toolboy's Corner: Ryobi 18v to 40v Adapter and Extended Runtime Battery Bank



This is a project I've been meaning to build for a very long time now. I've often wanted to use my vast collection of 18v batteries with Ryobi 40v tools, and I've also wanted to be able to use more than one 18v battery to extend the runtime of a Ryobi 18v tool. I'd hoped that Ryobi would release an Uninterruptible Power Supply which used 18v batteries, and I was very disappointed that when Ryobi finally released an 18v inverter it was a MSW unit (Modified Sine Wave), not a pure sine unit. At least Ryobi got it right when they released their 40v inverter. But of course the 40v inverter must be used with their 40v batteries.

Or can it be done with Ryobi 18v batteries?


The Ryobi "40v" batteries contain one or more strings of ten 18650 cells in series. Ryobi 18v batteries contain one or more strings of five 18650 cells. So the "40v" batteries are essentially two of their 18v batteries in series, or more correctly it's a 36v battery. What's to stop someone from placing two 18v batteries in series to make a 36v battery? From a technical standpoint, not much. But from a safety and usability standpoint, there are some concerns.

The biggest concern is charging. It's important to realize that each 18v Li-Ion battery contains a Battery Management System (BMS), so each 18v battery should be charged separately. Never try to charge two 18v Ryobi Li-Ion batteries in series! Most of the other concerns have to do with load balancing and ensuring that batteries at a higher charge level don't discharge into the batteries at a lower charge level. The BMS in each battery should prevent this, but I decided to incorporate a blocking diode on each battery in the system to be sure. The blocking diodes must therefore be hefty enough to handle the full discharge current of a battery, which can be substantial.


How it Works

The three batteries on the left work together to make 18v bank "A" and the three batteries on the right work together to make 18v bank "B". When the switch is in the left postition, banks A and B work together to make one big 18v bank of up to six batteries. When the swith is in the right position, banks A and B are put in series to make 36v. When in 18v mode, the bank can be used with just one battery. But when in 36v mode we must have at least one battery in bank "A" on the left and one battery in bank "B" on the right.

Batteries can be hot-swapped.

This means that the output power will be maintained while swapping batteries, so long as there's at least one battery in each bank as needed. So remove one discharged battery and replace with a charged battery before advancing to the next battery. When hot-swapping in 36v mode, leave at least one battery in each bank at a time!

This design provides for additional run time, but NOT additional power! Each battery is protected by a blocking diode, which means that the battery with the highest voltage in each bank will be discharged first. A charge will be drawn from additional batteries in the bank only after other batteries in the bank have been discharged down to the same level. If six identical batteries are used in 18v mode, this can mean 6x longer run time but will NOT mean 6x more power.

Extended runtime example

Let's say that you have lots of 18v 4Ah batteries. During a power outage you can use a Ryobi 18v 150W AC inverter with your modem and wifi router. These devices draw about 70W and a 4Ah battery can power them for about an hour before the 4Ah battery is depleted. At that point you must interrupt power briefly while you swap batteries. With the Extended Runtime Battery Bank you could use up to six 18v 4Ah batteries at once to increase run time to about six hours without interruption. If you have more than six batteries, you could at any time replace one of of the drained batteries with a fresh one to extend run time without interrupting power. You could even start with just one battery in the device. Press the battery's fuel gauge periodically and when it's down to just one bar, plug in a second battery and remove the first. Repeat as many times as you like to maintain uninterrupted power.

The Build

Fifteen months ago I purchased a "for parts" 6-port Ryobi P125 charger from eBay. The inside of the charger was scorched from failed components. I delayed starting on this project because I really wanted to use a more compact P135. A year ago I purchased a "for parts" P135 charger, but I was able to troubleshoot the problem and repair it. Two months ago I purchased a "for parts" OP4015 battery, and after determining that the board was bad I gutted it and created a pass-through "shell" battery. It was at this point that I figured it was time to get back to my original project.

Parts required:


Disassemble the P125 and remove the Printed Circuit Board (PCB). Carefully cut all board traces leading to the three battery contacts for each of the six battery locations. Desolder the six SB540 diodes (Schottky 5A). Cut off part of the PCB (Printed Circuit Board), removing the portion above the top two batteries which has the power input and rectifiers. Do not remove any portion the PCB where a mounting hole is located. The case's compartment above the top two battery sockets should be empty after cutting the PCB.

Why not include a few pictures?

This project is not for a beginner. If you don't fully understand what is being described in the written description then please don't attempt this. These Ryobi 18v batteries may seem harmless, but they contain a number of Lithium Ion cells which could start a fire in seconds or leak if connected improperly.

Install six MBR30100CT Schottky Diodes, one on each positive (+) battery terminal. Install a TO-220 heatsink on each diode. Wire together the output of the three diodes on each side using shielded 12GA wire of one color (red?). Wire together the three negative (-) battery terminals on each side using 12GA wire of another color (black?). Mounted the 3PDT On/Off/On Switch (rated 15A at 250V) to the cover, then mount two pairs of Anderson PowerPole connectors (one for 18v output and one for 36v output).


Wire the 3PDT switch (3PDT = 3 Pole, Double Throw). When in the "18v" position, the left and right sides are placed in parallel and the output goes to the 18v connector. When in the center position, there's no output to either connector. When in the "36v" position, the left and right sides are placed in series and the output goes to the 36v connector.


Battery "Shells"

The 18v to 40v Adapter and Extended Runtime Battery Bank is not quite ready for use. We need a way to connect this unit to Ryobi 18v and 40v tools. As mentioned before, I recently created a "shell" 40v battery from a 1.5Ah OP4015 battery. I selected the OP4015 because it's the smallest of the 40v batteries. After opening the OP4015 battery I carefully removed the ten 18650 cells. Next I cut the traces to the three battery terminals so components on the board would no longer be connected to the contacts. I mounted a pair of Anderson Powerpole connectors on the end where the fuel gauge had been located, and I added a yellow banana jack to connect to the third terminal. If you're just going to use this "shell" for powering tools, then the third connector is not needed.


I use a five-foot, 12 gauge Anderson PowerPole extension cable to connect the battery Adapter to the "shell" battery. I've had no trouble using this setup to power my 40v brushless blower and my 40v 300W pure sine inverter.

I already owned an 18v battery "shell" which I'd created over a year ago. I selected a P107 to use for the "shell" as it's among Ryobi's smallest 18 batteries. I'd been using this 18v battery "shell" in conjunction with the charger testing found elsewhere on this website. I put a second banana jack (in green) on the 18v battery adapter because dual chemistry 18v chargers require this additional contact. If you're just going to use this "shell" for powering tools, then the neither of the banana connectors are needed.


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Last revised 29-Nov-2020
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