What is a good internal resistance for a LiPo battery?

For FPV LiPos, internal resistance (IR) under 5 mΩ per cell is excellent (like new), 5-8 mΩ is good, 8-12 mΩ shows normal aging, and 12-18 mΩ indicates significant degradation. Measure IR at approximately 22°C for accurate readings. Cold cells show artificially high IR values.

When should I retire an FPV LiPo battery?

Retire a LiPo when: IR exceeds 18 mΩ per cell, cell-to-cell IR delta exceeds 3 mΩ, the pack shows physical puffing, IR has doubled from baseline, or the pack returns excessively hot after mild flights. Any puffing combined with IR above 12 mΩ means immediate retirement due to fire risk.

How do I measure LiPo internal resistance?

Use a charger with IR measurement capability (like the ISDT Q8, HOTA D6 Pro, or ToolkitRC M6D). Connect both the balance lead and main lead, start a 1C charge, and read IR values after 30 seconds. Record per-cell values — do not rely on averages. For professional accuracy, use a dedicated meter like the Wayne Giles ESR Meter.

Does high internal resistance cause voltage sag?

Yes. Internal resistance directly causes voltage sag under load. As IR increases with battery age, the pack drops more voltage when motors draw current. This manifests as sluggish throttle response, reduced punch, and faster heating. A pack with double its baseline IR has effectively lost half its C-rating.

FPV LiPo Internal Resistance Guide: Testing, Tracking, and Battery Retirement

# FPV LiPo Internal Resistance Guide: Testing, Tracking, and Battery Retirement

A LiPo battery does not suddenly fail — it gradually degrades, and internal resistance (IR) is the single best metric for tracking that wear. This guide explains what IR is, how to measure it, what values are normal, and when to retire a pack before it becomes a fire risk.

## What Is Internal Resistance?

Internal resistance is the opposition to current flow **inside** the battery cells. As a LiPo ages, chemical degradation increases IR, which has two critical effects:

– **Voltage sag under load**: Higher IR = more voltage drop when the motors pull current. This manifests as sluggish throttle response and reduced punch.
– **Heat generation**: Current flowing through resistance generates heat (P = I²R). A degraded pack runs hotter and degrades faster in a vicious cycle.

**All IR measurements should be taken at approximately 22°C (72°F)**. Cold cells show artificially high IR; hot cells show artificially low IR.

## How to Measure Internal Resistance

### Method 1: Charger with IR Mode (Recommended)

Most modern chargers include IR measurement:

**Procedure**:
1. Connect the balance lead AND main lead
2. Start a charge at 1C
3. Read IR values after 30 seconds of charging (first reading may be inaccurate)
4. Record all cell values — do not rely on the average

### Method 2: Dedicated IR Meter

Dedicated meters like the **Wayne Giles ESR Meter** provide the most accurate IR readings by applying a known current pulse and measuring voltage drop. These are used in professional battery testing but cost $80-120.

## Tracking IR Over Time

A single IR reading tells you current health. A log tells you trends.

| Date | Pack Name | Cell 1 | Cell 2 | Cell 3 | Cell 4 | Max Delta | Notes |
|——|———–|——–|——–|——–|——–|———–|——-|
| Jan 15 | CHL 1300 #1 | 3.8 | 3.9 | 3.7 | 4.0 | 0.3 | Good |
| Mar 15 | CHL 1300 #1 | 4.2 | 4.3 | 4.1 | 4.5 | 0.4 | Slight increase |
| May 15 | CHL 1300 #1 | 5.1 | 5.5 | 4.8 | 5.3 | 0.7 | Noticeable sag now |

Track IR for every pack, every 10-15 cycles. Rising IR across all cells is normal aging. But watch for:

## Warning Signs: When to Retire a LiPo

### 1. Cell IR Delta > 3 mΩ

When one or two cells have significantly higher IR than the others, the pack is failing unevenly. This causes the weaker cells to sag deeper and heat faster, accelerating failure.

### 2. IR Doubles from Baseline

If your pack started at 4 mΩ per cell and is now 8-9 mΩ, it has lost roughly half its effective C-rating. Time to demote it to bench testing duty.

### 3. Physical Puffing

Puffing = gas buildup from electrolyte decomposition. Any puffing combined with IR > 12 mΩ means immediate retirement. Puffed packs can rupture and catch fire under charge or discharge.

### 4. Heat After Mild Flights

A healthy pack returns warm (40-45°C) after aggressive flying. If the same pack returns hot (55-60°C) after cruising, IR has climbed too high.

## Retirement Protocol

| Step | Action |
|——|——–|
| 1 | Discharge to 0V per cell (using charger discharge function) |
| 2 | Cut off connectors (save XT60/XT90) |
| 3 | Twist balance leads together (shorts cells permanently) |
| 4 | Drop off at any electronics retailer with LiPo recycling |

**Never throw LiPos in household trash.** Even discharged cells can short-circuit and ignite under compression.

## Building a Healthy Battery Fleet

Quality chargers with accurate IR measurement are the foundation of battery health tracking. The **UAVModel HOTA D6 Pro** charger features dual-channel charging, per-cell IR measurement at every charge cycle, and automatic storage charging — making battery maintenance effortless. Check it out at [uavmodel.com](https://uavmodel.com).

Leave a Comment Cancel Reply