Battery group size chart: amp-hours for group 24, 27 and 31
A battery’s group number (24, 27, 31…) is a Battery Council International case size: it fixes the outside dimensions and terminal layout, not the capacity. So “how many amp-hours is a group 31?” has a range for an answer. Here are real figures from two manufacturers’ data sheets.
Amp-hours by group size (20-hour rate)
| Group | Battery (type) | Amp-hours, 20-hr | Weight |
|---|---|---|---|
| 24 | Crown 24HDC (flooded deep cycle) | 90 Ah | 50 lb |
| 24 | Crown 24MDC (flooded marine deep cycle) | 80 Ah | 40 lb |
| 24 | Crown 12CRV80 (AGM) | 80 Ah | — |
| 24 | Crown 12CE75 (sealed lead-acid) | 75 Ah | — |
| 24 | Trojan 24TMX (flooded) | 85 Ah | — |
| 24 | Trojan SCS150 (flooded) | 100 Ah | — |
| 24 | Crown V12-100 (LiFePO4, 12.8 V) | 100 Ah | 27.6 lb |
| 27 | Crown 27HDC (flooded deep cycle) | 115 Ah | 58 lb |
| 27 | Crown 27MDC (flooded marine deep cycle) | 105 Ah | 49 lb |
| 27 | Crown 12CRV100 (AGM) | 98 Ah | — |
| 27 | Trojan 27TMX (flooded) | 105 Ah | 55 lb |
| 27 | Trojan SCS200 (flooded) | 115 Ah | — |
| 31 | Crown 31MDC (flooded marine deep cycle) | 115 Ah | 54 lb |
| 31 | Crown 12CRV110 (AGM) | 110 Ah | — |
| 31 | Crown 12CE110 (sealed lead-acid) | 110 Ah | — |
| 31 | Trojan 31-AES (AGM) | 104 Ah | — |
| 31 | Crown V12-120 (LiFePO4, 12.8 V) | 120 Ah | 35.3 lb |
The short answers
- Group 24: about 75–100 Ah in these data sheets — most lead-acid models 80–90 Ah.
- Group 27: about 98–115 Ah.
- Group 31: about 104–120 Ah, the largest of the three. Crown also sells a 130 Ah deep-cycle battery, the 31HDC, listed under group 30H.
Lithium changes the picture: the same case can hold more usable energy. Crown’s group 24 lithium battery is 100 Ah at 12.8 V and weighs 27.6 lb against 50 lb for its group 24 flooded battery — and you can plan to use about 80% of it rather than 50%. That is why a 100 Ah LiFePO4 battery is often described as a drop-in for group 27 or 31 lead-acid: how many watt-hours in a 100Ah battery compares the usable energy.
Why one battery has several amp-hour ratings
Lead-acid capacity depends on how fast you take it out. Trojan’s 27TMX is rated 85 Ah at the 5-hour rate, 97 Ah at 10 hours, 105 Ah at 20 hours and 117 Ah at 100 hours. Trojan’s FAQ explains that the 100-hour rate uses a much lower current, so the battery delivers more amp-hours. Comparisons only make sense at the same rate — the 20-hour figure is the usual one in the US.
| Trojan 27TMX | Amp-hours | Average current |
|---|---|---|
| 5-hour rate | 85 Ah | 17 A |
| 10-hour rate | 97 Ah | 9.7 A |
| 20-hour rate | 105 Ah | 5.25 A |
| 100-hour rate | 117 Ah | 1.17 A |
An inverter running a microwave pulls far more than the 5-hour current, which is why lead-acid batteries deliver noticeably less than their label under heavy loads — see how long a 100Ah battery lasts with a 1000W inverter.
Deep cycle, starting and dual purpose
Group sizes are shared by car starting batteries and deep-cycle batteries, and they are not interchangeable in use. Trojan’s FAQ describes a starting battery as built for brief bursts of high current that can’t withstand more than a few deep discharges; a deep-cycle battery is built to be discharged deeply many times. Dual-purpose batteries are a compromise. For a solar, RV or boat house bank, buy deep cycle — and compare capacity in amp-hours, not cold cranking amps.
What else the group number fixes
- Case dimensions: a group 31 is longer than a group 27, which is longer than a group 24 — check your battery box or tray before upgrading.
- Terminal layout: posts, studs or both, and which side is positive.
- Not the voltage: these groups are 12 V, but 6 V golf-cart batteries (GC2) are another group entirely and are wired in pairs.
Turning amp-hours into runtime
Multiply amp-hours by 12 V for watt-hours, take the usable share — about 50% for lead-acid, 80% for LiFePO4 — and divide by your load. A group 31 flooded battery at 115 Ah gives about 690 Wh usable; that runs a 12 V compressor fridge averaging 25 W for about 28 hours. The battery runtime calculator does it for any load, and how long a 100Ah battery runs a fridge works the fridge example through.
CalculatorAmp-Hours to Watt-Hours Calculator →Convert any battery’s amp-hours into watt-hours and usable energy.Frequently asked questions
How many amp hours is a group 31 battery?
In Crown’s and Trojan’s data sheets, about 104–115 Ah for lead-acid (flooded and AGM) at the 20-hour rate, and 120 Ah for Crown’s group 31 lithium battery.
How many amp hours is a group 27 battery?
About 98–115 Ah at the 20-hour rate in these manufacturer data sheets — Trojan’s 27TMX is 105 Ah, Crown’s 27HDC 115 Ah.
How many amp hours is a group 24 deep cycle battery?
About 75–100 Ah in these data sheets; most flooded and AGM models are 80–90 Ah at the 20-hour rate.
Is a bigger group number a bigger battery?
Not necessarily — group numbers are case codes, not a size scale. Among 24, 27 and 31, though, the case does get larger in that order, and capacity tends to follow.
Sources
- Crown Battery — Deep Cycle Battery Product Specifications (effective Oct 1, 2024) (opens in a new tab) — 20-hour capacities by BCI group for flooded, AGM and lithium 12 V batteries.
- Trojan Battery — 27TMX data sheet (opens in a new tab) — Group 27 flooded: 85 Ah (5-hr), 97 Ah (10-hr), 105 Ah (20-hr), 117 Ah (100-hr); state of charge vs open-circuit voltage table; charger settings.
- Trojan Battery — 24TMX product specifications (opens in a new tab) — Group 24 flooded: 70 Ah (5-hr), 78 (10-hr), 85 (20-hr), 94 (100-hr).
- Trojan Battery — SCS150 and SCS200 product specifications (opens in a new tab) — SCS150, group 24: 100 Ah (20-hr). SCS200, group 27: 115 Ah (20-hr).
- Trojan Battery — 31-AES product specifications (opens in a new tab) — Group 31 AGM: 83 Ah (5-hr), 104 Ah (20-hr).
- Trojan Battery — Technical support FAQs (opens in a new tab) — “We generally recommend a discharge of 20 to 50% of the battery’s rated capacity even though the battery is capable of being cycled to 80%”; 20-hour and 100-hour ratings; freezing points by state of charge.