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. In addition to the nitty-gritty technology details, I'm including tips and tricks for selecting the most appropriate battery technology for your application (the first two tips appeared in my previous column), along with tidbits of trivia and nuggets of knowledge, as Max would say. In this article we consider lead-acid batteries.
Tip No. 3: Estimate your energy requirements.
After eliminating the technology types that don't fit your usage/storage environment (as discussed in Tip No. 1), I decide on primary or secondary types and then calculate the energy my load will need over the desired run time. A rough estimate is the average load voltage multiplied by the average load current multiplied by the required number of run time hours. Keep in mind that loads may be constant power, constant current, constant resistance, or a mix (including pulses). Next, we normalize the calculation to one hour to give Watt-hours at the load. Next time, I will show how to estimate all the losses and temperature derating to arrive at the minimum Watt-hours rating of the battery.
For the science nerd
An interesting team to watch over the coming years is to be found at the Battery Innovation Center.
Where can you find a place to charge your car, and how does it all work? Answers to these questions can be found at the website of the Car Charging Group, the largest electric-vehicle charging service provider.
The lead-acid battery
The lead-acid battery was invented in 1859 by the French physicist Gaston Planté. This is a secondary battery that provides a very low energy-to-weight ratio and a low energy-to-volume ratio, but it compensates for this by supplying high surge currents at low cost. Four main application categories have driven the development of this battery type: small consumer, starting/vehicle, deep cycle, and industrial.
Two reduced-maintenance varieties are known as gel cells and absorbed glass mat (AGM). These are collectively known as valve-regulated lead-acid (VRLA) or sealed lead-acid batteries. Some specific details:
- Specific energy: 30-40 Wh/kg
- Energy density: 60-75 Wh/L
- Specific power: 180 W/kg
- Charge/discharge efficiency: 50-92%
- Energy/consumer price: 7-to-18 Wh/$
- Self-discharge rate: 3-20%/month
- Cycle durability: up to 800 cycles (500 typical)
- Nominal cell voltage: 2.1 volts
- Float-charge voltage per cell: 2.23 (gel), 2.32 (flooded), 2.25(AGM).
- Cutoff voltage: 1.75 volts per cell (loaded)
- Charging temperature: -40°C (min) to +49°C (max); see specific manufacturer for more details
Discharge, negative plate reaction:
Pb(s) + HSO4-(aq) → PbSO4(s) + H+(aq) + 2e-
Discharge, positive plate reaction:
PbO2(s) + HSO4-(aq) + 3H+(aq) + 2e- → PbSO4(s) + 2H2O(l)
Discharge, the whole reaction:
Pb(s) + PbO2(s) + 2H2SO4(aq) → 2PbSO4(s) + 2H2O(l)
Charge: negative plate reaction:
PbSO4(s) + H+(aq) + 2e- → Pb(s) + HSO4-(aq)
Charge: positive plate reaction:
PbSO4(s) + 2H2O(l) → PbO2(s) + HSO4-(aq) + 3H+(aq) + 2e-
Except for vehicle batteries, the charge time of a sealed lead-acid battery is 12-16 hours, or up to 36-48 hours for large, stationary batteries. With higher charge currents and multi-stage charge methods, the charge time can be reduced to 10 hours or less. However, the topping charge may not be complete. Sealed lead-acid technology is sluggish and cannot be charged as quickly as other battery systems.
For optimal charging, lead-acid batteries should be charged in three stages. The constant-current charge applies the bulk of the charge and takes roughly half the required charge time. The topping charge uses a lower charge current and provides saturation. The float charge compensates for loss caused by self-discharge. This graph illustrates an example these three stages.
The two figures below show examples of VRLA discharge curves. These are for the UP-VW1220P1 battery made by Panasonic Corp. of America.
Duration of discharge verses discharge current for several temperatures.
Terminal voltage verses duration of discharge for several rates.
When it comes to disposal, lead-acid batteries cannot be put into landfills or incinerated. A very useful resource for the use and recycling of lead-acid batteries is the Battery Council International organization. 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.