---
title: Building High Capacity 18V/20Ah Battery for Makita Power Tools
date: 2026-10-08T08:34:26+00:00
modified: 2026-10-08T11:59:09+00:00
image:: https://kaspars.net/wp-content/uploads/2026/10/18v-makita-power-tool-batter-bms-protection-components-scaled.jpg
permalink: https://kaspars.net/blog/high-power-capacity-battery-power-tools
post_type: post
author:
  name: Kaspars
  avatar: https://reverse.kaspars.net/gravatar/avatar/92bfcd3a8c3a21a033a6484d32c25a40b113ec6891f674336081513d5c98ef76?s=96&d=mm&r=g
category:
  - Electronics
post_tag:
  - How to
---

# Building High Capacity 18V/20Ah Battery for Makita Power Tools

Note: work in progress (lab notes).

My Makita grass trimmer last about 15 minutes with two 18V/6Ah batteries (each rated at 108Wh). Makita now offers 12Ah/216Wh LXT packs (released in 2025) but I bought two 20-cell DIY kits on AliExpress instead. With 5Ah cells arranged as 5S4P, each pack has a nominal capacity of 20Ah and approximately 360Wh which is about 3x the capacity of my current packs or 50 minutes of runtime (hopefully).

## Batteries: 20pcs of 21700 Cells

These kits use 20 pieces of 21700 cells (larger and higher capacity than 18650 cells) arranged in 5S/4P configuration and I got 50 of 3.7V/5Ah for $160 (including shipping and taxes) which is a total of 925Wh (amazing value).

I did some tests and they actually did have the promised capacity:

![SWY 21700 5.0Ah 3.7V 18.00Wh 26C23D05](https://kaspars.net/wp-content/uploads/2026/10/swy-21700-5ah-18wh-cell.jpg?strip=all&quality=90&resize=1920,1273)SWY 21700 5.0Ah 3.7V 18.00Wh 26C23D05 cell capacity — 5049mAh during 0.5A discharge and 5363mAh during 1A charging.## Battery Protection System (BMS)

The BMS board that came with the kit was one of the most popular implementations found on AliExpress which are built around the [CellWise Microelectronics](http://www.cellwise-semi.com/en/) CC9053/CC1053 battery protection chips. It monitors the five series-connected cell groups (5S) and controls external power transistors (MOSFETs). These transistors connect or disconnect the pack when a protection condition is detected. The external charger still has to regulate charging current and voltage.

![18V 5S battery protection BMS board with CellWise chip](https://kaspars.net/wp-content/uploads/2026/10/18v-makita-power-tool-batter-bms-protection-components-scaled.jpg?strip=all&quality=90&resize=1920,1718)CW1053 provides overcharge, over-discharge, over-current, short-circuit and temperature protection; its published reference circuit does not include cell balancing and the exact protection thresholds (voltage, current, temperature) depend on the surrounding circuit.

To evaluate a board I would check the following:

- **Discharge path:** the components carrying current between the cells and the power tool terminals. The board above has 7 power transistors on the output path followed by three shunt resistors.
- **Charge path:** the components carrying current between the charger terminal and the cell groups. This is usually a separate circuit with a much lower current limit (unfortunately). The board above has two parallel diodes followed by two smaller charging transistors.
- **Balancing:** whether the board can discharge individual high-voltage groups or only monitor them and stop charging. The board above doesn’t have any.
- **Protection:** the actual voltage, current and temperature thresholds usually configured through different resistor values (defined in datasheet).

## Output Current and Power

The current required by the tool can be determined by dividing the maximum power output (such as 1kW in case of the trimmer) with the battery voltage which (in case of 2x18V packs) is 36V nominal:

```
1000W / 36V = 28A
```

Note that with a fully charged or slightly discharged battery the current would be lower or higher. Accounting for additional heat losses in the motor and the power supply, we can estimate the required output to be 40A.

With two packs connected in series, the current remains 40A per pack. Since the packs have 5 cell groups also connected in series, each of them must provide the same 40A.