Greg In The Desert

I purchased a reconditioned cordless Dewalt combo tool set (hammer drill, Circular Saw, Reciprocating Saw, Flexible Floodlight) well over ten years ago. The tools have been one of the best tool investments I made and they have performed perfectly over the years.

Except for the batteries. They no longer hold a charge and it’s time to replace them. My options are:

• New OEM batteries, throw out (recycle) the old batteries. A two pack of official DEWALT DC9096-2 18-Volt XRP 2.4 Amp Hour NiCad Pod Style Batteries from Amazon are $119 (about $60 each) and they hold 2100MAH of power.
• Have them rebuilt by someone like voltman batteries. They offer a higher capacity rebuilt NICAD battery of 3300MAH for $79.99 each.
• In 2007 Dewalt came out with the DEWALT DC9180 18 Volt Nano Tech Battery Pack. This battery is a replacement Lithium Ion battery that work in older Dewalt tools like I have. At Amazon, one battery costs about $111 (and I will need to buy a new charger).

I do not buy things on price along. So what performance metrics can I look at?

Comparing the two NICAD options, the rebuilt NICAD batteries are a better deal since I would be getting considerable more amp-hours for a little more money. Plus, I already have the chargers so there are no additional expenses.

Comparing the NICAD vs the Lithium Ion, the Lithium Ion weighs less than the NICAD. I love my Dewalt tools but they are heavy and the NICAD batteries are about 2.5 lbs vs 1.5 for the Lithium Ion batteries. Eliminating a pound off the weight of the tool would be good my tennis elbow.

For other performance information on NICAD vs Lithium Ion, I found a helpful FAQ on the Digital Reality Crew forums which has some comments from an experienced user. A few things I’ve noted is that the Lithium Ion battery will not loose power while being unused where a NICAD will, the Lithium Ion has slightly less capacity than the NICAD and the NiCad battery will fade as it looses power but the Lithium Ion will just shut off.

Based on this information I am heavily weighted toward the Lithium Ion option (one with a battery charger). Is there anything else I should consider?

A interesting project I’d like to try.

To start with I needed to design a mower that would fit my needs. I have a yard that takes about forty-five minutes to mow so it was important to size the battery accordingly. To do this I had to find out what kind of amperage the motor would be pulling for the forty-five minutes it takes to mow the grass. This is dependent on the type of motor used. Motors are rated in HP (horse power) and I wanted around the same HP as a standard internal combustion mower. The newer gas powered lawnmowers are about 4 to 6 HP and the older mowers are around 3 HP. According to the article in “Home Power” magazine 1 HP of an electric motor is equivalent to about 4 HP of an internal combustion engine. I was hoping to purchase a 1 HP motor but after pricing them I decided I needed to go with the least expensive motor that would get the job done. This turned out to be a 12 VDC, 3/4 HP, Dayton electric motor. (See link below for details on parts and cost.) The 3/4 HP Dayton motor is specified to pull 58A with a full load. I used this figure to help determine the size battery I needed. I knew I wanted a 12VDC battery, they are easy to find, use, and come in a variety of sizes. The next step was to figure out the amp-hours I needed to run this particular motor for 45 minutes. Amp-hours are a measurement of the length of time it takes to discharge a battery at a certain amperage. For example; a 35 amp-hour battery should give 35 amps for an hour before being discharged. I know I want to run my mower for 45 minutes but I’ll just round it to an hour to be safe. Although the motor is rated for 58A, that amperage is with a full load and typically while mowing I will not be running a full load. While just spinning the blade (not cutting grass) the amps will only be about half of the full load or 30A. So a 40 Amp-hour battery should allow me to mow moderately tall grass for about 45 minutes to an hour. It just so happens that I had a 40 amp-hour deep-cycle battery from a past project. (See below for details on parts and cost).