Battery charger reverse polarity help

Thread Starter

SimpleJoe

Joined Mar 22, 2016
38
TLDR: A diode for 3 amp forward current and possibly 50amp reverse

So i upgraded an old NiCd batteries for a makita drill to run on lithium. The batteries consist of 4x 18650s and a balancing charge controller rated to ~40 or 50 amp peak draw and takes anywhere from 14-17V input to properly charge the system. The batteries work great and were relatively cheap to make at the time.

However, i have had endless problems with the charger i made to go along with it, its a simple setup with a 19v 3.5amp laptop powersupply running into a generic CC CV buck converter to take the 19v down to 17v -ish that the BMS needs and caps it at 2.5 amps. Every so offen however, the charger will trip what i believe is an over current protection in the power supply which i would not think would be a problem. I have found that the battery then powers the light on the buck converter after the charger is unplugged (i had assumed that because the buck converter is setup for solar charging it would have reverse polarity protection, apparently not).

I have tried wiring in a diode i had on hand (a 1N4001 i believe, something small and commonplace) but this drops my charging current down to anywhere between 300-800 mA. I assume this is just because I am sizing the diode wrong so the main question i have is, what is a suitable diode for say 3 amp forward current, a peak reverse of 50 amp (this is the part that i am not sure about) and as the power supply input voltage and the charger output are relatively close, something with a smallish/suitable voltage drop. Of course on top of all of this, something that is also cheap which was the main motivation behind this whole project.

I believe it is as simple as just a diode but then again i have no actual education in electronics other than a single electrical engineering class in University so i may have made a massive mistake, any insight would be great.

Cheers,
SimpleJoe


EDIT, the parts:
CC CV buck converter:
https://www.banggood.com/3pcs-DC-DC...a5ZpOfyc995d2dRoCk-0QAvD_BwE&cur_warehouse=CN

The BMS (I think):
https://www.banggood.com/4S-30A-14_...Board-Balance-p-1140544.html?cur_warehouse=CN
 

dendad

Joined Feb 20, 2016
3,611
What do you mean by 50Amps peak reverse?

The 1N400x diode is way too small. It is a nominal 1A diode.
A good cheap source of high current diodes is to use a bridge rectifier.
Depending on your requirements, some of these...
Bridge1.jpg
Bridge3.jpg
Bridge2.jpg
The bigger ones makes it easy to mount as they have a hole.
Connect the two AC (~) pins together.
Your power supply +ve out to the bridge "-" and the charger +ve input to the bridge "+" connection.
This will give you 2 diodes in series so drop around 1.5V or more.

But the buck converter you link to can handle the 19V ok if you are using that.
The diodes are only for the charge controller without the buck converter. But I'm surprised the charge controller can't run straight on the 19V anyway.
 
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Thread Starter

SimpleJoe

Joined Mar 22, 2016
38
What do you mean by 50Amps peak reverse
That is the maximum peak current the battery can put out, I am trying to stop the battery reverse powering the buck converter/charge controller. My circuit is: laptop Power supply -> buck solar charge controller -> diode -> BMS -> battery cells

But I'm surprised the charge controller can't run straight on the 19V anyway
what do you mean by the charge controller here? The BMS or the solar charge controller because the solar charge controller IS running on 19v. But the BMS inside the battery is a generic thing that allows up to 30 amp charge current (which is like 10c charge rate, not ideal) also it was a while ago I made this but I do believe that the BMS has a rather narrow input voltage range.
 

Thread Starter

SimpleJoe

Joined Mar 22, 2016
38

dendad

Joined Feb 20, 2016
3,611
The diode will block the current from the battery. The battery will not force the "inverse current" into the power supply. That is the max allowable current the battery can handle without damage I would think. In fact, the power supply is probably quite safe to hook up to the battery without the diode, as long as you do not hook it up the wrong way around.
If you use a diode, it just has to handle the max current (plus a bit) that the power supply can output.
 

Thread Starter

SimpleJoe

Joined Mar 22, 2016
38
In fact, the power supply is probably quite safe to hook up to the battery without the diode, as long as you do not hook it up the wrong way around.
That is my major issue, I have found that the battery is powering the componentry on the buck converter while the power supply and I believe it has a power creep as the battery charges and trips the over current on the power supply, which is the whole reason for this thread
 

Dodgydave

Joined Jun 22, 2012
9,641
What is your reasoning for this? I just want to know will i need to get something that has an inverse current rating greater than that of the peak output of the batteries (which is 70amps!) but i have no easy access to something that big. Does anyone know how commercial battery chargers stop this?
The diode i selected is a Shotkley diode, low forward voltage drop and high current passing , why don't you just put the power supply straight to the bms pcb, and let it charge the cells for you.?
 

Thread Starter

SimpleJoe

Joined Mar 22, 2016
38
why don't you just put the power supply straight to the bms pcb, and let it charge the cells for you.?
To be honest, the power supply was a bit of an unknown, it was from a very early laptop and i wasn't sure of its condition. I also remember researching the BMS i used (im not 100% that it was the one i linked in the OP) but from memory it had a rather low input voltage range (if at all) and it was a very obscure value. Information which took alot of research to find as no mass seller has even the slightest clue about what they are selling anymore. I am also using the buck converter as a constant current source as the 3.5 amp from the power supply is slightly higher than recommended and i wanted to preserve the lithium as well as not completely load the power supply. However, with all that said, i may just remove the buck converter and give it a go, as its is quite noisy (electrically) and messes with my radio signal :(:(

EDIT: I am not certain about the input voltage rating of the BMS, after having a think back, i contacted the suppliers and no one could help so i believe i used a voltage just slightly above the nominal 16.8v fully charged rating just to be safe. Does anyone know what the input limits of these type of things are? that would be a great help
 
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