LiFeP04 Batteries for Solar & BMS

[Sunking]

if you can explain how you get 200% extra energy out of half the ah capacity, I'd be most appreciative.

I did not say that, I said 200% more efficient. Big difference.

Let's assume two simple circuits of a 12 and 24 volt battery 100 feet of copper 4 AWG wire, and 2 500 watt Gizmos one rated for 12 volts and the other rated for 24 volts. The 12 volt battery and 24 volt have the exact same capacity. The 12 volt is 320 AH = the 24 volt is 160 AH. We use the exact same two 12 volt 160 AH batteries.

On the 12 volt system the gizmo draws 42 amps and looses 1.2 volts or 10% of its power (50 watts)
On the 24 volt system the gizmo draws 21 amps and looses .6 volts or 2.5% of its power (12.5 watts)

Having said that we could make the 12 volt system just as efficient as the 24 volt system. We would have to use 2/0 AWG copper cable instead of #4 AWG. 100 feet of #4 AWG cost roughly $70, and 100 feet of #2/0 cost $220. Which system do you use?

I would not use either of them. I would use 48 volts and only loose 1.25% of the power or 6.25 watts. I could use a little smaller battery and panel wattage saving me more money. Now you know why utilities use very high voltages.

[tasman]

That is not possible. 24 volts is 200% more efficient than 12 volts and 48 volts is 400% more efficient than 12 volts.

You'll have to explain how that works, as I said I'm not that up on the technicalities of different voltage use, just go by what others have said and what I've picked up myself.

Was told using 24volts for the house would halve my pack ah capacity and on the surface that's how it looks. if you can explain how you get 200% extra energy out of half the ah capacity, I'd be most appreciative. Can understand the extra power for EV's by upping the voltage, but not how you can get 200% more energy out of a 24v system, by halving the amperage of a 12v one. Especially when using it through an inverter.

[Sunking]

A friend set his house up using 12v and an inverter, found it used less energy than running either 24v or 48v for his house,.

That is not possible. 24 volts is 200% more efficient than 12 volts and 48 volts is 400% more efficient than 12 volts.

[tasman]

4 Balance boards right? Connected like the diagram for ladder right? Sounds like 4S16P t o me making 12 volts. It will work but 12 volts was a very poor voltage selection unless you have a special reason like an RV.

Yep 4 active balance boards, connected to a dedicated off grid BMS.

My apologies, thought using "MH" for motorhome would be ok, but can see you call them RV's. My house pack is also 12v, because it uses a 3000-6000w inverter for all house requirements. A friend set his house up using 12v and an inverter, found it used less energy than running either 24v or 48v for his house, plus apppliances are cheaper and easy to get down here than dc ones. So changed all our lght globes to 240v LED and reconnected the house wiring to the inverter. Haven't had a problem since, The only things using power at night are the freezers, big LED TV and a linux pc. During the day we run our band studio on it with no problems at all, all it takes is good energy management.

Forgot, we also connect our MH to the house when not travelling, which gives us another 1000w of solar and 480ah extra lifepo4.

[Sunking]

In the photo, there are 64 x 40amp cells to make 12v x 640amps. The active balancers are connected to each cell line and about to take out my pack to make some adjustments and changes to it, by adding a couple more relays to switch back charge when it drops to 3.4v, from 3.6v. Instead of having to do it myself. But as I've said, yet to reach either top or bottom charge parameters so am really doing this for added security and improve my knowledge.

So what would you call my connection technique and is a good one, or not. To me it looks like a ladder because it connections across and along each cell line, rather than each line being separate until the end where they connect across the lines in parallel.

4 Balance boards right? Connected like the diagram for ladder right? Sounds like 4S16P t o me making 12 volts. It will work but 12 volts was a very poor voltage selection unless you have a special reason like an RV.

[tasman]

In the photo, there are 64 x 40amp cells to make 12v x 640amps. The active balancers are connected to each cell line and about to take out my pack to make some adjustments and changes to it, by adding a couple more relays to switch back charge when it drops to 3.4v, from 3.6v. Instead of having to do it myself. But as I've said, yet to reach either top or bottom charge parameters so am really doing this for added security and improve my knowledge.

So what would you call my connection technique and is a good one, or not. To me it looks like a ladder because it connections across and along each cell line, rather than each line being separate until the end where they connect across the lines in parallel.

[Sunking]

The Alternative method you looked at is LADDER connected. That is the proper way to connect lithium cells in parallel as it forces each paring of 2 or more cells in parallel to equalize. It also makes battery management much less complicated and less expensive to implement. For example in the Stickie is shown a 3S2P or 3 cells in series, 2 in parallel. From a BMS point of view only requires 3 monitor points. If you were to use conventional method would require 6 cells to monitor and control. In addition in a conventional method both charge and discharge currents would be unequal forcing the string with lower Ri to do the majority of the work. Connecting them in a ladder style eliminates all that trouble and complexity. For example the Nissan Leaf battery is 96S2P made up of 33 AH cells. Together makes a 355 volt 66 AH battery with a 24 Kwh capacity.

Your picture is quite unique and difficult to make out what you have. It appears you have 64 cells arranged S4P16 12 volts or most likely S16P4 for 48 volts I hope. Either way they should be Ladder Connected as shown in the Stickie.

[tasman]

Look at this Stickie, Alternative is ladder connected series/parallel

Thanks for the links, but it hasn't enlightened me much as can't see much difference between the conventional way and alternative. Here's a photo of my MH pack when I was putting it together, is this what you mean by ladder connections and is this a good or bad way to connect lifepo4 cells. My current setup is of smaller ampergage, found didn't need 600amps in the MH, so dropped it to 480ah and used the other 120ah for a second battery in our sahara which has heaps of elctronics to run.

