CCA, Ah and reserve capacity: the three numbers decoded
Every label carries three figures that measure three different things. Buying the wrong one for your application is how people end up with a battery that "should" work and doesn't.
Cold cranking amps (CCA)
CCA is the current a battery can deliver for 30 seconds at 0°F (−18°C) while staying above 7.2V. It measures starting ability in cold weather and nothing else. A higher CCA figure does not mean more usable energy — it usually means thinner plates, which are less tolerant of deep discharge.
Related: CA and MCA (marine cranking amps) are the same test at 32°F (0°C), so MCA numbers look roughly 20-30% higher than CCA for the same battery. Comparing a CCA figure with an MCA figure is one of the most common spec-sheet errors.
Amp-hours (Ah)
Ah is capacity: the current a battery can supply for 20 hours at 77°F before falling to 10.5V, for a 12V battery. A 100Ah battery can nominally deliver 5A for 20 hours. Real capacity varies with discharge rate — pulling 50A from the same battery gets you far less than 2 hours — which is why lithium, with a flatter discharge curve, appears to out-perform its rating in practice.
Reserve capacity (RC)
RC is minutes at 25A before dropping below 10.5V. It is the most honest number for an application like a boat with a failed alternator, or a car with a failing alternator limping home.
| Figure | Measures | Matters most for |
|---|---|---|
| CCA | 30s burst at 0°F | Starting engines in cold weather |
| Ah (20hr) | Slow capacity | House banks, golf carts, solar |
| RC | Minutes at 25A | Backup / limp-home capacity |
| Wh | Energy (Ah × V) | Comparing across voltages |
The four chemistries in one paragraph each
Flooded lead-acid
Cheapest per amp-hour, tolerant of abuse, needs watering and venting, and must not be discharged below about half. Best value where weight and maintenance do not matter.
AGM
Sealed, spill-proof, better vibration resistance, charges slightly faster, and costs 40-80% more. Good where the battery is hard to access or must not leak.
Gel
Similar to AGM but requires lower charge voltages and is easy to ruin with a standard charger. Rarely the right choice now that LiFePO4 prices have fallen.
LiFePO4
Deep discharge without damage, 3,000-5,000 cycles, near-flat voltage curve, no maintenance, and 3-5× the cost up front. Wins on cost per cycle wherever the equipment gets regular use; loses where the battery is rarely cycled or sits in freezing conditions.