Deep cycle batteries for backup power
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Quick answer
A deep cycle battery is built to deliver steady power for hours and be drained and recharged many times, unlike a car starting battery that gives a short, big burst. For backup and solar, the main choices are lead-acid types (flooded, AGM, gel) and lithium iron phosphate (LiFePO4). A Renogy 12 V, 100 Ah AGM is rated 500 cycles if you stop at 50%. A LiTime 12 V, 100 Ah LiFePO4 is rated 4,000 cycles at full discharge, holds 1,280 Wh and weighs about 38% as much.
Deep cycle vs starting batteries
A starting battery is designed for a huge burst to crank an engine, then gets recharged by the alternator. A deep cycle battery is designed for long, steady draws: running an inverter, a trolling motor or lights all night. LiTime says its 12 V, 100 Ah LiFePO4 battery is suitable for energy storage rather than start-up. Renogy lists a burst current of 1,100 A for 5 seconds on its AGM, so some deep cycle AGMs can handle short high loads too. Check the label before you use one for both jobs.
The main chemistries
| Type | Upkeep | Usable energy per 100 Ah, 12 V | Notes |
|---|---|---|---|
| Flooded lead-acid | Check water levels, vent | About 600 Wh at 50% | Heavy. Gas while charging. |
| AGM | Sealed, maintenance-free | About 600 Wh at 50% (Renogy guidance) | Spill-proof, works with lead-acid chargers |
| Gel | Sealed | Lead-acid limits apply | Needs a charger with a gel or lead-acid profile |
| LiFePO4 | Built-in BMS | About 1,280 Wh (LiTime) | Light, long cycle life, no charging below freezing |
Battery Tender lists lead-acid, AGM, gel and sealed maintenance-free types as compatible with its lead-acid maintainers. Lithium needs a separate profile.
Sizing a deep cycle bank
- Add up your daily watt-hours, including any inverter's idle draw.
- Divide by the usable energy per battery: about 600 Wh for a 100 Ah lead-acid at 50%, about 1,280 Wh for a 100 Ah LiFePO4.
- Round up and add a cloudy-day or extra-day buffer.
Example: 1,800 Wh a day needs three 100 Ah AGMs (1,800 / 600) or two 100 Ah LiFePO4 (1,800 / 1,280, rounded up). By weight, that is 191.7 pounds of AGM (3 x 63.9) against 48.5 pounds of LiFePO4 (2 x 24.25).
Charging deep cycle batteries
- Match the profile. Renogy lists 14.4 to 14.8 V cycle charging and 13.5 to 13.8 V float for its AGM. LiTime lists 14.4 V, plus or minus 0.2 V, CC/CV for its LiFePO4.
- Size the charger. CTEK's 4.3 A NXT 5 NA covers 1.2 to 160 Ah, but a fully drained 100 Ah battery takes many hours at that rate. Battery Tender points large batteries to its 4 A Power Tender series.
- Mind the cold. Both Renogy's AGM and LiTime's LiFePO4 list charging from 32 degrees F up.
- Store charged. Renogy recommends charging to 100% before storage and recharging after three months.
Marine and RV deep cycle batteries
Many deep cycle batteries are sold for boats and RVs, which need the same long, steady draw as home backup. The labels to read are the same: amp-hours, cycle rating at a stated depth of discharge, weight, and charge limits. For a dedicated home backup bank, LiFePO4's usable energy per pound is hard to beat. If you already own lead-acid chargers and gear, an AGM bank is the simpler fit.
What to look for when you buy
12 V 100 Ah LiFePO4 deep cycle (example: LiTime 12V 100Ah)
Best for: Solar banks and daily cycling where weight matters
LiTime lists 1,280 Wh, 4,000 cycles at 100% DOD, a 100 A BMS, 24.25 pounds and a 5 year warranty.
Watch out: No charging below 32 degrees F. Not for engine starting.
See options on Amazon (opens Amazon in a new tab)12 V 100 Ah AGM deep cycle (example: Renogy Deep Cycle AGM)
Best for: Sealed lead-acid banks with existing lead-acid chargers
Renogy lists 500 cycles at 50% DOD, under 3% monthly self-discharge and sealed, maintenance-free, spill-proof construction.
Watch out: It weighs 63.9 pounds. Stop at 50% discharge.
See options on Amazon (opens Amazon in a new tab)Multi-chemistry smart charger (example: CTEK NXT 5 NA)
Best for: Charging both AGM and LiFePO4 deep cycle batteries
CTEK lists AGM optimisation and LiFePO4 support for 1.2 to 160 Ah 12 V batteries, with temperature compensation.
Watch out: At 4.3 A, recovering a flat 100 Ah battery takes a long time.
See options on Amazon (opens Amazon in a new tab)Questions people ask
- What is a deep cycle battery?
- A battery built for long, steady discharges and many recharge cycles, such as running an inverter or lights for hours, rather than a short engine-starting burst.
- How long does a deep cycle battery last?
- It depends on chemistry and depth of discharge. Renogy rates its AGM at 500 cycles at 50% DOD. LiTime rates its LiFePO4 at 4,000 cycles at 100% DOD.
- Can I use a car battery as a deep cycle battery?
- It is not designed for it. Starting batteries are built for short bursts, while deep cycle batteries like LiTime's are made for energy storage rather than start-up.
- How do I charge a deep cycle battery?
- With a charger set for its chemistry. Renogy's AGM uses 14.4 to 14.8 V, LiTime's LiFePO4 uses 14.4 V CC/CV, and both should be charged above 32 degrees F.
Sources
- Deep Cycle AGM Battery 12 Volt 100Ah, Renogy
- LiTime 12V 100Ah LiFePO4 Lithium Battery (specifications), LiTime
- NXT 5 NA, CTEK
- Battery Tender Junior 12V 750mA, Battery Tender
- Battery Tender Product Comparison, Battery Tender