Lithium battery and protection board selection precautions

1: Lithium battery type and capacity selection.

First of all, you need to calculate the continuous current that the battery needs to provide according to your own motor power (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 provide 20a current for a long time and the temperature rise is very low (even if the temperature outside is 35 degrees in summer, the battery temperature is best controlled below 50 degrees). In addition, if the current is 20a at 48v, the overvoltage will be doubled (96v, such as ecpu 3rd gear), and the continuous current will reach about 50a. If you like to use overpressure for a long time, please choose a battery that can continuously provide 50a current (or pay attention to the temperature rise problem). The battery continuous current here is not the merchant's nominal battery discharge capacity. The nominal number of C (or dozens and hundreds of amperes) stated by the merchant is the battery discharge capacity, but if it is really discharged at this current, the battery will heat up very seriously. If the heat dissipation is not done well, the battery life will be very short. (And the battery environment of our electric vehicle is that the batteries are discharged together, basically leaving no gaps, and the packaging is very tight, not to mention how to do a good job of forced air cooling and heat dissipation). Our use environment is harsh. The battery discharge current needs to be derated. The evaluation of the battery discharge current capability is to see how much the battery corresponds to the temperature rise under this current.Also read:48v 400ah lithium battery

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), a continuous discharge current of 20a requires a capacity of more than 40ah (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 at 1C. Even if A123 ultra-low internal resistance power type lithium battery, it is usually best to discharge at 1C (not more than 2C is better, in fact, 2C discharge can only be discharged for half an hour, and it is of little use value). The choice of capacity depends on factors such as the storage space of the car, personal expenditure budget, and the expected range of activities of the car. (Small capacity generally must be powered 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. It doesn't matter as long as everyone is unbalanced. The problem is that this state is an urgent and unstable state. The self-discharge of a good battery is very small, the self-discharge of a bad battery is very large, and the self-discharge is not too small. state that 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 (generally, the self-discharge of qualified products is small, and the manufacturer has measured it. The problem is that many unqualified products enter 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, if 15 batteries have a capacity of 20ah and only one is 18ah, then the total capacity of the battery can only be 18ah. In the end, the battery will be dead, the protection board needs to be protected, and the voltage of the whole group of batteries is still relatively high (because the voltage of the other 15 batteries is normal and still has electricity). Therefore, the discharge protection voltage of the whole group of batteries can be seen whether the capacity of the whole group of batteries is consistent (the premise is that when the whole group of batteries is fully charged, each cell must be 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 assemblies with similar capacities.Also read:48v 300ah lithium ion battery

The assembled battery must have 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 large contact resistance will heat up, and this heat will be transferred to the inside of the cell along the electrode and affect the battery life. Of course, the performance of the 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 is the assembled contact resistance and wire resistance)

3: Protection version selection and charging and discharging matters (the data is for lithium iron phosphate battery, the principle of ordinary 3.7v battery is the same, but the data is different).Also read:eve lifepo4

The purpose of the protection board is to protect the battery from overcharge and overdischarge, to prevent the battery from being damaged by high current, and to balance the battery voltage when it is fully charged (the balance capacity is generally small, so if there is a large self-discharge cell protection board is very important. It is difficult to balance, and there are also protective boards that do balance in any state, that is, balance from the beginning of charging, it seems that there are few).