LiFeP04 Batteries for Solar & BMS

[Sunking]

Does it shut off charging or force the controller into a lower float voltage? My understanding is it would be ideal to have the charge controller drop down to a lower float voltage. This way the charge controller will still provide power to the loads without unnecessarily cycling the battery up and down. Just trying to comprehend what goes on after charging a LiFePo4 battery is complete?

That is where you have to get out of the Pb box thinking. When the first cell reaches 90 to 95% you terminate charge period.

Now you could switch to float providing the float voltage is lower than the pack resting voltage so all charging stops. The LFP battery would have to drain down to Float Voltage before the panels would supply any power. Ideally if you have a day with very little use say gone over night somewhere, is just turn the controller off and not recharge the next day. All that can be done with BMS. Its one less cycle or nail in the coffin. LFP does not need to be 100% SOC. In fact better if it is never 100%.

[Living Large]

Yes, latching would work, and then one could unlatch it at a lower voltage. It could easily happen with a slight imbalance with the cells, and differences in charging rates, but depends on the margins of safety you put in too.

If you disable charging with an HVD, with no one around at the time, you could run the batteries down and wind up with an LVD to follow.

It depends what you want it to do. Based on the discussion we have been having, I was anticipating that for me it would not be a typical event. But hey, if it could be an added part of normal operation, that would add flexibility in what is possible.

I saw it as a hard error, and was thinking of shutting the whole system (MPPT, inverter/charger) down for either an LVD or an HVD.

[northerner]

Regarding your UPDATE. So there evidently is a function to support this. I need to go back and look again. My impression was this event shouldn't normally be happening in my setup, and my inclination would be a shutdown that requires intervention. The signal could be latched externally if this was desired.

Yes, latching would work, and then one could unlatch it at a lower voltage. It could easily happen with a slight imbalance with the cells, and differences in charging rates, but depends on the margins of safety you put in as well.

If you disable charging with an HVD, with no one around at the time, you could run the batteries down and wind up with an LVD to follow.

[northerner]

This discussion is about HVD, a failure mode. Would not normally happen.

Regarding the MS Classic, I could not find the described functionality of shutting down based on an input at Aux 2 in the manual, and I have an email into MS to confirm if it does or does not support this. The Aux ports have a lot of options, so I may have missed it.

Would not normally happen, but if it did happen, it looks like it could alternate back and forth between charging and not charging. Would it not be better to have it forced to a lower float voltage, if possible?

I looked for this function as well and according to Midnite it is there, but I don't see a description on how to use it?

[Living Large]

Does it shut off charging or force the controller into a lower float voltage? My understanding is it would be ideal to have the charge controller drop down to a lower float voltage. This way the charge controller will still provide power to the loads without unnecessarily cycling the battery up and down. Just trying to comprehend what goes on after charging a LiFePo4 battery is complete?

I know that some users make use of opportunity or waste not modes using the Midnite Classic, where any power beyond what the battery uses is made use of. If the solar controller shuts off charging, would the extra power from waste not or opportunity modes still be available? I could see that being the case if the Classic dropped down to a lower float voltage?

Or maybe it is ideal that you set up the Classic to go to float after reaching a preset bank voltage, and having a HVD event is something that shouldn't normally happen? I also take it that the controller would resume charging once the HVD signal is no longer present?

This discussion is about HVD, a failure mode. Would not normally happen.

Regarding the MS Classic, I could not find the described functionality of shutting down based on an input at Aux 2 in the manual, and I have an email into MS to confirm if it does or does not support this. The Aux ports have a lot of options, so I may have missed it.

Regarding your UPDATE. So there evidently is a function to support this. I need to go back and look again. My impression was this event shouldn't normally be happening in my setup, and my inclination would be a shutdown that requires intervention. The signal could be latched externally if this was desired.

[northerner]

LL FWIW I know you are not looking at Midnite Solar Classic Controllers but any of the Classic controllers have an Auxiliary Input (Aux 2) that allows an external signal to shut the charge off. Perfect for using a BMS.

Classic Manual

Does it shut off charging or force the controller into a lower float voltage? My understanding is it would be ideal to have the charge controller drop down to a lower float voltage. This way the charge controller will still provide power to the loads without unnecessarily cycling the battery up and down. Just trying to comprehend what goes on after charging a LiFePo4 battery is complete?

I know that some users make use of opportunity or waste not modes using the Midnite Classic, where any power beyond what the battery uses is made use of. If the solar controller shuts off charging, would the extra power from waste not or opportunity modes still be available? I could see that being the case if the Classic dropped down to a lower float voltage?

