Micro Power Electronics has the answers to your questions about powering portable equipment. Many of the most frequently asked questions are answered below. Additional, detailed technical information can be found on this website on the education page, and up to date information on transportation and regulations can be found on our transportation page. If you can't find the answer to your question here, please submit your question.
Custom Batteries and Cell Chemistry:
Q: What is the best maintenance procedure for Li-ion batteries after a long time in storage?
A: Cell manufacturers recommend that the Li-ionbatteries are stored at 40% capacity and approximately room temperature (or a little below) conditions. Research shows that storage, at elevated temperatures of charged Li-ion batteries, is the by far the most detrimental condition; yet, cell manufacturers recommend refurbishment after one year of storage, regardless of the condition.
Q: What limits the max # series cells in a custom battery pack?
A: Balancing requirements become more stringent when many Li-ion cells are in series. If one cell is out of balance with respect to the others, it can present a safety issue as the pack is cycled, so usually the number of cells in series is limited to 7. New technology is emerging to balance high series cell count strings, so this rule of thumb may soon change which will enable the manufacture of many medical batteries.
Q: Does a full drain help prolong the Li-ion batteries' life?
A: Li-ion batteries do not exhibit the "memory effect". The lifetime delivered by li-ion batteries is a function of the number of coulombs delivered- provided that it is used within the specified temperature ranges, etc. There are some cautions with this issue, however – coulomb counting gas gauges can require a learn cycles to accommodate for the reduced capacity over the life of the battery, and Li-ion batteries do not like to be held at a high state of charge for a long period of time.
Q: Are Li-ion batteries appropriate for use as a primary power source for vehicle systems?
A: The engines in hybrid vehicles, available on the market today, utilize Nickel batteries. However, there are several exciting prototype vehicles, such as the Tesla roadster (www.teslamotors.com) and Wrightspeed X1 (www.wrightspeed.com), that use lithium batteries as the only power source. There are several developers of Lithium batteries that are designing large Lithium batteries for this market. Valence is one example (www.valence.com).
Q: What is the typical material used to separate the anode and cathode in Li-ion batteries?
A: The anode and cathode have a liquid electrolyte between them that conducts the Li-ion; this is usually the dissolved salt LiPF6. There is a polymer separator that electrically isolates the anode and cathode; it is a thin membrane of porous polypropylene or polyethylene film.
Q: How much can a cell expected to swell?
A: Cylindrical Li-ion batteries radially distribute any internal pressure, which is normal and due to the Li-ion intercalation in the crystal structure of the materials, preventing cell swelling. The design of prismatic Li-ion batteries is improving; however, swelling of these prismatic cells can reach up to 10%.
Q: Are there federal regulations regarding the quantity of lithium in a battery, specifically for shipping/transport?
A: All Li primary and Li-ion battery packs, regardless of their size, will need to be tested according to the UN manual of Tests and Criteria prior to their production shipments.
As summarized in the chart below, three categories of batteries are defined in the U.S. DOT's latest rule based on their "size."
| Small (no more than) |
Medium (between) |
Large (more than) |
||
| Cells | Primary | 1 g Li | 1 g and 5 g Li | 5 g Li |
| Secondary | 1.5 g ELC* | 1.5 g and 5 g ELC | 5 g ELC | |
| Batteries | Primary | 3 g Li | 2 g and 25 g Li | 25 g Li |
| Secondary | 8 g ELC | 8 g and 25 g ELC | 25 g ELC | |
| * ELC (Equivalent Lithium Content) | ||||
Q: How is a Li-polymer battery different from a Li-ion battery?
A: The major difference is that the lithium-salt electrolyte is not held in an organic solvent as in the Li-ion design, but in a solid polymer composite such as polyethylene oxide or polyacrylonitrile. This allows a semi rigid form factor and very thin cells. Li-polymer cells can be encased in aluminum foil laminate pouches that are just 0.1 mm thick, rather than the 0.25- to 0.4-mm thick aluminum or steel cans traditionally used with Li-ion cells. Li-polymer cells are constructed by stacking electrode and electrolyte materials in a flat sandwich, rather than winding them in a jellyroll fashion as is done with Li-ion cells.
Q: I've seen "high rate cells" based on traditional materials. How are the new high rate technologies different?
