4D and 8D Marine Batteries: Big Cases for Big Loads
4D and 8D are the heavyweights of marine batteries. You’ll find them on larger boats with diesel engines, big house loads, inverters and generators. They’re powerful, but lead-acid versions are among the heaviest objects you’ll ever lift on a boat. This guide explains the two sizes, typical ratings, and how lithium alternatives compare. Check product pages for exact dimensions and current specs.
Sizes
| 4D | 8D | |
|---|---|---|
| Length (approx.) | 20.75 in | 20.75 in |
| Width (approx.) | 8.75 in | 11 in |
| Height (approx.) | 9.9 in | 9.9 in |
| Typical voltage | 12V | 12V |
These are typical BCI dimensions; check the listing for each battery. The two share a length and height, but the 8D is about 2¼ inches wider. Interstate labels its marine 4D as 4DM.
Lead-acid 4D and 8D in our store
| Model | Type | Capacity | CCA | Reserve | Weight |
|---|---|---|---|---|---|
| Interstate SRM-4D | Flooded deep cycle | 195Ah | 1,314 | 390 min | Not listed |
| Trojan 8D AGM | AGM dual purpose | 230Ah (20-hr) | Not listed | 460 min | 161 lb |
The SRM-4D’s 1,314 CCA makes it suitable for starting large engines, while its 195Ah capacity supports house loads. Trojan’s 8D AGM is a sealed dual-purpose battery for big diesel starting and house banks.
Where 4D and 8D batteries are used
- Diesel starting: large inboard diesels on trawlers, sportfishers and sailboats often need the cranking power of a 4D or 8D.
- House banks: one or two 8Ds can run lights, refrigeration, pumps and an inverter on cruising boats.
- Windlass and thruster banks: high-current loads at the bow sometimes use dedicated 4D or 8D batteries.
The weight problem
At 161 pounds, a lead-acid 8D needs two people and good access to install, and a bank of them adds hundreds of pounds. For flooded batteries, there’s also maintenance and ventilation to consider. Many owners replacing an 8D bank look at lithium for that reason alone.
Lithium 8D-size batteries
| Model | Voltage | Capacity | Energy | Weight | Notes |
|---|---|---|---|---|---|
| EPOCH 12V 300Ah Pro dual purpose | 12V | 300Ah | ~3.8 kWh | 81 lb | 8D footprint, IP67, heated, 300A continuous |
| EPOCH 12V 460Ah Essential | 12V | 460Ah | 5.89 kWh | 84 lb | Heated, Bluetooth, IP54 |
| EPOCH 12V 460Ah V2 Elite | 12V | 460Ah | 5.89 kWh | Check listing | IP67, heated, Bluetooth |
| EPOCH 24V 230Ah Essential | 24V | 230Ah | 5.89 kWh | 84 lb | Heated, 200A continuous |
| EPOCH 24V 230Ah V2 Elite | 24V | 230Ah | 5.89 kWh | 97 lb | IP67, heated |
| EPOCH 48V 100Ah V2 Elite | 48V | 100Ah | 5.12 kWh | 97 lb | IP67, heated |
Comparing energy
A 230Ah lead-acid 8D used to 50% delivers about 115Ah, or roughly 1.4 kWh. An EPOCH 12V 460Ah Essential delivers most of its 5.89 kWh, about four times as much, at roughly half the weight. In practical terms, one lithium 8D can replace a bank of three or four lead-acid 8Ds for house use.
Can lithium start my diesel?
Only use a lithium battery for starting if it’s rated for cranking and your engine maker approves. EPOCH’s 12V 300Ah Pro is listed as a cranking and deep-cycle dual-purpose battery. Standard deep-cycle lithium batteries shouldn’t be used for engine starting. Many owners keep a lead-acid or AGM start battery and switch the house bank to lithium.
Higher-voltage banks
EPOCH’s 24V 230Ah and 48V 100Ah models put a high-voltage bank in one 8D-size case. These suit 24V boat systems, 48V electric propulsion and large inverters. They also support parallel connections for even bigger banks; check each listing for limits.
Installation considerations
- Mounting: 8D-size batteries need strong trays and hold-downs. EPOCH sells an 8D bracket for its V2 and V2-T models, and a tray for its 12V 460Ah and 24V 230Ah batteries.
- Cable size: 4D and 8D batteries often feed high-current loads. Cable must be sized for the current and run length.
- Fusing: use a Class T or other high-interrupt-capacity fuse close to large lithium banks, sized per the system design.
- Charging: upgrade to a lithium-compatible charger and alternator regulation, or add a DC-DC charger. Large banks may need a high-output charger.
- Professional help: converting a large bank can involve inverters, alternators and battery monitoring. Many owners have a marine electrician plan the job.
Maintaining lead-acid 4D and 8D batteries
Flooded 4D batteries need regular watering and a ventilated compartment. AGM 8D batteries are sealed but need correct charging voltages; overcharging shortens their life. Both should be kept fully charged when the boat isn’t in use.
Signs your 4D or 8D needs replacing
- The engine cranks slowly even after a full charge.
- The house bank drops in voltage quickly under normal loads.
- Resting voltage falls noticeably overnight with everything off.
- A load test shows the voltage falling below 9.6V at half the CCA rating for 15 seconds.
- The case is swollen or leaking, or terminals are badly corroded.
Because these batteries are heavy and expensive, test before replacing. A weak charger or a parasitic drain can make a healthy battery look bad.
Warranty
Interstate’s SRM-4D lists 6 months of free replacement. EPOCH’s 8D-size lithium batteries list 11 years. Trojan’s listing doesn’t show warranty terms, so check Trojan’s terms.
More reading
For smaller sizes, see group 31 and GC2 golf-cart batteries. Read what 24M, 27M and 31M mean for marine labeling, and how to measure a battery compartment before ordering. The full chart is in BCI battery group sizes explained, and the marine battery buying guide covers choosing a battery.
Products mentioned
- Interstate SRM-4D
- Trojan 8D AGM
- EPOCH 12V 460Ah Essential
- EPOCH 12V 300Ah Pro dual purpose
- EPOCH 8D battery bracket
Frequently asked questions
What’s the difference between a 4D and an 8D?
Both are about 20.75 inches long, but the 8D is wider and typically holds more capacity.
How much does an 8D battery weigh?
Trojan’s 8D AGM lists 161 pounds. EPOCH’s 12V 460Ah Essential lithium lists 84 pounds.
Can a lithium battery replace an 8D?
Yes, for house use. For starting, use a lithium battery rated for cranking and approved by your engine maker.
Are 8D batteries 12V?
Lead-acid 8Ds are 12V. Lithium 8D-size batteries come in 12V, 24V and 48V.
Do I need a special charger for a lithium 8D?
Yes. Use a LiFePO4-compatible charger sized for the bank.