Balancing LiFePO4s

[Dr Bob]

1 hour ago, nicknorman said:

On the point about the high current shunts, well it’s just a matter if of dissipating the power. 3.3v at 50A is 165 watts. I can’t think where you would find a heating element, powerful light bulb etc that runs on 3.3v.

 

Why not just do as you have done but a bit more - a metal plate or heat sink onto which you bolt some chunky resistors. You need 0.066 ohm to get 50A from 3.3v. So how about two 0.1 ohm in parallel, that gives 0.05 ohm but with a bit of loss in wires etc it will be close. These ones are rated at 100W each with suitable heat sinking. If there was no loss in wires you would be over 200W but here’s a thought, you are sitting on one massive heat sink aka the canal. Make the wires long enough so that you can dangle the contraption in the canal. Don’t burn any fish though!

 

https://uk.rs-online.com/web/p/panel-mount-fixed-resistors/1752437/

 

Thanks Nick.

Two of the 100mOhm resistors would cost £20.

As someone who is clueless about heat sinks, would having over 200W be near to destroying them? If so would it be better to buy 10 * 1 Ohm 50W and wire them in parallel for the same price

https://uk.rs-online.com/web/p/panel-mount-fixed-resistors/1311276/

That way you would have 100mOhm so not 50A but maybe 30A which is better than what i have. It would then handle 500W so could you just bolt it to a bit of metal?

[nicknorman]

25 minutes ago, Dr Bob said:

Thanks Nick.

Two of the 100mOhm resistors would cost £20.

As someone who is clueless about heat sinks, would having over 200W be near to destroying them? If so would it be better to buy 10 * 1 Ohm 50W and wire them in parallel for the same price

https://uk.rs-online.com/web/p/panel-mount-fixed-resistors/1311276/

That way you would have 100mOhm so not 50A but maybe 30A which is better than what i have. It would then handle 500W so could you just bolt it to a bit of metal?

Ok but don’t forget the resistors themselves can dissipate nothing like 50w on their own - they can only do so when bolted to a heat sink that takes the heat away. In other words, their thermal conductivity is such that they can pass 50w to a heat sink, but they can’t dissipate that on their own without a heat sink.

 

As you describe it, it will be about 110W so you will need a fairly big heat sink and obviously no point in bolting them all on to the heat sink close to each other - the ones in the middle would cook! Maximum temperature is 200C, enough to make you flinch!

If you don’t like the idea of sticking it in the canal, you could stick it in a bucket of water. And then get free tea afterwards.

[Dr Bob]

10 minutes ago, nicknorman said:

Ok but don’t forget the resistors themselves can dissipate nothing like 50w on their own - they can only do so when bolted to a heat sink that takes the heat away. In other words, their thermal conductivity is such that they can pass 50w to a heat sink, but they can’t dissipate that on their own without a heat sink.

 

As you describe it, it will be about 110W so you will need a fairly big heat sink and obviously no point in bolting them all on to the heat sink close to each other - the ones in the middle would cook! Maximum temperature is 200C, enough to make you flinch!

If you don’t like the idea of sticking it in the canal, you could stick it in a bucket of water. And then get free tea afterwards.

Thanks Nick.

[MtB]

Ok a thought about the resistors.

 

1W raises 1cc of water through 1c in one second, IIRC. 

 

So.... 

 

300W raises 300cc of water by 1c in one second.

 

So....

 

In 80 seconds, 300W will raise 300cc of water by 80c.

 

So....

 

To raise 300cc of water through 80c in 800 seconds, which will be 30w.

 

A resistor dissipating 30W at 12V draws a current of 2.5A.

 

The resistor is therefore 4.8 Ohms, by Ohm's Law.

 

Therefore, four of these devices in parallel, in the bucket of water,  will drag 10A out of a 3v cell. A cheap and convenient way of doing it perhaps?

 

https://www.ebay.co.uk/itm/12V-Car-Van-Cup-Mug-Hot-Water-Heater-Element-Kettle-For-Tea-Coffe-Soup-Drinks/123588401178?_trkparms=aid%3D555018%26algo%3DPL.SIM%26ao%3D1%26asc%3D20131003132420%26meid%3D7f45c5d238944d97a9724c81d4a3198f%26pid%3D100005%26rk%3D4%26rkt%3D12%26mehot%3Dco%26sd%3D133200089520%26itm%3D123588401178%26pmt%3D1%26noa%3D0%26pg%3D2047675&_trksid=p2047675.c100005.m1851

 

 

 

 

 

 

[Dr Bob]

Mike, you seem to be a factor of 10 out. To take 10A out at 3.3V you need a 0.3 ohm resistor, not 4.5 Ohm or 1 Ohm if you use 4 of them. That's the theory ....ands it's what I get in practice. I have 3 *1ohm in parallel. 

Not sure where you went wrong in your workings but it sounds like 4 of them would work. 