A: Designing a cell that can accommodate high discharge and charge rates is an effort to reduce the path length and resistance for the transport of ions and electrons. The resistance of the cells must be lowered by using thin materials, increasing the amount of current collectors and increasing the electrolyte concentration and reducing its viscosity with solvents. Traditional Li-ion cells are based on a LiCoO2 cathode compound. In this material, Li-ions can only be inserted through two dimensional paths, so the rate capability is fundamentally limited. However, rate capability for short pulses can be improved by making the aforementioned changes, and cells for high current pulses have been available for some time. The rate capability of cells based on traditional materials is only about 5C, whereas the cells based on new materials can support rates of more than 30C.
Q: What different types of high-rate cells are there?
A: The new cells have fundamental material changes in the cathode to a 3-dimensional insertion structure. Two 3-D structures have been researched extensively: Manganese Spinel (LiMn2O4) and Iron Phosphate Olivine (LiFePO4). In addition, the problem can be addressed physically by decreasing the particle size of the materials to as small as nano-scale. These materials offer great ionic conductivity and low resistance with a trade-off in capacity. The manganese material has been most notably commercialized by E-One Moli Energy and a nano-scale phosphate has been commercialized by A123 Systems.
Q: What applications are best suited for the high power technology?
A: Like other Li-ion cells, the high power cells have operating voltages roughly three times that of the Ni chemistries. The manganese operating voltage is about 3.6V, and the phosphate is about 3.3V. Hence, any application that would benefit from the current capability of NiCd and the voltage of Li-ion is a good target. Anything hand-held and motorized is a likely candidate, and the performance improvement in the new power tools is a great proof point.
Battery Chargers:
Q: What guidelines do you recommend in choosing safety timer durations during each of the major charge cycles?
A: Schemes vary widely in Lithium batteries, but here is an example: If Vbat < 2V try to wake up the battery with both current and voltage limits set (I < C/10, V < 3V) for < 1 minute, then show battery fault if the cell voltage remains below 2V. With 2.0V < Vbat < 3.0V, charge at < C/10 for max of 20-30 minutes. If Vbat still < 3.0V, show battery fault. If charging in CC or CV mode, timeout after ~2X the anticipated charge time if the low current cutoff is not reached.
Q: Will you improve charge cycle life by charging to a lower CV voltage?
A: Yes — If you charge Lithium batteries to 4.1V/cell rather than 4.2V/cell you get about 20% better cycle life according to our cell expert. We've done a couple of designs that charge to < 4.2V for this purpose. You do cut the Lithium battery capacity a bit. How much is very dependent on the specific cell used in the Lithium battery.
Q: What is the highest charge current that I can use without fear of Lithium battery damage?
A: Never exceed the max charge current given in the cell spec. For example, for a Lithium ion battery, 2400mAh cell, with a max specified charge current of 1C, never exceed 2.4A CC mode charge current. We rarely see Lithium batteries that can be charged at the maximum current in the cell spec because of heating. I've seen an amount of heating energy cause a 5C to 10C rise inside Lithium batteries. Since you must shut down charge at 45C, and you normally get 5C to 10C rise due to charger-generated heat, you're down to 25C - 35C max ambient temperature. In a software-controlled smart battery charger, you can vary the charge current relative to the Lithium battery temperature to optimize charge time.
Q: Is there a temperature at which charging becomes dangerous?
A: Never charge at Lithium battery temperature above the max in the vendor spec. This is normally 45C max for Lithium batteries. If you charge too hot, you will first reduce cycle life, then create dangerous conditions. The cell vendors don't tell us where the transition to dangerous conditions is. Remember, the 45C spec is for any point on the surface of the cell, so watch out for hot spots due to nearby circuit elements.
Q: Do I need to have high rate cells in my battery to do a fast charge?
A: Designers of portable devices, especially lap-tops, have made incredible efforts to reduce charge time; one cannot simply increase the charge current. Li-ion batteries need to be charged with a constant-current followed by constant voltage method. Increasing the current in the first portion only increases dwell time at constant voltage. Many modifications have been used, "express charge", as one example, with moderate success, but a truly fast charge requires a cell designed to accept high current. These new cells boast charge times as low as fifteen minutes.
Q: What guidelines do you recommend in choosing safety timer durations during each of the major charge cycles?
A: Schemes vary widely in Lithium battery charging for the three generally accepted significant charge cycles or stages; charge initiation or wake-up, constant current or constant power and the final cycle, constant voltage or top-off. If Vbat < 2V try to wake up the battery with both current and voltage limits set (I < C/10, V < 3V) for < 1 minute, then show battery fault if the cell voltage remains below 2V. With 2.0V < Vbat < 3.0V, charge at < C/10 for max of 20-30 minutes. If Vbat still < 3.0V, show battery fault. If charging in CC or CV mode, timeout after ~2X the anticipated charge time if the low current cutoff is not reached.
