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flyingfishfinger Senior Member Joined: 12/09/2020 Location: United StatesPosts: 113
Posted: 06:37pm 13 Dec 2024
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Hi, I got hold of some bulk-priced LiFePo batteries! I'm considering upgrading my setup to these ones, but I wanted to see how I need to change the code for the MPPT.
The easy one is ending voltage (14.4V * 4 -> 57.6V), that's obviously trivial to change, as well as turning off the temperature compensation.
Additionally, I have a "standby load" at my site (electric fence) that I wish to keep connected, so I can't completely power off the setup when I am gone.
What I'm unsure about now is usage vs storage voltage. Since LiFePo are not supposed to be float charged, I would think we need a way to select between "use" and "storage" voltage.
I've seen that you can keep them charged to 3.3V / cell (so, 52.8V) for long-term storage, would people agree with that?
Is anyone using this MPPT for LiFePo batteries? If yes, what settings are you using?
Cheers, R Edited 2024-12-14 04:45 by flyingfishfinger
oreo Regular Member Joined: 11/12/2020 Location: CanadaPosts: 51
Posted: 07:17pm 13 Dec 2024
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I am using LiFePo batteries made by A123 and they do specify a float voltage for these at 3.4v per cell. I see others using in the range of 3.35-3.4v float voltage.
On my cheap china MPPT, when I select lithium it does not allow me to set the absorption voltage (just float). This means that with 3.4v charge/float voltage, when the batteries approach full charge (over 90%?), the battery current starts to drop.
Typically I see other people use 3.45-3.55 absorption voltage (maybe that is the ending voltage with the Poida MPPT, IDK) when supported to give them a quicker time to charge that last 10%. If you really needed to charge this last portion of the battery fast, you may want to go as high as 3.65v.
I would start out safe, and see if you are happy with the results.Greg
Revlac Guru Joined: 31/12/2016 Location: AustraliaPosts: 1037
Posted: 10:54pm 15 Dec 2024
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I have tried this MPPT on LFP just for a couple of days to charge some large cells, just to experiment I didn't change any settings ( I should, will later ) from what I had running the FLA battery in the shed. I did find the charge controller behave differently when charging the LFP batteries, it stayed in mppt mode then cut power for a moment then continued charging and repeating, this went on most of the day, it eventually went into absorb mode most of the next day so I should change a few settings to get it right The voltage on the FLA batteries used to rise fast and charge in absorb very quick, the LFP battery voltage didn't move, just barley, so might be some voltage differential setting. When I get the rest of the system built I will set it up better. Regardless of battery chemistry we stay within the manufactures specifications, a lot of people have there own personal preferences within those margins.Cheers Aaron Off The Grid
poida Guru Joined: 02/02/2017 Location: AustraliaPosts: 1425
Posted: 03:03am 17 Dec 2024
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I think this is the result of the mppt being in MPPT mode. This mode will include a mppt scan every minute, and this is a search for the PWM width that gives the highest output power WHILE NOT EXCEEDING absorb Bv and max output current. But it does exceed it, not for long, maybe only 0.1 second due to the search needing to receive the output current and battery voltage as soon as the new PWM width is programmed. There are time lags and I designed the code to work with lead acid which has a large capacity for these things. LI might not like it.
I think the quick search, connecting the battery to short periods of different PWM widths and large output currents might be bringing the LI battery above absorb voltage, and so then the mppt sees this and shuts down. Maybe a minute later Bv is less than Absorb voltage and it restarts, using MPPT mode as the starting point. so more mppt scans, more over voltage for a moment and more shutdowns. this is just my first idea of what might be going on.
I could make a few new settings to look after LI batteries. LI battery owners can help here.
maybe a "is it LI", if so, then no mppt scans once Bv gets close to absorb and we choose how close is close.
Maybe also if it's LI, then the mppt scan will go really slowly, but only every 10 minutes, when Bv is close to absob.
