Cheap LiFePO4 BMS?

[WotEver]

I spy space for a couple of FETs. 

31 minutes ago, nicknorman said:

My PCBs arrived today.

 

[nicknorman]

1 minute ago, WotEver said:

I spy space for a couple of FETs. 

 

Yes that's to operate the Tyco latching relay. Or maybe 2 latching relays if I go for the dual bus system (one relay to isolate the charging bus, one for the load bus). Originally I thought that resetting the relay would be done by the micro (hence the 2 FETs) but I now think it probably better for the reset to be entirely manual (ie a button to momentarily push near the relay, independant of the software).

[1st ade]

34 minutes ago, nicknorman said:

the bit bottom left is to be cut off and mounted near the batteries, for the purpose of fusing the battery feeds to the board.

At £2 for 5 PCB's couldn't you have splashed out £4 for 10 and put the fuses on a seperate board? (realising I'm going to hit the "going green" debate here but it can't be worth £2 of your time to gently fretsaw out that corner)

[WotEver]

1 minute ago, nicknorman said:

Yes that's to operate the Tyco latching relay. Or maybe 2 latching relays if I go for the dual bus system (one relay to isolate the charging bus, one for the load bus). Originally I thought that resetting the relay would be done by the micro (hence the 2 FETs) but I now think it probably better for the reset to be entirely manual (ie a button to momentarily push near the relay, independant of the software).

I think that’s an eminently sensible approach. If it’s triggered then manual input to have a look and ascertain why prior to a reset would indeed seem to me to be the best system. 

2 minutes ago, 1st ade said:

At £2 for 5 PCB's couldn't you have splashed out £4 for 10 and put the fuses on a seperate board? (realising I'm going to hit the "going green" debate here but it can't be worth £2 of your time to gently fretsaw out that corner)

Makes sense

[nicknorman]

1 minute ago, 1st ade said:

At £2 for 5 PCB's couldn't you have splashed out £4 for 10 and put the fuses on a seperate board? (realising I'm going to hit the "going green" debate here but it can't be worth £2 of your time to gently fretsaw out that corner)

I'm retired so my time is free (I keep telling myself!) No it was a special offer for a first order. One design per order. A second order will be around £8

[1st ade]

3 minutes ago, nicknorman said:

I'm retired so my time is free (I keep telling myself!) No it was a special offer for a first order. One design per order. A second order will be around £8

Fair comment.

[MtB]

5 minutes ago, nicknorman said:

I'm retired so my time is free (I keep telling myself!) No it was a special offer for a first order. One design per order. A second order will be around £8

 

Rip off China!!!

 

 

[peterboat]

On 25/01/2020 at 16:25, WotEver said:

Nope.

Wrong answer, I had 2 solar controllers cooked by a disconnect from the batteries before the solar, okay they wernt expensive ones but they didnt like it ☹️

[WotEver]

2 minutes ago, peterboat said:

Wrong answer, I had 2 solar controllers cooked by a disconnect from the batteries before the solar, okay they wernt expensive ones but they didnt like it ☹️

I thought you also told us that you couldn’t have two controllers feeding the same bank because they get very confused and blow each other up?

Was that someone else?

[peterboat]

53 minutes ago, nicknorman said:

My PCBs arrived today. I am still mightily impressed to get 5 PCBs manufactured to my own design and shipped from China, all for £2. Let’s hope i didn’t get a free gift of coronavirus!

 

the bit bottom left is to be cut off and mounted near the batteries, for the purpose of fusing the battery feeds to the board.

 

Brilliant! When its up and running I think you easily market these boards people are scared of Lithiums and overcharging, Kris would be a definite customer for one

[WotEver]

4 minutes ago, peterboat said:

Brilliant! When its up and running I think you easily market these boards people are scared of Lithiums and overcharging, Kris would be a definite customer for one

However, as it involves alternator surgery I’m not so sure that folk would be as keen as you suggest. And I’m sure Nick won’t want to go into business modifying other folks’ alternators. 

[peterboat]

2 minutes ago, WotEver said:

I thought you also told us that you couldn’t have two controllers feeding the same bank because they get very confused and blow each other up?

Was that someone else?

No that was me I could post pictures of that one I have taken the battery box out to fit the Drive battery bank and the old box is rotten where the acid spilled into it!

The solar controllers were one in my old boat, it was a PWM version and I for some reason pulled the fuse out whilst working in the engine compartment!

The second was on my Chevy day van, the batteries were under the sink unit and the sink unit could be moved in the van, so wires to the solar controller had a nice multiplug on it, which I disconnected whilst the sun was shining brightly!! it never worked again.

Moral of the story is dont play with solar when the sun is shining

[WotEver]

7 minutes ago, peterboat said:

No that was me

Then you had some seriously dodgy controllers. They should have no problem with two of them connected to the bank. 
 

8 minutes ago, peterboat said:

Moral of the story is dont play with solar when the sun is shining

Always good advice. 

[nicknorman]

17 minutes ago, peterboat said:

Brilliant! When its up and running I think you easily market these boards people are scared of Lithiums and overcharging, Kris would be a definite customer for one

Still very early stages.  I have a lot of software to write even if the circuit diagram / PCB has no mistakes!

