MPPT or PWM charger? For LiFePO4 battery?

[William]

Hi, Everybody,

Is anybody here experienced with LiFePo4 battery bank in off grid solar system?
Can the LiFePO4 battery bank be working with the MPPT charger or PWM charger?

Thanks

William

[Mike90250]

( moved post to it's own thread )

The MPPT is always more efficient when you have a higher voltage PV array. Like a low loss DC-DC transformer to match high voltage array to low voltage batteries.

BUT that's not what LiFe batteries care so much about, as they need a very precise way to balance the charge to each cell in the battery, and not to overcharge. So being able to set precise voltage set points is the most critical thing.

[Sunking]

I doubt you will get any answers, because there are so few if any using LFP batteries for solar. It is not that it cannot be done, there is just no demand for it right now and thus no one is manufacturing charge controllers targeted at LFP batteries. I recall one manufacture that has a LFP setting, but I would not recommend using it because it is not accurate enough. Lithium batteries are very expensive and need Battery Management Systems (BMS) to charge each cell or group of cells to precise voltages. No one makes that for Solar yet and it is very expensive.

You can buy BMS charging systems but they are designed for the Electric Vehicle markets and require a very hard power source, and solar systems are not hard power sources and never will be.

So for now the only way is a custom design/build which very few are capable of, and those that can will not give any clues.

[Thomas Lai]

LiFePO4 battery charger for solar panel

Hi Williams,

I'm also looking for a charger solution or provider. Would you please share with me once you get any update? I'll do the same for sure.

Since I'm designing a standalone solar generator for disaster recovery, I am keen to integrate LiFePO4 into my system in order to replace the Lead acid type.

with kind regards
Thomas

[Mike90250]

....I am keen to integrate LiFePO4 into my system in order to replace the Lead acid type. ...

The easiest way for you to do that, is to obtain the 12V automotive "drop in" replacement batterys that have the BMS built into their case. They are quite pricey !

[john p]

The 12 v "drop in" replacement lithium ion phosphate batteries are available in 12.,24.,48. and 144v

If you arenot wanting to spend much money you can use a PWM charger with an adjustable end of charge voltage.14.8v that also allows you to turn off the "equalize " setting.. IE the old Steca ones and their new chinese copies.and if you are using the smaller cylinder 10a cells you can make a simple BMS using a zenner 3.9v and resistor across each cell to balance against overcharge and a low voltage cut off switch to prevent over discharge.. set point about 10v..

This does work ok as I have done 3 systems like this and they have now worked perfectly for over 3 years .

[T1 Terry]

PWM is best for LiFeP04

Hi, new here, found this forum while looking up PWM v MPPT. I have a 360ah Winston Battery prismatic LiFeP04 battery pack I charge with a PWM style Plasmatronic PL20 with an expansion relay modification. There are 12 x 80w poly Solarex panels, 6 X 50w mono "Rich Solar" from evil bay and a 120W mono no name panel also from evil bay. 1380w in total, not angle optimised at present but still puts out 65 plus amps at mid day.
The PWM type controller is better suited to Li batteries if the sun is good and the ambient temp short sleeve or better during the sun periods. I've tested heaps and they just don't live up to the makers claims when it comes to Li batteries. I think the reason for this is the Li batteries in a 12v nom. configuration hold 13v or higher till deeply discharged, the difference between the MPP of a 12v nom panel in full sun with a temp of 40degC or more isn't enough to be of any value. The claim of better output in poor light or rain conditions has never happened either, the PL20 starts putting amps into the battery long before any of the MPPT unit do. I really wish their 30% better output claim was true but unfortunately it must only happen in a lab with cold panels into a load cell held at 12v or lower, it doesn't happen in the real world with Li batteries that's for sure.

T1 Terry

[john p]

TI TERRY What you have said is mostly what ive been trying to tell people or ages .MPPT in many cases put less than 5%extra into batteries compared to good PWM ones and in all cases ive found adjustable voltage PWM ones work just fine with lithium phosphate batteries. The same cant be said for MPPT chargers and they cost at least double of PWM ones (in Australia)and come with shorter warranties..

