DIY electric bike: battery

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A modern e-bike battery consists of the following main parts:

  • lithium-ion (Li-ion) cells connected by contacts;
  • management board (BMS, battery management system);
  • frame.

Often buyers are only interested in two battery parameters:

  • capacity, measured in ampere-hours (Ah);
  • operating voltage, measured in volts.

However, with this approach, one of the main characteristics of the battery escapes from the field of view — its

As a rule, the cheaper the battery, the higher its internal resistance, and the worse it delivers current. How does this affect and in what cases is it critical, and in what not?

Let’s take as an example a low-power e-bike with a 36V, 250W motor. We recall Ohm’s law and we find that the maximum current that the battery should produce is 250/36 = 6.94 amperes.

The optimal discharge current of a lithium-ion battery is 1C, that is, a value equal to its capacity. For example, for a battery with a capacity of 2500 mAh, the optimal discharge current will be 2500 mA, that is, 2.5 amperes.

Now we consider how many such batteries need to be connected in parallel to get the current we need (6.94 A).
b = 6.94 A / 2.5A = 2.8, round up to b = 3.

The battery assembly scheme is usually expressed in the form aSbP, where a is the number of series-connected blocks of cells, b is the number of parallel-connected cells within one block.

We have already found the value «b». The value «a» is defined as the nominal voltage of the battery divided by the nominal voltage of one cell. That is, in our case, b = 36 V / 3.6 V = 10.

Thus, the battery diagram, which is sufficient to power a 36 V 250 W motor, will look like: 10S3P, that is, the battery will consist of 10 series-connected blocks of 3 parallel-connected cells inside each block (30 cells in total).

Now let’s get back to inner resistance. In inexpensive Chinese batteries, cells with an internal resistance of about 60 mΩ are usually installed. Accordingly, three parallel-connected elements in our circuit will have a total internal resistance of 60 mΩ / 3 = 20 mΩ.

It is easy to calculate that when a current of 6.94 A flows on a block of three elements, a power equal to 6.94 A * 6.94 A * 0.02 Ohm = 0.96 W will be dissipated. This means that a battery of 10 such units connected in series will produce 9.6 watts of power.

If we consider more expensive batteries, for example, which have an internal resistance of 30 mΩ, that is, 2 times less, then obviously the power going to heat the battery will be 2 times less, that is, 4.8 watts.

But it’s not even about the heating of the battery. When a load is connected, cheaper elements have a greater voltage drop, which means that such a battery will turn off during discharge earlier than a similar one with a lower internal resistance, that is, it will give up a lower capacity.

Why does a battery disconnect occur and how does it work?

Responsible for disconnecting the battery

BMS boards usually have the following findings:

  • power: «P-» (to the minus of the discharge connector), «B-» (to the minus of the battery) and «C-» (to the minus of the charging connector)
  • balancing, b-, b1 … bn (to each of the battery cell blocks)
  • On / Off — to the lock or button to turn on the battery

The task of the BMS is to prevent overcharging and overdischarging of battery cells. Unlike lead-acid batteries, lithium-ion batteries operate strictly within a certain voltage range: from 3 to 4.2 volts. Only within these limits is a stable and long-term operation of the battery possible.

Thanks to the connected balancing connector, the BMS «sees» the voltage on each block of cells, and if the voltage on one of them reaches 4.2 V when the battery is charging, the BMS breaks the charging circuit, and charging stops.

Similarly, with the discharge — when the voltage on one of the units dropped to 3 volts (sometimes slightly lower), the BMS breaks the discharge circuit and the battery is disconnected from the load.

Another function of BMS is the so-called balancing, that is, equalizing the voltages on the cell blocks when the battery is charging, so that in the charged state the voltages on all cells are exactly 4.2 V (in some versions, 4.1 V). BMS uses resistors for this, connecting them to cells with a higher voltage during the charging process.

If the batteries from which the battery is assembled are of poor quality, then the battery is more difficult to balance, or rather, the battery will always give out the capacity that the worst block of cells in its composition possesses.

That is, if all blocks of cells have the same real capacity 2.5 * 3 = 7.5 Ah, and one block has a capacity, for example 2 + 2.5 + 2.5 = 7 Ah, then the entire battery will give out a capacity of 7 Ah. , since the problem unit will be charged before the others, and the BMS will turn off the charge, and discharge before the others, and on it the BMS will turn off the discharge.

There are still so-called SmartBMS on sale, which can be connected to a computer and manually set voltage thresholds and a huge number of other parameters. But, as practice has shown, during operation SmartBMS «hang» and stop working, and sometimes the configured parameters simply «fly off». In this regard, I would recommend using hardware BMS.

But let’s get back to the Li-ion cells that make up the battery. How is it that they are of different qualities?

Large manufacturers of lithium-ion batteries «18650» (the most common size, 18 mm in diameter and 65 mm in length) use better materials and a well-organized technical process to obtain cells with low internal resistance.

Smaller manufacturers save on materials, and lower quality of production leads to scatter in the characteristics of the elements in the output. Therefore, after production, each element is tested — and depending on the characteristics obtained, one or another level is assigned to it (A, B, C, D). And then the items are sold at prices in accordance with the assigned level.

In addition to the quality of Li-ion cells, the quality of the battery as a whole is influenced by the cross-section of the contacts that connect the cells. Insufficient cross-section leads to an increase in the internal resistance of the battery.

And of course, the quality of the welding has a huge impact. If the welding is of poor quality, and contact with the element is broken, the battery capacity will immediately decrease by the value of the capacity of this element. If, during welding, the tightness of the element is broken, the battery will also lose capacity.

There are craftsmen who, when assembling a battery, instead of welding, use soldering with a powerful soldering iron. Unfortunately, the reliability and durability of such a battery is extremely low.

What happens if you use

If the BMS in the battery is rated for low current, it will simply disconnect the battery. It looks like this: you turned on the bike, but when you press the throttle, it jerks and turns off.

If a powerful BMS is installed on a small battery, then at first glance everything will work, but the battery will not give out the full capacity, because with a heavy load (powerful motor), the voltage drop will be significant, and the battery will turn off earlier. That is, the mileage on one charge of such a battery will be much less than when using it with a low-power motor.

If the load is too heavy, lithium-ion batteries will become very hot, which will lead to their rapid degradation. In other words, this battery will only last you one season.

If there is no thermal contact or thermistor installed in the battery, there is a danger of ignition of the battery. Such cases can be found on YouTube.

Now let’s talk about

Often, in order to increase the reliability of mounting the battery on the bike frame, enthusiasts use metal cases, the weight of which sometimes reaches several kilograms. However, at the same time, they do not care much about the reliability of the battery fastening inside this case.

In cases where the battery is of significant size, and, accordingly, power, special attention should be paid to the safety issue of its fastening. The battery must not dangle inside the case, otherwise, if damaged, a short circuit and fire may occur.

Let’s let you down

Assembling a battery for an e-bike with your own hands is a rather difficult task.

To assemble a high-quality battery, you need to have high-quality batteries, a BMS designed for the required current (in accordance with the power of the motor and controller), a spot welder (ideally, a friend with such a device and experience in working on it), insulating materials, wires of a suitable cross-section.

In addition, it is necessary to consider the case for the battery and its reliable attachment to the bike frame.

Is it realistic to assemble a high-quality battery for an e-bike with your own hands — absolutely real, because

If you don’t want to assemble the battery yourself, you can always buy a ready-made solution on Aliexpress (although most often the actual capacity turns out to be less than the declared one), as in

volume_up

Thank you for reading the article to the end!

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