What are the precautions for choosing lithium battery and protection board?

1: Lithium battery type and capacity selection. First, you must calculate the continuous current to be supplied by the battery according to your own motor power (requires actual power, generally the riding speed will correspond to a corresponding actual power). For example, if the motor has a continuous current of 20a (1000w motor at 48v), the battery must be able to supply 20a current for a long time and the temperature rise is very low (even if the temperature is 35 degrees outside in summer, the battery temperature is best controlled below 50 degrees). In addition, if the current is 20a at 48v, the overpressure is doubled (96v, such as ecpu 3) and the continuous current will reach about 50a. If you like to use over-pressure for a long time, please choose a battery that can continuously supply 50a current (still pay attention to the temperature rise). The continuous current of the battery here is not the nominal battery discharge capacity of the merchant. Merchants claim that a few C (or hundreds of amperes) are the battery discharge capacity, but when the battery is really discharged under this current, the battery heats up very seriously. If the heat is not properly dissipated, the battery life will be very short. (And the battery environment of our electric vehicles is that the batteries are piled up and discharged, basically leaving no gaps, and the packaging is very tight, not to mention how to force air cooling to dissipate heat). Our use environment is very bad. The battery discharge current needs to be derated for use. Evaluating the battery discharge current capability is to see how much the battery's corresponding temperature rise is at this current. The only principle discussed here is actually the temperature rise of the battery during use (high temperature is the deadly enemy of lithium battery life). It is best to control the battery temperature below 50 degrees. (Better between 20-30 degrees). This also means that if it is a capacity type lithium battery (discharged below 0.5C), the continuous discharge current of 20a should be more than 40ah capacity (of course, the most important thing is to look at the internal resistance of the battery). If it is a power-type lithium battery, it is normal to discharge continuously according to 1C. Even the A123 ultra-low internal resistance power type lithium battery, it is usually best to discharge at 1C (no more than 2C is better, 2C discharge can only be used for half an hour, and it is not very useful). The choice of capacity depends on factors such as the size of the car storage space, personal expenditure budget, and the expected range of car activities. (Small capacity generally requires power-type lithium batteries) 2: Screening and assembly of batteries The major taboo of using lithium batteries in series is the serious imbalance of battery self-discharge. As long as everyone is the same and unbalanced, it doesn’t matter. The problem is that this state is abruptly unstable. A good battery has a small self-discharge, and a bad battery has a large self-discharge. The self-discharge is not small and the state generally changes from good to bad. State, this process is unstable. Therefore, the batteries with large self-discharge should be screened out, and only batteries with small self-discharge should be kept (generally, the self-discharge of qualified products is small, and the manufacturer has measured it. The problem is that many unqualified products flow into the market). On the basis of small self-discharge, select series with similar capacity. Even if the capacity is not similar, it will not affect the battery life, but it will affect the usable capacity of the entire battery pack. For example, 15 batteries have a capacity of 20ah, and only one battery is 18ah, so the total capacity of this group of batteries can only be 18ah. At the end of the use, the battery will be dead, the protection board needs to protect, and the voltage of the entire battery is still relatively high (because the voltage of the other 15 batteries is normal, and there is still electricity). Therefore, the discharge protection voltage of the entire battery pack can tell whether the capacity of the entire battery pack is the same (provided that each battery cell must be fully charged when the entire battery pack is fully charged). In short, the unbalanced capacity does not affect the battery life, but only affects the capacity of the entire group, so try to choose an assembly with a similar capacity. The assembled battery must achieve good ohmic contact resistance between the electrodes. That is, the smaller the contact resistance between the wire and the electrode, the better, otherwise the electrode with a large contact resistance will heat up, and this heat will be transferred to the inside of the battery along the electrode and affect the battery life. Of course, the large assembly resistance is manifested by the large voltage drop of the battery pack under the same discharge current. (Part of the voltage drop is the internal resistance of the battery cell, and the other part is the assembled contact resistance and wire resistance) 3: Protection version selection and charging and discharging usage matters (data is for lithium iron phosphate batteries, the principle of ordinary 3.7v batteries is the same, but the data is different) protection The purpose of the board is to protect the battery from overcharging and over-discharging, to prevent high current from damaging the battery, and to balance the battery voltage when the battery is fully charged (the balancing ability is generally small, so it is difficult to protect the battery if there is a self-discharged battery cell protection board. Balanced, there is also a protection board that is balanced in any state, that is, it is balanced from the beginning of charging, which seems to be very rare). Disclaimer: Some pictures and content of the articles published on this site are from the Internet. 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