LifePO4 batteries for use with Midnite Classic 150?

[karrak]

If this poor guys wires his cell in parallel will make a 400 AH pack. Assuming 50% SOC just how long does it take to charge them with 6 amps? 200 AH / 6 amps = 33 hours. Hell of a lot better waiting days for the Balance Chargers to Top Balance in series. While he is waiting he can paint a wall, and then watch paint dry to keep him entertained.

Maybe he is happy to wait the 33 hours and use a power supply that he already has rather than pay a couple of hundred dollars for something that would do it quicker but might only be used once. Anyway if you charge then in series first, then in parallel it cuts down the time to probably around half that time.

[karrak]

If you use your hobby charger and decide to Top Balance, it is going to be a very slow process and you will not be able to detect when they are fully charged with a 6-amp charger with all 4 cells in parallel. At 400 AH you hold 3.6 volts until charge current Tapers to 3% of C. 3% of C on a 400 AH battery is 12 amps. See the problem? Only way to Top Balance with your Hobby Charger is one cell at a time and terminate when charge current tapers down to 3 amps. Either way will take a couple of days with 4 cells. See a problem with that?

Where did the magic figure of 3% come from. As long as the charge current is larger than 'leakage current' of the cells they will charge to full and reach 3.6 volts

[karrak]

In Bottom Balanced system at 2.5 volts, all cells will have the exact same capacity of 0 AH per cell, and all SOC voltages of 2.5 volts per cell. Both Capacity and SOC are equal at the Bottom in a Bottom Balanced System. Thus you have no need to monitor cell voltages to protect over discharge. Setting the Inverter to 10.5 volts or greater does that for you.

What happens if the coulomb efficiency of the individual cells is not the same and the battery goes out of balance at the bottom end over time. Not all cells will reach 2.5 volts under these conditions

In a Top Balanced system all cells are charged to 100% SOC and capacity AH is not equal and unknown. Only SOC is known, not AH capacity .No two cells are equal in AH capacity and Chi-Com cells AH capacity tolerance is sloppy -3%/+15%. So a 4S pack weak cell can be as low as 97 AH, and the highest of 115 AH. The 4S pack capacity is dictated by the weakest cell in the string just like links in a chain. In this example is 97 AH would be maximum AH capacity. The 115 Ah cells can eat the 97 AH cell if discharged down to the PACK 20%SOC voltage of 12 volts. In a Top Balanced system you must have cell monitors with a LVC circuit to operate if any cell reaches 2.5 volts which wil be the weak 97 AH cell. The other three cells stil have capacity left in them to drive the discharge cell to reverse polarity. Game over.

The weakest cell in the battery will always dictate the capacity of the battery, doesn't matter whether you top balance or bottom balance. With top balance, when charging all the cells will get to 3.6volts at the same time and you can use overall battery voltage to stop charging, when discharging the weakest will get to 2.5 volts first and you will have to the use individual cell voltage to shut down the load. With bottom balance, when charging the weakest cell will reach 3.6 volts first and you will need individual cell voltage to shut down the charger, when discharging all cells will reach 2.5 volts at the same time and you can use overall battery voltage to disconnect the load. If the battery goes out of balance individual cell voltage monitoring at both the high and low level is needed to protect the battery.

The maximum Ah capacity of the battery is equal to the Ah capacity of the weakest cell. If you bottom balance you can measure the number of Ah going into the battery while charging until the weakest cell reaches 3.6 volts, that is the Ah capacity of your battery. For top balancing you can measure the number of Ah being drawn from the battery while discharging until the weakest cell reaches 2.5 volts, that is th Ah capacity of your battery

[Sunking]

when charging the weakest cell will reach 3.6 volts first and you will need individual cell voltage to shut down the charger, when discharging all cells will reach 2.5 volts at the same time and you can use overall battery voltage to disconnect the load.

Nope and that is the part you do not understand. You never take the weakest cell to 3.6 volts. Proves you have not read one thing over the year since you arrived. If you do you will unbalance the pack. Lithium batteries do not go out of balance. The only metric that throws them out of balance is difference in leakage current which takes months to accumulate.

However I do agree you need to use top balance because you do not know the technology and how it works. It is not for consumers like yourself who need to rely automation to protect your batteries.Bottom balance is very simple and requires very little automation. Manufactures do not like or approve because it does not make them money if they cannot sell you a BMS.

Karrak you came here for one purpose and one purpose only. To harass me. You even conspired on another forum to bring others here to help you. You found one poor soul. At least he had enough common sense to give up and leave.

[Willy T]

What happens if the coulomb efficiency of the individual cells is not the same and the battery goes out of balance at the bottom end over time. Not all cells will reach 2.5 volts under these conditions



The weakest cell in the battery will always dictate the capacity of the battery, doesn't matter whether you top balance or bottom balance. With top balance, when charging all the cells will get to 3.6volts at the same time and you can use overall battery voltage to stop charging, when discharging the weakest will get to 2.5 volts first and you will have to the use individual cell voltage to shut down the load. With bottom balance, when charging the weakest cell will reach 3.6 volts first and you will need individual cell voltage to shut down the charger, when discharging all cells will reach 2.5 volts at the same time and you can use overall battery voltage to disconnect the load. If the battery goes out of balance individual cell voltage monitoring at both the high and low level is needed to protect the battery.