I am open to the idea of making the code work well with LI.wronger than a phone book full of wrong phone numbers
Murphy's friend Guru Joined: 04/10/2019 Location: AustraliaPosts: 656
Posted: 08:28am 17 Dec 2024
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I have been using Poida's MPPT ever since he published it first about 3 1/2 years ago.
This is with my 25KWh Lithium battery bank. There were no problems with the MPPT once I had it set up to run reliably.
My settings are, as per the menu posted back then:
G - zero Iin sensor 123 ADC counts H - zero Iout sensor 119 ADC counts I - max battery Volts 57.5V J - Absorb voltage 56.0V L - maximum Current 50A M - night power 5W
O - NO Equalisation
S - Float voltage 55.5V
The remaining menu letter settings are left as Poida posted them in his message, dated 08 Sep 2020
I have never observed battery bank voltages higher than 56V, the MPPT changes to float once that is reached.
Revlac Guru Joined: 31/12/2016 Location: AustraliaPosts: 1037
Posted: 08:17am 18 Dec 2024
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Thanks Peter, At the moment its not a problem as it still gets the battery charged, will add a bit more info, this is a new 16.5Kwh battery and its very first charge since getting the terminals welded on, mostly these start at about 50% SOC or less, so it took a long time to reach 54v with the solar available, the charge controller was left set at 54v float and 56v absorb, I never reached 56v I intended to stop and pull the solar input Fuse before then as the batteries would be sitting, didn't need them full.
When charging the FLA batteries the scan (just after)would easily exceed the set BV voltage even go up to the 60v max for a moment then it would switch to absorb, I admit I haven't watched it close enough for perfect moment by moment description as it settles moments after this and runs perfectly so all ok, also the FLA batteries are small and have a lot of charge resistance.
The LFP battery has a very low resistance, and its not even close to hitting the absorb voltage when I started, I would need to hit it with lot more Amps to get that voltage up.
Seen as this is the LFP's first charge it might be a bit different with day to day running because normal use the SOC will likely be over 60% and start charging at 53v, will have to see later.
As yet I don't think the code needs changes, its probably some settings need tweaking for my particular setup, we all have different builds and batteries so there is bound to be a few differences and personal settings to play with.
I think it would be great to have a lot more of these controllers running in the Wilde with different battery types, and see how they go.
Will start a new thread for that later.
EDIT SOLVED. I think it might have hit the current limit that was set in the menu, did see a star * or ? mark show up while it was doing the MPPT sweep etc. So the difference is that the while running the FLA batteries in the shed it never got to the current limit that was set it always hit the voltage limit first and switched to absorb mode. When the same setup was used on the LFP batteries it hit the current limit first not the voltage, also take into account I connected it up when there was plenty of solar available instead of the steady sunrise that would have been normal practice.
so I expected to increase the current limit to 50Amp later anyway.
Hope this makes sense. Edited 2024-12-19 07:12 by RevlacCheers Aaron Off The Grid
flyingfishfinger Senior Member Joined: 12/09/2020 Location: United StatesPosts: 113
Posted: 07:30pm 20 Dec 2024
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This is one of my concerns. My inverter has a hard max of 64V in that I learned the hard way (see my thread on overvoltage protection options), so I don't want to get anywhere near that even "accidentally"...
R
Revlac Guru Joined: 31/12/2016 Location: AustraliaPosts: 1037
Posted: 10:47pm 21 Dec 2024
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Ok did a little experiment today, this only happens when the batteries are full and I connected the charge controller and powered up on a good sunny day, most of the time this would not be the case for a house battery as it would be used overnight. Tried draining the FLA shed battery just a little and or add some more capacity to it (more batteries) connected the charge controller and run it........No more brief over voltage excursion, beauty, it really ramps down fast compared to my more expensive PCM60X charge controllers that usually go to fault mode. Shouldn't be a issue for LFP, as they aren't at 100% SOC anyway. Hope this clears things up. Cheers Aaron Off The Grid
Notice. New forum software under development. It's going to miss a few functions and look a bit ugly for a while, but I'm working on it full time now as the old forum was too unstable. Couple days, all good. If you notice any issues, please contact me.
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