A battery backup system provides much-needed power for lights in a maternity ward.
|It also provides power for laboratory test equipment (below) in a Haitian hospital.|
The Caribbean country of Haiti has electricity available to less than 25% of the population. Even in areas with utility-provided electricity, power outages are frequent and typically last for many hours. Operating health-care facilities under these conditions is difficult. Most hospitals have backup diesel generators, but fuel shortages, poor power quality, and breakdowns are common.
Reliable electricity is essential for the medical testing laboratories. Voltage surges can damage sensitive equipment and poor power quality can result in inaccurate medical test results. In rural areas, transporting fuel for the generators can be impossible during the rainy season due to impassable roads, limiting the number of hours generators can operate before they run out of fuel. Some hospitals have PV arrays, but their high initial cost has kept the systems small. They are mostly used to power specific loads, such as vaccine refrigerators.
As part of the U.S. Agency for International Development’s (USAID) Improving Health Facilities Infrastructure (IHFI) program, 20 hospitals have been equipped with updated inverter-battery systems. Each uses diesel generators or grid power (where available) to charge batteries to provide high-quality inverter power to the laboratory and other essential loads 24 hours a day. An assessment of these systems was completed as a follow-up to an ongoing training and support program for the hospitals.
The systems were evaluated using several different methods to solicit feedback from the hospital staff about how well the systems were working, and also included inspections and testing of the backup power systems—which included a full battery assessment.
Although the majority of the systems include a battery temperature sensor (BTS), most systems had problems with them that compromised their accuracy. At one site, the BTS was plugged into the secondary (slave) inverter instead of the primary (master) inverter, with the result that no temperature adjustment occurred. At other sites, the BTS was installed incorrectly—either lying on top of the battery lid or at the end of a battery bank, which also reduced its accuracy. At many sites, the adhesive on the BTS failed.
The team corrected the failed peel-and-stick adhesive used on the sensors by wedging soft packing foam between two batteries, forcing the temperature sensor against the side of one of the batteries. Though rudimentary, this mechanical fix can also be used as a redundant measure instead of relying only on the sensor’s adhesive. It also insulates the sensor from the ambient airflow, making the measurement of the battery electrolyte’s temperature more accurate and less affected by other heat sources.
The highest electrolyte temperatures, which ranged from 26°C to 38ºC, were found in battery banks installed on the south side of a building without shade, in rooms with little ventilation, or in systems with battery chargers set at a high charging current. Batteries that operate at high temperatures on a regular basis will have reduced cycle life.
System Charging Setpoints
It was determined that most of the batteries were being regularly overcharged, causing them to operate at higher-than-normal temperatures. The inverter/charger battery setpoints were adjusted to reduce the amount of internal battery heating and to reduce gassing. Changes were made to the absorb, float, and equalization voltage setpoints.
The total charging rate for the systems was also reduced for the same reasons. The original design was calculated to charge the battery at a C/6 rate, which means that the battery would go from a completely discharged level to a fully charged level in six hours. In this case, the battery manufacturer suggested changing to a C/10 rate to reduce the heat generated during charging from a generator or the grid.
On the inverter/chargers that were used in these systems, the setpoint controlling the maximum charge rate is programmed in AC amps:
It was also important to ensure the charging rate was appropriate for the existing battery bank. At one hospital, the system was originally installed with three paralleled strings of eight 6-volt batteries. In the interim, as individual batteries failed, they had reduced the battery system to two strings, and then one string in an attempt to delay the inevitable replacement of the batteries. However, when the number of batteries was reduced, the charger’s maximum output setpoints were not adjusted. Battery electrolyte temperatures measured 50°C (the highest reading that the glass thermometer could indicate). Once the AC charging amps setpoint was reduced from 18 A to 6 A AC per inverter, the battery’s electrolyte temperature dropped to 35°C—still hot, but not as dangerous or as damaging.
For systems operating in hot climates like Haiti’s, high battery temperatures can be a big problem. The following list of measures is highly recommended:
- Properly locate the BTS. It should be placed on the side of a battery, two-thirds up the side of one battery and on a battery that’s located toward the middle of the pack. The BTS should not be adhered to the battery top.
- Putting some insulating foam material over the sensor makes the sensor reading more accurate and less affected by ambient air temperature changes or other heat sources. This can also keep the BTS in position if the adhesive fails. Additional protective sleeving may be needed to prevent rodent damage to the BTS wiring.
- Verify that the BTS is functioning correctly during the system commissioning and during annual maintenance/inspections.
- A charging rate that is too high can result in overheating the battery. Calculate the maximum charge rate and consult with the manufacturer to verify the C-rate recommended for your temperature range.
- For backup systems that are connected to high-power charging sources, consider using slightly lower charging-voltage setpoints. This may reduce battery temperatures, eliminate excessive gassing, and minimize the frequency that water needs to be added to the batteries.
- Keep batteries in a shaded, cool area with good airflow, and keep them spaced apart to reduce the temperatures of the batteries in the middle of the pack.
A thorough inspection of installation practices can identify problems like this poorly crimped cable lug, which was pulled apart by the assessor after noticing the crimping methods.
In hot climates, this rat-chewed battery temperature sensor cable will cause erroneous temperature readings and result in overcharged batteries.