As we promised back in the spring, the aim of this series is to look at batteries and chargers from the point of view of the end user.
These are not accreditation tests; they go a long way beyond simple compliance with international standards. These will test suppliers’ batteries under real-life and laboratory conditions, using performance criteria relevant to the end user and their related market demands. Using state-of-the-art battery test equipment supplied by Digatron Power Electronics; the in-house laboratory facilities of UK Powertech and real field conditions, testing has never been so targeted or relevant to customer requirements. With a complete technical report, the beneficiary will have a unique and unrivalled insight into their product and its fitness for purpose, which will not be available from any other source. The lifetimes of experience and knowledge in battery technology, together with the commercial and financial expertise that go into these tests and reports, cannot be matched by any other team or testing organisation. You could be one of the beneficiaries.
The series kicks off with an agent-sponsored programme aimed at determining the suitability of flooded lead-acid leisure batteries for use in solar energy storage
applications and comparing their performance with a lithium-ion equivalent. The programme began 20 June with the collection of 95Ah 12‑volt flooded lead-acid monobloc batteries and 12V 100Ah lithium iron phosphate batteries.
Determination of 12V 100Ah flooded lead-acid monobloc leisure batteries for solar energy storage and comparison with lithium-ion 12V 100Ah batteries.
Lead-acid: flooded monobloc leisure battery 12V 95 ah (C20) sealed design, lead calcium grid alloy, 500 cycle life, Figs 1 and 2.
Lithium-ion: Lithium iron phosphate chemistry, 12V 100Ah leisure battery
Energy storage from a domestic solar array currently feeding directly into the house and grid supply Fig 3. The purpose of the installation is to reduce power consumption from the grid at peak periods (5pm to 7pm) and reduce energy bills. Excess energy from the solar panels is diverted from battery charging to the electric immersion heater.
The critical features for this solar application are:
- Availability of power from the solar arrays to recharge the battery
- Efficiency of energy conversion for energy storage
- Efficiency of battery charging
- Battery capacity and discharge characteristics related to the application
- Speed of battery recharge
- Current and power draw on recharge
- Payback and amortisation of batteries based on round trip efficiencies
- Battery attributes which facilitate their installation and operation from a customer and maintenance perspective
- Optional high temperature corrosion test with tear down analysis and metallographic analysis of grid corrosion to predict battery life.
These fall into four categories for both battery chemistries:
1. Test protocols for batteries
1.1 Delivery: Packaging, external condition, cleanliness (any acid, dirt, grease etc.), any damage, state of charge, instructions, safety, maintenance and disposal/recycling instructions.
1.2 Design: Terminal size shape and position, SoC indicators, vents and caps, ease of maintenance, manual handling, weight and dimensions.
1.3 Laboratory: Establish basic performance criteria of capacity, voltage, impedance, DC-IR, charge acceptance, volts drop on load, charge-discharge energy efficiency. BMS characteristics and limitations. Reproduce operating parameters of field tests to predict battery response in the intended application.
1.4 Field trials: Ease of installation and monitoring when installed in their applications. Typical aspects monitored will be state of charge, amount of maintenance, completion of duty cycle, energy balance and efficiency, operating temperature, the duty cycle, customer feedback.
2. Project status:
100Ah (C20) and lithium-ion 100Ah batteries have been obtained for testing.
The lead-acid batteries have been sourced from a UK battery agent for purposes of product performance and ascertaining their suitability for their chosen application. The ‘as received’ status has been ascertained and the performance tests have started, the initial results are reported here.
The laboratory tests completed so far are for the lead-acid batteries using constant power discharge and recharge algorithms, based on field data obtained from the solar installation to be tested. Battery design attributes have been noted and comments made. The application data and the basic parameters of charge and discharge have been measured and recorded at the field installation.
- Section 1, 1.1. 1.2 and part of 1.3 of the test schedules have been completed.
- The initial condition of the batteries and their SoC has been ascertained.
- The design features of the battery and their benefits in this application have been evaluated.
- The label data has been noted for predicting performance and comparisons with actual test results.
- The expected charge and discharge algorithm has been identified and is the basis of the test regime for the Digatron equipment to be used in this project.
- Performance tests based on application requirements have started. Basic constant power discharge and recharge data has been completed using existing UK Powertech calibrated test equipment.
