Yeah!  Solar off grid upgrades! - Page 1
Forum section: electronics / general

author: paulca
author_rank: Super Contributor
timestamp: February 26, 2023, 11:09:41 am
content: Finally took the plunge/risk and ordered some LiFePO4 cells from AliExpress. £145 - free shipping - for 4 105Ah cells.In the process of swapping the battery over from the old lead acid, I accidentally left the panel connected. When the system didn't work afterwards I assumed the worst. I was going to upgrade panel next and then the solar charge controller, but alas, the "Buy now" button on a £125 EPEver 40A (common neg) MPPT controller happened. Well, it was payday and I figured if I'm in 60% the way, I might as well buy the panel, the 10AWG wire for the battery side, crimps, crimper... and a 330W panel.50W w/ 100Ah Marine starter battery --> 330W w/ 105Ah Lithium is quite an upgrade.The panel is arriving Wednesday. In the meantime I'm trying to get used to the new voltages and what they indicate. Seems anything from 13.0V to 13.8V at rest is good. 14.6v MAX charge. 12V is flat. Which is fine for my setup. The only thing I might need is a low end 'crowbar' or cut out. In the past I have found the charge controller itself, while switching off the load at 11V, will carry on consuming it's quescent current ~50mA until the voltage drops below 9V!Are LiFePO4 cells not less damaged by being flattened?The panel might look a bit silly sitting on a table against a wall for now, but I'm not ordering the full rail mounts for the garage roof this month! That will have to wait. I might just end up paying some roofer £200 to fit the lot instead of messing around for days myself and falling off the roof.Question. The Lifepo4s came charged at 13.0V (around 30-40%). I put them straight into service, got 2 sunny days with 4 hours direct panel light and it went up to 13.2V. I ran them on a mains charger for most of yesterday evening at 10 amps and it's still only resting at 13.5V. Is it advised to give them a full 14.6V charge, once, since they are new.... although they may just be "new to me". I don't know how to verify they have been used before.

author: paulca
author_rank: Super Contributor
timestamp: February 26, 2023, 11:11:12 am
content: Oh... and as it's a 4S pack, balancing will need addressing soon enough.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 26, 2023, 12:34:02 pm
content: Quote from: paulca on February 26, 2023, 11:09:41 amLFP cells cannot take any overdischarge, just like other li-ion chemistries. Copper dissolution mechanism is the same.If they are over about 1.5V/cell, you are likely fine and can slowly (say, C/50) charge them to normal range. The closer they are to zero, the higher the chances of permanent failure, possibly a safety issue, too.But if you can bring cells up to say 3.0V with C/50 slow charge and they do not then self-discharge, they are likely fine. Discard any that fail to rise in voltage with very small rates, or start to self-discharge.

author: paulca
author_rank: Super Contributor
timestamp: February 26, 2023, 10:04:51 pm
content: These arrived at 3.25V so fine.I suppose it might be a good idea to give them a survey/test charge.Maybe 13.8V at 10A and then from there, with a bit more supervision up to 14.6V, with rest periods while I check I am getting a reasonable top balance.... no cells are lagging behind or close to overcharging. etc. etc. Then leave it idle for 12 hours and check none of the cells have fallen.

author: paulca
author_rank: Super Contributor
timestamp: March 02, 2023, 05:09:55 pm
content: Update. After manually top balancing the cells and letting the panel work with it for a few days, it would seem it's sorted itself out pretty much, at the top end anyway. The cells are all sitting within 7mV of each other under 5 amps of load.Under minimal load, < 1W, the pack settles around 13.3-13.4V, which I understand is fine, there is such little voltage variance from 99% to 10% it's pretty vague. Under 6 amps that comes down to 13.2V. I only tried 10A briefly as I have a 10A fuse and it was still 13.2V.Interesting behaviour noted with a nod at how conveniently it works out... The "Lead Acid mode" on the charge controller boosted the pack to 14.40V for 2 hours, then dropped to 13.60V float mode. The lithium pack said, "13.6V" piss off mate promptly ending it's own charge cycle. Eventually some hours later when the pack settled down it accepted another bit of current and the charge controller went back to boost. I think that's fine. If I can't disable "float" mode, I can at least make it work to my advantage, meaning the pack is not loaded and is fully charged it will just get disconnected from the panel effectively.The new panel arrived and I've installed it sitting upright against a wall, temporarily. I will need to address a way of securing it to the solid brick work before the next windy day. Ultimately, before long, it will get mounted on the garage roof. I just need to order all the bits and pay someone stupid or skilled enough to get up the roof with their own insurance.I have yet to see what it will do. It was cloudy today and by the time I got everything hooked up it was a bit dull. I think it managed 65W for a few seconds. I'm trying to take a 25% chunk out of the battery pack tonight to make room for daylight tomorrow to see what it will do.You know it's not going to sunny here now for weeks and weeks and it's all my fault?

