Many friends are enthusiastic about DIY lithium battery packs, which are used to modify battery packs for electric toys and electric vehicles. In the process of communicating with them, I found that many people are still too optimistic and lack some detailed knowledge. How to do a safe and reliable DIY battery pack, I have the following suggestions:
The contents include:
- How to choose the battery cells
- Handle to balance battery cells
- Common problems in spot welding
- Environmental issues of battery pack use
- Lithium battery pack management board selection
- Charger selection
- Electric Vehicle brake matching
First, How to choose the battery cells
Before to choose something, we should really know something.
At present, the cells of the DIY lithium battery pack are mainly divided into two types according to the material:
- 3.2V LiFePO4
- 3.7V ternary lithium battery (Li（NiCoMn）O2)
This is a simple distinction.The main advantages and disadvantages and electrical parameters of the two are as follows:
LiFePO4: refers to a lithium ion battery that uses LiFePO4 as the positive electrode material. Its characteristic is that it does not contain precious elements such as cobalt, the price of raw materials is low, and phosphorus and iron are abundant in the earth's resources, so there will be no problem of feeding. Its working voltage is moderate (3.2V), large capacity under unit weight (170mAh / g), high discharge power, fast charging and long cycle life, high stability in high temperature and high temperature environment.
Advantages: higher safety, longer service life, and do not contain any heavy metals and rare metals ( Low cost of raw materials), support fast charging, wide operating temperature range.
Disadvantages: low tap density and compact density, resulting in low energy density of lithium ion batteries; the cost of material preparation is higher than the battery manufacturing cost, and the battery yield is low, poor product consistency.
Li（NiCoMn）O2: a lithium battery in which the cathode material uses lithium cobalt manganate (Li (NiCoMn) O2)"ternary" material. Its working voltage is 3.7V.
Advantages: Li（NiCoMn）O2 battery has high energy density and better cycle performance than normal lithium cobalt oxide. At present, with the continuous improvement of the formula and the perfect structure, the nominal voltage of the battery has reached 3.7V, and the capacity has reached or exceeded the level of the lithium cobalt oxide battery.
Disadvantages: The ternary material power lithium battery mainly includes nickel cobalt lithium aluminate battery, nickel cobalt manganese lithium battery, etc. Due to the unstable high temperature structure of nickel cobalt aluminum, the high temperature safety is poor, and the high pH value tends to make the monomer bloating, This in turn causes danger and is currently expensive.
Here is a diagram to compare the parameters for the technical difference.
Cell LiFePO4 Li（NiCoMn）O2
Rated voltage 3.2V 3.7V
Working voltage 3.0-3.3V 2.4-4.2V
Energy density ~125 ~ 170
Charging temp -10℃ -55℃ 0-45℃
Discharging temp -20℃ -60℃ -20℃ -60℃
Standard discharge C 2-5C 0.2C
From the practical experience: It’s not easy to buy a good battery from retail purchase for freshman. Good batteries are preferentially supplied to electric vehicles, and only those screened can flow into the market. Large merchants in the market will be screened again. Good batteries are used for battery packs, but poorly enter the retail market. Normally inferior ones enter the retail market, even the first-tier brands are no exception. Of course, the higher the premium you pay, you have the chance to get good ones.
1. So how do DIY people judge the quality of retail batteries?It’s a must to use internal resistance tester to do electricity test. Test the nominal MH and the actual MH. If the gap from the nominal parameter is larger, the quality is worse. The lower the internal resistance of the battery, the better. Generally speaking, it is less than 50. Above 50 may be old batteries or poor batteries.
2.Cycle life problem
Many friends assembled it by themselves, the capacity did not meet the standard, and it was obvious after a period of power failure. Most of this is a battery cell quality problem, and it is difficult for retail customers to bypass this pit. After all, there is no channel to get good goods and no equipment to check the quality of the battery.
Second, Handle to balance battery cells
After we get the battery cells, it is not that we can diy it in pack immediately. It must be processed again before it can be used. (For power battery pack)
The battery pack must be assembled with a matching protection board!