The equipment and test rig are shown schematically in Fig 4. This is a replica of the expected load and input for the batteries in the domestic solar power set up for the field trials.
Tests to be completed
1.3 Laboratory: Establish basic performance criteria of capacity, voltage, impedance, DC-IR, charge acceptance, volts drop on load at different SoCs, charge-discharge energy efficiency for the lead-acid batteries. For the lithium-ion batteries the same as the lead-acid plus the BMS characteristics and limitations along with the field simulation tests.
1.4 All the field trials: SoC of battery, heating issues, max charge rate, efficiency, charging time and customer feedback for both the lead-acid and lithium-ion chemistries.
These are for schedules 1.1 and 1.2. and part of 1.3. The physical examination of the delivered lead-acid batteries and the design attributes have been noted. The charging pattern and load for a domestic solar storage system has been ascertained and the discharge and charge characteristics for a lead-acid battery have been replicated in the test lab of UK Powertech. The results of using two batteries in series are given along with the annual and daily power input from the solar array. The recharge characteristics of the batteries and their behaviour under a constant power load have been measured.
3.1 Lead-acid batteries visual attributes, schedule 1.1 and 1.2.
Figs 1 and 2 give a top and side view of the lead-acid batteries supplied for testing. The as received condition and packaging plus the design features noted on the batteries are given below:
Packaging as received condition:
- The lead-acid batteries were collected at the distributor’s premises. There was no packaging.
- Casing was clean with no dust or terminal grease on the lid. The labelling was clear and on straight. Looked solid and well made, overall first impressions were very good.
- The terminals were dual design providing both a lead SMMT automotive terminal and a male 6mm screw thread to provide flexibility for connecting to the appliance.
Design features/ customer aids:
- Integrated flat carrying handle
- Magic eye electrolyte state of charge indicator
- Dual terminal take off, SMMT taper and threaded male screw
- Dimensions: 302L x 175W x 220H
- Weight: 21.3 Kg
- Rated energy density (C20): 53.5 watt hours per kilo and 95 watt hours per litre.
3.2 Charge and discharge tests lead-acid batteries
These tests were based on the conditions expected in the field trials. The maximum measured load is 1.04kW and the charging is via a three-stage taper charger rated at 40 amps maximum output. Two batteries were connected in series to provide a 24 V, 2.4 kWh supply based on a C20 discharge rating. The load is supplied by a domestic fan and places a constant 1.04 Kw load on the battery. The inverter/charger between the battery and load is an identical specification to that used in the field trials Fig 4.
The discharge results are shown in Table 1 and are the measured and calculated parameters of volts, amps, watts and kWh are provided by the battery to 20V (1.67) VPC. Fig 5 is a graphical representation of the test data which shows the clear trends and limits of the battery performance during the discharge test. The choice of constant power discharge is more representative of real application conditions than conventional constant current capacity tests.
Discharge results summary:
Batteries lasted for one and a half hours and gave a total of 1.38 kWh before reaching the cut-off voltage.
The voltage dropped from 24.15V at the beginning of the test to 20V at the end of the test.
The current varied from 41.59 to 50.10 amps from the beginning to the end of the test respectively.
Average current on discharge was 44.05 amps, Average voltage was 23.74V (1.04kW). This represents an average discharge rate of 9C20. From this we could expect a run time of 0.95 hours compared with an actual run time of 1.38 hours, far higher than expected from the catalogue rating.
Table 2 and Fig 6 are the tabulated and graphical representation of the charge results respectively. The results are for the first 50 minutes of the charge period. The maximum output from the charger was 38 amps, this can be compared with the label rating or 40 amps. During this period the charger did not reach the secondary stage which is triggered at 2.4 VPC (28.8V). During this incomplete bulk phase charging the batteries absorbed 0.84 kWh in 50 minutes.
Charge results summary
The batteries draw the full current output from the inverter/charger for at least 50 minutes i.e. a little under 38 amps. This gives a charge/discharge ratio of around 0.86 with the equipment supplied. This is a high ratio which is beneficial in this application where recharge time is critical.
The kWh returned are 0.841 in 50 mins compared to 1.63 removed in 93 mins. This represents a 52% return of the energy removed in under an hour. Again, this is a high figure for lead-acid batteries.
