How to choose battery capacity, what sets AGM, gel, and LiFePO4 technologies apart, how to properly charge and store a battery — plus answers to the most frequently asked questions.
Battery capacity is given in amp-hours (Ah) and tells you what current the battery can supply for one hour before it discharges. To size it to your needs, first calculate your daily energy requirement — list your devices, their power (W), and expected daily operating time (h).
Ballpark figures to compare your result against:
These are three different technologies, suited to different applications. AGM (Absorbent Glass Mat) has electrolyte trapped in a glass-fiber mat and performs best in float/standby duty — where it's rarely deeply discharged. Gel batteries handle cyclic use and deeper discharges better, but cope less well with very high currents. LiFePO4 is the most modern technology — far more cycles, lower weight, and higher efficiency, at the cost of a higher purchase price.
| Technology | Cycles at 50% DOD | Weight | Deep discharge | Best use |
|---|---|---|---|---|
| AGM | 200–400 | Higher | Poorly tolerated | Starting, float duty, UPS |
| Gel | 400–600 | Higher | Well tolerated | Cyclic use, solar power |
| LiFePO4 | 4000–6000 | Low (~1/3 of AGM weight) | Very well tolerated (up to 80–90%) | Camper, boat, long-term use |
A single LiFePO4 battery can outlast several sets of lead-acid batteries in terms of cycles — so despite the higher upfront cost, it often turns out cheaper in the long run.
A "battery" isn't a single, uniform product — a completely different construction hides inside depending on the chemistry. Lead-acid batteries (AGM, GEL, PB, EFB) essentially share the same core and differ mainly in the state of the electrolyte and minor construction details. LiFePO4 is a completely different technology — lithium cells instead of lead plates.
Same core — electrolyte and construction differ
Inside each of them are lead plates (grids coated with lead paste, alternating positive and negative) submerged in electrolyte — dilute sulfuric acid. Between the plates lies a porous separator, which prevents short-circuiting while still letting ions through. The whole assembly is sealed in a case with safety valves that vent excess gas (hence the name VRLA — Valve Regulated Lead-Acid).
What sets the individual types apart is mainly the form of the electrolyte: in flooded batteries (PB) it's liquid and flows freely around the plates (hence the need to top up water); in AGM it's absorbed into a glass-fiber mat compressed between the plates (capillary effect — maintenance-free, spill-resistant); in GEL it's turned into a gel with a silica additive (even better resistance to shocks and tilting); in EFB the plates and fleece separator are reinforced to better withstand frequent, shallow cycles (e.g. start-stop in cars).
A different chemistry — lithium cells instead of lead plates
A single LiFePO4 cell consists of a cathode made of lithium iron phosphate coated onto aluminum foil, an anode made of graphite coated onto copper foil, and a thin polymer separator that lets lithium ions through while blocking electrons. The whole assembly is soaked in electrolyte — a lithium salt dissolved in an organic solvent — and wound or stacked in layers inside a sealed case (cylindrical or flat, so-called prismatic).
The crystal structure of iron phosphate is exceptionally thermally stable, which is why LiFePO4 is considered the safest of the widely used lithium chemistries — it's much harder to push into uncontrolled overheating (so-called thermal runaway) than, say, the cobalt cells used in laptops or phones. A BMS (Battery Management System) watches over every cell — electronics that balance the voltage of individual cells and cut off the battery in the event of overcharging, a short circuit, or overheating.
When a single battery isn't enough — because you need higher voltage (e.g. 24V instead of 12V) or more capacity (more Ah at the same voltage) — several batteries are connected into a bank. How you connect them determines what adds up: voltage or capacity. It's one of the questions we hear most often from customers, so we've diagrammed both variants below.
Voltage adds up — capacity stays the same
You connect the "−" of the first battery to the "+" of the second (a bridge in the middle), and connect the remaining free terminals — the "+" of the first and the "−" of the second — to the load or charger. Two 12V/100Ah batteries give you 24V this way, with capacity unchanged at 100Ah. This is how 24V systems are built from 12V batteries.
Connect batteries with the shortest cable possible.
Every few charges, we recommend disconnecting the batteries from the series/parallel bank and charging them separately to full — this extends their service life.
Capacity adds up — voltage stays the same
You connect all the "+" terminals together and all the "−" terminals together (two shared buses). It's best to take the power for the load or charger diagonally — the "−" from one battery and the "+" from the other (marked with a dot on the diagram) — this way both batteries share the load more evenly than drawing current from a single cell. Two 12V/100Ah batteries give you 12V this way, with 200Ah capacity.
| Current | 10A | 20A | 30A | 50A | 70A | 100A |
|---|---|---|---|---|---|---|
| Cable cross-section | 1.5–2.5 mm² | 4 mm² | 6 mm² | 10 mm² | 16 mm² | 25–35 mm² |
These are approximate values for short (up to ~50 cm) inter-battery connections. For longer cable runs (e.g. to a distribution panel, charger, or load), choose a thicker cross-section to limit voltage drop — use a cable cross-section calculator or the wire manufacturer's table for the specific length.
Each technology has a different recommended charging profile — different voltages and a different number of stages. That's why an ordinary car battery charger isn't a good choice: it doesn't adapt to the battery's state, and undercharging or overcharging is easy. A multi-stage charger (like the Toyama TBC models) automatically recognizes the charging stage and selects the right voltage and current — from initial plate desulfation, through main charging, to float mode.
