CHARGING METHOD OF Li-ion SECONDARY BATTERYAIMING AT SPACE USAGE
Kiyokazu Koga, Yoshitugu Sone, Hiroaki Kusawake , and Saburo KuwajimaNational Space Development Agency of Japan, Office of Research and Development
2-1-1, Sengen, Tsukuba-shi, Ibaraki 305-0047
AbstractUsing 50 Ah cells designed for H-2 orbiting plane-X (denoted as HOPE-X), we compared CC-CV charge
controlling method with CC-multistep way. This study revealed that both methods realized almost the samecapacity of the cells. To know the further influence of the constant voltage (CV) level where constantcurrent (CC) is switched to constant voltage (CV), commercial cells with different negative electrodes werecharged at different CV level. This measurement revealed that the capacity of cells increases proportionallyto the level of the constant charge voltage. All these measurements were performed at various temperature.The higher capacity of the cells were obtained with increasing temperature.
1. IntroductionLi-ion secondary cells (Li+/C) are widely used
for cellular-phone, notebook-type computer andothers, since it has high energy density. Thesedays, specially, electrical vehicles (denoted as EV)request cells with large capacity. The applicationof these large cells to a space battery is verypromising, since the required performance for theEV's battery is quite adequate to that for satellites inGEO. However, the performance of the Li+/C isdegraded drastically under the overcharge and overdischarge condition.
To our knowledge, today, all kinds of Li+/Ccells has common problems to over chemicalreactions. The most famous problem is caused bythe overcharge of the cell. Once the cell voltage israised more than 4.2 V, Lithium dendrite is formedon the surface of the negative (carbon) electrode toresult in short circuit inside the cell. If the cell isover discharged (less than 2.8V), it also starts tohave a problem of melting Cu which is usually usedas current collector on the positive (LiMO2, M=Co,Ni etc.) electrode. These nonconformance may notonly cut off the life cycles of the cell but also befollowed by the highly increasing temperature andpressure inside the cell and be concluded by theburst of the cell case. To prevent this accident,commercial cells usually have a safety valve, aPositive Temperature Coefficient (denoted as PTC)element, a shut down separator, and etc. These arethe functions to prevent the accident mechanically.
Another way of using Li+/C safely is tocontrol the cell voltage properly. Li+/C requestsus a more careful control of the cell voltagecomparing to that of the current space batteries likeNi-Cd, Ni-MH, and Ni-H2. This enhanced us totest the charge controlling methods aiming at a safe
application of Li+/C to space crafts with highcapacity.
There are two different methods of controllingcell voltage. One is, so-called, CC-CV control.First, the cell was charged by the constant current(denoted as CC). Once the cell voltage reaches toa value above which a Li dendrite is formed, theconstant current (CC) is switched to a constantvoltage (denoted as CV). This is one of the mostordinal way, today, to charge Li+/C.
The other is CC-multistep method. First, thecell was charged at constant current (CC). Oncethe cell voltage reached to a point decided, the cellwas ceased to be charged. After several min., thecell started to be charged, again, using lower current.Different constant level are followed repeatedlyuntil the cell is fully charged.
Today, H-2 orbiting plane-X (denoted asHOPE-X), which is planed to use Li+/C as a powersource, is going to adopt this CC-multistep chargingmethod. In this paper, we will show an effect ofcharging methods as well as temperature on the cellcapacity.
2. Experimental50 Ah Li+/C cells were used to see an effect
of charging methods. The cell specification islisted in Table 1. The photograph of the cell isshown in Fig. 1. Using CC-CV and CC-multistepway, these cells were charged. 25 A (C/2) was theconstant current (CC) until the cell voltage wasreached to 4.05 V.
When the cell was charged by CC-CV control,the cell voltage was kept being 4.05 V. 4 hourswere totally spent in order to charge the cell.
In the case of CC-multistep control, the
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charging current was once ceased when the cellvoltage reached to 4.05. After 5 minutes interval,the cell was charged again with 5 A chargingcurrent. After charging untill 4.05V, the cell wasinterrupted to be charged for 5 minutes, again.Finally, the cell was charged using 1 A chargingcurrent to obtain 4.05V.
After the above charging, the cell wasdischarged at 25 A (C/2). These measurementswere performed at 0, 25, and 40°C to see theinfluence of temperature on realized capacity.Test condition is shown in Table 3.
