toolboy's Corner: Ryobi 18v Circular Saw Battery Test

I've read many reviews online where individuals claim that performance of a Ryobi 18v circular saw is poor. While I agree that longer battery life would be better, I think a lot of the unfavorable reviews are the result of consumers selecting the wrong battery. Ryobi has sold quite a few models of 18v batteries over the years, and I think that they are not all well suited for use with a circular saw. The circular saw is one of the most demanding Ryobi 18v tools, especially if your cut is deep or your wood is wet as these conditions cause more friction on the blade and therefore require more energy to continue.

In April 2016 I set up an experiment to gather some objective data on the use of a Ryobi 18v circular saw with various Ryobi batteries. My saw of choice was a Ryobi P500 circular saw with the original Ryobi 5-1/2" blade (several years old and well used). I selected an old pressure treated 2x6 I'd reclaimed from a deck demo and which had been left outside but sheltered from the rain. It's moisture content was 13%, so it was fairly dry. I then clamped it steady in a Black & Decker Model 200 Portable Work Center and proceeded to make 90 degree cuts while holding a Swanson Speed Square as a guide, producing 1/2" slices. I then made as many cuts as possible with various different batteries and counted the resulting cookies. I allowed the blade to stop after each cut and before starting the next cut. After every 15 cuts (-ish) I paused to clear the sawdust, reposition the board, and collect the cookies before proceeding.

The first battery I tried was a used P108, the newest one I owned. It cut 55 cookies and seemed the most energetic of all the batteries I tried. It started to show signs of bogging down somewhere in the 40s.

Next I tried a new P107. It cut 14 cookies, though it seemed less energetic than the P108 from the get-go. It had to cut through a couple of tough knotholes, which may have skewed the results.

Next I tried a P104, the very first one I ever owned, from 2007 (9 years old!). It cut 22 cookies, and seemed to start bogging down in the late teens.

On to a P103 from 2008, It cut 1 cookie. So I tried another P103, also from 2008. It could not cut one cookie. I guess the older cells in the P103 packs simply cannot drive a circular saw anymore.

On to a P100 from 2004. It could not cut one cookie, though I know this battery was ready for the recycle bin. I'd been keeping it around only for use with a P740 radio and for future size/weight comparisons.

Then I tried another new P107. This one cut 20 cookies and seemed to start bogging down in the teens.

I meant to test some other batteries but ran out of time and patience, and I figured that I probably had enough data already. After putting everything away the last thing I did was to fully recharge every battery and then analyze them on my computerized battery tester. I know each battery's rated capacity, but I want an actual measurement so I can make accurate calculations. Below you'll see a table I created which shows each battery's measured capacity and the number of cookies cut. In the last column I divided the measured capacity by the # cookies to estimate the energy required per cut.

Battery Battery Age Rated Capacity Measured Capacity Measured as % of Rated # Cookies Cut mAh/Cut Comments
P108 4 yrs old 4,000 mAh 3,421 mAh 86% 55 62  
P107 #1 New 1,500 mAh 1,328 mAh 89% 14 95 Several knotholes encountered, may have skewed results.
P107 #2 New 1,500 mAh 1,322 mAh 88% 20 66  
P104 9 yrs old 2,400 mAh 1,512 mAh 63% 22 69  
P103 #1 8 yrs old 1,300 mAh 1,049 mAh 81% 0 N/A  
P103 #2 8 yrs old 1,300 mAh 1,037 mAh 80% 1  
P100 12 yrs old 1,700 mAh 419 mAh 25% 0 N/A  

Some of you may be looking at the above chart and wondering how the P103 batteries did so poorly. How could two batteries combined cut just one cookie, even though they tested so well on the battery analyzer?! My guess is that the older cells on these packs simply cannot deliver the amount of current required to drive a circular saw anymore. I've configured the tester to draw a steady 2.5 Amps until the pack's voltage drops to 14v, and these older P013 can manage that load just fine. The circular saw draws MUCH more current, which these older P103 batteries cannot deliver. So these P103 batteries work just fine in low-draw tools like a the radio or a fan, but they're useless in a circular saw.

Is there something about the P103 that makes it a poor choice for use with the circular saw?

