I have seen a lot of people on the internet repurposing old laptop batteries and batteries from electric vehicles. This is a great way to acquire low cost, high quality batteries, but it does require a lot of work to disassemble, test, and sort them. I think this will become a common method in the future because a battery that doesn’t work well for an electric vehicle might still be a good fit for a battery energy storage system (BESS), since the charge and discharge rates are so much lower in a BESS compared to an EV. Typically a BESS uses much lower power compared to an EV that needs to accelerate quickly.
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Building a lithium battery pack from used cells is a great way to save money and get more life out of something that would otherwise be discarded. Do not underestimate how useful a BMS can be to monitor and protect you from an unsafe condition, especially when using used cells. Please note that since a BMS set up incorrectly will not protect your pack from thermal runaway, you need to contact an expert to ensure the BMS is properly configured to protect the specific cells you are using.
Every battery cell is as unique as a snowflake, with microscopic variations on the cathode and anode material that result in slight functional variations. When manufacturers build new packs they grade and sort cells so that packs have nearly identical characteristics. They consider energy capacity, internal resistance, and manufacturing date. When you are building your pack from used cells, I recommend you do the same. If you want to build a 16 cell pack, you should buy more than 16 cells and “cherry pick” the best to match. If you use a high quality BMS, you can manage the variations and rebalance the pack after each cycle.
When batteries age their available amp-hour capacity decreases. Sometimes the internal resistance will increase too, which means the voltage drops proportionally to the current. If the voltage drops too low, then the system loses power. You can imagine this like a clogged pipe where the narrowing pipeline allows less flow than before.
Building a battery from used cells requires a strong foundation in battery and electrical knowledge and I only recommend it for experts.
I sell a 4P busbar kit for Nissan LEAF for this configuration, see the details on the link above or on Off Grid Solar Store.
Design Guide for building a Nissan LEAF pack for energy storageNissan Leaf has a very unusually (and sometimes frustrating) way of stacking all of the modules together. There are holes in which you push a long rod through and fasten down on an end plate. If you get one piece backwards, then you have to start all over again, hence the frustration.
Stack a P-group of modules together usually 2–6 modules using the module type “A” with the positive terminals on one side, then alternate with module type “B”. This makes it easier when you are attaching bus bars in the series connections.
Why is a Battery Management System (BMS) so Important? Lithium-ion batteries always require some electronics to protect the cells from extreme voltage, current, or temperatures. In many cases, a proprietary Battery Management System (BMS) comes with a battery pack to equalize and protect the individual battery cells. But you can also build a battery pack by assembling cells and adding a BMS. Most batteries other than lithium-ion do not require a BMS for safe usage. Lithium batteries are unique in this way because they can easily catch on fire if the voltage, peak current, or temperature for an individual cell is not kept under control.
A BMS monitors each cell and ensures each remains in a safe voltage range. Some mistakenly think they can ensure safety by simply keeping the overall pack voltage below a safe limit, ignoring the individual cell voltage. The problem with this approach is that it assumes that all the cells are in exact balance. In reality all battery cells have unique variations and they rarely have the same voltage and internal resistance. This causes each cell to drift on their state of charge and specific voltage. As a battery pack is charged and discharged many times the cells can become out of balance.
For example, if a pack voltage is measured at 28.8V for 8 cells in series, you might think that all cells are at 3.6V by taking the average of the pack voltage per cell.
EXAMPLE OF TWO DIFFERENT PACKS WITH THE SAME PACK VOLTAGE28.8V / 8 cells = 3.6 average V per cell
If the cells are in perfect balance, then each cell would be at 3.6V. But all cells do not function the same. Every battery cell (lithium, lead-acid, etc.) is a unique snowflake. Its capacity, resistance characteristics, and aging patterns are all slightly different.
The magic of a BMS is that it can help you with these variations between cells. It can actively rebalance or discharge the highest cells so the average cell voltage and each individual cell’s voltages are close. It can also disconnect the pack if any one cell gets into an unsafe condition.
In the example above with a pack voltage of 28.8V, without a BMS there is no way to tell if any one cell has reached its maximum voltage limit. Below is an example of two packs with the same pack voltage but with very different cell voltages. Without a BMS there is no way to stop Cell #5 from overcharging.
Some people on the internet recommend managing the peak voltage on a cell by “top balancing” each cell before assembling the pack. This refers to manually charging all cells up to a peak voltage so that they all match. After they “top balance” the cells, they only monitor the pack voltage. What they fail to consider is that over time, all cells will naturally drift from each other and cell voltages will eventually not match at the top of the charge. As I described above, measuring exclusively pack voltage is a dangerous practice, as top balancing only reduces but does not eliminate the chance of thermal runaway.
