate reached absorption voltage at around 65% state of charge. From there, accepted current tapered from more than 40 amps to about half an amp near full, and reaching 100% took about five and a half hours. A larger charger does little to shorten this stage because the battery limits the current.
LiFePO₄ has a much shorter voltage-limited finishing stage and generally maintains a high charging current through more of the cycle. In practice, lithium primarily needs charging watts; AGM needs watts plus enough time for the low-current absorption stage that completes the charge.
The honest comparison
Neither chemistry wins every row, and a fair comparison should say so.
| Comparison | AGM (sealed lead-acid) | Lithium (LiFePO₄) |
|---|---|---|
| Usable energy (100Ah nameplate) | ≈600–780 Wh, depending on the designed state-of-charge window | ≈1,020–1,150 Wh, planned across a 10–100% window |
| Capacity under heavy load | Shrinks at high draw (Peukert effect); voltage sags | Nearly rate-independent; voltage stays flat until almost empty |
| Recommended charge rate | 0.1–0.25C (10–25A per 100Ah) | 0.5–1C (50–100A per 100Ah) |
| Time to a true 100% | Hours of absorption regardless of charger size | Much shorter voltage-limited finishing stage; maintains a high charging current through more of the cycle |
| Round-trip efficiency | Representative design range: ≈80–85% | Representative design range: ≈95–98% |
| Life at partial charge | Needs regular full recharges; chronic undercharge sulfates the plates | Normal operation; no penalty |
| Charging below freezing | Accepts charge without a heater (at reduced efficiency) | BMS blocks charging near/below 32°F unless the pack is heated |
| Weight | Baseline | 60–75% lighter for the same usable energy |
| Upfront cost | Lower per nameplate amp-hour, though the gap has narrowed | Higher upfront; lower cost per delivered kilowatt-hour over its life |
| What it reports | External voltage only; a shunt improves the estimate | Measured state of charge, cell voltages, temperature, current, cycle count, fault logs |
AGM is often a strong fit for backup power, generator- or AC-supported platforms, deployments that return regularly for service, and applications that must charge below freezing without battery heaters. LiFePO₄ is often better suited to solar-only deployments, heavy continuous loads, weight-sensitive platforms, and fleets where detailed remote monitoring drives operations.
What each chemistry asks of the deployment
AGM requires regular opportunities to reach a complete charge. Sulfate crystals form during normal discharge, and prompt, complete recharging converts most of that material back into active material. If a conventional AGM battery remains partially charged, some crystals harden and become difficult to reverse, reducing available capacity.
The charging architecture determines whether that becomes a problem. AC- and generator-supported platforms can complete both bulk and absorption charging on schedule. Solar-only platforms have less control, especially during winter, when production is lower, days are shorter, weather varies, and continuous equipment loads consume power before the battery receives any surplus. The bank may appear to recover each day while repeated undercharging and the accumulation of hardened sulfate crystals gradually reduce capacity. The system therefore needs additional solar margin, scheduled generator or shore-power charging, or a rotation plan that periodically returns the bank to a controlled charger.
Lithium asks for warm cells and a good battery management system (BMS). A LiFePO₄ BMS refuses charging near or below 32°F to protect the cells — the one place AGM holds a genuine chemistry advantage. Northern systems therefore use heated batteries or enclosures. Lithium also requires a correctly configured charge profile, and manufacturer quality varies widely; the BMS is where good and bad lithium batteries diverge.
Neither requirement is a defect. Both are constraints the manufacturer must design around. “How did you engineer around the chemistry’s constraint?” is more useful than simply asking which chemistry it uses.
What monitoring looks like on each chemistry
An AGM battery reports one external signal: voltage. At rest, voltage is a workable proxy for state of charge; under load or charge, voltage sag and surface charge distort the reading, so a shunt-based monitor provides a better estimate. Chronic undercharging is otherwise easy to miss while capacity gradually declines. Remote monitoring makes that trend visible before a cloudy stretch exposes it as an outage.
A LiFePO₄ battery measures itself. Its BMS counts every amp-hour entering and leaving the pack and can report true state of charge, cell voltages, temperature, current, cycle count, and fault logs. Fleet operators can confirm recovery after cloudy weather and see degradation early enough to plan a replacement instead of responding to an outage.
Mobile Pro Systems therefore treats remote power and health monitoring as standard practice regardless of chemistry. Monitoring is not a lithium feature; it is an operating discipline. Lithium simply gives it more data.
Four questions to ask when evaluating stored power in your mobile surveillance trailer.
Either chemistry can power a deployment well when the system is designed around it. When evaluating any vendor, ask four questions:
- How many usable watt-hours does the bank deliver — across what planned state-of-charge window, and at my actual load current?
- What does a recharge look like in my hardest month — and what pushes through the absorption tail if solar can’t?
- What happens to the bank if it lives at partial charge for a month — and how would I know?
- What does the platform report about its battery remotely — a measured state of charge, or a voltage estimate?
The label says amp-hours and watts; the mission runs on usable energy, recharge behavior, season, and what operators can see remotely. Compare on those terms and choose the chemistry on purpose. Mobile Pro Systems configures platforms both ways and can run the numbers against your actual sites and seasons.
Talk to Our Team About Reliable Off-Grid Surveillance
If you’re evaluating mobile surveillance trailers for remote, temporary, or long-duration deployments, our team can help you compare power architectures, deployment requirements, and operational tradeoffs. Speak with Mobile Pro Systems sales to see how the Commander 3400 supports dependable uptime in the field.
