AGM batteries for backup and solar storage
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Quick answer
An AGM (absorbed glass mat) battery is a sealed lead-acid battery that is spill-proof and maintenance-free. For backup power it is simple and handles high current. But it is heavy, and you should only use about half its capacity. Renogy's 12 V 100 Ah deep cycle AGM is rated 500 cycles at 50% depth of discharge and weighs 63.9 pounds. That is about 600 usable watt-hours per cycle. A 100 Ah LiFePO4 battery like LiTime's is rated for 4,000 cycles at full discharge and weighs 24.25 pounds.
What AGM means
In an AGM battery, the electrolyte is held in glass mats instead of sloshing as liquid. Renogy describes its deep cycle AGM as a sealed, low-maintenance design that is spill-proof. It credits lead-acid chemistry with high chemical stability compared with lithium-ion. It also says you do not need to add water. Renogy recommends placing it upright.
Usable energy: the 50% rule
Lead-acid batteries last longer when you do not drain them deeply. Renogy says not to discharge its AGM below 50% to avoid shortening its cycle life, and it rates the battery at 500 cycles at 50% depth of discharge. Compare usable energy per cycle:
| 12 V, 100 Ah battery | Usable depth | Usable energy | Rated cycles | Weight |
|---|---|---|---|---|
| Renogy deep cycle AGM | 50% | About 600 Wh (12 x 100 x 0.5) | 500 at 50% DOD | 63.9 lb |
| LiTime LiFePO4 | 100% | About 1,280 Wh (12.8 x 100) | 4,000 at 100% DOD | 24.25 lb |
So for the same amp-hour label, the lithium battery gives you roughly twice the usable energy per cycle, at about 38% of the weight (24.25 divided by 63.9). AGM still makes sense when you want sealed, simple lead-acid that works with older chargers and gear.
AGM specs worth checking
Renogy's 12 V 100 Ah AGM listing shows what to compare across brands:
- Self-discharge: under 3% per month at 77 degrees F.
- Charging: 14.4 to 14.8 V cycle charge, 13.5 to 13.8 V float, 30 A maximum continuous charge.
- Burst current: up to 1,100 A for 5 seconds.
- Temperatures: charge from 32 to 122 degrees F, discharge from -4 to 140 degrees F, store from -4 to 140 degrees F.
- Banks: connects in series, and up to 4 in parallel.
Sizing an AGM bank for an outage
Work backward from what you need each day, then double it for the 50% rule. Say your essentials use 1,200 Wh a day, for example a router, LED lights and a small inverter's own draw. Following Renogy's 50% guidance, each 12 V, 100 Ah AGM gives about 600 Wh, so you would need two in parallel. That is 127.8 pounds of battery (2 x 63.9). The same 1,200 Wh fits in one LiTime 100 Ah LiFePO4 rated at 1,280 Wh. Renogy allows up to 4 of its AGMs in parallel, so a lead-acid bank can grow, but weight and floor space grow with it. Whatever you choose, add a charging plan: solar, a generator run outdoors, or grid power before the storm.
Charging and storing AGM batteries
Use a charger with an AGM or lead-acid profile that hits the maker's voltages. CTEK's NXT 5 NA, for example, includes an AGM optimisation mode. Battery Tender's lead-acid maintainers list AGM compatibility. For storage, Renogy recommends charging to 100% before storing and recharging after three months. A smart maintainer can hold it at float instead. Renogy notes its AGM battery has no self-heating, so cold-weather charging limits apply.
Where AGM fits in a prepared home
- Sump pump backup and small inverters that were designed around lead-acid.
- Generator, vehicle and equipment starting, where high burst current matters.
- A first, simple solar bank when weight is not a concern and you will respect the 50% rule.
For long daily cycling or anything you carry, LiFePO4 gives more usable energy per pound. See our LiFePO4 and deep cycle battery guides.
What to look for when you buy
12 V 100 Ah deep cycle AGM (example: Renogy Deep Cycle AGM)
Best for: Sealed, simple backup and solar banks built around lead-acid gear
Renogy lists 500 cycles at 50% DOD, under 3% monthly self-discharge, 1,100 A burst for 5 seconds and series or parallel (up to 4) banks.
Watch out: It weighs 63.9 pounds and should not be discharged below 50%.
See options on Amazon (opens Amazon in a new tab)Smart charger with an AGM mode
Best for: Recharging AGM banks to the right voltage
CTEK's NXT 5 NA lists an AGM optimisation mode, 4.3 A output and 1.2 to 160 Ah coverage.
Watch out: Large AGM banks need a bigger charger to recover quickly.
See options on Amazon (opens Amazon in a new tab)12 V 100 Ah LiFePO4 as the lighter alternative
Best for: Daily solar cycling or portable setups
LiTime lists 1,280 Wh, 4,000 cycles at 100% DOD and 24.25 pounds for its 12 V 100 Ah battery.
Watch out: Needs a LiFePO4 charge profile and will not charge below 32 degrees F.
See options on Amazon (opens Amazon in a new tab)Questions people ask
- What is an AGM battery?
- A sealed lead-acid battery with its electrolyte held in absorbent glass mats. Renogy describes its AGM as spill-proof and maintenance-free.
- How deep can I discharge an AGM battery?
- Renogy recommends not going below 50% to protect cycle life, and rates its 100 Ah AGM at 500 cycles at 50% depth of discharge.
- AGM or lithium for a backup battery?
- Lithium gives about twice the usable energy per 100 Ah and weighs far less. AGM is sealed, simple and suits lead-acid chargers and equipment.
- How often should I charge a stored AGM battery?
- Renogy recommends charging to 100% before storage and recharging after three months, or keeping it on a maintainer.
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