The lowdown on batteries: Alkaline batteries
Tip No. 4: Selecting batteries
After computing the watt-hours at the load, we need to know how to find the losses a given battery will have, so that a run time can be found. Internal power loss percentage can be computed using Pi = (IL2 x Ri)/(VL x IL) x 100, where Pi is the percentage ratio of lost power in the battery to the power delivered to the load, IL is the average load current, Ri is the average internal resistance of the battery, and VL is the average load voltage. If maximum load power is paramount along with low terminal voltages, we choose a battery (Ri) and load (RL) combination such that Ri is approximately equal to RL. If run time (low losses) is paramount, choose a battery (Ri) very much less than RL.
To find the specific run time on a manufacturer's discharge curve, I use what I call the eyeball interpolation method. As an example, suppose I want to find the run time for a 7mA constant-current load, but this isn't on one of the curves. To create my own curve, I simply note the non-linear rate between the two given boundary curves and eyeball in my own estimate (see the black curve I drew on figure 1). Then notice that, at the end-of-life voltage of 0.8 V, the service hours reads 480 on the horizontal axis. We'll continue discussing this topic in my next post.
The alkaline battery
Non-acid batteries were invented by Waldemar Jungner in 1899 and by Thomas Edison in 1901. The patent for the zinc/manganese version was granted and assigned in 1960 to Union Carbide Corp. This is a primary battery, since it is ready for use after assembly. As an inexpensive primary battery goes, the alkaline type has a long history. It has high reliability, high specific energy, and high energy density. However, its internal resistance doesn't permit high-current or high-power discharge. Shelf life is very good, and its temperature performance is adequate. Some manufacturers have made versions that sacrifice capacity and reliability for being able to be recharged. I talk a little bit about rechargeable types below. There are lots of variations on this, so you'll have to look at individual data sheets to see how this works. A good guide on alkaline batteries can be found here. And here's another useful datasheet guide from Rayovac.
Specific energy: 85-190 Wh/kg
Energy density: 250-434 Wh/L
Specific power: 50 W/kg
Discharge efficiency: 45-85% (low-rate discharge)
Energy/consumer price: 0.5 Wh/dollar
Self-discharge rate: 0.17%/month (newest types)
Cycle durability: not applicable (primary battery, but see manufacturer for rechargeable versions)
Nominal cell voltage: 1.3-1.5 V
Cut-Off voltage: 0.8 V per cell, loaded
Zn(s) + 2OH-(aq) → ZnO(s) + H2O(l) + 2e- [e0 is -1.28V]
2MnO2(s) + H2O(l) + 2e- → Mn2O3(s) + 2OH-(aq) [e0 is +0.15V]
Zn(s) + 2MnO2(s) ←→ ZnO(s) + Mn2O3(s) [e0 is 1.43V]
The following illustrations are for Duracell's MV1500 Quantum AA battery. Figures 1-3 show examples of alkaline discharge curves in the form of voltage at various loads and temperatures. Figure 4 shows the battery at two load rates and illustrates the effect on internal resistance.
Figure 1: Voltage versus service hours for several constant currents. Observe the interpolated black curve that I added.
Figure 2: Voltage versus service hours for several constant-power loads.
Figure 3: Voltage versus service hours for several temperatures with a 100mA load.
Figure 4: Voltage and internal resistance versus depth of discharge for the Duracell MV1500.
Rechargeable alkaline batteries
Rechargeable alkaline batteries are sometimes called RAM batteries. The early technology was developed by Battery Technologies Inc. and licensed to Grandcell, Rayovac, Pure Energy, and EnviroCell. Additional patents have been developed to expand on the technology. The most common sizes are AAA, 9V, AA, C, and D. These batteries are manufactured fully charged and have much better charge-holding characteristics than regular rechargeable batteries like NiMH and NiCd.
These alkaline types are specifically designed to be rechargeable, with reasonably high recharging efficiency, for up to about 20 cycles. They have been designed for low-drain, periodic use, where the depth of discharge is slow and no more than 25% on average. Deep discharging reduces the cycle life substantially.
With regard to capacity, rechargeable alkaline batteries are about two-thirds as strong as their non-rechargeable counterparts. Also, terminal voltage after recharge is somewhat lower, typically recovering only to 1.4 V or so with the best brands. Also, you should recharge these batteries only using the specific charger recommended by the manufacturer.
I haven't personally tried to recharge ordinary alkaline batteries, but I believe it is technically possible. I have heard reports that, so long as the battery hasn't been discharged too deeply, it is possible to recover up to 90% of capacity with a special type of pulse charging using the Battery Xtender.
Last but certainly not least, when it comes to the disposal of alkaline batteries, this is generally easy, but you should refer to individual manufacturers for specific instructions. In my next column, we'll look at some more tips and tricks, and we will consider another battery technology. In the meantime, I welcome any questions or comments.
About the author
Ivan Cowie is the Chief Engineer at MaxVision.
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