back to back MOSFET's as a switch

[DavidAlfa]

It's also your work to try avoiding these things.
Suddenly the charger breaks down, outputting lower voltage, and your board burns out?
I would only expect that with something from AliExpress.
Meh. You'll earn a bad reputation...avoiding a simple voltage detector to save few cents.
Specially when playing with batteries, that things can catch fire!
You can do it with few resistors, transistors and zeners.
That's assuming you want to make it as simple as possible, the best would be a voltage divider, sampling it with the ADC.

[Simon]

Uh, OK. The input/ouput is gated by the transistors rated for 60V. The terminal as I call it is indeed connected to an ADC that checks the voltage before it will do anything!!!! As I said I have states galore in the software of this thing as it is either in charge mode or discharge mode which is decided at startup and you cannot go from mode to another. If some idiot wants to fashion their own power supply solution and jamb it on I cannot help that. We supply the power supply, they are not encouraged to use their own, they follow the manual! when did you last buy an appliance with a power supply that said in the manual that you are OK to connect to it anything you like?

[DavidAlfa]

Don't think only on the customer's fault,  your own power supply could also fail for whatever reason.
It's not a toy or a simple thing that will just make a little bang and end of the story.
Batteries must be treated with caution and within safety limits.
You could get sued if they find your device was poorly made,
avoiding any safety systems, not to mention if it causes personal damage.
Why did the battery explode? -I don't know, it's not covered by the warranty. Doesn't sound like a good argument!

For the overvoltage protection, you simply put a fuse and a clamp diode.
If the voltage is exceeded, the diode shorts the line and burns the fuse. Simple, and safe.
The customer will send back the device, you'll see what happened, deny warranty and quote a reparation. Everyone happy.
But you know you made a good job by not letting things to the luck.

[dietert1]

Fuses and relays are a bit oldfashioned. Nowadays people use PTC resistors instead of fuses and mosfets instead of relays. Also it will be easier to find a SO8 dual mosfet than a SO8 relay.

Regards, Dieter

[Simon]

Don't think only on the customer's fault,  your own power supply could also fail for whatever reason.
It's not a toy or a simple thing that will just make a little bang and end of the story.
Batteries must be treated with caution and within safety limits.
You could get sued if they find your device was poorly made,
avoiding any safety systems, not to mention if it causes personal damage.
Why did the battery explode? -I don't know, it's not covered by the warranty. Doesn't sound like a good argument!

For the overvoltage protection, you simply put a fuse and a clamp diode.
If the voltage is exceeded, the diode shorts the line and burns the fuse. Simple, and safe.
The customer will send back the device, you'll see what happened, deny warranty and quote a reparation. Everyone happy.
But you know you made a good job by not letting things to the luck.

Yes I will be adding those as well. All I am saying is that for normal use the mosfets are already rated and the gate resistors worked out so it is a case of bulk protection from incidents like you mention. I have to protect from an automotive output anyway so will need a TVS. Each battery is also fused on it's own.

[Simon]

Fuses and relays are a bit oldfashioned. Nowadays people use PTC resistors instead of fuses and mosfets instead of relays. Also it will be easier to find a SO8 dual mosfet than a SO8 relay.

Regards, Dieter

I can look at PTC's but they seem to be harder to get when the current goes over 1 amp. As for SO8 P channel mosfet's when was the last time you tried to get some, not easy right now to get with a channel resistance of 14m Ohm. 55+ yes.

Relays are abundant, it's the silicon fabs that are overworked and failing to supply.

[langwadt]

Fuses and relays are a bit oldfashioned. Nowadays people use PTC resistors instead of fuses and mosfets instead of relays. Also it will be easier to find a SO8 dual mosfet than a SO8 relay.

Regards, Dieter

I can look at PTC's but they seem to be harder to get when the current goes over 1 amp. As for SO8 P channel mosfet's when was the last time you tried to get some, not easy right now to get with a channel resistance of 14m Ohm. 55+ yes.

Relays are abundant, it's the silicon fabs that are overworked and failing to supply.

what voltage?  will 40V do?


https://www.lcsc.com/product-detail/MOSFETs_Vishay-Intertech-SI4401DDY-T1-GE3_C72317.html
https://www.lcsc.com/product-detail/MOSFETs_Vishay-Intertech-SI4401BDY-T1-E3_C142561.html

[Simon]

No, 40V is cutting it a bit fine, I can use 40V in some places but one or two need to be 60V as the maximum battery voltage is 38V.