The battery requires matching consistency, the more consistent the better and more stable. If the matching is inconsistent, there will be insufficient capacity, which will seriously cause the detection of the battery protection board to fail and the battery pack will not work. How to deal with it?
- If there is a condition, go to the machine for screening. If there is no machine, use an internal resistance tester to measure one by one. Test each cell’s voltage, internal resistance and capacity and let it stand for a while (7 ~ 15 days). If it is still consistent, it can be processed. This is also a physical method to judge the stability of the cell.
- Why is it so troublesome? Can we use it directly? It can be used directly, but in the case of a mismatch, the battery pack will abide by the wooden barrel theory, and the capacity is limited to the lowest board. The worst cell in the battery pack is the upper limit. In layman's terms, the capacity of the battery pack will drop by 10 ~ 40%. Seriously, a bad cell in the battery pack will cause the whole group to not work. Inspection and maintenance work is more cumbersome. So we suggest that don’t use it directly before you check each one is in similar status: a cell full charged voltage tolerance should be in 0.002-0.005V range
Three, Common problems in spot welding
The processing and assembly of battery packs are prioritized by machine processing, and manual assembly is not recommended. (Multiple strings and other battery power packs are not required for single parallel)
2. Why is manual spot welding not recommended? The battery spot welding strength is not well controlled, the battery spot penetrates the liquid, and the nickel sheet spot is not solid and then virtual welding.
Manual soldering iron welding, too high solder temperature will damage the pole piece, it is recommended to solder at low temperature. (Most households are high temperature solder)
3.The thicker the battery core connection line, the better. The thicker the nickel sheet, the better.
4. As a charge and discharge container, the battery will also release heat during energy conversion. Irregular processing may cause problems with the battery pack. The battery pack must be treated with caution. Many industry personnel suffer a lot. PACK manufacturers even caught fire due to its careless.
Four, Environmental issues of battery pack use.
1.The battery pack is required to be waterproof! Probability of short circuit fire!
2. The battery pack requires shockproof! In the use of electric vehicles, can not be used as off-road vehicles! Nor can the battery pack case be used to cushion the collision, and deformation will cause a short circuit! Try to use grid bracket + cushion + outer box. Without the grille bracket, the battery pack will fall off under the squeeze of years of activities!
3. The battery pack should be provided with heat conduction channels. In energy conversion, timely heat conduction can optimize the battery. Steel outer box + fan assist are all conventional solutions.
4.Anti-corrosion problem, the battery is rusted for a long time, and the anti-corrosion measures should be taken as far as possible.
Five, lithium battery protection board selection
1.Two electric wheels below 40A are enough, and three wheels above 80A. The specific choice should be based on your own motor power.
2.The voltage should be matched. Most of 60V is conventional. It is necessary to distinguish ternary, lithium iron, and the number of strings.
3.As large capacity and low power as possible, stable and durable. If you have a small battery with high power, you must calculate whether the discharge rate is supported.
4. If possible, choose a protection board with low protection voltage, and give the battery a certain margin. For example: 4.25V protection is not as good as 4.2V ~ 4.1V, because the battery will not be placed at 100% power, do not close to the limit. However, most protection board designs have not paid attention to this, and a few manufacturers are doing it.
Six, charger selection
1. According to the protection voltage of the protection board, choose how many cells of lithium iron / ternary lithium
2. According to the total capacity, choose how many A / H chargers are suitable. 8A / H is currently recommended on the market.
3.The charger should also pay attention to protection against water and shock, and it will break down quickly if you don't take care every day.
Seven, electric vehicle brake matching
Try to choose an electric car with dual disc brakes. The lithium battery has a high discharge rate and fast acceleration. Without good brakes, it is prone to accidents.
Summary
It’s interesting to DIY battery pack yourself, it meets your needs of learning and hobby use. But for commercial use, it is necessary to consider a series of problems such as after-sales. It does not support the use of traditional manual spot welding to make electric vehicle battery packs. At the same time, the battery cycle life is also a key factor that easily be overlooked. A lot of self-made battery packs, the capacity does not reach the standard, and it takes a while to power down quickly, which is a common problem encountered during self-assembly. So All above suggestions provided before you consider to DIY a lithium battery pack.
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