Convert Makita DC18RC Li-Ion battery charger from 110V/220V AC to 12V/24V DC - Page 1
Forum section: electronics / projects

author: Matse.P
author_rank: Newbie
timestamp: October 03, 2016, 11:22:36 am
content: Hey guys,I just need a possibility to charge my Makita 18V Li-Ion batteries not only on AC at home but even on 12V DC via power-terminal inside my car to make the operation of my Makita 18V-tools independent from AC.Yes, sure, one way would be to buy a Makita DC18SE car charger but I have a secondMakita DC18RC AC chargercoming from a complete tool kit which I don't need anymore and which is ready to bemodified to a DC chargerby this conversion.I've been made up a few considerations how to do this conversion and that resulted in two realistic ways it shoud go:1. DC-AC sine wave converter and some additional safety circuits connected to the mains terminalsThe first thing is to integrate a sine wave converter from 12V DC into 220V AC 50Hz and bring it on to the internal terminals of the AC plug. By that, the 12V DC would be converted to 220V AC and then supplied into the charger the way the original AC current from the mains cable would do it. The charger itself won't ever notice that the supply source is not 220V AC but 12V DC and would work in normal condition as is.The biggest problem of this way is that there would be AC at the pins of the mains plug then which is awful dangerous and in no way acceptable. So I thougt of adding a relais between the mains terminal inside the charger and the mains cable which completely switches of the connection if used at 12V DC. The relais is directly triggered by the 12V DC input. There would be even a 1s switch-on delay for the sive wave converter to go sure that the relais has switched of the connection to mains cable safely before the converter starts working.Question for this solution is:Is there a cheap but good ready-to-manufacure type of sine wave converter available which could be integrated into the Makita DC18RC body and would this be an intelligent sollution for such a conversion?2. Supply 12V DC inside the charger directly to the low voltage DC section of the Makita DC18RC chargerI just took the Makita DC18RC charger apart and made two pictures:PCB top side:PCB bottom side:If you look at the picture of the bottom side of the PCB, where would be the best point to supply a DC voltage directly to the PCB?I just thougt this would be ideal at the point of the rectifier but then, I realised that there is anyway a type of sensing circuit next to the DC output of the rectifier, no idea what this is sensing. Maybe, it senses the amplitude of the AC part but for what?Is it to realise some type of soft start functionality for the main tranformer or for power factor correction reasons or something like that or for anything comletely different?And what, if you add some DC to this point without having any AC before on that circuit? Maybe it lets the charger not starting because the controller sees no AC voltage on the sensing pins?In this case choice nr. 1 would be better or not?Please have much comments below and give me many hints how to realise this in a most intelligent way!Thanks a lot and sorry for my english, I'm a foreigner ...Greetings from germany,Matthias

author: Paul Price
author_rank: Super Contributor
timestamp: October 03, 2016, 02:10:29 pm
content: Does look hackable. I can read the PCB and there are actually two different supplies. One high current for charging and one for possibly powering the control circuit.You could apply the high current 20-24 DC to the underside of the PCB right in the middle of the board where the the rectifiers D203 D204 cathodes are.There is also a small auxiliary supply point where the cathode of D202 connects.If you applied somewhere between 20 to 24V to the D203 point and another yet unknown voltage at the D202 point(upper left hand corner of the bottom board view), then the charger would probably just work like it was plugged into the mains, if the voltage applied to the battery was creating a constant current with voltage limiting. The idea would be to make the voltage vary here to limit the current charging the drill battery. Takes some more sensing-feeback to do this, all this is probably too complicated.The question remains is what would be the right voltages for each point.I built my own charger for my drill and just bypassed the internal circuitry entirely.I used used high current rated DPDT relays(free, salvaged from an old washing machine or microwave oven control board) to disconnect the AC charging circuit from the charger connections to the battery and the I apply a voltage limited constant-current charge to the Li-Ion batteries at C/2 where C is the capacity of the individual Li-Ion cells. One PNP transistor senses charge current across a small-valued resistor in series with the charging voltage and limits the current accordingly by lowering the 12-24V converter output voltage(this is done by connecting the current sensing PNP collector to an optocoupler through a resistor and the optocoupler's output transistor is connected across the voltage setting pot on the converter.Any charger output voltage must be set to limit the max voltage to <4.2 volts per Li-Cell. I just measured the charging cutoff voltage of the original AC main circuit and made my circuit match. No bells, no whistles, just a LED on the box I built it in to show its getting power. I just leave the drill battery connected to the charger in my car for a few hours and its done.It is easy enough to make a DIY 12V to 24V converter or else buy one on EBay to get the voltages you need from your car battery.

author: ajb
author_rank: Super Contributor
timestamp: October 03, 2016, 02:16:56 pm
content: There's no secondary-side regulation happening in the charging path, so there's no point where you can just inject a low DC voltage and expect it to work. This is clear from the fact that the secondary of the main transformer is connected straight through a couple of rectifier diodes and a sense resistor to the batter connector (CN203). You could conceivably create a 12VDC->320VDC boost converter to feed the HVDC bus inside the charger. That's simpler to build than a sine wave inverter, but still not exactly a beginner project. Buying an off-the-shelf inverter is certainly the easiest and safest option.

author: jitter
author_rank: Frequent Contributor
timestamp: October 03, 2016, 03:33:03 pm
content: I'd have to agree, this looks like the regulation takes place on the primary side. A feedback from the secondary side (through an optocoupler) is used by the control-IC on the primary side.I'm no designer, but there is a dead giveaway and it can be found in the silkscreen near one of the optocouplers:

author: jitter
author_rank: Frequent Contributor
timestamp: October 03, 2016, 03:47:28 pm
content: Quote from: ajb on October 03, 2016, 02:16:56 pmI'm wondering, would this charger work off a 12 VDC > 230 Vac modified sinewave inverter?If so, the lighter ones (100, 150 W or so) are usually pretty cheap, about a quarter to a third of what the DC18SE charger would cost.

author: djQUAN
author_rank: Frequent Contributor
timestamp: October 03, 2016, 07:39:31 pm
content: I believe it should work fine on a MSW inverter. There is no active PFC in it so even straight HV DC should work too.