[MtB]

3 minutes ago, Dr Bob said:

Mike, you seem to be a factor of 10 out. To take 10A out at 3.3V you need a 0.3 ohm resistor, not 4.5 Ohm or 1 Ohm if you use 4 of them. That's the theory ....ands it's what I get in practice. I have 3 *1ohm in parallel. 

Not sure where you went wrong in your workings but it sounds like 4 of them would work. 

 

Odd coz that's what I thought too, so I posted my workings! 

 

 

V = IR.

 

12 = 10 x R

 

12/10 = R

 

1.2 Ohms. 

 

It didn't work out like that last night after several Bunnhabhains!

 

[rusty69]

19 minutes ago, Mike the Boilerman said:

 

Odd coz that's what I thought too, so I posted my workings! 

 

 

V = IR.

 

12 = 10 x R

 

12/10 = R

 

1.2 Ohms. 

 

It didn't work out like that last night after several Bunnhabhains!

 

Should the 12 bit not be 3.3?

[MtB]

17 minutes ago, rusty69 said:

Should the 12 bit not be 3.3?

 

Yes it should for one cell, which is how Dr Bob gets to 0.3 Ohms...

 

 

[Dr Bob]

7 minutes ago, Mike the Boilerman said:

 

Yes it should for one cell, which is how Dr Bob gets to 0.3 Ohms...

 

 

Yea!

The muppet got it right!

[MtB]

On 11/11/2019 at 00:06, Dr Bob said:

I have now bought one of the modules that Tom (T&B) wrote about to do it automatically.

 

I saw the other day and now can't find a link to it, despite spending 30 mins trawling through T&B's old posts and Goggling. Do you have a link to it, pretty please Dr Bob? (Or anyone else?)

 

Whilst Goggling, I chanced upon this, which seems similar. The page flogging a related device has some interesting stuff to say on balancing, although some of it seems a bit woo.

 

https://electriccarpartscompany.com/Lithium-Lead-Acid-Battery-Balancers-Equalizers-12V

 

 

[Detling]

At these currents you must be using very thick cable to connect the resistor because of volt drop. Maybe just a coil of thinner cable (30 meters or so long) in the bucket of water to keep it cool will give you the required resistance. Someone with a better brain at the maths may be able to work it out, but we are talking arc welding currents here.

[MtB]

4 minutes ago, Detling said:

At these currents you must be using very thick cable to connect the resistor because of volt drop. Maybe just a coil of thinner cable (30 meters or so long) in the bucket of water to keep it cool will give you the required resistance. Someone with a better brain at the maths may be able to work it out, but we are talking arc welding currents here.

 

No definitely not arc welding currents, just tens of Amps are needed for adjusting the SoC of individual cells.

 

 

 

[Dr Bob]

36 minutes ago, Mike the Boilerman said:

 

I saw the other day and now can't find a link to it, despite spending 30 mins trawling through T&B's old posts and Goggling. Do you have a link to it, pretty please Dr Bob? (Or anyone else?)

 

Whilst Goggling, I chanced upon this, which seems similar. The page flogging a related device has some interesting stuff to say on balancing, although some of it seems a bit woo.

 

https://electriccarpartscompany.com/Lithium-Lead-Acid-Battery-Balancers-Equalizers-12V

 

 

I found it in 30 seconds!

https://www.ebay.co.uk/itm/254003975899?ul_noapp=true

 

I got it 3 weeks ago but not fitted it yet. Bought a car/caravan plug so I can plug in when I need it. I was planning to start wiring it in today.

Not sure how it is going to work. @Tom and Bex leaves his in circuit all the time and says it works great but I am a bit worried that my low cell when in the middle of the 20-80% range is the highest when getting to full charge....so it wont be good if under normal operating conditions the low cell is charged up more. I am thinking I will connect it only when charging and I have gone over 80%. Tom says he never measures more than 1A flowing so its not going to be quick. I think it will be more of a fine tune. That's what this Lithium thing is all about though. Trying new things out and having fun!

[MtB]

1 minute ago, Dr Bob said:

I found it in 30 seconds!

https://www.ebay.co.uk/itm/254003975899?ul_noapp=true

 

Fanx! But how beeeezarre. I searched ebay using the search term "LiFePO4 battery balancer" and it didn't show in the about 8,000 results! 

 

 

 

3 minutes ago, Dr Bob said:

That's what this Lithium thing is all about though. Trying new things out and having fun!

 

Exactly. We sure know how to live the dream, don't we!!!!

 

 

 

[Dr Bob]

19 minutes ago, Detling said:

At these currents you must be using very thick cable to connect the resistor because of volt drop. Maybe just a coil of thinner cable (30 meters or so long) in the bucket of water to keep it cool will give you the required resistance. Someone with a better brain at the maths may be able to work it out, but we are talking arc welding currents here.

Now that is a good idea. What thickness wire and how long to get a current of 20-30A at 3.3V (or even 10A)?

