Feb 15

1. Limit the depth of discharge (DOD) by swapping when battery first shows signs of loss of power.

2. Remove from the charger early to limit the charging level.

3. Keep your batteries in the shade when possible.

4. Transporting them in the coolest place in your vehicle.

1. Limiting Depth Of Discharge

“At a given temperature and discharge rate, the amount of active chemicals transformed with each charge – discharge cycle will be proportional to the depth of discharge. The relation between the cycle life and the depth of discharge is logarithmic as shown in the graph below.

In other words, the number of cycles yielded by a battery goes up exponentially the shallower the DOD. This holds for most cell chemistries. (The curve just looks like a logarithmic curve. It is actually a reciprocal curve drawn on logarithmic paper).”

“The above graph was constructed for a Lead acid battery, but with different scaling factors, it is typical for all cell chemistries including Lithium-ion. This is because battery life depends on the total energy throughput that the active chemicals can tolerate. Ignoring other ageing effects, one cycle of 100% DOD is roughly equivalent to 2 cycles at 50% DOD and 10 cycles at 10% DOD and 100 cycles at 1% DOD. See also Cycle Life which shows how cell performance diminishes due to deterioration of the active chemicals as the battery ages.

There are important lessons here both for designers and users. By restricting the possible DOD in the application, the designer can dramatically improve the cycle life of the product. Similarly the user can get a much longer life out of the battery by using cells with a capacity slightly more than required or by topping the battery up before it becomes completely discharged. For cells used for “microcycle” applications (small current discharge and charging pulses) a cycle life of 300,000 to 500,000 cycles is common.

Mobile phone users typically recharge their batteries when the DOD is only about 25 to 30 percent. At this low DOD a lithium-ion battery can be expected to achieve between 5 and 6 times the specified cycle life of the battery which assumes complete discharge every cycle. Thus the cycle life improves dramatically if the DOD is reduced.
Nickel Cadmium batteries are somewhat of an exception to this. Subjecting the battery to only partial discharges gives rise to the so called memory effect (see below) which can only be reversed by deep discharging.

Some applications such as electric vehicles or marine use may require the maximum capacity to be extracted from the battery which means discharging the battery to a very high DOD. Special “deep cycle” battery constructions must be used for such applications since deep discharging may damage general purpose batteries. In particular, typical automotive SLI batteries are only designed to work down to 50% DOD, whereas traction batteries may work down to 80% to 100% DOD.”

2. Charging Level

“The cycle life of Lithium batteries can be increased by reducing the charging cut off voltage. This essentially gives the battery a partial charge instead of fully charging it, similar to working at a lower DOD as in the example above. The graph below shows the typical cycle life improvements possible.

Reducing the charging voltage cut off voltage avoids the battery reaching its maximum stress point.”

3. Charging & Sitting Tools Go In The Shade

3. When you are not using your tools with rechargeable batteries keep them under a sheet of plywood or in the shade. It takes a while to get used to doing this, but just like learning the hard way to keep your power saw under a sheet of plywood in the rain the shocking price of having to replace your batteries earlier can help you remember this.

You can’t do this every time you lay your tool down but at least make sure they are not left lying in the sun at break or over lunch.

The same with your charger. Keep your battery charger and the batteries it is charging in the shade or shaded by a sheet of plywood so they are never expose them to the direct sun rays.

4. Transport Your Tools In A Cool Place

As soon as you are done for the day put your battery operated tools into a covered tub and put them into the coolest place in your vehicle. Also don’t leave the tub you put the tools in lying in the sun all day.

5. Seasonal Storage

If you use your battery operated tools for seasonal work make sure you store them with a full charge. I put a full charge on both my lawnmower and weed eater batteries. Then in February I brought them out and recharged them. This will also help to lengthen the life of your batteries.

6. Bottom Line

For power tools that you are use often on your job you should have at the minimum two batteries, one in the charger and one in the tool. If you find that you are using your power tools to the point where both batteries are almost dead by the end of the day then get a third battery.

If you are using battery operated power tools in your work place daily try to buy tools where the you can use the same batteries for each of the tools.

Batteries are not cheap and by applying these principles you can easily double or triple the life of your batteries.

Note: Quotes and pictures in this post were taken from Battery Life on the Battery and Energy Technologies Website which is owned by Woodbank Communications Ltd a battery consultancy company based in Chester, UK.

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