The maximum Ah capacity of the battery is equal to the Ah capacity of the weakest cell. If you bottom balance you can measure the number of Ah going into the battery while charging until the weakest cell reaches 3.6 volts, that is the Ah capacity of your battery. For top balancing you can measure the number of Ah being drawn from the battery while discharging until the weakest cell reaches 2.5 volts, that is th Ah capacity of your battery

Karrak, thanks for posting this Information, it's great to get concise and accurate facts about LiFepo4. Your the one consistent poster with the longest experience using them on a PV offgrid system.

[createthis]

Maybe he is happy to wait the 33 hours and use a power supply that he already has rather than pay a couple of hundred dollars for something that would do it quicker but might only be used once. Anyway if you charge then in series first, then in parallel it cuts down the time to probably around half that time.

That was my logic, yeah, but I ended up buying a PowerLab 8 v2 to aid in automatically pulling the battery voltage down to 2.5V. Didn't feel like babysitting it, and I figure I'll probably make more battery systems in the future, so why the hell not?

[createthis]

Big problem with this approach is that if the cells are balanced at the bottom end they will not be balanced at the top end unless their capacity is the same you are in real danger of overcharging some cells and damaging them.

You and SK are going back and forth on this one because you are using the same words with different meanings. The quote above can be made accurate by simply substituting SOC for capacity.

If capacity means how many AH can be withdrawn from a cell starting at its current state and going to a stopping point which is determined on a per cell basis, then SK is correct that in a bottom balanced bank all cells will always be at equal capacity. But at no point but the bottom will they be at the same SOC, measured on a per cell basis.

If by capacity you mean the total AH capacity of a cell, not a bank, as determined on a per cell basis, then no amount of balancing will produce a bank with equal cell capacities.

I don't see how you could take karrak's quote to mean anything other than the capacity of the cell. He very clearly said "if the cells [...] they will not [...] their capacity". Therefore I still don't see how his quote is inaccurate.

[inetdog]

I don't see how you could take karrak's quote to mean anything other than the capacity of the cell. He very clearly said "if the cells [...] they will not [...] their capacity". Therefore I still don't see how his quote is inaccurate.

I was not referring to any specific quote, such as karrak's. I was referring to the overall discussion, in particular your posts and SK's posts going back and forth.
The text of yours that I quoted was probably not the best example for that purpose.

[createthis]

I had a 48 volt 100 AH set of them. I liked them mechanically, but performance lacked due to very high internal resistance of the cells and higher than expected capacity loss. Mechanically I liked the covers and 4-point bussbars. But for the 23% higher price than CALB's was very disappointing performance. So got rid of them and replaced them with Leaf cells.

The 12V 100AH GBS battery I bought was $700 shipped to my door. I live on the East Coast. It looked like shipping for the CALB was going to be more. This was the only for sale page I could find for CALBs: http://www.electriccarpartscompany.c...ails_p_38.html and they talk about pallets and lift gates. I just didn't want to deal with it.

I was aware of the greater internal resistance of the GBS cells before I made the purchase, though I admit I don't fully understand how it will affect my system in practice yet. Worst case, I buy a CALB set later. No big deal. I'll probably end up with lots of different batteries before my curiosity is sated.

[createthis]

Never heard of a RC hobby charger limited to 50 watts, especially if you are running RC heli's. Those guys use large 6S 6000 mah packs and larger. A 50 watt charger would take 3 hours to charge a 6S@6000 mah pack. Ever consider getting a Cellpro Power Lab 8? It would make short work of maintaining and Gross Balance your new cells with either top or bottom balance @ 60 amps charge or discharge current.

250 sized racing quads typically use 1000mah 4S batteries, and the 50W B6AC V2 works great for those: http://www.getfpv.com/lumenier-1000m...tery-xt60.html
250s are actually aging now too. It's actually been a while since I've charged any of my batteries (which may have destroyed them), so I don't remember exactly, but I think I used to charge those at 1A, just to be safe. Yeah, it took a long ass time, but better that than a bloated pouch cell. The charger is capable of 6A charging, so this was nowhere near the upper limit.

150s and even 122 sized quads are quickly becoming the standard, and these quads use even smaller ah batteries.

The B6AC top balances, BTW. There's that.

Interestingly enough, I hadn't read this post before I made the purchase, but I did indeed buy a Power Lab 8. Weird that you and I are on the same page for once, eh?

[createthis]

No sir it is not

"Top balance means capacities are off at the top and over charging is a risk."

That part is half correct, At the top, all cells are at maximum capacity, but capacity is not equal. The only thing that is equal at the top is SOC voltage of 3.6 volts. Both over charge and over discharge are is highly likely without a BMS.


"Bottom balancing means capacities are off at the bottom and over discharge is a risk. It's really a personal preference thing."