We are then talking 30-90W which is a lot of heat. My 25W soldering iron gets a big warm when turned on! .....but its nowhere near arc welding currents.

[MtB]

The balancing function in the BG8S thing seems to be having an effect though... will post a photo in a sec...

 

 

[Dr Bob]

1 minute ago, Mike the Boilerman said:

 

Fanx! But how beeeezarre. I searched ebay using the search term "LiFePO4 battery balancer" and it didn't show in the about 8,000 results!

 

 

 

I typed 'Tom and Bex balancing' into the thread search here and it was the first post!

Just now, Mike the Boilerman said:

The balancing function in the BG8S thing seems to be having an effect though... will post a photo in a sec...

 

 

Ooooo. Thats encouraging! Let us know how full you think you are....although that should be estimatable from the cell voltages.

[MtB]

[nicknorman]

3 minutes ago, Mike the Boilerman said:

 

Fanx! But how beeeezarre. I searched ebay using the search term "LiFePO4 battery balancer" and it didn't show in the about 8,000 results! 

 

That’s because it says it is for lead acid batteries. It doesn’t mention LiFePO4.

[MtB]

10 minutes ago, nicknorman said:

That’s because it says it is for lead acid batteries. It doesn’t mention LiFePO4.

 

Aha! So unless we can figure out how it works/what it does exactly, we don't know if it will work with Li, or do we? 

 

 

17 minutes ago, Dr Bob said:

Ooooo. Thats encouraging! Let us know how full you think you are....although that should be estimatable from the cell voltages

 

So the high reading on cell 8 has been reduced, and the low cell 1 seems to have come up a bit, but not a lot yet. 

 

This photo is with the Whispergen running and pushing 25A in...

 

Its powering down now, will take a photo with it OFF in a few mins.

 

 

 

[MtB]

 

 

Here is the bank disconnected (i.e. with the isolator switch turned OFF) after an hour of charging. No current going in or out.

 

 

 

[MtB]

47 minutes ago, Dr Bob said:

Let us know how full you think you are....although that should be estimatable from the cell voltages.

 

Well the cell voltages are the only way to know how full, shirley?

 

I reckon this bank is at about 85% despite the (must be ignored) fuel gauge saying 29%...

 

Yes/no?

 

[PeterF]

12 hours ago, Mike the Boilerman said:

Ok a thought about the resistors.

 

1W raises 1cc of water through 1c in one second, IIRC. 

 

So.... 

 

300W raises 300cc of water by 1c in one second.

 

So....

 

In 80 seconds, 300W will raise 300cc of water by 80c.

 

So....

 

To raise 300cc of water through 80c in 800 seconds, which will be 30w.

Sorry no, 4.186W raises 1 cc of water through 1 degC in 1 sec. The specific heat capacity of water is 4,186 Joules per kg per degC, so 4.186 Joules per cc per degC. I believe that you are confusing it with calories, the specific heat capacity is 1 calorie per cc per degC.

Edited November 12, 2019 by PeterF

[Dr Bob]

30 minutes ago, PeterF said:

. I believe that you are confusing it with calories, the specific heat capacity is 1 calorie per cc per degC.

If I ate an extra sausage roll that would be 150 calories more so you woud have to apply the resistor thingy for an extra 30 mins.

[Dr Bob]

2 hours ago, Mike the Boilerman said:

 

Well the cell voltages are the only way to know how full, shirley?

 

I reckon this bank is at about 85% despite the (must be ignored) fuel gauge saying 29%...

 

Yes/no?

 

Ok, those voltage data are useful.

I dont think you can infer much from them other than the one with the 25A going in but even then at 3.38V it is a bit low and not up to the knee. It is clear though from the screen shots from yesterday with cell 8 up at 3.6V (almost) that cell 1 is the lowest....and it is also the lowest by quite a bit at the 60-80% charge. That means any balancer should work to get cell 1 up in voltage so worth trying with the Bg8s meter even at these 'not high' state of charge. How long have you had it in balancing mode? Why not just leave it on for 24 or 48 hours and then post the rested cell voltage data. You will get a feeling if it is better but the true test will be to take the bank up to near 100% again and show us the voltages when the first cell hits 3.6V. If the BG8S has worked then the cell delta will be less than 250mV. How much lower will be a test of how well that meter is balancing.

 

One piece of detail to watch .....but the effect is small....is that the BG8S meter seems to take the power to run the meter from cell  1....ie the meter works on 3.3V. I see a drop of 4mV on cell 1 when I push the button to light up the display. This is not a big issue as the meter display is only on for 60secs or so. HOWEVER - I also have the older meter (the BC8S) which has the display on all the time.....I couldnt work out how to turn it off...and this also takes power from cell 1. Over a month or so that could take a bit of capacity out of cell 1. I have disconnected my BC8S.

 

Just ignore the SoC value on the display. It has to be set for a different type of Li battery and certainly doesnt fit with LiFePO4s. You must ignore it.