Q: Will you improve charge cycle life by charging to a lower constant voltage (Vbatt)?
A: With typical Li Ion cells, yes — If you charge Lithium batteries to 4.1V/cell rather than 4.2V/cell you get about 20% better cycle life according to our cell expert. We have developed several battery packs that charge to < 4.1V per cell for this purpose. You do cut the Lithium battery capacity, this is the design trade-off. How much capacity is lost depends on the specific cell used in the Lithium battery.
Q: What is the highest charge current that I can use without fear of Lithium battery damage?
A: Never exceed the max charge current given in the cell spec. For example, for a Lithium ion battery, 2400mAh cell, with a max specified charge current of 1C, never exceed 2.4A CC mode charge current unless the cell specification clearly states this is an acceptable rate. Besides the maximum allowable charge rate many manufacturers provide a "suggested" charge rate to maximize battery life. In addition, we rarely see Lithium batteries that can be charged at the maximum current in the cell spec because of heating in the pack and surrounding electronics. In our experience, it is not unusual to see a 5C to 10C rise inside Lithium battery pack during charge. Since you often must shut down charge at 45C, and you normally get 5C to 10C rise due to charger-battery generated heat, you're down to 25C - 35C max ambient temperature as the practical charging window. In a software-controlled smart battery charger, you can vary the charge current relative to the Lithium battery temperature to minimize temperature rise and optimize charge time.
Q: Is there a temperature at which charging becomes dangerous?
A: In general never charge at Lithium battery temperature above the max in the vendor spec. This is normally 45C for Lithium batteries. If you charge too hot, you will first reduce cycle life, then create dangerous conditions which could lead to thermal runaway. The cell vendors don't tell us where the transition to dangerous conditions is. Remember, the 45C spec is for any point on the surface of the cell, so watch out for hot spots due to nearby circuit elements. Most cells also come equipped with a CID or current interrupt device. This is one of the cell's primary internal defenses against dangerous charging conditions.
Q: What compliance issues must be addressed when specifying a battery charger?
A: Too many to answer in one or two paragraphs. "Compliance" can cover many things but in our experience we break it down into several sub-catagories that include safety, electro-magnetic compatibility (EMC or emissions and susceptibility) and the newly emerging "Green initiatives" such as RoHS, China RoHS, WEE, Energy Star and CEC. A standard practice is to have the compliance requirements of the primary appliance flow-down to all accessories. While good as a rule of thumb this practice can unduly burden accessories with development and recurring costs along with costly and resource consuming validation testing.
With regards to safety testing there are few Li Ion specific requirements that a charger is burdened with but in general safety measures are pushed to the battery pack. Like any other switching power supply country specific and harmonized IEC safety standards must be followed for insulation from AC mains when applicable. In addition good design practices should be followed to equip chargers with fault protection for cases of over-voltage to the output and over-current (short circuit) at the output. The heart of most battery chargers is one or more switching converters. As with any switching converter design both conducted and radiated EMI must be dealt with and usually this is best done by snubbing noise at the source. All good design practices regarding PCB layout and filtering must be followed so extraordinary measures such as metal shielding and large expensive inductor filters can be avoided or at least kept to a minimum.
Q: Does higher charge current complicate compliance issues?
A: In general yes. With regards to safety higher current translates into larger and usually hotter power sub-systems and/or components. Mechanical fixes for removing heat from the system often end with vents and fans. In cases where AC/DC conversion is done in the same enclosure safety concerns related to a device as a "fire enclosure" can seriously complicate enclosure design and cost when venting or fan apertures are required. In addition higher current going through the power switch infers more work and countermeasures will be required to meet limits of both conducted and radiated EMI.
Power Supplies:
Q: When should I consider a custom power system vs. an off-the-shelf power supply?
A: A custom power system (AC/DC or DC/DC bulk converters, constant voltage DC/DC converters and battery chargers) should always be considered vs. designing around or with off-the-shelf power converters. It is most likely true that the development time and cost (particularly third party compliance testing: UL, TUV, MITI, NOM, FCC, etc.) will not achieve an acceptable ROI when production quantities are low (below 10,000 units per year).