As you may recall, a few weeks ago, Max Maxfield roped me into his ongoing robot project. This led to my writing this series of articles on the various battery technologies available to us. Along the way, in addition to the nitty-gritty technology details, I'm including tips and tricks on selecting the most appropriate battery technology for your application, along with tidbits of trivia and nuggets of knowledge, as Max would say. In this article we consider alkaline batteries, but first...
Tip No. 4: Selecting batteries (continued from this post)
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 nonlinear rate between the two given boundary curves and eyeball in my own estimate (see the black curve I drew on Figure 1 below). 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
- Cutoff 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]
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