FPV LiPo health: storage voltage, internal resistance and when to retire a pack

The voltages that matter

A LiPo cell is full at 4.20 V. The numbers below are per cell, so multiply by the cell count for the whole pack (a 6S pack is full at 25.2 V).

Landing around 3.5 V per cell leaves margin. Below that you trade pack life for a few extra seconds in the air. Set the OSD warning and the low-voltage alarm with this in mind, because "one more lap" is how packs end up at 3.3 V.

Store at 3.8 V, not full

At 3.80-3.85 V per cell a LiPo holds roughly 40-50% charge. That is its most stable state, which is why new packs usually arrive at about this voltage. Leaving packs full for long periods raises internal resistance and can make them puff.

The FPV Freedom Coalition makes a useful point: the time at full charge adds up. Ten separate days left full do about the same harm as ten days in a row. Most pilots accept a day or so at full charge. If you charged for a session that got rained off and you will not fly in the next day or two, run the charger's storage mode.

Where you keep the packs matters too. Use a LiPo bag or a metal ammo box, and keep them at room temperature. Heat and cold both shorten battery life.

Track internal resistance, and track it the same way

Internal resistance (IR) is the most honest health number you have. C ratings on the label are marketing; IR is something you can measure. Higher IR means more voltage sag under load and more heat. As a worked example from the FPV Handbook, a 6S pack with 21 mΩ total at 120 A sags about 2.5 V and dissipates roughly 300 W as heat inside the pack.

Most decent chargers report IR, but the numbers are only comparable when you control the conditions. Oscar Liang lists what changes the reading:

• the charge state (measure at a fixed point, such as 3.8 V or 4.2 V per cell);
• temperature (IR reads lower when the pack is warm);
• the charger itself, since hobby chargers are not calibrated instruments. Use the same one every time.

Larger and higher-rated packs have lower IR, so compare a pack to its own history, not to a different model. Record each pack's IR when it is new. That baseline is what makes later readings mean something.

When to retire a pack

Retire it right away if:

• it is puffed, or feels soft, and the swelling stays after it cools;
• it is dented or punctured, or the leads are damaged;
• it gets noticeably warm while charging at a sensible rate. Oscar Liang warns that a common cause is an internal short, which can be extremely dangerous.

Plan to retire or downgrade it (to a slower build, or bench use) when:

• total IR has roughly doubled from its new baseline;
• one cell reads well above the others and the pack keeps coming back unbalanced after flights. Small spreads such as 3.55 / 3.59 / 3.61 V are normal;
• the charger puts back about 80% of rated capacity or less after you land at your usual voltage;
• the low-voltage alarm goes off much earlier than it used to, or flights are clearly shorter than on sibling packs of the same age.

You will often read that a LiPo lasts around 300 cycles. Treat that as a ballpark. A pack that has been stored well and landed at 3.5 V can outlast that, and one that sat full in a hot car can fail far sooner.

Disposal

Do not throw a dead LiPo in household waste. Discharge it following the manufacturer's guidance and take it to a battery recycling point. Until then, keep a puffed pack in the same fire-safe container you use for storage, and do not charge or fly it.

Charging habits that avoid most problems

• Never charge unattended.
• Let a pack cool after flying before you charge it, and do not charge one that took a hard crash.
• Balance charge, so the charger watches each cell.