DoD is a percentage, not a feeling
Wikipedia’s depth-of-discharge article defines how much of the rated capacity you use before you recharge. Daily 80 percent is a different life from daily 30 percent. A trolling motor that dies at noon on a 36-volt string is often a DoD problem plus age, not a mysterious amp-hour sticker.
Flooded vs AGM vs lithium
BU-201a cites AGM around 80 percent depth of discharge versus about 50 percent for flooded. Lithium iron phosphate can use more of the rating, with a BMS that will disconnect when empty. Using flooded like lithium — emptying it every trip — is how a cheap group 31 lasts a season on the Harris Chain.
Runtime ratings sit underneath
Reserve capacity (minutes at 25 A, BU-105) and 20-hour amp-hours describe the tank. DoD describes how much of the tank you drink. Two batteries with the same Ah last different years if one owner deep-cycles and the other only cranks.
Charging has to catch up
A deep cycle that never gets a full charge is also a sulfation cycle (BU-802). DoD math assumes you refill. A trailer boat that runs Saturday and charges for twenty minutes in the driveway is not refilling an 80 percent hole.
How we use this on the visit
We ask how long the motor runs and how you charge. We do not lecture a number we did not measure. We do pick chemistry that matches the way the boat is actually used in Leesburg and Orlando.
A practical way to stay shallower
A larger Ah bank at the same voltage, or a second parallel 12-volt on a 12-volt motor, reduces how deep each cycle goes. That is a tray and weight conversation, not a moral one. If the motor died at noon on a 36-volt string, we look at age and cables first, then at whether the bank was ever big enough for that grass and wind. Chemistry comes after those two.
Sources
- Wikipedia — Depth of discharge
- Battery University BU-201a — Absorbent Glass Mat (AGM)
- Battery University BU-105 — Battery Definitions (RC at 25 A)
- Battery University BU-802 — What Causes Capacity Loss
If this is the job on your boat, see how we replace batteries at the boat. We come to you — Florida towns and counties.