Makita Dual Battery Charger Repair - Page 1
Forum section: electronics / repair

author: RyanG
author_rank: Regular Contributor
timestamp: May 26, 2025, 06:37:26 pm
content: Someone asked me to look at their battery charger for their tools. The charger will charge one battery normally and the other one just flashes the lights saying the battery is bad even on a known good battery. I took it apart and it has independent boards for each battery charger. I noticed someone broke the inside of the holes for hanging the charger on a wall out and the screws they were hanging it from scraped 2 components off the board. One of them is a 100 ohm resistor and the other is a small, I'm assuming SOT323, transistor. I need some help identifying the transistor. My guess is an N MOSFET, but I figured someone here would know. I'm only including the pictures of the good board with the parts circled that are missing from the bad board. The red line on the one picture is the connection to the header that goes to the battery connector.

author: Kean
author_rank: Unknown
timestamp: May 26, 2025, 07:48:49 pm
content: An SMD code of "40" (I think) plus an SOT323 package is not much to go with.The guess of N MOSFET does seem to fit... but that is without any pre-existing knowledge of the circuit.Do you have a transistor tester? Like those common ATmega based devices that can identify various components?As you have a working part, I'd suggest (carefully) removing it from the PCB and hooking it up to one of those testers.If not, I might be able to check on my charger but I won't have time in the next few days.

author: RyanG
author_rank: Regular Contributor
timestamp: May 26, 2025, 10:29:23 pm
content: I do have one of those testers somewhere. I'll look for it. As for the circuit, pin 1 goes directly to the microcontroller, pin 2 goes through the resistor to the battery connector, and pin 3 goes to ground. Pin 2 is also connected to what looks like a voltage divider going to another pin on the microcontroller. I'm assuming each of the pins is connected to one of the cells in the battery pack for voltage monitoring and making sure it's the correct type of battery.

author: RyanG
author_rank: Regular Contributor
timestamp: May 27, 2025, 12:36:39 am
content: I decided to just throw some stuff in there and see what happens. I couldn't find any 100 ohm resistors that size so i threw a 180 and 220 in parallel, which read almost exactly the same as the one on the good board. And, I had some N MOSFETS from some project I did years ago that were the right footprint. It seems to work fine.

author: Kean
author_rank: Unknown
timestamp: May 27, 2025, 10:17:54 am
content: Good to hear it was that easy.I found this post where someone thinks pin 3 is like a chip select signal used as part of the charger to battery communications.https://www.laptopu.ro/community/power-tools-battery-pinout/makita/paged/2/#post-10049

author: RyanG
author_rank: Regular Contributor
timestamp: May 27, 2025, 11:57:11 pm
content: It would make sense that it has some form of communication because that specific charger can do 2 different battery voltages. I already gave it back to the guy that asked me to look at it but I think it could do 18v and 7.2v batteries.