Inverters convert direct current (DC) to alternating current (AC), to render solar or battery power into energy usable for your home. But in some circumstances, it is useful, also, to be able to convert AC to DC as well (for example, to use a generator to charge batteries on cloudy winter days). AC to DC conversion is called rectifying; a rectifier changes AC to DC.
A Bi-Directional Inverter, also called a Hybrid or Battery Inverter, converts between both DC and AC. It can rectify (convert from AC to DC) and invert the power (convert from DC to AC). All that means is that the inverter also has a battery charger built into it. Some of these solar battery inverters also have the ability to convert high voltage DC from the solar array to a lower voltage for the battery. In this case it is simply an inverter with a battery charger and a charge controller in one box. It could be beneficial to buy all of these components in one box rather than have three separate components.
Battery research is focusing on lithium chemistries so much that one could imagine that the battery future lies solely in lithium. There are good reasons to be optimistic as lithium-ion is, in many ways, superior to other chemistries. Applications are growing and are encroaching into markets that previously were solidly held by lead acid, such as standby and load leveling. Many satellites are also powered by Li-ion.
Lithium-ion has not yet fully matured and is still improving. Notable advancements have been made in longevity and safety while the capacity is increasing incrementally. Today, Li-ion meets the expectations of most consumer devices but applications for the EV need further development before this power source will become the accepted norm. BU-104c: The Octagon Battery – What makes a Battery a Battery, describes the stringent requirements a battery must meet.
As battery care-giver, you have choices in how to prolong battery life. Each battery system has unique needs in terms of charging speed, depth of discharge, loading and exposure to adverse temperature. Check what causes capacity loss, how does rising internal resistance affect performance, what does elevated self-discharge do and how low can a battery be discharged? You may also be interested in the fundamentals of battery testing.
The lithium-ion battery works on ion movement between the positive and negative electrodes. In theory such a mechanism should work forever, but cycling, elevated temperature and aging decrease the performance over time. Manufacturers take a conservative approach and specify the life of Li-ion in most consumer products as being between 300 and 500 discharge/charge cycles.
In 2020, small wearable batteries deliver about 300 cycles whereas modern smartphones have a cycle life requirement is 800 cycles and more. The largest advancements are made in EV batteries with talk about the one-million-mile battery representing 5,000 cycles.
Evaluating battery life on counting cycles is not conclusive because a discharge may vary in depth and there are no clearly defined standards of what constitutes a cycle(See BU-501: Basics About Discharging). In lieu of cycle count, some device manufacturers suggest battery replacement on a date stamp, but this method does not take usage into account. A battery may fail within the allotted time due to heavy use or unfavorable temperature conditions; however, most packs last considerably longer than what the stamp indicates.
The performance of a battery is measured in capacity, a leading health indicator. Internal resistance and self-discharge also play roles, but these are less significant in predicting the end of battery life with modern Li-ion.
Figure 1 illustrates the capacity drop of 11 Li-polymer batteries that have been cycled at a Cadex laboratory. The 1,500mAh pouch cells for mobile phones were first charged at a current of 1,500mA (1C) to 4.20V/cell and then allowed to saturate to 0.05C (75mA) as part of the full charge saturation. The batteries were then discharged at 1,500mA to 3.0V/cell, and the cycle was repeated. The expected capacity loss of Li-ion batteries was uniform over the delivered 250 cycles and the batteries performed as expected.
Eleven new Li-ion were tested on a Cadex C7400 battery analyzer. All packs started at a capacity of 88–94% and decreased to 73–84% after 250 full discharge cycles. The 1500mAh pouch packs are used in mobile phones.
Although a battery should deliver 100 percent capacity during the first year of service, it is common to see lower than specified capacities, and shelf life may contribute to this loss. In addition, manufacturers tend to overrate their batteries, knowing that very few users will do spot-checks and complain if low. Not having to match single cells in mobile phones and tablets, as is required in multi-cell packs, opens the floodgates for a much broader performance acceptance. Cells with lower capacities may slip through cracks without the consumer knowing.
Similar to a mechanical device that wears out faster with heavy use, the depth of discharge (DoD) determines the cycle count of the battery. The smaller the discharge (low DoD), the longer the battery will last. If at all possible, avoid full discharges and charge the battery more often between uses. Partial discharge on Li-ion is fine. There is no memory and the battery does not need periodic full discharge cycles to prolong life. The exception may be a periodic calibration of the fuel gauge on a smart battery or intelligent device(See BU-603: How to Calibrate a “Smart” Battery)
The following tables indicate stress related capacity losses on cobalt-based lithium-ion. The voltages of lithium iron phosphate and lithium titanate are lower and do not apply to the voltage references given.
