diy solar

diy solar

usability of lithium with lead acid charge profiles ?

WoodsieLord

New Member
Joined
Oct 4, 2019
Messages
145
Location
Buenos Aires, Argentina (230V monophasic, 50Hz)
expensive and custom built LiFePO4 batteries are SLOWLY becoming available here in Argentina. Before I get too carried on putting my sky mask grabbing the rolling pin and heading to the bank asking for donations...

I have a cheap MPP solar clone inverter that is not compatible with lithium. Available charge profiles are AGM, FLA or USER. The later one allows you to set
- BULK charge voltage
- FLOAT charge voltage
- low voltage disconnect
- Enable or disable Equalization (and it's respective parameters, if enabled)

I also have an EPEVER MPPT CC that features lithium (if I recall correctly) that I'm not currently using.

I've read the LiFePO4 charging profile sticky and the BULK charge is not mentioned ( Absorption is used ).
Should I forget about lithium with the cheap MPP clone? What would you do?
Thanks!
 
You'll have no problem with the MPP unit and LFP. The biggest issue with Lithium" is not LFP but NMC, NCA and such because their voltage ranges are not "typical" with regards to being close to FLA voltage ranges.
* ABSORB is short duration.
* Equalize is either set to OFF or to Absorb Voltage for as short a period as possible.

Here are the Settings I use with my Midnite Solar Classic 200 and similarly set for the Samlex EVO Inverter/Charger
All equipment MUST BE Voltage Corrected & Calibrated (VERY IMPORTANT) see link in my signature on how to do it.
Divide Values X2 for 12V. Multiply X2 for 48V.
Absorb: 28.2 for 15 minutes (3.525vpc) (some call this boost)
Equalize: OFF
Float 27.9V (3.4875vpc)
MIn Volts: 22.0 (2.750vpc)
Max Volts: 28.7 (3.5875vpc)
Rebulk Voltage: 27.7 (3.4625vpc)
End Amps: 14A (*1)

(*1): End Amps is calculated from the Highest AH Battery Pack in a Bank. IE: 200AH X 0.05 = 10A 280AH X 0.05 = 14A.
NB: Victron Forum discussion says EndAmps = TailCurrent
This get's the bank charged to full with high amps (Constant Current) and then float (Constant Voltage) tops off so the cells are on average between 3.475-3.500. I am running 7/24/365 so float is used up by the Inverter + provides whatever the packs will take to top off.

** Coulumbic Efficiency for LFP is 99%
 
The User profile on your cheap controller will allow you to set those parameters to be compatible with LiFePo4. The biggest issue is likely temperature cut-offs, I don't recall whether the Epever's have those. Keeping the batteries in a continuously temperature controlled living space is the easiest of many solutions.
 
I finally did it.
Bought a pylontech UP2500.
Seems working fine. But I have some questions.

pylontech.jpg



SHORT story: Battery seems to work OK. There are lot of different approaches to BULK and floating voltage limits and I'm afraid to set them to somthing that might reduce the lifespan. First day I set 28.7V bulk and charged 65Ah reaching 89% before going to float. Second day I lowered the bulk voltage to 28.2 as suggested in the battery manual, out of fear, and this time the Bulk phase only charged 31Ah reaching only 66% before switching to Float. No absorption phase detected in any case. Charger goes stright from bulk to float.




EXCRUCIATINGLY DETAILED version (the devil is in the details, they say?).
It was a great investment for me so I wanted to configure it accordingly to make it last as many years as possible.
Searching for more info I kind of discovered that this subject might be a can of worms!.

DAY0:
Battery arrived barely charged (between 0% and 25 %, according to the simplistic LEDs on the front).

BEFORE switching banks (still using lead):
Disabled PV input, and load. Set the inverter to "utility first" (so the battery just charges while loads use the grid, Limit set to 2A max utility charging) and used the next parameters:

- Program 05: Battery type -> User defined
- Program 12: Back to utility -> 22.5V
- Program 13: Back to battery -> 26.5V
- Program 26: Bulk Voltage -> 28.7V (*1)
- Program 27: Float Voltage -> 27.2V
- Program 29: Low DC cut-off -> 23V (*2)
- Program 30: Equalization -> OFF (*3)
*don't remember which program is it but I limit the charging current to 2A when on utility and 60A from solar.

My criteria for those values was based on a compromise between will's Sticky thread (29V bulk, 27.2 float) and the battery manufacturer info:
1639762818956.png

(*1) Will's suggests 29V (sticky thread) while the battery manufacturer states "28.2 ~ 28.5" as "charging voltage". I chose something slightly above the specifications while staying below Will's value.
(*2) I wanted to set 23.2V as stated in the manual but I kind of forgot the decimal places.
(*3) I previously enabled it and set every value to the safest values possible. Then I disabled it.

After this, I switched everything off, removed the lead bank, connected the LiFePO4 battery and turned everything ON.
I let the battery charge: AC overnight ( max 2A ) and solar as soon as sun rises (max 60A).

DAY1 (yesterday):
As the Sun was rising, the "Bulk charge" started growing until it reached it's maximum at 7:59 AM: 24.15A while at 27,61V
At this point, it switched to Float and dropped to 1.5A at 26.9V. Voltage was kept stable (I was expecting that) but current also remained fairly constant until 17:32hs. At this point The LED indicators for the battery were already marking 100%. Set my monitoring solution to 100Ah capacity and SoC to 100%...
...finally I decided to change "utility first" to "SBU" (Solar>Battery>Utility, the ususal setting). Inverter switched to offgrid, everthing went OK!

A total of 76Ah were drained from the battery at around 6:35hs. Only 1 led was lit on the battery SoC indicator. This was the lowest charge, and from this point onwards the battery began to charge.

1639764575812.png

Bulk phase reached its peak at 10:03hs with 41.73A at 28.17V. Then, once again, went straight to Float at 26.9V ~4A. At this point the Battery had been charged 65Ah and was at 89% (Only if it REALLY was 100% charged the day before. Which I'm not sure but I think it was).
Voltage remained stable between 26.90V and 26.87V and the last hour until the sun went down the current was only 0.6A.

After reading lots of threads, that same afternoon I decided to change the BULK voltage to 28.2V to respect the manufacturer's value. This changed A LOT....



DAY2 (today):
Lowest SoC reached at 6:35Am. 31%. 69Ah were drained from the Bank. Then it started charging. BULK peak was reached around 9:30hs: 34A at 27,6V. Way early and quite lower peak values. As usual, system switched back to float and kept the same values I'm now familiarized with: 26.9V starting with 4A and decreasing with time.

1639766503374.png



I have a feeling that the previous BULK limit (28.7V) that I set arbitrarily between the manual's and the sticky thread's threshold recommendations was fine and that it should be safe. But as an argentinian idiom goes "Safe was detained". (that's a literal translation that more or less states that Someone called "Safe" was not safe at all and was arrested for something, in the end nothing is safe enough. I'm seriously questioning who the heck invented that. Anyway, I think the equivalent idiom would be "Better safe than sorry".)

Thanks for any input! I'm really unsure the effects that these settings might do in the long term.
 
Back
Top