Home / Guides / Depth of Discharge Explained — Why Half Your Battery Is Off Limits
Lead-acid tubular batteries are rated for roughly 50% depth of discharge: draining past half state-of-charge sulphates the plates and shortens life sharply. So a 150Ah 12V battery holds 1800Wh nominal but only about 900Wh is safely usable, before inverter losses. LiFePO4 cells tolerate 80-90% depth of discharge, which is why a smaller lithium pack matches a much larger tubular battery.
A lead-acid tubular battery discharged repeatedly below about half state of charge suffers accelerating plate sulphation, and its capacity falls permanently. Battery Buddy therefore applies a 50% depth of discharge to every lead-acid calculation. It is not pessimism; it is the condition under which the warranty cycle life is quoted.
| Battery | Nominal energy | Usable at 50% DoD | Usable after 70% efficiency |
|---|---|---|---|
| 100Ah | 1200Wh | 600Wh | 420Wh |
| 120Ah | 1440Wh | 720Wh | 504Wh |
| 135Ah | 1620Wh | 810Wh | 567Wh |
| 150Ah | 1800Wh | 900Wh | 630Wh |
| 165Ah | 1980Wh | 990Wh | 693Wh |
| 180Ah | 2160Wh | 1080Wh | 756Wh |
| 200Ah | 2400Wh | 1200Wh | 840Wh |
| 220Ah | 2640Wh | 1320Wh | 924Wh |
| 250Ah | 3000Wh | 1500Wh | 1050Wh |
LiFePO4 cells tolerate 80-90% depth of discharge and return 92-95% of what goes in. That is why a 100Ah lithium pack matches a 180-200Ah tubular battery in delivered energy while weighing a fraction as much. The catch is the charger: a lithium pack needs an inverter with a LiFePO4 charging profile, which many older Indian units simply do not have.
Compare batteries on usable watt-hours, not on the Ah number printed on the sticker. A 200Ah tubular at 50% depth of discharge and a 100Ah lithium at 90% are far closer than the labels suggest — and the honest comparison is the one that stops you overpaying in either direction.
The fraction of a battery's rated capacity you may safely use in a cycle. Lead-acid tubular batteries are limited to about 50%; LiFePO4 lithium cells tolerate 80-90%.
Discharging a lead-acid battery past roughly half state of charge sulphates the plates and permanently reduces capacity. The rated cycle life assumes you stop at 50%.
A 150Ah 12V battery holds 1800Wh nominal, about 900Wh after the 50% depth-of-discharge limit, and roughly 630Wh once 70% system efficiency is applied.
On delivered energy per kilogram and per cycle, usually yes. On upfront cost and compatibility with an existing Indian inverter, often no. Compare usable watt-hours and check whether your inverter has a LiFePO4 charging profile before switching.
Directly. It is a multiplier in the usable-energy formula, so halving it halves your backup time. Any calculator that omits it will overstate runtime by roughly 2x.
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