To know the further influence of the constantvoltage (CV) level for CC-CV control, commercialcells with different negative electrodes (coke andgraphite) were charged at several CV levels. Thecells we used is listed in Table 2. Test condition isshown in Table 3. These measurements wereperformed at -10, 0, 25, and 50°C. This test isperformed to know that the Li+/C cell has VT curveor not. VT curve indicates the charge limit voltageat battery operated temperature.
3.Results and discussion3.1 CC-CV vs. CC-multistep
Fig. 2 and 3 shows charge and dischargeprofiles of 50Ah cell at 0, 25, 40°C under CC-CVcharge condition. Fig. 4 and 5 shows charge anddischarge profiles at 0, 25, 40 °C under CC-multistep condition. Horizontal line showscharge/discharge time. Fig. 6 shows capacitycharacteristics at each temperature. Both CC-CVand CC-multistep method realized almost the samecapacity at each temperature. But, capacity whichis obtained using CC-multistep is little less than thatof CC-CV at 25, 40°C on one side, and little morethan at 0°C on the other. In the case of 0 °C,from Fig. 2, charge current during CV-charge wasnot decreased sufficiently. It seems that the chargingtime of 4 hours were not enough to fill up thecapacity at 0°C.
Table 5 shows the time of charging at eachcurrent rate under CC-multistep condition. At anytemperature, charge time of CC-multistep is longerthan 4 hours. Especially, under 0°C, charge time is2 times as longer than that of 25, 40°C. But, itmay be improved if the number of multistep isincreased.
Commercial cell was tested under same way.Fig. 7 shows capacity characteristics of commercialcell at -10, 0, 25, 50 under CC-CV and CC-stepcondition. This cell is graphite type as same as50Ah cell for HOPE-X. Capacity is almost samebetween CC-CV and CC-step like 50Ah cell at anytested temperature. At -10°C, charge voltagebecame to 4.1V as soon as test start where the
charge current is 1/2C of first step. This indicatesthat impedance of Li+/C cell increase rapidly at lowtemperature.
3.2 CC-CV charge at several CV levelFig. 8 and 9 shows charge and discharge
profiles at 25°C about graphite type. Fig. 10 and11 shows about coke type. Charge amountincreased in proportion to CV level. And capacityincreased according to this charge amount. Theratio of capacity variation and CV level is about8%/0.1 V about graphite type. That of coke type is17%/0.1V. Capacity variation of coke type isbigger than that of graphite type. Fig. 12 and 13show charge and discharge characteristics ofgraphite type at 0, 25, 50°C. Fig. 14 and 15show that of coke type. Charge voltage of bothtype increased in proportion to CV level. Thisreversed capacity characteristic of tempararure is assame as the battery which has alkalinity electrolytelike Ni-Cd cell. Fig. 16 and 17 show capacitycharacteristics at each temperature. Fig. 16 showsabout graphite type and Fig. 17 shows about coketype. Temperature characteristics is almost sameabout graphite and coke type. But, it seems thatthe ratio of capacity variation and temperature isquasi linear about coke type, and nonlinear aboutgraphite type.
4.ConclusionWe found that both CC-CV and CC-multistep
method realized almost same capacity of the cell.Charging time of CC-multistep is longer than that ofCC-CV especially at low temperature. But it maybe improved if we increase the number of multistep.
Using CC-multistep charge method, we cansimplify charge control circuit, because currentcontrol of CV charge is not necessary. In the caseof HOPE-X, battery is charged on the ground only.We must evaluate cycle life of the cell under CC-multistep condition when we adopt this method tothe satellite which needs long life cycle of the cell.
From CC-CV changing test at several CV level,capacity increases in proportion to CV level.Under 0°C, capacity decrease quickly. To increasecapacity, increase CV level is useful. It semmsthat Li+/C cell has VT curve like Ni-Cd cell. Butwe have not checked tolerance zone of CV level atlow temperature. Furthermore we must evaluatethis CV level which is applicate to life cycle test ofthe cell at low temperature.
For future work, we have been planning thetests concerning life cycle to compare CC-CV andCC-multistep. Furthermore we evaluate the effect tothe life cycle when we applicate different CV level.
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