Or maybe it is ideal that you set up the Classic to go to float after reaching a preset bank voltage, and having a HVD event is something that shouldn't normally happen? I also take it that the controller would resume charging once the HVD signal is no longer present?

Update: Looks like the charging function is disabled as long as the signal is present on the AUX 2 input to the Midnite Classic. When voltage of the cell that triggered the HVD drops down, it would no longer give a signal to disable the CC, and the Midnite Classic would then resume charging. This could alternate back and forth.

[codo1982]

How much is average for 1ah of the LiFeP04 these day? Where you buy them from?

[tasman]

Ouch. You want to disconnect the SOLAR PANELS from the charge controller. The controller wants to be powered by the battery bank all the time.

Sorry getting my way of putting things wrong again, assumed eveyone would know you disconnect the panles from the controller, just like at night as it requires full battey connection to function. Just like a BMS. The LVD works after the charger as well, have no idea how it will work if it every reaches the top or bottom paramaters. Only tested it when it was put together, been in use since and never need to work.

Put my systems together with no knowedge to start with and no access to any help being as remote as we are and until a while ago, no real communications other than constant drop out mobiles. So just begining to learn and discuss, seems haven't yet learnt how to phrase things properly so most understand.

Got my MH pack out now changing a few thing around from what Iv'e picked up here and researched, which has given me answes to some questions and knowledge to solve some myself. So hang in with me for awhile whilst I get the gist of how this forum works best and fit in with it. When I post something, it's really learning how others see my systems and addng that to better my knowledge and approach. Some times may post something I haven't really thought deeply enough about so gives the wrong impression, so it's good it gets noticed and corrected.

[Mike90250]

....My BMS sends a 12v signal to a 150amp relay when the cell pack reaches 14.4v and that disconnects the charger controller from the pack.....

Ouch. You want to disconnect the SOLAR PANELS from the charge controller. The controller wants to be powered by the battery bank all the time.

[tasman]

You just cannot use a relay to disconnect a CC as they need battery power applied at all times. Failure to do that can damage the controllers. However you can disconnect the solar panel with no problem All you gotta do is either open the circuit, or short them out.

The relays I use, switches to another contact which disconnects the battery pack, when this happens it connects to a 12v supply that takes over from the upper paramater signal and supplies power to the relay constanly, to keep the charge disconnected. To switch on charge, I have a switch which turns of the power to the relay and charge returns. I have a led light which tells me without checking the bms that charge is off, then I chek the bms to see the pack charge state and switch of the relay supplying power to the main relay keep charge dosconnected and it charges.

It works well when I tested it, but haven't need it because the packs stays within the top and bottom voltage parameters all the time. Because there is always a frdge and freezer going even when no one is home.

[PNjunction]

Sadly there doesn't seem to be any 3.2v cell chargers that have high amperage inputs, no good trying to charge 700ah with 4x4.5 amp chargers coming from a 60-120 amp supply.

Tas - here is one CC/CV that might do with some warnings about usage. Admittedly a bit expensive if you are only doing 4 cells but it kind of proves my point:

http://www.batteryspace.com/Smart-Charger-40A-for-3.2V-1cells-LiFePO4-Battery-Pack.aspx

Note that this 40A / 3.2 (3.65v hvd) single cell charger is really intended for cells no larger than 400ah. See the capacity column for this 3.2v model:

http://www.batteryspace.com/smartchargerforlifepo4batterypacks.aspx

So why is it limited to no larger than 400ah cells? Because at this rate of about 0.1C, it will reach 3.65v with minimal electrolyte heating.

BUT, you *could* use it, provided you watched it like a hawk, and when it stalls between 3.45 and 3.5v, (or better yet just starts to drop below 40a) you pull it (essentially you are *starting* out at .05C, which is typically considered the current you stop charging with). You DON'T allow the charger to finish and shut off by itself. If you used this on a 700ah cell, eventually it would try to go to zero amps and stall, but NEVER reach 3.65v and merely heat the electrolyte because you are just under the wire at .05C. EVENTUALLY, yes it will reach 3.65v, but that is due to electrolyte heating, (applied high voltage but no current flowing) and not from charging. Hence the common recommendation not to go lower than 0.1C, unless you know up front that you should set your hvd low.

At 700ah, I don't know how long or how much degradation from electrolyte heating would cause, but I can see it happen pretty rapidly on my 40ah cells.

For your 12v application, the same consideration is given to solar. It is quite feasable that you could be at 95% SOC on your first sunny day, but the next - bad weather arrives and you are charging with <0.1C. If your high cutoff voltage is set too high, then the bank slowly finishes charge at a lower voltage, and spends the rest of the day heating the electrolyte *trying* to get to say 3.6v. And it will, but that is due to parasitic reactions.

