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Rebuilding Ryobi 18v Batteries

So you have some older Ryobi 18v batteries that are no longer working as well as they used to. You guess the best thing to do is to buy some new batteries -- from The Home Depot or on eBay. Or you could buy some reconditioned batteries from one of a variety of sources. Or, if you're skilled with a soldering iron, you could rebuild the packs yourself.

Over the years I've rebuilt numerous battery packs and replaced the "non-replaceable" batteries in various devices. Sometimes I've done this because it's the cheapest option, but more often it's because I wanted to upgrade to higher capacity cells.

If you're not skilled with a soldering iron and knowledgeable about the cell technologies used, please do not try this yourself.

Some points to consider before rebuilding a pack:

1. Do you fully understand what you're doing and the associated risks?
   The cells used in the Ryobi 18v packs can be very dangerous if not handled correctly and can leak a corrosive acid, explode, or start a fire in mere seconds.
2. Do you understand the charge/discharge characteristics of the old and new cells, and are they compatible?
   The cells used in Ryobi drill battery packs have high charge and discharge rates. If you use replacement cells that have lower charge/discharge rates, the pack could overheat and explode, leak a corrosive acid, or start a fire when charging or in use. And your high-current tools may work fine without a load, but when a load is applied the tool will seem weak even on a fully charged battery pack.
   It's easy to think that you can double (or more!) the capacity of your Lithium Ion pack by replacing the 18650 cells with much higher capacity cells. P104 packs contain ten 1200mAh 18650 cells, and P103 packs contain five 1300mAh cells. Currently (Dec 2009) it's not hard to find vendors selling 2600mAh cells, or even 3000mAh 18650 cells. So by replacing the ten cells in a P104 with ten 3000mAh cells you could more than double the capacity of the pack from 2.4Ah to 6.0Ah, right? Yes, but ONLY if you're going to use the pack in low discharge rate tools, like a flashlight. The 3000mAh cells have a charge/discharge ratings of around 1C/2C (3A/6A) whereas the original cells are rated 5C/10C (6A/12A) So at best you'd be able to drain current half as fast from the new pack as from the old. That might be fine for a flashlight or fan, but no way could you operate a circular or reciprocating saw under load. And at that high discharge rate, the new pack's capacity would be far less (maybe 25% less?) than the apparent rating of 6.0A, because that high rating was based on a 0.2C discharge rate, or 1.2A for the pack.
3. Can your charger properly charge the new cells?
   Most chargers can charge new cells of the same chemistry and larger capacity. But it'll take longer, or the pack may not be at full capacity at the end of the charge cycle.
   Ryobi 18v chargers are designed to work with Nickel Cadmium (NiCd) or Lithium Ion cells. If you replace your NiCd cells with Nickel Metal Hydride (NiMh) the charger's peak detection circuitry may think that a pack is fully charged long before it is. When Ryobi 18v chargers think a battery is at capacity, the green light comes on and a "trickle" charge is applied. This trickle charge can be used to fully charge a NiMh pack, but count on it taking overnight.
   You cannot replace NiCd cells with Lithium Ion cells, because there's some fancy electronics in each Lithium Ion pack that's not in the NiCd packs.

Which is correct: "NiCd" or "NiCad"?

Technically, "NiCd" is correct. From the Periodic Table of the Elements, "Ni" stands for Nickel and "Cd" for Cadmium. Putting these together gives "NiCd" for Nickel Cadmium, the chemistry of these cells.

The abbreviation "NiCad" is a registered trademark of SAFT Corporation, although this brand name is commonly used to describe all nickel-cadmium batteries.

But I really wanna replace the cells in my NiCad pack!

OK, so do it! The NiCd packs each contain fifteen Sub-C cells. You can buy new Sub-C cells with solder tabs in capacities up to 2200mAh. The older black packs contains 1500mAh cells and the newer yellow packs contain 1700mAh cells, and I recommend not buying cells with a lower capacity that the originals. I'm not going to talk you through disassembling your pack, rewiring, and reassembling, but I'll say that, having done it several times, it's not too difficult. I find it easiest to remove the old cells and set them aside to look at as an assembly guide (remember there's one in the "stem"). I like to "aim" the solder tabs so that each positive tab "aims" towards the center of the next cell's negative, and I double over the negative tab to solder the previous cell's positive to it. Before bundling and soldering, tape the thermistor (or whatever type of temp sensor it is) to one cell, and arrange 14 new cells of the pack as they're going to fit in the shell. I like to stick them together with tape or hot melt glue -- makes them easier to handle before soldering.

