1. The range degradation curve – one picture tells you everything

A lot of users just go by gut feeling: “It doesn’t seem to last as long as it used to.” But you can easily draw a simple “range degradation curve” for yourself.

How to do it:

1. During the first month of use, on a standard lawn (fixed grass height, terrain, and speed), record how many minutes or square metres you can mow on a full charge. Call that value 100%.
2. Every 2‑3 months of use, test the runtime again under the same conditions and calculate the percentage.
3. Plot those percentages on a piece of paper and connect the dots.

What a normal degradation curve looks like: The first 100 cycles drop fairly quickly (10‑15%), then the middle 300 cycles decline slowly and steadily, and the final 200 cycles accelerate downward. Overall it’s a shallow “S” shape.

When should you replace the battery?

• When runtime drops below 60% of the original value, you should start thinking about a replacement. At that point, the mower often stops mid‑job and hurts your efficiency.
• If the curve shows a sudden “cliff” – for example, last month it was still at 80% and this month it suddenly falls to 50% – that suggests a damaged cell or serious imbalance inside the battery. Get it checked and replace it soon.

Practical tip for farms with several mowers: Keep a small notebook next to the charger and jot down “how long did I mow this time” before each recharge. After a month, the average runtime tells you a lot. You don’t need expensive instruments – your phone’s stopwatch is fine.

2. Internal resistance testing – the “thermometer” of battery ageing

The range curve shows the result, while internal resistance is one of the causes. As a battery ages, its internal chemical activity slows down and ions have a harder time moving. That shows up as increased “internal resistance”. The higher the internal resistance, the more the voltage sags under high current (like when starting the mower or hitting thick grass). That’s when the mower may suddenly stop or the battery protection kicks in.

What is internal resistance?

In simple terms: it’s the resistance inside the battery. A new battery has very low internal resistance – like a wide, fast road. An old battery has high internal resistance – like a narrow alley with a traffic jam. For current to flow through the battery, it has to overcome this resistance. The higher the resistance, the more heat is generated, and the less usable energy you get out.

How can a regular user measure internal resistance?

Method 1: Use a charger or battery tester that has internal resistance measurement
There are handheld battery internal resistance testers available for a few tens to around two hundred dollars (like the kind RC hobbyists use). Operation is simple:

• Fully charge the battery and let it sit for half an hour.
• Touch the two probes to the positive and negative terminals of the battery (watch polarity).
• Read the internal resistance value, shown in milliohms (mΩ).

Reference values (for a typical 40V lithium‑ion mower battery pack as an example):

• New battery: individual cell internal resistance around 15‑25 mΩ. The whole pack, depending on series/parallel configuration (e.g., 5‑series, 10‑series), is usually between 50‑100 mΩ.
• Healthy: internal resistance increased by up to 50% from new – OK for normal use.
• Keep an eye on it: increased by 50‑100% – runtime noticeably shorter. Plan to replace soon.
• Must replace: increased by more than 100%, or individual cells show abnormally high resistance (you can see voltage differences with a balance charger). At this point the battery can’t deliver enough current, runs very hot, and may be a safety risk.

Method 2: The “poor man’s” voltage sag test
No special tester? You can still get a rough idea:

• Measure the open‑circuit voltage of a fully charged battery (e.g., a 40V battery fully charged is about 42V).
• Connect a known load (for example, run the mower at maximum speed with no grass – just the blades spinning), and measure the voltage at the same time.
• If the voltage instantly drops below 36V under load (normally it should stay above 39V), the internal resistance is too high. Test it a few times to confirm, then replace.

Reference values for different battery types

• Lead‑acid (older mowers): internal resistance varies a lot with temperature. A typical 20Ah battery has an internal resistance of 10‑20 mΩ. When that number doubles, the battery is essentially dead.
• Lithium iron phosphate (LiFePO₄): very long cycle life, internal resistance increases slowly. Consider replacing when resistance rises more than 80% above new.
• Lithium‑ion (ternary / NMC): common in lightweight mowers. Internal resistance is more sensitive – a 50% rise already gives noticeable range loss.

3. Besides the curve and internal resistance, here are other signs you need a new battery
4. Charging time suddenly shortens – used to take 2 hours to fully charge, now it shows full in 30 minutes. That means capacity has dropped sharply.
5. The mower suddenly cuts out while mowing – the display shows two bars left, but as soon as you hit thick grass it shuts down. This happens when internal resistance causes voltage to drop below the protection threshold.
6. The battery gets very hot – normal use makes it warm. If it’s too hot to touch (above 50°C) after just 10 minutes of mowing, internal resistance is likely very high.
7. The mower shows a “battery fault” error – many smart mowers have a BMS (battery management system) that reports errors. Don’t ignore repeated error codes.
8. Practical advice for farm owners and dealers

If you are a farm owner:

• Give each battery a number and keep a simple record: “purchase date + runtime per charge”. Look at the degradation curve every couple of months – you’ll know where you stand.
• Don’t wait until the battery is completely useless to replace it. When runtime drops below 60% of new, buy a fresh battery ahead of time. The old one can serve as a backup or for lighter jobs.
• Store batteries over winter at 40‑60% charge, at room temperature. Extreme cold or heat accelerates internal resistance rise.

If you are a dealer:

• Offer a “battery health check” service for your customers. An internal resistance tester gives results in a few minutes. That’s a value‑added service and helps you anticipate when customers will need replacement batteries – driving parts sales.
• Help customers understand that battery ageing is not a quality defect – it’s normal physics. Teaching them about the degradation curve and internal resistance testing actually reduces complaints about the mower itself.
• Suggest that customers buy an extra battery when they purchase a mower. Rotating two batteries cuts the number of cycles on each in half, and overall pack life often increases.

Final thoughts

You don’t need advanced engineering to decide when to replace a mower battery. A home‑made range degradation curve plus an inexpensive internal resistance tester (or even a simple voltage‑sag test) is enough to make a sound decision. Remember two key thresholds: runtime below 60% of new, or internal resistance increase above 100% – if you hit either one, it’s time to start shopping for a new battery. Replacing it in time keeps your mowing efficient and avoids unexpected shutdowns and safety risks from an aged battery. We hope this article helps. Next time we’ll talk about “how to extend battery life” – stay tuned.