[T1 Terry]

Hi John,
I found the Plasmatronic PL series really good for any sort of battery charging. I'm into converting a bus into a mobile home and I've been experimenting with running household 240vac appliances via an inverter for around 5 yrs. I discovered the Li batteries by accident while browsing electric vehicle stuff, found the AEVA forum and later the DIY EV forum and that lead to an interest in Li batteries.
I have been researching these batteries and cells for some time before lashing out and buying them, $10,000 takes a while to save
From that I have set up a system for a mate who lives full time on the road, it's a 180ah 24v system and it will be my mobile test bed. We have 720w of the evil bay 120w no name panels fitted to the roof.

From my testing so far I've found that 13.8v to 14v is a good flat constant voltage for charging a 12v nom. system as the cells are fully charged by 3.4v or 13.6v in a 12v nom, battery pack.
Not trying to pick a fight on my second post but, your previous post recommending 14.8v made me wince a bit, even though 3.7v per cell is within their tolerance range it doesn't leave much room for safe if a cell goes into runaway before the other cells are fully charged.
Do you have a (shudder) BMS?

T1 Terry

[john p]

Terry if you had read all my post you would have seen I put on each cell a 3.9v zenner and a 10 ohm resistor to make sure no cell goes above that voltage and use a low voltage cut off switch set to 10v..This is more important as the cell voltage drops very quick below 2.8v and if a cell drops to below 2.2v it will die a sad death.Im only using 10ahr cells so 1w zenners are more than adequateas voltage levellers.. Make sure your charger cant do an :equalizing" charge.

I have found when you ask the people that are always saying how wonderful MPPT chargers are ,if they have done a direct comparison between a MPPT and a PWM on their panels and batteries and what were the differences,The answer is NO but they "know" they are better.

[T1 Terry]

The answer is NO but they "know" they are better

Even the electrical engineers have problems accepting that what happen in a lab under unrealistic conditions won't duplicate in the real world. The theory is correct, if you leave out the fact that the MPP drops as the panel voltage increases, if you use 12v or less as the acceptance load, iff you ignore the fact that there is no free lunch, all these electronics are going to convert some of the energy to heat because nothing is 100% efficient..... if you ignore all those things the it will work as described. When you try to tell them that panel in full sun on a 30degC plus day will be over 50 degC and the MPP is now around 14v they refuse to believe you, they refuse to test it just incase you might be correct and are much happier to live in their world of denial, they are engineers, how could they be wrong.
Eventually you just shake your head and walk away.

T1 Terry

[john p]

Terry at work we have tested almost every MPPT and PWM and I can tell you . in real life the differences are not great.. in many cases no improvement at all. just a lot of extra expense. one of the good PWM chargers is the Steca copy sold by Jaycar for about $130 its available in 12 or 24v and 20 and 30a .they are what I use for lithium phosphate cells.

[steveg]

The easiest way for you to do that, is to obtain the 12V automotive "drop in" replacement batterys that have the BMS built into their case. They are quite pricey !

Hi,

Have been using LiFePO4 for 1.5yrs now - using TS-MPPT-60 on custom mode 60A current limit. I have a 300Ah pack at nominal 48V. Empty voltage 44V full charge 56.4 (I dont charge to 100%)

My Blog is here : offgrignz.livejournal.co.nz

More folks should consider it - I use no gas and cook with electricity - currently in summer we use 20kWh per day on average - we have a 4kW array

I got my pack wtih BMS from Seiden in China for US$7k - have under and over charge protection and balancing.

Regards,

Steve.

[john p]

Steveg it hard to believe you can use 20kwh a day with a 4kw solar array??
If the 4kw array is the rated figure its never going to produce 4kw in just about any real life scenario.and even if it did there are going to be losses ,and you are never going to have a totally cloudless day.Will not anywhere I know on this planet of that is a livable area.

My calculated figure would be about 13kwh per day in summer. but then you have charger losses and battery losses and inverter losses..

[russ]

PV Watts shows data for three locations in NZ - they vary between 4500 and 5000 kWh/year projected average production - a far cry from 20 kWh/day.

With annual average insolation figures of 4.2 to 4.6 high production would be difficult.