The 2200mAh NiCad cells have 29% more capacity than the original yellow P100 pack, and generally have a higher discharge rate so tools feel more powerful, though YMMV.

The heck with NiCad packs, I wanna rebuld my Lithium Ion pack!

Not so fast, bub. Read above about cell selection. No battery likes to be overcharged or overly discharged, but doing so to Lithium Ion cells can really be disastrous. Lithium Ion cells will become irreparably damaged if discharged too much, for most 18650 cells it's around 2.8v. And they'll explode nicely if overcharged, for most the maximum is 4.2v. Many 18650 cylindrical cells are assembled at the factory with a protection circuit built right into the "button" of the cell. The circuit automatically disconnects the battery if the voltage goes over or under the "safe" range. The Ryobi packs use "unprotected" cells and instead utilize complex circuitry within the battery pack itself to manage this. If you have P103 or P104 packs you may have noticed this behavior already -- while operating a tool with a Lithium Ion battery near the end of its charge the tool stops abruptly. By contrast, the same tool operated by a P100 NiCad pack will slow down significantly as the battery nears the end of its charge, perhaps even "crawling" along for awhile before it finally stops.

At this time (Dec 2009) the highest capacity 18650 cells I've been able to find with suitable charge/discharge characteristics for use in P103 or P104 packs are rated 1600mAh, and they can be purchased for maybe $10 each plus shipping in small quantities. Do the math -- assuming $7.50 to ship 5 or 10 cells that's $57.50 to rebuild a P103 or $107.50 to rebuild a P104. The Home Depot sells new P103 Packs for $49.97+tax and P104 packs for $89+tax, so while you'd get a little more capacity doing the rebuild yourself it's cheaper, easier, and safer to buy new.

You're still reading on, so you must still be interested. And yes, I've rebuilt both P103s and P104s. I rebuilt one P103 using Tenergy 2600mAh 18650 cells. I carefully marked this pack, set it aside for use only with flashlights and fans, and sure enough it outlasts P104s in these tools. For the P104 rebuilds I decided that buying and assembling individual cells was too expensive and too much of a hassle, so I started looking into what cells are used by Ryobi's competitors. See this thread in the DeWalt Owner's Group for an informative discussion on this topic. The bottom line is that the Makita BL1830 is the most cost effective pack, containing ten 1500mAh cells for $99 (that's $9.90/cell). I found a vendor on Amazon.com selling these for about $65 each, so I ordered two. The Makita BL1830 packs came apart easily. The Sony cells inside were already arranged and wired as needed for use in the P104 shell, though spaced more closely so fitting them in P104 shell was just a tad tricky. Nonetheless, in about an hour I converted two dead P104s into two 3.0AH P104s, 25% more capacity than the original packs!

Let's see what the inside of the P103 and P104 batteries look like.

Has anyone ever actually tested the capacities of these packs?

Yup, I sure have. I have a West Mountain Radio CBA II and I've tested all my packs at the 1C rate (e.g., P100 capacity is 1700mAh, so it was tested at 1700mA rate). There's no telling how many charge/discharge cycles each of these packs has gone through, though it's a safe bet that older packs have seen more use. I originally generated results in late fall 2009, then re-tested the same batteries about a year later in November 2010..

What does the discharge curve look like?

See the chart below for an example of 12 batteries.

The drill batteries were dischanged at the 1C rate, or 1.3AH for the P103s and 2.4AH for the P104s. When the battery voltage reached 14v the test stopped, which is why the curves don't generally go below 14v. In the above chart you can see three P103s as the shortest lines in light green, yellow, and dark green and ending in the 1200mAh-1350mAh range. The P104s with date codes CS0938 and CS0939 in red and light green end around 2300mAh. All the rest of the curves are the P104s with date codes CS0949 and CS0950, ending in the range beyond 2400mAh. (Update 29-Apr-2010: I received some CS0948 batteries today which also tested above 2400mAh!) To my eye, the shape of the curves of the P103s and the newer P104s is the same, only elongeated for the P104s. They discharge evenly to around 17v (except for the "hump" at 19v) and then drop off rapidly. By contrast, the older P104s seem to hold their voltage a little better, discharging evenly to around 18v before dropping off rapidly.