Precautions for selection of lithium battery and protection board

1: Lithium battery type and capacity selection.

First, calculate the continuous current that the battery needs to provide based on the power of your own motor (requires actual power, and the general riding speed will correspond to a corresponding actual power). For example, the continuous current of the motor is 20a (1000w motor at 48v), then the battery needs to be able to provide 20a current for a long time with low temperature rise (even if the temperature outside is 35 degrees in summer, the battery temperature should be controlled below 50 degrees). In addition, if the current is 20a at 48v, the overvoltage is doubled (96v, such as ecpu 3 gear), and the continuous current reaches about 50a. If you like to use overvoltage for a long time, please choose a battery that can continuously provide 50a current (still pay attention to the problem of temperature rise). The battery continuous current here is not the merchant's nominal battery discharge capacity. The merchants claim that a few C (or tens of hundreds of amps) is the discharge capacity of the battery, but if it is actually discharged at this current, the battery will heat up very badly. If the heat dissipation is not done well, the battery life will be very short. (The battery environment of our electric vehicles is that the batteries are stacked and discharged, basically leaving no gaps, and the packaging is very tight, not to mention how to do a good job of forced air cooling). Our operating environment is very harsh. The battery discharge current needs to be derated. Evaluating the battery discharge current capability is to see how much the battery temperature rises under this current.Also read:Lifepo4 Battery 24V

The only principle discussed here is actually the temperature rise of the battery during use (high temperature is the enemy of lithium battery life). It is best to control the battery temperature below 50 degrees. (best between 20-30 degrees). This also means that if it is a capacity-type lithium battery (discharge controlled below 0.5C), it needs a capacity of more than 40ah to provide a continuous discharge current of 20a (of course, the most important thing is to look at the internal resistance of the battery). If it is a power lithium battery, it is normal to continue discharging at 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 discharged for half an hour before it runs out of power, which is not of much use value). The choice of capacity depends on factors such as the size of the car storage space, personal expenditure budget, and the expected range of activities of the car. (Small-capacity general power type lithium battery)

2: Screening and assembly of batteries

The taboo of using lithium batteries in series is that the battery self-discharge is seriously unbalanced. As long as everyone is unbalanced, it doesn't matter. The problem is that this state is in an unstable state. A good battery has a small self-discharge, and a bad battery has a large self-discharge. The state of self-discharge is generally from good to bad. state, the process is unstable. Therefore, it is necessary to screen out the batteries with large self-discharge, and only keep the batteries with small self-discharge for matching (generally, qualified products have small self-discharge, and the manufacturer has measured them. The problem is that many unqualified products flow into the market).

On the basis of small self-discharge, choose a series connection with similar capacity. Even if the capacity is not similar, it will not affect the battery life, but it will affect the available capacity of the whole set of batteries. For example, the capacity of 15 batteries is 20ah, and only one is 18ah, so the total capacity of this set of batteries can only be 18ah. At the end of use, the battery will be dead, the protection board needs to be protected, and the voltage of the whole battery pack is still relatively high (because the voltage of the other 15 batteries is normal and still has power). Therefore, the discharge protection voltage of the entire battery pack can be seen to see whether the capacity of the entire battery pack is consistent (the premise is that each 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 whole group, so try to choose an assembly with a similar capacity.

Assembled batteries must achieve good ohmic contact resistance between 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 transmitted along the electrode to the inside of the cell and affect the battery life. Of course, the performance of large assembly resistance is that the voltage drop of the battery pack is large under the same discharge current. (Part of the voltage drop is the internal resistance of the cell, and part of it is the assembled contact resistance and wire resistance)

3: Protection version selection and charge and discharge usage (the data is for lithium iron phosphate battery, the principle of ordinary 3.7v battery is the same, but the data is different)

The purpose of the protection board is to protect the battery from over-charging and over-discharging, prevent high current from damaging the battery, and balance the battery voltage when fully charged (the balancing ability is generally relatively small, so it is very important to have a battery protection board with large self-discharge. It is difficult to equalize, and there are also protection boards that do equalization in any state, that is, equalization is done from the beginning of charging, which seems to be rare).Also read:

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