Lead-acid batteries (GEL/AGM/PB/EFB) are usually charged at 14.4–14.7V. LiFePO4 batteries require a dedicated, shorter profile with a BMS activation stage at the start — a procedure that "wakes up" the cell-protection electronics before the actual charging current flows. Charging a LiFePO4 battery with a charger meant only for lead-acid batteries (without a dedicated lithium mode/profile) can damage it or trigger a BMS cutoff — always check that the charger supports the given chemistry.
A battery is best stored in a dry, ventilated place at 5–20°C — avoid unheated rooms where the temperature drops below freezing. Cold genuinely reduces available capacity: at 0°C it drops to about 80%, at -10°C to about 70%, and at -20/-25°C to as little as 50–60%.
If a battery isn't used for an extended period (e.g. seasonal equipment out of season), it's worth running a 4–6 week float charge lasting 8–12 hours. This prevents deep self-discharge and plate sulfation in lead-acid batteries. Terminals and contacts should be cleaned regularly and kept dry — corrosion on the terminals is one of the most common causes of starting problems.
This is a topic few people think about during actual use — not just during storage. Temperature affects how much power a battery can actually deliver at any given moment: the further you get from the optimal ~20–25°C (in either direction), the less energy it can supply, or the faster it wears out.
Cold slows down the chemical reactions inside the cell, which reduces the available capacity of lead-acid batteries (AGM, GEL) — at 0°C it drops to about 80%, at -10°C to about 70%, and at -20/-25°C to as little as 50–60% of the rated value. In LiFePO4 batteries, freezing temperatures mainly limit power (the maximum current the cell can deliver) rather than capacity itself — but charging LiFePO4 below 0°C is usually impossible or automatically blocked by the BMS, because charging a frozen cell risks permanent damage.
High temperature works paradoxically — it briefly increases the available capacity of lead-acid batteries a little, but drastically shortens their service life: it's generally accepted that every 8–10°C above 25°C can cut a battery's lifespan in half. A battery that would last 10 years at 25°C might only last around 5 years at a constant ~33°C.
| Temperature | Available capacity / power | Effect on lifespan |
|---|---|---|
| Below 0°C | AGM/GEL: drops to ~70–80%. LiFePO4: power heavily limited, charging usually blocked | Charging a frozen battery (esp. LiFePO4) can permanently damage it |
| -20 to -25°C | AGM/GEL: drops to as little as 50–60% | Deep frost accelerates plate degradation |
| 20–25°C (optimal) | Full, rated capacity | Longest lifespan |
| Above 25°C | Briefly somewhat higher capacity (lead-acid) | Lifespan can be cut in half for every 8–10°C rise |
That's why mounting a battery near an engine, a heater, or a sun-exposed locker on a boat is often a worse idea than it seems — even if the battery "works," at high temperature it loses service life much faster than in a cooler, well-ventilated spot.
It depends on the technology: AGM typically handles 200–400 cycles at 50% discharge, gel batteries 400–600 cycles, and LiFePO4 as many as 4000–6000 cycles. In practice, a standard lead-acid battery lasts 3–5 years, while LiFePO4 can serve several times longer.
We advise against it. LiFePO4 batteries require a dedicated charging profile with a BMS activation stage and a different final voltage than lead-acid batteries. A charger without a lithium mode may fail to fully charge the battery or cause damage — always use a multi-stage charger with a profile dedicated to LiFePO4.
The most common symptoms are weaker starting, dim or flickering lights, unstable electronics, and faster discharging than before. It's worth periodically checking the voltage with a multimeter — a fully charged 12V battery should read about 12.5–12.85V at rest.
The BMS (Battery Management System) is electronics that protect lithium cells from overcharging, overheating, or short circuits. If a battery is deeply discharged, the BMS can go into sleep mode — dedicated lithium chargers have a first stage that "wakes up" the BMS with a low current before the actual charging current flows.
An ordinary charger supplies a fixed current/voltage regardless of the battery's state, risking overcharging. A multi-stage charger automatically moves through successive phases (including desulfation, main charge, absorption, and float), adapting its parameters to the current state of charge — this charges faster and more safely, extending the battery's service life.
No. A starter battery delivers a short, very high current to start the engine and spends most of its time fully charged. A deep-cycle battery (e.g. GEL, LiFePO4) is designed for repeated, prolonged discharging and charging — it performs well in campers, on boats, or in solar installations.
In a dry, ventilated place at 5–20°C, ideally fully charged. Every 4–6 weeks it's worth running a float charge (8–12h) to prevent deep self-discharge and plate sulfation in lead-acid batteries.
Yes, provided it's a multi-stage charger with a float mode — once charged, it automatically switches to a low maintenance current and protects against overcharging. Don't leave an ordinary charger connected unattended for extended periods.
For a typical compressor fridge in a camper/caravan, a good starting point is a 110–120 Ah battery — it will cover the fridge and minor electronics for a weekend without recharging. It's worth calculating the exact value based on your devices' actual power draw — see the "Choosing capacity" section above.
Get in touch with us or browse the full range of Toyama batteries and chargers — we'll recommend the best solution for your application.