None of this statement is correct and leading you to the wrong conclusions. When you BB all cells both AH capacity and SOC voltages are equal in all cells. Capacity = 0 AH, and SOC Voltage = 2.5 volts. Over discharge risk is minimized to almost impossible. Like Top Balance, over charge is possible with BB system if you try to push the weak cell to high. The magic of BB systems is capacity is equal in all cells at all SOC levels. You stop charging when the weak cell reaches 80 to 90% SOC. So in the proceeding example if the weak cell is 97 AH and you charge the weak cell to 90% capacity, all cells have 87.3 AH. The weak cell will have ever so slightly higher voltage than all others when charged up. But the voltage difference is very small roughly .05 volts or less. The weak cell should be up around 3.41 to 3.43 volts at rest, and the others around 3.36 3.38 volts so in a 4S pack totals roughly 13.5 volts.

I think I was tired at this point. My second statement is clearly not correct. In bottom balancing, the capacity and SOC of each cell is zero at the bottom. Over discharge is less of a risk than top balancing because the other cells have no opportunity to cannibalize the lowest voltage cell (a process I admittedly do not fully understand, but I've heard you talk about a lot). It's still possible to damage the cell if bottom balanced by over discharging. It's just not as much of a risk as with top balancing. Over charging is the greatest risk with bottom balancing, which we mitigate by setting the charge controller to a voltage lower than 14.6V, thereby providing a buffer area.

In top balancing, the greatest risk is over discharge because of the (admittedly not well understood) cannibalization effect. Over charging is still a risk, if the BMS current shunt things cannot offload enough current to keep up, but it's less of a risk than in bottom balancing situations because all cells have equal SOC at the top. In addition, top balancing features the added risk of BMS failure. If this happens, the battery will be destroyed by overcharging.

I decided to start out with bottom balancing because I know LifePO4 and lithium cells in general do not like to be charged 100%, so I'm hoping to get a little extra life out of my cells by avoiding that SOC range.

I may try top balancing at some point in the future if I feel like, and that's no ones business but my own, especially if I end up destroying my pack in the process.

Am I actually starting to sound like I know what I'm talking about?

[inetdog]

Am I actually starting to sound like I know what I'm talking about?

You are definitely acquiring a dangerous thing.

[createthis]

However I do agree you need to use top balance because you do not know the technology and how it works. It is not for consumers like yourself who need to rely automation to protect your batteries.Bottom balance is very simple and requires very little automation. Manufactures do not like or approve because it does not make them money if they cannot sell you a BMS.

I don't think it's so sinister as that. I don't think it's about money. I think it's about uneducated consumers not understanding the technology, like you said.

Frankly, I would have been pretty content to remain one of those uneducated consumers. I don't really mind paying a little extra to have the technology manage itself and stay out of my way. Again, I think if Midnite Solar had a LifePO4 charging profile, we wouldn't be having this conversation and I'd be happily using one of those automated batteries.

I watched a video on Youtube a few months ago about a guy who made a sandwich from scratch. Like, he milked the cow and made the cheese and grew the lettuce and baked the bread. I think it took him something like six months to make the stupid thing. When he finally ate it, he hated the way it tasted. Six months of his life, just to be dissatisfied.

I don't think consumerism is the problem here. I think it's a lack of adoption.

[SunEagle]

You are definitely acquiring a dangerous thing.

Yeah but he is into a cool hobby. I enjoy flying RC but never got into racing quads.

Mine goes up and has come down hard a couple of times.

[Sunking]

I don't think it's so sinister as that. I don't think it's about money. I think it's about uneducated consumers not understanding the technology, like you said..

True, very true, but that makes consumers easy pigeons to pluck.

Here is what Top vs Bottom Balance comes down too pure and simple Risk and picking the lessor of the two evils.

Top Balance maximizes pack capacity and is best used for infrequent applications like power tools. The down side is stressing the cells charging cells to 100%, and you destroying weaker cells if fully discharged. To run Top Balance requires cell level monitor and control provide by a BMS.

Bottom Balance eliminates the risk of over discharging cells. Down side is it has a risk of over charging the weakest cell if you push the pack SOC level too high. It requires no cell level monitoring or extra equipment to implement, just sound strategy and sacrificing of few percentage points of total pack AH capacity.

LFP batteries are sensitive to over discharge (cell reversal) and over charge. However LFP is fairly tolerant of over charge up to 4.0 vpc before you risk thermal runaway, whereas LiCo is extremely sensitive and cannot tolerate any over charge. Take them to 100% or 105% you only loose cycle life. On the other hand no LFP type will tolerate over discharge to cell reversal. One time is all it takes.

So you choose which path you like. I choose the low risk option and sacrifice 3 to 5 AH out of each 100 AH of battery. With a cutoff voltage of 3 vpc per pack, I never ever get close to going over the cliff at 2.0 vpc. On a 12 volt system I would set LVC at 12 volts, the cliff is at 8 volts. No Inerter or gizmo is going to work down to 8 volts. On the charge side on a 12 volt system, you stop at 13.4 volts and hold. Well below 14.6 volts upper limit..That only cost you 3 to 5% capacity and doubles cycle life.

It is not that hard to understand.