High-current charging protection without fuses?

[ChrisW]

Hi,

I am building a LiPo 7S bike battery charger. Rather than taking the conventional approach of charging the bank from its two ends and bleeding individual cell banks if they rise above the average, I want to charge all cell banks individually with a TP4056 or similar charge controller.

I am providing isolated/floating 5V for each controller by using 7 Wall wart USB chargers. This means I can connect them in series without shorting anything.

This works but - because of the large almost-flat voltage part of the LiPo charge curve - the charge controller is extremely sensitive to the sensed battery voltage and will throttle back the charging current greatly if even a tiny rise in battery voltage is detected. This is of course, doing what it's meant to do. However, if I try to charge through the balance charging leads, which are thin and high resistance, the controller takes forever trickling in charge, because these balance leads drop significant voltage for reasonable charge currents. They were never meant to carry charge current - only to sense voltage at no current - so again, all is as it should be. So I want to replace them with beefier low-resistance cables which can feed higher charging currents without dropping voltage - so the charge controller can sense each cell bank's charge state more accurately and charge as fast as possible.

BUT this approach is dangerous. If the charge controller fails short-circuit then a cell bank is shorted out and the thicker wires will not blow so the whole battery could burn up.

Fuses would be the obvious approach - a fuse in each charge line. The problem is that these also drop significant voltage so we're back with very slow charging.

I suppose I could fit Hall effect sensors and relays and add a microcontroller to sense the current and turn off the relay if current becomes too high (or probably hold ON the relay as long as current is NOT too high) but this adds a great deal of complexity.

So my question is - is there an easier/cheaper/better way to provide this protection?

Any other comments on this proposal are also welcome.

Thanks
Chris

 

[danielrlee]

Your plan sounds unnecessarily complex. Why have you decided against a conventional 'passive bleed' BMS?

 

[vfr400]

You're playing with fire - literally.

Get yourself a normal 7S BMS, then, if you want to charge above its rating, double up the charge MOSFET. Some BMSs don't have separate charge MOSFETs. Instead they switch the main ones. You can recognise them by the fact tgat they don't have a C- pad or wire on the pcb. I guess you want to discharge at a high rate too. Often the high current BMSs allow high charge currents anyway.

With a BMS, it's plug-and-play with your battery. No more panicking when it approaches a low state of charge. Set and forget the charging. Why would you want anything else?

Same Port 24V 7S 16/25/35/45/60A BMS PCB PCM Electric Battery protection Board | eBay

1 x BMS board with wires. 60A: For 800W max electric bike. 25A: For 250W max electric skateboard. 35A: For 350W max electric skateboard. Suitable for 7 string 3.7V Lithium Ion battery. 45A: For 500W max electric bike.

www.ebay.co.uk

 

[Andy-Mat]

Hi,

I am building a LiPo 7S bike battery charger. Rather than taking the conventional approach of charging the bank from its two ends and bleeding individual cell banks if they rise above the average, I want to charge all cell banks individually with a TP4056 or similar charge controller.

I am providing isolated/floating 5V for each controller by using 7 Wall wart USB chargers. This means I can connect them in series without shorting anything.

This works but - because of the large almost-flat voltage part of the LiPo charge curve - the charge controller is extremely sensitive to the sensed battery voltage and will throttle back the charging current greatly if even a tiny rise in battery voltage is detected. This is of course, doing what it's meant to do. However, if I try to charge through the balance charging leads, which are thin and high resistance, the controller takes forever trickling in charge, because these balance leads drop significant voltage for reasonable charge currents. They were never meant to carry charge current - only to sense voltage at no current - so again, all is as it should be. So I want to replace them with beefier low-resistance cables which can feed higher charging currents without dropping voltage - so the charge controller can sense each cell bank's charge state more accurately and charge as fast as possible.

BUT this approach is dangerous. If the charge controller fails short-circuit then a cell bank is shorted out and the thicker wires will not blow so the whole battery could burn up.

Fuses would be the obvious approach - a fuse in each charge line. The problem is that these also drop significant voltage so we're back with very slow charging.

I suppose I could fit Hall effect sensors and relays and add a microcontroller to sense the current and turn off the relay if current becomes too high (or probably hold ON the relay as long as current is NOT too high) but this adds a great deal of complexity.

So my question is - is there an easier/cheaper/better way to provide this protection?

Any other comments on this proposal are also welcome.

