SOAS University of London

Power Solutions in the Field: Using an Accumulator

Thomas Castle and David Nathan, ELAR

1. Introduction

We conducted research and tests in regard to the situation of a field linguist who needs to power a laptop computer and possibly other equipment (recorders, charging devices, lighting etc) where mains power is not available, or inconsistently available.

Unless you have mains power available, or use only very low powered devices for which you can bring enough alkaline batteries, you will need an accumulator (battery/batteries) and strategies for replenishing it (/them). Lead-acid batteries, such as those used in cars, are currently the best accumulator solution as a form of "appropriate technology" for remote or poor locations. The disadvantage of lead-acid batteries is that they are heavy. However, this is currently the price we pay for a relatively high capacity, flexible, robust, locally-manageable, low-tech and low-cost power supply.

We recommend the following planning strategies for powering in the field:

  • amount: think about power (capacity, replenishment etc) in orders of magnitude, not increments
  • approach: be flexible
  • adapt: adapt to the local situation – and its opportunities

Here is a summary of the findings:

  1. the accumulator (battery or batteries) is the centre of the powering system. These will almost always be available in-country (UK cost GBP35-70)
  2. the accumulator can power a laptop at mains voltage using an inverter (available in-country, UK cost approx GBP40), or at low voltage using a special computer transformer (UK cost GBP80)
  3. a mains voltage/inverter solution is more flexible, because it can power most devices without needing special converters, configurations, knowledge or skills
  4. a mains voltage/inverter system is less efficient than a low voltage solution (around 30% less efficient), but this is very much less significant than the differences among strategies for managing replenishment of the accumulator (see section 5)
  5. lead-acid accumulators (i.e. car/truck batteries, or deep-cycle versions) are easily available in-country, relatively cheap, have a large capacity, are robust, very predictable, and (importantly) are flexibly replenished (using vehicles, mains charger, generator, solar, or other batteries)
  6. alternative strategies for replenishing the accumulator are needed
  7. a digital multimeter (or voltmeter, UK cost GBP15) is required to monitor and predict power usage

Our tests focused on the downstream side of the accumulator; i.e. the options, capacity and behaviour of an accumulator for powering a laptop computer. We used a deep-cycle, 85 amp-hour lead-acid battery (for boats and caravans) purchased from a local automotive store. While this type of battery would better suit field powering situations and last longer than an ordinary car battery, the difference between the two types may not be too important over a period of a few months (the main care required is not to discharge lead-acid batteries too far – however, it is easy to prevent this using a multimeter or an inverter that cuts out when the battery becomes low).

We only tested one replenishment option – charging from the mains, using a 5 amp car battery charger (available in-country, UK cost GBP25). It took about 12 hours to fully charge the battery; we would recommend that a higher-powered (e.g. 15 amp) charger should be considered in situations where mains power is available only for a short time (e.g. town visits are short, or when using a generator that runs sporadically). Charging the battery by fitting it into a car would take from half an hour up to 2 hours depending on the car and how it is used. We did not investigate solar replenishment.

2. Test results

2.1. Mains voltage setup using inverter

An inverter converts low voltage direct current sources to higher voltage alternating current such as found in normal house mains outlets. Tests A and B compared the length of time a Dell Latitude 100L laptop could run using:

  • Test A: the laptop's internal battery
  • Test B: a 12 volt, 85 Ah (amp-hour) deep-cycle battery, and a SkyTronic 150watt inverter
Test A

This test measured the length of time the laptop can run on its own internal battery. To record times and to create computer activity, a macro was set up (using MJT Macroer) which wrote the current time to a file on disk every 10 minutes. The test ran until the laptop stopped running due to low battery power (its sleep mode having been disabled).

Time Action
11:28 booted up and macro started
14:38 last recorded entry
Total time: 3hrs 10mins.
Test B

This test measured the length of time the laptop can run on the 12 volt deep-cycle battery and inverter. The 12 volt battery was connected to the inverter using alligator clips (supplied with inverter). This particular inverter offers two standard UK 240V outlets (similar inverters purchased in other countries will have the appropriate local voltages and sockets, of course). The laptop was then plugged into one of the outlets. The laptop's internal battery was removed. The test ran until the power supply failed; in this case, when the inverter's built-in cut-off mechanism operated. This mechanism cuts off the unit and sounds an alarm when the loaded voltage drops to 10.5 volts, in order to protect the battery (see section 4).

Time Action/battery voltage
Before test unloaded 12.98V
17:19 booted up and macro started
03:09 next day inverter cut off
After test unloaded 11.5V
Total time: 9hrs 50min
2.2 Low voltage setup

The 85 Ah deep-cycle battery was used to power an IBM 42P laptop. This test used a low-voltage system, with the laptop powered by a Targus 70U transformer connected to the battery via a cigarette-lighter socket. The laptop's own internal battery was removed.