author: paulca
author_rank: Super Contributor
timestamp: March 03, 2023, 05:54:45 pm
content: Well, day one, not impressed. Maybe I'm being over critical, it was a broken overcast morning, but literally as the sun came up over the house and onto the panel it became overcast. The panel caught a few seconds of direct sunlight, then the clouds got progressively thicker stratus for the remainder. Seems the weather is for the same unless Tuesday.I'm less impressed with the charge controller. To me it seems overly reluctant to engage into MPPT mode and prefers to just tie the panel to the battery.If the graph you can see it hourly reset the duty cycle to give an open circuit voltage on the panel and then the MPPT algorithm works it to find peak power. However in several of those tests it's found more power, substancially more power, but opted not to lock in on it, but revert to direct battery.Maybe it's more aware than I give it credit for and those extra 3 or 4W might just be lost by the conversion process and it's better off just direct connect for efficiency... but I'm not convinced of that either.Remotely... it could just be an aritfact of measurement or the output capacitance etc. Maybe it isn't producing more power, but by lifting the voltage it does drain the cap? Doesn't sound right.It did hit close to 100W and in fairness, compared to the 50W panel on a similar day the 330W is producing consistent single figure watts. The 50W panel was producing around 1-2W.

author: paulca
author_rank: Super Contributor
timestamp: March 10, 2023, 04:02:37 pm
content: So in full sunlight on a blue sky winters day with snow fall, it hit 302W.More over it is more than capable of producing 1 kWh per day+. I only have 1.2kWh storage.I tried to bring the battery down by running some network gear for 24+ hours and burnt a kWh out of it. Today, sunny day, battery full again.These are good problems to have.

author: hojnikb
author_rank: Contributor
timestamp: March 11, 2023, 02:39:38 pm
content: Which seller did you get your cells from? 13c$/Wh is a very good price for LFP, provided your cells are actually that capacity. Did you do capacity test at all?Does your charge controller support lithum or CC-CV charge mode? Ideally, you'd want that, even for LFPs.

author: paulca
author_rank: Super Contributor
timestamp: March 11, 2023, 03:40:50 pm
content: Quote from: hojnikb on March 11, 2023, 02:39:38 pmThey claim to be "Class A", but no I haven't capacity tested them. Well not properly. I wasn't really interested in hunting for OEM grade suppliers, such as on alibabu. Mine could even be second hard for all I know.QuoteFirst CC-CV is a myth. Literally. It's so miss understood it's often humorous. The CC part is just a limit, it doesn't have to be constant. It's often found in Lipo chargers and small batteries which can easily exceed the maximum recommended charge current. The CV portion doesn't have to be constant either, in fact it's usually nothing like constant, it's more of a limit again. With LiFePO4s you have so much headroom between 99% and 100% the voltage can go anywhere from 14.40 to 16V if you want.If you are charging a small Li-ion on a charger with enough current and voltage to murder the cell then ultimately you will have a current limiter response at your max change current, say 1 Amp. Whatever voltage the cell takes at 1Amp... up until you hit the max cell voltage, say 4.2V, when the voltage becomes the limiter.This, outside of more advanced chargers, is basically just a bench power supply set to 1 Amp and 4.2V. Which is exactly how I charge individual cells... although maybe 10 or 20 amps for a 105Ah cell.In reality with a large pack of LifePO4 cells, my 330W panel is not going to exceed 0.2C charge current anyway, even though the controller can supply 40A or 0.4C. Which is still fine, should I get 3 panels.... it might then start current limiting, but going up to 24V or 48V pack solves that pretty quick. Of course if the sun goes behind a few clouds the charge current changes. It's certainly not going to be CC.Similar the final charge voltage varies a lot, which I'm actually going to change the settings as I believe there is still an equalisation charge phase enabled which will allow the pack to get up to 15V for up to an hour. Its supposed to only do it once every N days, but I need to check... or disable it and shorten absortption phase down to just 1 hour, I think it's defaults to 4.

