- What depth of discharge changes
- Charge stages are not the same as depth
- How to talk about depth without a fake percentage
- Frequently asked questions
- What depth of discharge is safe for a solar battery?
- Do lithium batteries need bulk, absorption, and float?
- Does a deeper discharge always ruin the battery?
- Sources and references
Depth of discharge is the fraction of a battery’s capacity that you have used. A full battery is at zero depth of discharge. A battery you have emptied halfway, by the manufacturer’s definition of empty, is at 50 percent. The fraction you are allowed to use is on the battery sheet. It is not a universal 50 percent for lead-acid and 80 percent for lithium. Those round numbers appear in brochures. They are not this article’s recommendation, because the sheet in front of you can be stricter, and a warranty cycle count is a contract, not a measurement of how deep you may go tonight.
What depth of discharge changes
Usable energy is the battery’s rated energy times the fraction you may cycle, at the current and temperature the manufacturer states. A 10-kilowatt-hour label is not 10 kilowatt-hours of nightly backup if the sheet tells you to stop at a higher state of charge, or if the inverter’s cutoff arrives first. Home backup duration is this arithmetic plus the circuit list, not a slogan: home battery backup for outages.
Deeper cycling, on chemistries that care, uses up the warranty’s cycle budget faster. The budget is whatever the warranty counts, at the depth it specifies. A lab cycle on a cell is not that budget. This page does not import a cycle life from the battery-types article. It tells you to read the cycle table that matches the pack you own, including the depth column. Chemistry, and why a starter battery is the wrong object, is in deep-cycle batteries for solar and solar energy storage battery types.
Voltage is a poor depth gauge on a flat lithium curve unless the manufacturer published the table you are using. A lead-acid battery’s voltage moves more, and it still depends on the load and the temperature. Do not invent a “12.0 volts means 50 percent” rule for a battery whose sheet does not say it. The controller’s state-of-charge estimate is only as good as its calibration to that battery.
Charge stages are not the same as depth
Depth is how far you went. Charge stages are how the controller fills the battery on the way back. Flooded and many sealed lead-acid batteries are charged in bulk, then absorption, then float. Bulk is the high-current stage while the battery is down. Absorption holds a voltage so the last part of the charge finishes. Float holds a lower voltage so a full battery is not boiled. Equalization, a controlled overcharge, is allowed only for some flooded types, at the voltage and duration the manufacturer prints. It is not a monthly ritual for every solar battery.
Lithium iron phosphate usually does not want that sequence. The sheet often gives a charge voltage and an end of charge, and it forbids equalization. A float meant for flooded lead-acid can be the wrong place to hold a lithium pack. Temperature cutoffs, especially a block on charging below the temperature the sheet names, are part of the lithium profile. Defeating them is not a deeper discharge. It is damage. Settings live on the controller: solar charge controller parameters. Maintenance by chemistry is in solar battery maintenance.
A charge controller applies these stages. An inverter that also charges from a generator or the grid has its own charger settings, which must match the same sheet. Two chargers that do not agree will fight, or one will finish a charge the other immediately undoes. Solar controllers and DC-DC chargers are different paths into the same battery: DC-DC charger versus solar charge controller.
How to talk about depth without a fake percentage
| Phrase | What to require before you believe it |
|---|---|
| “Use only 50 percent” | The battery sheet’s recommended maximum depth, at a stated rate. If the sheet says something else, the sheet wins. |
| “Lithium can go to 100 percent” | The usable window the manufacturer warrants. Many packs are not operated from absolute empty to absolute full. The BMS cutoff is part of the product. |
| “Equalize every month” | A sentence in the manual of this flooded battery. Absent that sentence, do not equalize. Never equalize a lithium pack that forbids it. |
| “The voltage says it is half full” | A voltage-to-depth table for this chemistry, at this temperature, at rest or under the load the table names. A generic table is not that. |
Off-grid sizing uses usable energy, the days of autonomy you chose, and the loads. A deeper allowed discharge makes a smaller bank look adequate and can shorten the life the warranty describes. A shallower discharge does the reverse. Neither choice is free. Living with that bank is in living off grid with solar storage. Voltage class, 24 or 48, does not set the depth. It sets the current. A 48-volt pack can be used gently or abused. The class is in 24 volts versus 48 volts.
A flow battery does not use this depth language in the same way. Energy is the volume of electrolyte, and power is the stack. Discharging it is not a solid plate being cycled to a percentage a lithium warranty would recognize. If that is the machine in the yard, read its own limits: what a flow battery is.
Frequently asked questions
What depth of discharge is safe for a solar battery?
The maximum depth on that battery’s sheet, at the conditions the sheet names. There is no universal safe percentage in this article. A warranty that counts cycles at a stated depth is a contract about that depth, not permission to go deeper because the battery “seems fine.”
Do lithium batteries need bulk, absorption, and float?
Only if their manual says so. Many lithium iron phosphate batteries use a charge voltage and a stop, and they forbid equalization. Applying a flooded lead-acid profile because the controller’s default is that profile is a settings error. Load the profile the battery maker published.
Does a deeper discharge always ruin the battery?
It uses the cycle life the manufacturer associated with that depth, which may be shorter. It is a problem immediately if you pass the low cutoff and the manual says not to. It is not a problem merely because a forum’s favorite percentage was crossed, if your own sheet allows it. Read the sheet.
Sources and references
Charge-controller behavior depends on the battery voltage it is allowed to see, which is why a lead-acid profile and a lithium profile are not interchangeable settings: Victron, on the controller’s relationship to battery voltage. Flow batteries store energy in tanks rather than in a solid depth-of-discharge percentage: Sandia redox-flow chapter. No universal depth of discharge and no cycle-life number are stated. Both belong to the battery document for the pack in question.
