Test of internal resistance of power tool battery packs

Test of internal resistance of power tool battery packs

(Zeng Guanglun)

Because of battery power tools, I keep growing in the number of battery packs. Because I was afraid that the power tool would run out of power during use, I also made a lot of battery packs. And because I bought a battery internal resistance tester, I decided to conduct an internal resistance test on all battery packs, in an attempt to get some of the things I needed from the test.

The battery on Emma's e-bike is the most capable of the batteries I use. 48V/20Ah。 How big is its internal resistance? With the powerful ability of LQ1060S, one measurement, the internal resistance is 40mΩ, and the voltage is 50V. A 13-string battery pack is only 40mΩ, and the internal resistance of a single cell is only 3.07mΩ. If this battery is used as a battery for power tools, the internal resistance after 5 strings is less than 16mΩ.

In my battery pack, there are 5 strings, 5 cells, 21V; There are also 5 strings of 2 parallel, 10 knots, 21V; There are also 5 strings of 3 parallel, 15 cells, 21V (this 21V, refers to the nominal value of the battery fully charged. Same below).

To distinguish between different battery packs:

The letter A series represents 5 strings of 3 parallel, 15 cells, 21V battery packs;

The letter B series represents 5 strings of 2 parallel, 10 cells, 21V battery packs;

The letter C series represents the 5-string, 5-cell, 21V battery pack.

The battery pack bought is referred to as (bought); Homemade battery pack abbreviated (system).

The measured data of the LQ1060S battery internal resistance tester for A series battery pack is shown in the table:

sequence

Internal resistance (mΩ)

Voltage(V)

A1 (Buy)

40

20.4

A2 (system)

40

20.4

A3 (system)

60

20.9

A4 (system)

41

20.7

A5 (system)

62

19.2

A6 (system)

77

20.54

A7 (system)

73

21.17

A8 (buy, Makita bayonet)

49

20.7

A9 (15 knots, buy, Makita bayonet 84 yuan) Makita bayonet

80

20.1

A10 (system, 15 sections, 5 strings 3 parallel) Makita bayonet

70

19.7

A11 (buy 15 sections, 5 strings of 3 and 124 yuan) Makita bayonet

36

20.7

A12 (buy 10 knots 5 strings 2 and) Makita bayonet

46

17.2

A13 (self-made 15 cells of 5 strings lithium battery bayonet, February 5, 2023)

48

18.6

The A6 and A7 battery packs in the table are made by dismantling the battery pack of electric bicycles, and the internal resistance of a single 18650 battery is around 30mΩ. The theoretical internal resistance should be at 50mΩ. The internal resistance distributed to each battery is more than 40 milliohms, which is too large.

B series (10 batteries, 5 series and two parallel) 5 battery packs with LQ1060S battery internal resistance tester measured data is shown in the table:

sequence

Internal resistance (mΩ)

Voltage(V)

B1 (Buy)

81

20.2

B2 (Buy)

104

20.7

B3 (system)

47

20.2

B4 (system)

53

20.6

B5 (system)

67

20.7

B6 (Buy)

112

20.8

B7 (system)

117

20.8

B8 (buy, 10 knots 2 and Makita bayonet)

46

17.2

B9 (homemade, 10 knots 2 and large art bayonet)

39 (single cell internal resistance less than 12mΩ)

19V

The internal resistance can be as small as 46mΩ, indicating that the internal resistance of each cell must be less than 20mΩ.

C series (5 cells 5 strings) 6 battery packs with LQ1060S battery internal resistance tester measured data is shown in the table. If the internal resistance of a single cell is 24mΩ, 5 strings are 120mΩ.

sequence

Internal resistance (mΩ)

Voltage(V)

C1 (Buy)

0.22Ω

20

C2 (Buy)

0.28Ω

20

C3 (Buy)

105

20

C4 (system)

104

20

C5 (system)

0.37Ω

20

C6 (system)

0.31Ω

20

The good internal resistance of a single cell is 14mΩ, and 5 strings is 70mΩ. If such a battery is used to make a 15-cell, 5-string 3-parallel battery pack, the theoretical internal resistance is 70÷3=23.3mΩ. This should be a very good battery pack.

When I made my own battery pack, most of the batteries I used were purchased from Pinduoduo, of which Panasonic's 3400mAh 18650 battery was the most. This kind of battery, also uses a LQ1060S internal resistance tester to test the internal resistance of a single battery, most of the internal resistance meets the 24mΩ promised by the owner.

According to this calculation, 5 cells are connected in series, and the internal resistance is not less than 120mΩ.