author: Siwastaja
author_rank: Super Contributor
timestamp: March 11, 2023, 04:28:23 pm
content: You can just skip the CV step: just stop charging when you hit maximum voltage, if that is easier for you to arrange. How much unused capacity remains depends on charging current. At very high currents, voltage drop I * ESR is higher so cells end up at lower voltage and SoC. Driving charge into cells requires voltage differential U = R*I. CV phase is merely to top the cells off without exceeding maximum voltage.But the LFP chemistry has an interesting characteristic not present on other li-ion chemistries, namely the largest acceptable voltage is higher than the equivalent 100% SoC Open-Circuit Voltage. A full cell is something like 3.50V open circuit IIRC, but can be charged by applying 3.65V. This means unless the charging current isverylarge, you can get them very close to 100% even without a CV phase.Something like this: a LCO/NCA/NMC cell charged at C/2 until 4.20V and then stopped will be around 85-90%, but a LFP cell is easily maybe 95%.You can always consider the voltage and current limits of li-ion cellsstatelesslimits; never exceed the limits and you'll be fine. Thinking about CC and CV as two distinct steps is not necessarily helpful. They just normally happen in this order.

author: hojnikb
author_rank: Contributor
timestamp: March 11, 2023, 04:39:06 pm
content: Quote from: paulca on March 11, 2023, 03:40:50 pmYou're right; obviously with solar, you're not gonna be CC in the traditional sense. But its still a good idea to have a limit in place not to overcoock the cells; obviously in your case your panels are small enough that them themselfs are the limit.I guess if you have a BMS, you're safe enough if your dumb Pb charger keeps float charging.

author: hojnikb
author_rank: Contributor
timestamp: March 11, 2023, 04:43:01 pm
content: Quote from: Siwastaja on March 11, 2023, 04:28:23 pmyephttps://lygte-info.dk/info/BatteryLiFePO4Charging%20UK.html

author: paulca
author_rank: Super Contributor
timestamp: March 11, 2023, 05:05:47 pm
content: Quote from: hojnikb on March 11, 2023, 04:39:06 pmFloat doesn't really matter. Mine does have a float mode. After a while at 14.40V it drops to 13.6V which is below the fully charged resting voltage and so no current flows unless the battery is under load for which it's still fine at 13.60V or 14.40V.The only slight irritation with this is it messes up my "Daylight" detection as when it switches from 14.40V to 13.60V there is a period with 0W produced. My automation system thinks it's night time.Note, this is not the case for li-ion. It's hard to match their voltages up to a pb charger. I believe Adam Welch on YT ended up with something weird like 7S being somewhat equivalent to a 24V lead acid. The min/max voltages need to be much tighter too.Anyway. The charge profile I set in the EPEver is not working exactly as intended, I need to lower the maximum charge voltage. It is set to 14.40V but there may be a small equalisation phase at up to 15V enabled still, or so it seems. I will remove that or set the equalisation phase voltage to 14.40V also. I am going to keep the float phase and keep it at 13.6V. The reason being the battery will always be under a small load and I could like "solar power path" through put during daylight. If I disable or lower the float voltage anymore it will effectively become a pack disconnect, solar input will drop to 0, panel will go OC and all the load will be coming off the battery until it gets pulled down to a lower float voltage... or switches back into boost. The float mode at 13.6V sits nicely at the corner point around 99% charge. If the battery is idle it will end charge, if it's under load it will take current equal to the load minus losses at 13.6V, effectively solar power through path.