Fig 1 shows that the rate of voltage drop varied according to the type of activity on the laptop. However, the voltage drop was reasonably smooth over a range of loads, and the progress of battery discharge (as measured simply by its voltage) was quite predictable. Note that the battery voltage recovers significantly, but gradually, over a period of several minutes, when the battery is unloaded. Therefore, to interpret battery voltage readings, the load status must be taken into account.

The battery was full at the beginning of the test. The test was stopped when the laptop was still running but the battery voltage dropped to less than 12.0V under load (11.96V). This kept a considerable safety margin, both for the laptop and the battery, since it is important not to discharge the battery too low (see XX).

In this test, the battery powered the laptop for 11.5 hours. Extrapolation of the test results to a final battery voltage of 11.8V would (at an average rate of -0.056 volts per hour) yield a total running time of nearly 14 hours.

Graph of voltage change under discharge.

Fig 1. Battery capacity test, April 2006, 85 Ah battery - Targus transformer - IBM T42P

3. Discussion

In Test A, using the inverter, the 12 volt battery powered the laptop for approx 10 hours, or more than 3 times the time provided by the internal battery. However, 3 hours is quite a good result for a laptop internal battery and may not be achieved with other laptops. In general, we would conclude that the 12 volt battery with inverter would run a laptop from 3 to 6 times the length of time provided by its own internal battery, depending on the type of laptop and the size and condition of its battery.

Although a 12 volt battery is larger and heavier than 3, or even 6, laptop batteries, it is more flexible. Laptop batteries are very expensive, and usually only suit a particular model. The cost of 6 laptop batteries would exceed the price of the laptop itself, while car batteries are cheap, and can be bought in-country as required. Additionally, replenishing (recharging) laptop batteries must typically be done using the laptop itself, so flexible approaches to replenishment are not available. Therefore, laptop batteries are not likely to be a good option for the powering system's accumulator.

Test B showed that the low voltage solution yields a longer running time and is perhaps 30-40% efficient. However, it requires a special laptop transformer that is expensive and which may not be readily available. This method does not offer the flexibility of the inverter, which can be used as a multipurpose power supply since it provides normal mains power up to its rated capacity.

Obviously, the running time available will also depend on the size and type of laptop. Not did we test the laptops' running time in relation to the specific activities carried out on them. As the Dell laptop ran, its moment-to-moment power consumption fluctuated between 2 and 8 amps, largely depending on the level of hard-drive activity. Other settings, such as screen brightness, use of wireless devices etc, will also affect power consumption, so laptop users should make themselves aware of the power implications of various tasks and settings.

4. Battery discharge level and checking voltage

Depending on the consuming device, a 12 volt battery will continue to supply power and to discharge until it has no power left at all, i.e. its voltage is zero volts. This will then destroy the battery. Discharging it too low will also damage it or reduce its life. Recommendations for minimum voltage vary. Typically, the loaded voltage should not be lower than 10.5 to 11.5 volts, and the stabilised unloaded voltage not lower than 11.5 to 11.8 volts.

The fieldworker should become familiar with how her/his equipment works and how the voltages change with use. It is necessary to carry a digital multimeter to check voltages. Digital multimeters are cheap, light, and make it very easy to check the capacity of a battery, calculate average power consumption rate, and predict the amount of time left to power devices. In addition, they can be used to check all kinds of batteries and to identify a variety of faults with electrical equipment.

5. Thinking strategically

Field situations will typically involve compromise, extra burdens, and unpredictabilities. Therefore it is important to have realistic expectations, think laterally, make best use of local situations, and choose options that are available to you that you may not normally choose at home.

Applying these principles to the field powering situation means that there are going to be many alternative ways to provide power, most of them involving concessions, effort, flexibility and creativity on your part. Nevertheless, local situations will provide some opportunities. For example, car batteries and chargers are things that will exist in many places, even in remote and poor communities. People may be able to either provide batteries for you to use, or, perhaps better, they might be pleased to use a battery (or batteries) that you buy locally and ultimately give to them in return for helping you with your replenishment strategy. A possible scenario would be that you bought two batteries and arrange to have them swapped every other day to be charged, either when a generator is running, by fitting into a vehicle, or by transporting to a source of mains power and charging it there. In such a scenario, the batteries (and charger, if relevant) would be useful to people after you leave. According to our tests, if you can arrange to have a battery charged every other day, you can probably use a laptop computer as much as you need – i.e. from 3 to 6 hours a day depending on manner of use.

References and useful information

There are many web pages with advice for managing 12 volt batteries, see, for example Bosch Automotive.

This document

Author: Tom Castle and David Nathan

Version: First draft

Date: 5 May 2006

Acknowledgements: Thanks to the following people for helpful input: Rob Kennedy (Language Centre, SOAS), Bernard Howard (Linguistics, SOAS).