[peterboat]

1 minute ago, WotEver said:

Then you had some seriously dodgy controllers. They should have no problem with two of them connected to the bank. 
 

Always good advice. 

Tracers Tony, thats why I now have proper Midnite ones that can be connected together by a lead so they talk to each other

[Dr Bob]

3 hours ago, nicknorman said:

My PCBs arrived today. I am still mightily impressed to get 5 PCBs manufactured to my own design and shipped from China, all for £2. Let’s hope i didn’t get a free gift of coronavirus!

 

the bit bottom left is to be cut off and mounted near the batteries, for the purpose of fusing the battery feeds to the board.

 

Put it into quarantine for a few weeks?

[nicknorman]

43 minutes ago, Dr Bob said:

Put it into quarantine for a few weeks?

Too late, I already licked them.

[MtB]

2 hours ago, nicknorman said:

Too late, I already licked them.

 

Did it kill you?

 

 

[WotEver]

51 minutes ago, Mike the Boilerman said:

Did it kill you?

You’d have to ask the butler. 

[TheBiscuits]

4 hours ago, WotEver said:

You’d have to ask the butler. 

... in the library, with the candlestick.

[Tom and Bex]

Another video comparing the safety of different lithium cells during overcharge, short circuit, mechanical damage, and fire.

 

https://youtu.be/Qzt9RZ0FQyM

[peterboat]

45 minutes ago, Tom and Bex said:

Another video comparing the safety of different lithium cells during overcharge, short circuit, mechanical damage, and fire.

 

https://youtu.be/Qzt9RZ0FQyM

Great video good to know that we have the safe batteries ?

[Dr Bob]

45 minutes ago, peterboat said:

Great video good to know that we have the safe batteries ?

Well no!

The video is good in that it shows some batteries being destroyed and shows what happens in the  experiments they did but it doesnt help with another failure route of the safer types.

The only thing that is clear here is that Li-ion are dangerous and can go on fire at a whim. I am not so sure the message on the other types though is correct - suggesting perhaps that they are totally safe.  If you look at all the non-Li -ion batteries, you will see them swell and then vent liquid/vapour but do not go on fire with impact, shorts, fire, charging etc. That vapour is the electrolyte boiling and venting ....but at too low a temperature for it to ignite. Once the vapour is out then the battery is 'dead' and its power cannot be released. This is how they are designed. They then are pretty safe.

Unfortunately however this is not the way to set one on fire. Any 'general' heat blows the safety valve so the electrolyte escapes. What happens however if there is a point source short (not the mechanism they used in the second set of tests which is to diffuse). In practice dendrite growth is the thing to be worried about. Under overcharging scenarios, a dendrite (a very thin ...microns....finger of metallic iron) can grow from the anode which when it gets to the cathode can then produce an extremely high temperature at a very small spot which can ignite the electrolyte without triggering the safety valve. This ignition can then heat the next set of molecules causing them to ignite and starts a thermal runaway. The safety valve has not yet blown but the cell is now burning. This is the key thing that you have to avoid on a LiFePO4. I keep banging on this thread that we must avoid overcharging. Dendrites dont just grow willy nilly. They grow if you mistreat your batteries and continually overcharge. They will not grow if you do not overcharge (or keep them above 5°C......there is a method of dendrite growth at very low temps, sub zero but I dont know enough about them...so keep your batteries above 5°C). One or two excursions of overcharging are very unlikely to cause problems but regular overcharging is bad.  Do a search on dendrites and lithium batteries. You can also see in the videos how these batteries swell under these external inputs and you may as well chuck 'em once that happens.

This all says to me that the risk of fire from LiFePO4s is almost zero and easily as safe as Lead Acids if you dont overcharge.

 

You also have to be careful with videos like this. Who is the firm who made it? Someone who sells the 'safer' batteries? How do we know the batteries were charged to 100% before the testing (apart from the first test obviously). The energy released in a fire from a 100% charged Li battery is far far worse than one that is only 30% or even 80% SoC. It would be easy to rig a video of this sort. Maybe a voltage reading from each prior to their destruction would have been instructive but even that can be fiddled.

 

Please all, dont believe everything you read or see on the internet (including this post!) without doing a bit of checking.

 

LiFePO4s are known to be a lot safer than Li ion cells due to the stability of the PO4 ground so oxygen cannot easily be released for combustion.

[Richard10002]

1 hour ago, Dr Bob said:

This all says to me that the risk of fire from LiFePO4s is almost zero and easily as safe as Lead Acids if you dont overcharge

For the sake of clarity, what do you mean by overcharge?

[peterboat]

7 minutes ago, Richard10002 said:

For the sake of clarity, what do you mean by overcharge?

Richard the BMS on out batteries will balance the cells over time, however, if you are forcing lots of amps in once past 13.9 volts, it will struggle to cope with some cells going over voltage and their lies the problem! Because we dont have the master BMS, which would turn off the charging source, giving batteries and cells time to balance. We if we are sensible, only charge to the 80%, saving ourselves lots of grief and big bills