Note:
Tables 2, 3 and 4 indicate general aging trends of common cobalt-based Li-ion batteries on depth-of-discharge, temperature and charge levels, Table 6 further looks at capacity loss when operating within given and discharge bandwidths. The tables do not address ultra-fast charging and high load discharges that will shorten battery life. No all batteries behave the same.Table 2 estimates the number of discharge/charge cycles Li-ion can deliver at various DoD levels before the battery capacity drops to 70 percent. DoD constitutes a full charge followed by a discharge to the indicated state-of-charge (SoC) level in the table.
Depth of Discharge
Discharge cycles
NMC
LiPO4
100% DoD
~300
~600
80% DoD
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~400
~900
60% DoD
~600
~1,500
40% DoD
~1,000
~3,000
20% DoD
~2,000
~9,000
10% DoD
~6,000
~15,000
Table 2: Cycle life as a function ofdepth of discharge** 100% DoD is a full cycle; 10% is very brief. Cycling in mid-state-of-charge would have best longevity.
Lithium-ion suffers from stress when exposed to heat, so does keeping a cell at a high charge voltage. A battery dwelling above 30°C (86°F) is considered elevated temperature and for most Li-ion a voltage above 4.10V/cell is deemed as high voltage. Exposing the battery to high temperature and dwelling in a full state-of-charge for an extended time can be more stressful than cycling. Table 3 demonstrates capacity loss as a function of temperature and SoC.
Most Li-ions charge to 4.20V/cell, and every reduction in peak charge voltage of 0.10V/cell is said to double the cycle life. For example, a lithium-ion cell charged to 4.20V/cell typically delivers 300–500 cycles. If charged to only 4.10V/cell, the life can be prolonged to 600–1,000 cycles; 4.0V/cell should deliver 1,200–2,000 and 3.90V/cell should provide 2,400–4,000 cycles.
On the negative side, a lower peak charge voltage reduces the capacity the battery stores. As a simple guideline, every 70mV reduction in charge voltage lowers the overall capacity by 10 percent. Applying the peak charge voltage on a subsequent charge will restore the full capacity.
In terms of longevity, the optimal charge voltage is 3.92V/cell. Battery experts believe that this threshold eliminates all voltage-related stresses; going lower may not gain further benefits but induce other symptoms(See BU-808b: What causes Li-ion to die?) Table 4 summarizes the capacity as a function of charge levels. (All values are estimated; Energy Cells with higher voltage thresholds may deviate.)
Every 0.10V drop below 4.20V/cell doubles the cycle but holds less capacity. Raising the voltage above 4.20V/cell would shorten the life. The readings reflect regular Li-ion charging to 4.20V/cell.
Guideline: Every 70mV drop in charge voltage lowers the usable capacity by about 10%.
Note: Partial charging negates the benefit of Li-ion in terms of high specific energy.
* Similar life cycles apply for batteries with different voltage levels on full charge.
** Based on a new battery with 100% capacity when charged to the full voltage.
Experiment: Chalmers University of Technology, Sweden, reports that using a reduced charge level of 50% SOC increases the lifetime expectancy of the vehicle Li-ion battery by 44–130%.
Most chargers for mobile phones, laptops, tablets and digital cameras charge Li-ion to 4.20V/cell. This allows maximum capacity, because the consumer wants nothing less than optimal runtime. Industry, on the other hand, is more concerned about longevity and may choose lower voltage thresholds. Satellites and electric vehicles are such examples.
For safety reasons, many lithium-ions cannot exceed 4.20V/cell. (Some NMC are the exception.) While a higher voltage boosts capacity, exceeding the voltage shortens service life and compromises safety. Figure 5 demonstrates cycle count as a function of charge voltage. At 4.35V, the cycle count of a regular Li-ion is cut in half.
Besides selecting the best-suited voltage thresholds for a given application, a regular Li-ion should not remain at the high-voltage ceiling of 4.20V/cell for an extended time. The Li-ion charger turns off the charge current and the battery voltage reverts to a more natural level. This is like relaxing the muscles after a strenuous exercise(See BU-409: Charging Lithium-ion)
Figure 6 illustrates dynamic stress tests (DST) reflecting capacity loss when cycling Li-ion at various charge and discharge bandwidths. The largest capacity loss occurs when discharging a fully charged Li-ion to 25 percent SoC (black); the loss would be higher if fully discharged. Cycling between 85 and 25 percent (green) provides a longer service life than charging to 100 percent and discharging to 50 percent (dark blue). The smallest capacity loss is attained by charging Li-ion to 75 percent and discharging to 65 percent. This, however, does not fully utilize the battery. High voltages and exposure to elevated temperature is said to degrade the battery quicker than cycling under normal condition. (Nissan Leaf case)
* Discrepancies exist between Table 2 and Figure 6 on cycle count. No clear explanations are available other than assuming differences in battery quality and test methods. Variances between low-cost consumer and durable industrial grades may also play a role. Capacity retention will decline more rapidly at elevated temperatures than at 20ºC.