[Sunking]

You'll have to explain what you mean by paralleled, in my meagre knowledge thought they were all parallel series connections,

Look at this Stickie, Alternative is ladder connected series/parallel

[tasman]

I do not think that is true. Large format cells are not paralleled. If they are they are ladder connected. I know a lot of EV guys who run EV's Bottom Balanced and do not have BMS and running fine for 20 months. With everything in series and bottom balanced capacity remains equal.

You'll have to explain what you mean by paralleled, in my meagre knowledge thought they were all parallel series connections, otherwise you'd never get above 3.x volts out put. Do you mean by ladder connected, that all parts of the cell lines are connected series parallel, instead of just one end and how it would have a different effect for charging.

I believe I've set my MH pack up as full series parallel connections, as it was easier and meant the pack was secure, nice and tight for being in the MH. We've come a cross a number of people with big lifepo4 packs on the road, they don't use a BMS and just meters, manually control their charger rates and have problems balancing no matter how they are connected. That's why I'm considering connecting both negatives and positives ends pf the pack together to get a better distibution of charger and discharge.

[northerner]

The Aux 2 input on the Classic turns out to suspend charging until its state changes. The active polarity can be set.

One could easily build a circuit to have that latch on once HVD triggers, and then have it latch back off again when battery pack drops below a preset level (ie rebulk voltage). If you don't latch it, the charging voltage may get stuck there (at the HVD trigger point) for a while. It's too bad you couldn't trigger the Classic to go to Float mode.

[Sunking]

The Aux 2 input on the Classic turns out to suspend charging until its state changes. The active polarity can be set.

Perfect.

[Living Large]

The Aux 2 input on the Classic turns out to suspend charging until its state changes. The active polarity can be set.

[Sunking]

I am not stuck in Pb mentality, because remember, I am brand new to this. What I am stuck with is the current state of the art, and that is why I am looking at it the way I am. While a BMS can monitor to the cell level, the question is what can be done with that information. That was the angle I was coming from.

You didn't specifically critique what won't work in what I proposed. I told you one potential issue with your proposed solution - the XW, which appears to be a pretty flexible unit, will shut down using the Aux input. It's definition is "REMOTE POWER OFF". It doesn't suspend activity and wait for the input to change state. So I am stuck with equipment function limitations, and how to operate within the confines. You're the one who taught me this.

I think where we are stuck is you need to make some decisions. So here is my thoughts.

If you are going to use LFP batteries, I highly recommend using some sort of BMS. That BMS will control the charger and a LVD device. The challenge is how to do that with what equipment is on the market. Unfortunately there is nothing really usable on the Solar side for LFP except some toy equipment, but none for serious energy management. Everything for solar is geared for Pb and operates on total battery voltage and not Cell Voltages where LFP has to be controlled.

I assume from this point forward you are going to Bottom Balance as that is the easiest and least expensive way. For charging you are going to have to use the BMS to terminate the charge. For me it is real easy I modified a telecom 48 volt rectifier to accept a logic 1 signal from the BMS when the first cell hits set point. So now we, more specifically you have to determine a way to terminate the charge. I am not familiar with Xantrex aux inputs if that can be done or not. I know MS can be controlled. One work around would be to use a 48 volt 50 amp DPDT relay to open the panels from the input of the controller, and shorting out the panels. You don't even need to short the panels, just open them up and fool the controller thinking it is night time. I do not care much for mechanical operations and one could use an electronic relay. Bottom line is you gotta figure out how you want to handle it.

On the disconnect side you need to figure out how to protect those batteries. Since you are Bottom Balancing and assume you reference at say 2.5 vpc you could set the inverter built-in disconnect to operate at 2.5 volts x 16 = 40 volts. Then reconnect at 48 volts. Personally I would not do that. I would have something redundant. For example what I do on my EV is I have two fail safes and you can employ the same idea. Install a 200 to 400 amp contactor between the battery and inverter. The contactor operates from the BMS signal at say 2.6 vpc first cell which should be a pack voltage of around or 42 volts. Then have the Inverter built in LVD operate at a lower voltage of say 40 volts.

Bottom line here is LFP batteries are expensive. Fortunately LFP are fairly tolerant of Over Charge so that operation is not real critical. Just keep in mind to maximize cycle life you really do not want to go to 100% SOC 90% on the weakest cell is good enough because every LFP cell is somewhat under rated so if you buy 100 AH cells you get something of 101 to 115 AH. What LFP and all lithium is very sensitive to is over discharge and you have to guard and never let it happen. The best and easiest way is to Bottom Balance so all cells reach 2.5 volts at the same time.

[Mike90250]

You didn't specifically critique what won't work in what I proposed. I told you one potential issue with your proposed solution - the XW, which appears to be a pretty flexible unit, will shut down using the Aux input. It's definition is "REMOTE POWER OFF". It doesn't suspend activity and wait for the input to change state.

While the XW does not have a good way to easily do remote shutdown or restart (without manual button pushing on a control box) it does have a programmable LVD both time and voltage settings.

If you bottom balance a pack, and set the LVD for something you are comfortable with, it should stop inverting till the battery recovers and then resume.

The top voltage for the charge controllers can also usually be set for 1 minute absorb and then drop back to Float voltage.

Or the BMS can manage a Contactor or Solid State relay and disconnect the battery bank in case of a violation. That would require manual intervention to restart the system.