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A deep-cycle battery for solar is a battery built to be discharged and charged as a regular job. A starter battery is built to deliver a short, high current and then be refilled at once by an alternator. Those are different machines. Putting a car battery on a house bank is how a solar system “works” for a few months and then will not hold the night. The chemistry inside the deep-cycle case can be flooded lead-acid, a sealed lead-acid, or a lithium iron phosphate pack. The duty is what the name deep-cycle is pointing at. The chemistry comparison, including which cycle figures come from a lab and which come from a warranty, is already in solar energy storage battery types. This page stays on the job.

What a deep-cycle battery is asked to do

In an off-grid system the battery is the supply after sunset. The modules refill it the next day. How many hours of load that bank must cover is a design choice, not a property of the word deep-cycle. Living with that arithmetic is in living off grid with solar storage. On a grid-tied house the battery’s job is different. It may move midday solar into the evening, or it may carry selected circuits through an outage. The Department of Energy’s description is the modest one: batteries store energy and make it available later. Later has to be specified. A battery that is never cycled is not doing the deep-cycle job, whatever is printed on the case.

Depth of discharge is the share of the stored energy you actually use before recharging. Lead-acid banks are usually operated in a shallower window than the nameplate suggests, because the usable energy is not the whole nameplate. Lithium iron phosphate packs are sold for a deeper daily cycle. The exact window is on that battery’s sheet. A blog cannot print one percentage for every brand. Ask what capacity remains at the end of the warranty, at what depth, and whether the clock is years, cycles, or energy throughput, whichever comes first. A warranty cycle count is not a lab measurement of life. The master distinction is worth repeating: the paper is a remedy, not a survival curve.

Why a starter battery is not a deep-cycle battery

A starter battery is optimized for the engine’s few seconds of cranking. The plates are built for that burst. A solar bank does the opposite: a long, slower discharge, then a charge that may be unfinished if the afternoon clouds over. Using the starter as a house bank shorts its life in a way the car never asked of it. On a boat or an RV the two batteries often sit near each other, which makes the mistake easy. The starter stays the starter. The house bank is the deep-cycle battery, on its own charge profile. One controller left on a flooded-lead voltage will not correctly charge a lithium house bank, and it should not be asked to cook the starter either. Bank separation is a wiring design. It is described from the boat side in a solar charge controller on a boat and from the vehicle side in solar panels for an RV.

Charge control is part of the battery, not an accessory you add if there is money left. A PWM controller pulls the array down near battery voltage. An MPPT controller converts the array’s higher-voltage power into charging current. Either one still has to stop at the voltage the battery maker allows, and the temperature compensation has to be the battery’s, not a default. Settings faults look like “the battery will not fill.” They are controller problems, covered in solar charge controller problems.

What a deep-cycle battery does not decide

It does not decide the inverter size. The inverter has to start the motors and run the overlapping load. A large battery behind an inverter that cannot start the well pump is a dead pump with a full battery. Sizing that forming inverter is in how to size an off-grid inverter. A grid-tied inverter will not become a deep-cycle system because you added a battery in the same room. Backup requires a transfer scheme. IEEE 1547-2018 still requires a grid-connected resource to stop energizing an island within 2 seconds.

It does not decide long-duration storage for a factory. A flow battery keeps energy in tanks and power in a stack, so the two can be sized apart. That architecture is attractive for multi-hour stationary storage and awkward in a hallway. It is a different machine from a deep-cycle pack, explained in what a flow battery is. If the quote in front of you is a wall battery for a house, you are in the deep-cycle conversation. If the quote is tanks and pumps, you have left it.

Temperature still writes the result. A pack at the temperature the test assumed is not a pack in a closed shed in summer or an unheated room in January. Follow the battery’s operating window. Do not invent a wider one because the chemistry is popular. Ventilation for flooded lead-acid is a gas question, not a preference. Lithium packs have their own installation rules from the manufacturer and from the local code. This page will not replace either.

Frequently asked questions

Is a deep-cycle battery always lithium iron phosphate?

No. Deep-cycle describes the duty. Flooded and sealed lead-acid batteries have been built for that duty for decades. Lithium iron phosphate is the chemistry many new house banks use because it is sold for daily cycling. Compare the sheet in front of you with the chemistry table on the battery-types page, and do not treat a warranty cycle count as a lab result.

Can a deep-cycle battery start an engine?

Some can supply the current, and the manufacturer may say so. A starter battery still should not be the house bank. If one battery is asked to do both jobs, you have combined two duties and you should be able to point to a design that allows it, not to a convenient post.

How many days should a deep-cycle battery cover?

As many as you decided the site must survive without sun or a generator. There is no standard autonomy. Off-grid design states the days. Grid-tied storage states the evening or the outage circuits. A number copied from another house is not either of those.

Sources and references

Storage as energy held for later use: US Department of Energy, grid modernization. Cease-to-energize within 2 seconds: NREL and Sandia, IEEE 1547-2018 primer. PWM versus MPPT charge control: Victron technical note. Flow batteries as a separate architecture, with power and energy sized apart: Sandia, DOE electricity storage handbook, redox flow chapter. Chemistry-by-chemistry cycle figures are not repeated here. They are sourced on the battery-types article this page links to.