Only a full cycle provides the specified energy of a battery. With a modern Energy Cell, this is about 250Wh/kg, but the cycle life will be compromised. All being linear, the life-prolonging mid-range of 85-25 percent reduces the energy to 60 percent and this equates to moderating the specific energy density from 250Wh/kg to 150Wh/kg. Mobile phones are consumer goods that utilize the full energy of a battery. Industrial devices, such as the EV, typically limit the charge to 85% and discharge to 25%, or 60 percent energy usability, to prolong battery life(See Why Mobile Phone Batteries do not last as long as an EV Battery)
Increasing the cycle depth also raises the internal resistance of the Li-ion cell. Figure 7 illustrates a sharp rise at a cycle depth of 61 percent measured with the DC resistance method(See also BU-802a: How does Rising Internal Resistance affect Performance?) The resistance increase is permanent.
Figure 7: Sharp rise in internal resistance by increasing cycle depth of Li-ion [4]Note: DC method delivers different internal resistance readings than with the AC method (green frame). For best results, use the DC method to calculate loading.
Figure 8 extrapolates the data from Figure 6 to expand the predicted cycle life of Li-ion by using an extrapolation program that assumes linear decay of battery capacity with progressive cycling. If this were true, then a Li-ion battery cycled within 75%–25% SoC (blue) would fade to 74% capacity after 14,000 cycles. If this battery were charged to 85% with same depth-of-discharge (green), the capacity would drop to 64% at 14,000 cycles, and with a 100% charge with same DoD (black), the capacity would drop to 48%. For unknown reasons, real-life expectancy tends to be lower than in simulated modeling(See BU-208: Cycling Performance)
Li-ion batteries are charged to three different SoC levels and the cycle life modelled. Limiting the charge range prolongs battery life but decreases energy delivered. This reflects in increased weight and higher initial cost.
Battery manufacturers often specify the cycle life of a battery with an 80 DoD. This is practical because batteries should retain some reserve before charge under normal use(See BU-501: Basics about Discharging, “What Constitutes a Discharge Cycle”) The cycle count on DST (dynamic stress test) differs with battery type, charge time, loading protocol and operating temperature. Lab tests often get numbers that are not attainable in the field.
Environmental conditions, not cycling alone, govern the longevity of lithium-ion batteries. The worst situation is keeping a fully charged battery at elevated temperatures. Battery packs do not die suddenly, but the runtime gradually shortens as the capacity fades.
Lower charge voltages prolong battery life and electric vehicles and satellites take advantage of this. Similar provisions could also be made for consumer devices, but these are seldom offered; planned obsolescence takes care of this.
A laptop battery could be prolonged by lowering the charge voltage when connected to the AC grid. To make this feature user-friendly, a device should feature a “Long Life” mode that keeps the battery at 4.05V/cell and offers a SoC of about 80 percent. One hour before traveling, the user requests the “Full Capacity” mode to bring the charge to 4.20V/cell.
The question is asked, “Should I disconnect my laptop from the power grid when not in use?” Under normal circumstances this should not be necessary because charging stops when the Li-ion battery is full. A topping charge is only applied when the battery voltage drops to a certain level. Most users do not remove the AC power, and this practice is safe.
Modern laptops run cooler than older models and reported fires are fewer. Always keep the airflow unobstructed when running electric devices with air-cooling on a bed or pillow. A cool laptop extends battery life and safeguards the internal components. Energy Cells, which most consumer products have, should be charged at 1C or less. Avoid so-called ultra-fast chargers that claim to fully charge Li-ion in less than one hour.
[1] Courtesy of Cadex
[2] Source: Choi et al. (2002)
[3] B. Xu, A. Oudalov, A. Ulbig, G. Andersson and D. Kirschen, "Modeling of Lithium-Ion Battery Degradation for Cell Life Assessment," June 2016. [Online]. Available: https://www.researchgate.net/publication/303890624_Modeling_of_Lithium-Ion_Battery_Degradation_for_Cell_Life_Assessment.
[4] Source: Technische Universität München (TUM)
[5] With permission to use. Interpolation/extrapolation by OriginLab.
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