Yes. I think the problem is that as a general rule the compact Li-Ion batteries are a poor choice for use with a circular saw or other high current drain tools. The compact Li-Ion batteries contain a single string of 18650 cells, so the full current required to drive a tool must be supplied by every cell in the string. The full-height Li-Ion batteries contain two strings of 18650 cells in parallel, so each cell must only provide HALF of the full current required to drive a tool. The cells in the full-height batteries encounter HALF the stress of the cells in the compact Li-Ion batteries, given the same load. And this means that full-height Li-Ion batteries can potentially deliver TWICE the current as the compact Li-Ion batteries, so high current draw tools bog down less easily. To be clear, the compact Li-Ion batteries are the P102, P103, and P107 and the full-height Li-Ion batteries are the P104, P105, and the P108.

As batteries age, both their capacity and their ability to deliver their rated current decreases. For this application, the decreased capacity is not as much a factor as the decreased current. The compact Li-Ion batteries are no longer able to sufficiently drive a circular saw long before the full-height Li-Ion batteries. Think about it -- let's say the cells age until they can deliver only half their rated current. A compact Li-Ion battery which can deliver half its rated current may be able to run the circular saw out of wood just fine, but stops immediately once a load is applied. This is like the behavior observed in the test, where the old P103 compact Li-Ion batteries were unusable with the circular saw. But the old full-height P104 Li-Ion battery performed about the same as the brand new compact P107 Li-Ion batteries! That's probably because the cells are arranged inside such that each needs to deliver only half the total current requirement of the load. So the aged full-height battery remains usable in the high current tool, though it's runtime will have decreased.

I find it very interesting that each of the batteries which created 20+ cookies required about the same amount of energy per cut. The numbers 62,66, and 69 means an average of 66mAh per cut. If we extrapolate this to Ryobi's current line of batteries, we can estimate the # cuts per new battery as follows:

Battery Rated Capacity # Cuts (est)
P108 4,000 mAh 61
P105 2,400 mAh 36
P100 1,700 mAh 26
P107 1,500 mAh 23
P102 1,300 mAh 20

The amount of energy per cut is a meaningless number by itself, because too many variables would be involved to apply this number to other circumstances. However, the consistency of these numbers supports the idea that the load was about the same for each battery tested.

Can any reasonable conclusions be drawn from this test?

I conclude that full-height Li-Ion batteries are a better choice than compact Li-Ion batteries for use in high current tools. When compared to compact Li-Ion batteries, the full-height Li-Ion batteries offer longer run times and get bogged down less easily, And as they age one may find that the full-height Li-Ion batteries continue to function reasonably in a high current tool, whereas the aged compact Li-Ion batteries may no longer allow the tool to function at all.

There's also the issue of stress. The compact Li-Ion battery must deliver TWICE the current to do the same job as the full-height Li-Ion battery. This is twice as stressful on the cells and likely means additional heat. These factors will shorten the life expectance of the batteries. (Though I have no data at this time to suggest how much the cell life expectance could be shortened.)

So are you saying that one should not use compact Li-Ion batteries?

Absolutely not. The P102 and P107 compact Li-Ion batteries are great and make an excellent choice for use in many of Ryobi's expansive One+ line of tools. I'm just saying that they're not the best choice for regular use in tools with a high current drain.

Exactly which tools are those with "high current drain"?

I haven't used them all, so I cannot give a complete list. My rule of thumb is that if a tool completely drains a new and fully charged battery in less than 15 minutes of continuous use under load, then it's a high current drain tool. So my list of high current draw tools includes the P3200 Canister Vac, Leaf Blowers, Circular Saws, Chop Saws, and the Lawn Mower. The list could easily be expanded to include a variety of other tools which can draw significantly more current when under load such as drills, impact drivers/wrenches, reciprocating saws, chainsaws, etc.

Final Wrap-Up

The results show that the battery one selects can greatly influence the results when evaluating a tool. If I had only ever used a circular saw with a P102 battery I'd probably be fairly disappointed. With a P108 battery the saw can make an estimated THREE TIMES more cuts than with a P102, and it will bog down a lot less too. If I had only ever used a circular saw with a P108 battery I'd probably be fairly impressed.

If you use your circular saw occasionally and for a few cuts only, then the P102 may meet your needs. But if you intend to build a deck or use your tool regularly, then do yourself a favor and get a couple of P108 batteries.

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Last revised 18-Jan-2021
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