Introduction: Ryobi Battery Capacity

About: Lazy Old Geek

So Home Depot sells a good line of tools called Ryobi One+. They’re easily identified with their lime green color. While not very decorous, the advantage is that they usually stand out in the field and are easy to find. Well, this Lazy Old Geek (L.O.G.) owns a lot of them. See pictures. One advantage is that they all (all of the recent ones) use 18V lithium ion batteries so you can interchange batteries between tools.

A somewhat confusing aspect is that, while all are 18V some are called One+, some One+ Lithium and some One+ High Performance. I haven’t found any technical information to explain the differences.

While all are 18V, the batteries also come in different capacities rated in AmpHours and several generic batteries are available, usually much cheaper. While I’ve had really good luck with Ryobi batteries, I decided to build a tester to measure a ‘Ryobi’ battery and get a rough idea of its capacity.

Technical Note: I’ve read that official Ryobi tools and batteries can only be sold by Home Depot. I know Walmart and Amazon sell Ryobi tools and batteries and I’m pretty sure they’re legitimate. I do not know if Home Depot will cover the warranty though.

Now I’ve done battery capacity testers before including one for the 18650 battery:

Technical Note: 18650 batteries are used in many things including most laptops and most Lithium Ion tools including the Ryobi One+ batteries. These 18V batteries use 5-18650 batteries of one capacity or a multiple of 5.

Theory of Operation:

Using a fully charged battery, hook it up to a fixed load resistance and measure how much time it takes to be discharged. Using Ohms law you can figure out the capacity of the battery.

For the 18650 the fully charged voltage is about 4.2V and fully discharged is usually estimated at 2.8V. For the 5 cell Ryobi that would make fully charged around 21 Volts and discharge voltage at 14V.

When a battery is listed as 2Ahr, that means if the tool draws 2amps it should run for one hour. Well, that rate is a little extreme as you don’t want to have change batteries every hour. I selected a load of 50 ohms which calculates to 0.36A as a more realistic load.

How my tester works: I use a 5V Arduino to control the tester. The Arduino turns on a FET to put the 50 Ohm load across the battery and starts a timer. Instead of measuring current, I measure the voltage drop across the 50 Ohm load and calculate the current. I just keep adding the mAh (milliamp hours) until the voltage reaches 14V.

Step 1: Construction

So I just got some generic 3aHr batteries in and wanted to test them while I could return them.

First issue was they worked in most of my stuff but not in my project lamp.

Anyway, I wanted to get the tester working quickly.

PROBLEM: 21V across a 50 ohm load generates a lot of heat about 9 watts.

SOLUTION: I used two 100 ohm 5 watt resistors connected in parallel and put them on a heat sink. See picture.

PROBLEM: How to connect to the battery terminals. The – and + terminals are labeled on the top of the batteries.

SOLUTION: I don’t have a 3D printer so I bolted two boards together. (The reason those bolts are so long is because they’re the only ones I had that would fit.

Next I made some battery contacts out of aluminum. The aluminum was thinner then the indent so I added some wooden spacers cut from a tongue depressor. I used double sided sticky tape to hold them together.

I attached the PCB (next step) and heat sink to a piece of plastic and attached the plastic to one side of the fixture.

Step 2: Arduino PCB

I used a 5V Arduino Pro Micro purchased from and an LCD5110 for display.

PROBLEM: A 5V Arduino cannot measure 21V directly

SOLUTION: I used two voltage dividers to bring the voltage down to 5V or less, 8.2K and 2.4K resistors in series.

I put the circuit onto a PCB using this:

Schematic and Eagle Cadsoft files attached.

The Arduino sketch, Ryobi18.ino is attached.

Step 3: Tester Instructions and Conclusions

PROBLEM: When I connected a battery to the tester it would sometimes crash the program so I added an on/off button right on the Battery + wire. See picture


Charge the battery to be tested.

Make sure Battery connect button is off.

Connect power to the PCB.

Loosen the bolts and slip fixture over the battery, making sure polarity is correct.

Align the battery contacts with the battery and tighten the bolts.

The display should read 0V and Capacity 0mAh.

If not unplug and plug power adapter.

Press the Battery connect button.

The display should read ‘Paused’ and about 20V.

Press and hold the Start button on the PCB.

When display says ‘Running’ release the Start button.

The second voltage will start dropping off and Capacitance will increase.

After a few hours, the display will show ‘Paused’ and Cap will show the final battery capacitance.

Turn off Battery connect button.


I tested a couple of old Ryobi 1.3Ahr batteries, the both read about 0.9Ahr.

My Ryobi 2Ahr battery read 1.7Ahr.

I would say close enough.

The two supposed 3Ahr batteries read about 1.7Ahr.

If they read over 2Ahrs I would probably keep them. I am returning them.

Tester works pretty good.

By the way, sometimes you can guess the battery capacity by weight and for these batteries I tested that is true. My 2Ahr weighs more than the 1.3Ahr. However the generic 3Ahr weighs less than the 1.3Ahr. I believe the 4Ahr batteries have 10 18650 batteries instead of 5 so will weigh more.

However, I think Ryobi claims their 4Ahr High Performance battery weighs less that their regular 4Ahr battery.

NOTE: If you have a Ryobi battery that will not charge, do not give up. There are a lot of articles/videos on how to restore a battery that fails to start charging.

NOTE: There are also a lot of videos comparing various Ryobi batteries under real world testing. While somewhat helpful, it's hard for the real world testing to be exactly the same for both tests.