That's why my moral with solar is to set your hvd low to no more than 3.5v since we can't control the environment. In some cases with low current, the only time the bleedoff circuits would activate is when the cells are actually finished charging, and are now rising in voltage due to electrolyte heating! No balancing is taking place at low currents.

You've got a lot more to lose with 700ah than I do with 40ah. However, because I run conservatively, and started out sanely, I run bare with NO balance boards whatsoever because in a "Sub C" application, the cells tend to stay in the condition of balance they were first left in. Running >1C regularly, or abusing them with higher or lower voltages than necessary can change that. If you stay within a window of sanity, I can get away with going bare.

This allows me to use off the shelf chargers and controllers that operate on the pack-level (two terminal), as long as temp comp is defeated, the HVD is no more than 14.1v (common "gel" setting if I can't set it lower myself), and a float at 13.6v (3.4v per cell), is benign as it is well under the higher-voltage that can cause electrolyte heating when used in a relatively low current application.

Anyway, that's my take on being conscious of differentiating between EV / RC applications, and relatively low-current solar.

[Living Large]

LL if the XW controller has an input to stop charging you are good to go. I cannot help you with that, well I could if I was not lazy tonight, so that is a Home Work assignment for you. Should be in the documentation or a quick e-mail to Xantrex. I just know it can be done with any MS Classic and Wizbang Jr.

As for most controller, in fact all that I know of there is a correct order for connection Battery first, then panels. To disconnect panels first, then battery.

Yes, that is my homework - I didn't expect you to do it. The manual says "shuts down the system". So it is left to the reader to determine if "system" means both the charger and inverter. Common sense would be it is both. I'll write Schneider and ask them, because it didn't occur to me to ask today.

Yep, disconnect panels first, then battery is very clear.

[Sunking]

LL if the XW controller has an input to stop charging you are good to go. I cannot help you with that, well I could if I was not lazy tonight, so that is a Home Work assignment for you. Should be in the documentation or a quick e-mail to Xantrex. I just know it can be done with any MS Classic and Wizbang Jr.

As for most controller, in fact all that I know of there is a correct order for connection Battery first, then panels. To disconnect panels first, then battery.

[Living Large]

LL FWIW I know you are not looking at Midnite Solar Classic Controllers but any of the Classic controllers have an Auxiliary Input (Aux 2) that allows an external signal to shut the charge off. Perfect for using a BMS. Check the product you are looking at and see if they have something similar.

As for whatever controller you use, you are just going to use the Bulk portion which is Constant Current set to a higher voltage than needed. 58.4 volts should work just fine. When your first cell hits 3.45 volts, you terminate charge. That will put the weakest cell at approx 95% SOC and the rest slightly lower.

Classic Manual

Thank you for this suggestion. At this point in time, considering the current desired functionality, it would be very wise to revisit the MS. FWIW, I did look at it a bit in the beginning, before I fell for the XW, and started leaning towards the Xantrex CC. The only synergy I was anticipating looking at Xantrex for both was sharing of settings, and having faith the MPPT and inverter/charger would be on the same page to an extent. This was largely as a noobie - thinking there would be an advantage to using the same manufacturer. If the MS has settable thresholds like the Xantrex, and the modes can be programmed as desired for LFP, then giddy-up!

You just cannot use a relay to disconnect a CC as they need battery power applied at all times. Failure to do that can damage the controllers. However you can disconnect the solar panel with no problem All you gotta do is either open the circuit, or short them out.

I was wondering about exactly where and how relays can be used, and what happens if a circuit is broken. I appreciate this information. I assume somewhere in the installation or user manuals this is covered. With an Aux input to shut down the XW, and an Aux input to shut down the MS MPPT, it is starting to almost sound like Nirvana. One thing I need to verify is the Remote Power Off of the XW shuts down both the inverter and charger - to account for both LVD and HVD faults.

I'll take another look at the MS. Thanks again.

[Sunking]

LL FWIW I know you are not looking at Midnite Solar Classic Controllers but any of the Classic controllers have an Auxiliary Input (Aux 2) that allows an external signal to shut the charge off. Perfect for using a BMS. Check the product you are looking at and see if they have something similar.

You just cannot use a relay to disconnect a CC as they need battery power applied at all times. Failure to do that can damage the controllers. However you can disconnect the solar panel with no problem All you gotta do is either open the circuit, or short them out.

As for whatever controller you use, you are just going to use the Bulk portion which is Constant Current set to a higher voltage than needed. 58.4 volts should work just fine. When your first cell hits 3.45 volts, you terminate charge. That will put the weakest cell at approx 95% SOC and the rest slightly lower.

Classic Manual