10 batteries use 5 strings of 2 parallel battery internal resistance, the internal resistance should not be less than 60mΩ.

15 batteries use 5 strings of 3 parallel battery internal resistance, the internal resistance should not be less than 40mΩ.

I also once bought 15 18650 disassembly batteries on Pinduoduo. The owner said that his battery has an internal resistance of about 14mΩ and a capacity of 2000mA. The price is also about 8 yuan. Buy it back and test it, sure enough.

According to this calculation, the internal resistance of 5 strings of 21V battery packs should not be less than 70mΩ.

5 strings of 2 and 21V battery pack, the internal resistance should not be less than 35mΩ.

5 strings of 3 and 21V battery pack, the internal resistance should not be less than 25mΩ.

Why use words like "internal resistance should not be less than"? Because of the battery pack composition process, there will be many connections, and the resistance will only increase and never decrease. For example, the nickel sheet connected between the batteries has obvious resistance. I have measured the resistance of common nickel sheets: how much resistance is the nickel plated sheet?

From the nickel-plated strip (for spot welding machines) with a thickness of 0.1mm and a width of 8mm just bought, cut out 3 meters and measure its resistance with a milliohmmeter (LQ1060S) and a four-digit digital multimeter. The milliohmmeter measured resistance of 0.4018Ω. Two digital multimeters (ZOYI ZT100; UT33D+) measured resistance by two digits: 0.4Ω.

The above data is the resistance of a 3-meter-long nickel-plated sheet, and if the length of the nickel-plated sheet is only 10mm (the distance between the two solder joints), the resistance is 1.34mΩ. For handsome guys who have made battery packs, you can imagine how many nickel-plated plates of more than 10mm are used in a battery pack.

Once in the milliohm level, what material is used to connect the wire, and how to test how to ensure reliable contact have become problems that cannot be ignored. I was even worried that connecting the battery with a nickel sheet would increase the internal resistance of the battery pack. When it comes to milliohm tests, accuracy is difficult to guarantee. The same item, measured many times, the data is different, get used to it. Therefore, the above measurements are for reference only.

A completely new 18650 battery, if the capacity is 3400mAh, what is its internal resistance? Some say, 4mΩ. I haven't played and don't dare to say right or wrong. Some people ask: How much is the internal resistance of 18650 normal? Netizens said: The internal resistance of the current excellent 18650 battery is about 12 milliohms, and generally about 13-15 milliohms. The minimum internal resistance I have measured from the disassembled 18650 battery is 14mΩ.

Personal judgment: Most of the 18650 batteries currently appearing on the market are disassembled. It should be said that most of the batteries disassembled can still be used. To be honest, brand new is good, but the vast majority of people will be expensive and can't afford to buy or use it. I see, these disassembled used 18650 batteries are best suited for people like me.

Recently, I remembered that the 48V12A battery pack that had been removed from the electric bicycle that was not used was significantly degraded. After dissection, it was found that it was assembled by 78 18650 batteries in 13 strings of 6. Decided to disassemble them and assemble several 21V5 string 2 parallel lithium battery packs for power tools. When measuring the internal resistance, it was found that most of these batteries had internal resistance of about 28mΩ (the internal resistance of the disassembled 18650 battery that I bought before was 14mΩ and 24mΩ respectively). It can be seen that the internal resistance of 28mΩ is relatively large, and the quality of these batteries is not too good). The nominal capacity is 2000mAh. According to the calculation of the internal resistance of a single cell cell of 28mΩ, the internal resistance of 5 strings of 2 parallel battery packs should be above 70mΩ. After completing the assembly and fully charging, the internal resistance of the two battery packs was tested with a LQ1060S battery internal resistance tester, and the internal resistance of the two battery packs was: 72mΩ; 80mΩ。

These battery packs of mine are cheap. It is said that a brand new 18650 battery, even if it is 2000mAh, the price is about 40 yuan. If you use it to form a 15-section, 5-string 3-parallel battery pack, the cost of the battery alone is 600 yuan! How many people are willing to buy? How many people can afford it? Battery power tools, I use almost every day, but basically play, most of them only work for a few minutes or half an hour. Therefore, I have never encountered a power loss during use. With battery power tools, it is very convenient to do things. For example, to punch a hole in the wall, you don't need to pull the wire, stand on a chair, and use the battery impact drill for a few minutes to get it. Another example is sawing wood, or even opening the lid of the wooden box, using a battery cutter plus a backer, and the cutting can be completed in an instant. The electric drills I use now are all battery. Battery power tool, it's so convenient. (Zeng Guanglun)