Figs 7 and 8 show the energy available by month for 2017 and for one day in December 2017.
The highest monthly average is obtained in May with almost 440kWh generated from both banks of solar panels. The lowest value is in December, which provides a little under 28kWh in total. This gives a daily average energy input of 14 and 0.9kWh respectively. The requirement for this peak shaving application is 4.4kWh, i.e four batteries in a 24V, 200 ah series-parallel connection. This energy input requirement will only be achieved in six months of the year. For efficient operation the battery must recharge within six hours to take advantage of the available light in the low light months. The output depends on the usage but will have a peak draw of 1.048kW.
4. Discussion of results
The visual inspection of the lead-acid batteries showed a well-made product with three useful features: a dual connection terminal, a carrying handle and an optical state of charge indicator. The handle worked well and folded flush into the lid which ensured maximum flexibility in fitting into tight spaces. It’s not possible to comment on the effectiveness of the SoC indicator as it is early into the testing period with insufficient data to comment on its reliability. The batteries had no leakages when tipped on their side and visually at least the lids appeared to be sealed. This type of flooded sealed battery should not be confused with VRLA valve regulated sealing.
The weight of the batteries was within 3% of the catalogue rating and the dimensions accurate so there would be no surprises when the batteries are fitted into confined spaces. However, because these are flooded designs, venting will always be necessary. In mitigation, gas production would be negligible with the right charge regime. The grid alloy used in the plates is a lead calcium alloy which gives a high hydrogen overpotential compared with a medium or high antimony alloy used in many deep cycle applications. The right charging regime is vital to both minimise gassing and prevent the ills of PSoC cycling that can occur with this application. This will be determined during the field trials.
Whilst in the early stages of the test programme, the first simulation tests of the solar application have started using a typical constant power load and an inverter-charger commonly used for domestic solar energy storage. From Figs 5 and 6, the first indications are that the battery has exceeded the label rating for capacity and has good charge acceptance having drawn the maximum current from the charger for almost an hour. The return of 0.84kWh of the 1.38kWh removed in the discharge test is a significant result, particularly for those occasions when the battery is not fully recharged in the darker months after the daily cycle. This would indicate that the batteries would be capable of storing at least the minimum available energy (0.9kWh) from the darkest days in less than two hours.
This is 52% of the capacity whilst the battery is still below the gassing voltage of 28.8V. This is a very good rate of energy return for a lead-acid battery. This should easily provide the energy input required by the autonomy and recharge periods available in the field trials. This would also indicate that the batteries would be capable of storing at least the minimum available energy (0.9kWh) from the darkest days in the year in under two hours.
The visual appraisal shows design features which enable the user to easily handle the battery and fit it into restricted spaces.
Addition of the optical SoC sensor is a very useful tool, making it easy to ensure that the battery is properly maintained.
Although incomplete, the tests are showing very good results for this battery in a solar energy storage application.
The results have shown the battery to have:
- Good charge acceptance with fast recharge times
- Discharge curve has an almost linear voltage decay for most of the high rate discharge period
- The discharge period to the cut-off voltage is longer than expected from the published capacity rating
- The charge acceptance and autonomy characteristics under constant power load are more than adequate for the intended application
- Despite the testing being at a very early stage, a lot has been gained in understanding the characteristics of this battery and its suitability for the intended application
- This information cannot be obtained by conventional testing nor from label or commercially available catalogue data
- The full test results including the field trials and remaining laboratory tests will provide a definitive and unique review for use of this battery in domestic solar storage applications.
Although the review is limited in results, at this stage the team is on track to finish the testing and publish the report in the autumn issue of BEST magazine. The full recharge capability, charge acceptance and operational efficiency will be completed using the Digatron equipment, the field trials will be completed on site at the domestic solar installation.
This article has shown that the testing programme is under way and on track to deliver unique results and information to thoroughly assess a particular battery’s suitability for an intended application. The nature of the tests, the specific targeting and real field trials represent a unique and highly relevant insight into a battery’s capabilities. This is possible only because of the long and particular experience of the team devising the procedures and tests.
Once completed, the endorsement has value that cannot be matched or reproduced by any other organisation. For those who may have missed the announcements in the last issue or the BBB leaflets, the following appendix gives a brief summary of the aims, benefits and terms and conditions of this testing programme.