Thanks
Chris

You need for each circuit for safety, what is called a "Crowbar". This will block the curent, till the device is turned off, as it locks on once activated.
Also an indicator/sensor for each crowbar that it has been activated, and probably a microprocessor that notes exactly which circuit was activated, and then the MP removes the power completely, but posts a fault code that you can still read, even after power off.
Not difficult to do nowadays!!
There are many useful and quite small and cheap MPs, that could do the job, my personal favourite (made in the UK!) is the PICAXE series. The books have many examples of cheap, easy to build interfaces, making it easy to sense which crowbar has been activated.
They only need an old PC or Laptop, plus a very simple interface that you can either build or buy for either a serial or USB port, to program them, and as the programs are in a special version of BASIC, they can easily be read and programmed by an almost complete beginner.
The chips start in the around £2.26 including VAT. So if you blow one up, you will not be upset.
All the needed software and all the (huge) books are free to download, or simply buy a starter pack and the CD ROM will come with all the stuff needed on it. Useful if you have a slow data rate for example to the web.
Regular updates and improvements are made available, also free....
Go here and have a good look around,I think you will be amazed:-

Home - PICAXE

www.picaxe.com

regards and if you need further infos about anything, just ask me.
Andy

 

[ChrisW]

Your plan sounds unnecessarily complex. Why have you decided against a conventional 'passive bleed' BMS?

Hi Daniel, a couple of reasons. Firstly charger working on this principle are very slow becuase the MOSFETs which do the bleeding are whimpy and you can't charge the pack any faster than the highest cell bank can bleed or it would "overtake". Secondly, it wears out batteries, since the charge cycle incorporates discharging. It just seems like an uncomfortable compromise (although my own solution is now looking a bit that way too).

 

[ChrisW]

You're playing with fire - literally.

Get yourself a normal 7S BMS, then, if you want to charge above its rating, double up the charge MOSFET. Some BMSs don't have separate charge MOSFETs. Instead they switch the main ones. You can recognise them by the fact tgat they don't have a C- pad or wire on the pcb. I guess you want to discharge at a high rate too. Often the high current BMSs allow high charge currents anyway.

With a BMS, it's plug-and-play with your battery. No more panicking when it approaches a low state of charge. Set and forget the charging. Why would you want anything else?

Same Port 24V 7S 16/25/35/45/60A BMS PCB PCM Electric Battery protection Board | eBay

1 x BMS board with wires. 60A: For 800W max electric bike. 25A: For 250W max electric skateboard. 35A: For 350W max electric skateboard. Suitable for 7 string 3.7V Lithium Ion battery. 45A: For 500W max electric bike.

www.ebay.co.uk

Thanks for the advice vfr.

 

[ChrisW]

You need for each circuit for safety, what is called a "Crowbar". This will block the curent, till the device is turned off, as it locks on once activated.
Also an indicator/sensor for each crowbar that it has been activated, and probably a microprocessor that notes exactly which circuit was activated, and then the MP removes the power completely, but posts a fault code that you can still read, even after power off.
Not difficult to do nowadays!!
There are many useful and quite small and cheap MPs, that could do the job, my personal favourite (made in the UK!) is the PICAXE series. The books have many examples of cheap, easy to build interfaces, making it easy to sense which crowbar has been activated.
They only need an old PC or Laptop, plus a very simple interface that you can either build or buy for either a serial or USB port, to program them, and as the programs are in a special version of BASIC, they can easily be read and programmed by an almost complete beginner.
The chips start in the around £2.26 including VAT. So if you blow one up, you will not be upset.
All the needed software and all the (huge) books are free to download, or simply buy a starter pack and the CD ROM will come with all the stuff needed on it. Useful if you have a slow data rate for example to the web.
Regular updates and improvements are made available, also free....
Go here and have a good look around,I think you will be amazed:-

Home - PICAXE

www.picaxe.com

regards and if you need further infos about anything, just ask me.
Andy

Thanks Andy, I'll research the crowbar.

 

[ChrisW]

Thanks Andy, I'll research the crowbar.

Hmmm. I watched this video: https://www.pedelecs.co.uk/forum/threads/high-current-charging-protection-without-fuses.34602/ which explains that a crowbar circuit is used to protect a voltage/power-sensitive circuit form over-driving, but it does it by short-circuiting the power supply. Unfortunately, in my application, this would also short circuit the battery bank - which is exactly what I'm trying to avoid. If I have this wrong, I'd appreciate your comments. Either way - thanks again for trying to help me.