Need a protection circuit

[EasyGoing1]

Hello,

I have a situation where a battery charger will be destroyed if the battery that it is charging is accidentally connected backwards. So Id like to come up with some kind of circuit that will not allow current to flow from the battery being charged back into the charger if it is accidentally connected backwards. A diode will not work because the voltage drop across the diode is significant enough that the battery being charged will not be charged properly.

I realize that people will question the application and wonder why this or why that, but I've been through all that, I just need to know if there is something out there or some kind of circuit I can apply to this function that will produce the results I am looking for.

I have attached a small simple diagram.

Thank you,

[petehall347]

Re: Need a protection circuit

diode across charger output . fuse on plus wire .
like this . choose suitable diode and fuse .

[eccerr0r]

Re: Need a protection circuit

Technically just a fuse is enough, there will be a lot of current flowing if you connect it backwards that will be easy to blow a fuse. It also helps against shorted charger output/shorted battery. However I don't know how sensitive that charger is to self destruction if you reverse connect.

I accidentally hooked up my car charger to my car battery wrong, yeah that was a lot of current. I noticed it right away and fixed it, luckily no damage.

I noticed in one of HFT's battery chargers that it had an intricate polarity detector and would cut off the charge if it were hooked up backwards - mostly to protect the battery from getting reverse charged, not protecting the charger. I suppose most of today's smart chargers can completely disconnect the charge outputs, likely via MOSFETs, and test the terminal connections with a voltmeter before attempting to charge.

[NeedsMoreFlux]

Re: Need a protection circuit

I have a ton of diodes laying around.

If you want I can check to see if I have a 5408 in my pile and mail it to you.

Let me know.

[EasyGoing1]

Re: Need a protection circuit

diode across charger output . fuse on plus wire .
like this . choose suitable diode and fuse .

Interesting ... Perhaps a resettable fuse would be sufficient. The charger is one of those inexpensive TP4056 LiIon chargers ... They fry pretty quickly when you connect the battery backward but if the fuse is fast enough, I'm sure it would protect it. Im curious does the diode serve to divert the current away from the charge ciurcuit?

[EasyGoing1]

Re: Need a protection circuit

However I don't know how sensitive that charger is to self destruction if you reverse connect.

The charge circuit is a TP4056 ... they fry pretty quickly if the battery has enough charge on it when you connect it backward. I've connected batteries to them backwards that were too weak to fry it.

[EasyGoing1]

Re: Need a protection circuit

So I found this Texas Instruments N-MOSFET protection circuit and I gave it a shot and it actually works, but for some reason, the MOSFET that I'm using gets super hot while the battery is charging ... not sure why though ... it is, however, hot enough to be worried that it might cause issues depending on what it's resting on. It's certainly too hot to touch without some protection on the skin.

[eccerr0r]

Re: Need a protection circuit

You need a low Vth MOSFET for this if you're working with single cell lithium ion batteries, else it just won't turn on hard enough. Plus you need to worry about damaging the gate from static...

[momaka]

Re: Need a protection circuit

So I found this Texas Instruments N-MOSFET protection circuit and I gave it a shot and it actually works, but for some reason, the MOSFET that I'm using gets super hot while the battery is charging ... not sure why though ... it is, however, hot enough to be worried that it might cause issues depending on what it's resting on. It's certainly too hot to touch without some protection on the skin.

I don't see how that circuit will protect anything. The body diode in the MOSFET will allow reverse current to flow through it and still damage the charger.

Meanwhile, this circuit is simple, but it *will* protect your charger just fine. However, if the diode shorts due to connecting the battery in backwards and you don't happen to notice it, you can end up with a case where the battery gets completely discharged through the diode. With Lithium batteries, that can be dangerous.

So here is a slightly safer circuit that I just threw together. It probably can be refined, if anyone cares to do that. It's only downside is that it uses a lot more parts and considerable amount of power from the charger to run its relay. But for multi-cell Lead-Acid and Lithium batteries, this may not be a problem, as they are charged until reaching a constant voltage level per cell. That said, if anyone finds any mistakes, please point them out. So far, I don't see anything obvious.
https://www.badcaps.net/forum/attach...1&d=1510117776

[EasyGoing1]

Re: Need a protection circuit

I don't see how that circuit will protect anything. The body diode in the MOSFET will allow reverse current to flow through it and still damage the charger.

And yet I actually tested it by attaching a battery backward and it was fully charged... now SOME current got back to the charger, but it was not enough to damage it.

[EasyGoing1]

Re: Need a protection circuit

https://www.badcaps.net/forum/attach...1&d=1510117776

I couldn't see this cheap little LiIon charger actually being able to work this circuit and charge the battery ... I like it though ... maybe I could use it in my Arduino LiIon charger project that's been ongoing from time to time as I am able.. :-)

[EasyGoing1]

Re: Need a protection circuit

You need a low Vth MOSFET for this if you're working with single cell lithium ion batteries, else it just won't turn on hard enough. Plus you need to worry about damaging the gate from static...

So the MOSFET Im using is a STP16NF06L

[momaka]

Re: Need a protection circuit

And yet I actually tested it by attaching a battery backward and it was fully charged... now SOME current got back to the charger, but it was not enough to damage it.

Hmmm... not sure why that is. But again, given the body diode of the MOSFET, I don't think this circuit will work. If it appears to be working, there may be something else going on with your charger that we are not seeing here.

Either way, that is still a poor design for the reasons eccerr0r mentioned.

I couldn't see this cheap little LiIon charger actually being able to work this circuit and charge the battery

Relays typically only require about 1/2 Watts to run, so I imagine most Li-ion batteries should be able to supply that. The only thing that might stop this from working is if your charger is made for charging a single Li-ion cell, as then you will need a relay that is rated to run at that low voltage.

That said, you could also have an external adapter to power the relay circuit, and only use the battery and charger to enable the relay logic (which should take only several mA at most). I'll see if I can draw a circuit for that later tonight.

[budm]

Re: Need a protection circuit

http://handstronix.blogspot.com/2017...-teardown.html

HT3582DA universal charger control chip.

[R_J]

Re: Need a protection circuit

Why don't you put a diode in series with the + lead?

[petehall347]

Re: Need a protection circuit

Why don't you put a diode in series with the + lead?

voltage drop maybe ..schottky is pretty low .

[eccerr0r]

Re: Need a protection circuit

Actually none of these solutions work on the charger side, they all need to be implemented on the device/battery side. I still think it's a design flaw of the charger if it will instantly fry if connected wrong or shorted, but putting the diode/fet on the charger side doesn't quite work... only a fuse *may* work.

Since they all need to be on the device/battery side, then the other option might well be mechanical reversal prevention: don't use alligator clips, use one-way mating connectors...

[momaka]

Re: Need a protection circuit

That said, you could also have an external adapter to power the relay circuit, and only use the battery and charger to enable the relay logic (which should take only several mA at most). I'll see if I can draw a circuit for that later tonight.

There! Attached below.
Just don't call this circuit convoluted.