Does my MPPT SCC and 120V charger choice make sense for my LiFeMnPO4 batteries?

[ASprinter]

Hi all,
I am designing a solar electric system for my small Sprinter van for camping and could use some help. I am interested in installing a 12V 200-amp/hr LiFeMnPO4 GBS battery that includes cell-level balance boards and a BMS based on the Elite Power Systems kit. Charging will mostly come from a single 305W LG solar panel (40V open circuit voltage, ~10 amps max). I have limited space inside so I am attempting to keep the size of the other components on the smaller side. I would like to use the Victron 75/15 MPPT solar charge controller, which has user-adjustable absorption and float, so I was thinking I could set the absorption to 13.9V and float to 13.2V (or something like that--I don't think the float time is adjustable to less than one hour, so maybe it should be less) to keep the battery from hitting 100% all the time and preserve battery life. The BMS applies a 0.5A load when cell voltage hits 3.55V to cell balance, so these settings would not allow a 100% charge and cell balancing to occur. Question 1, does this logic make sense and can I acheive what I want with the Victron 75/15?

Screenshot of slightly adjustable charge settings on Victron 75/15 solar charger controller:


Question 2 - I do want the ability to use 120V shore power on occasion, mainly to charge up the battery in my driveway prior to a trip, which also means turning on the refrigerator and letting it come down to temperature. I have been unable to find a compact, affordable charger that has user-adjustable settings, however Victron has a Blue Smart IP22 30-amp charger with a pre-programmed lithium setting (I know, I know...) that deletes the 6-stage AGM charge profile for a 2-stage 14.2V bulk and 13.35V float setting. Because shorepower will be infrequently used, I thought this might work okay and would probably allow my batteries to do the cell balance. Is there a better product or way to do this?

[Mike90250]

Better set EQ to = float voltage, and auto EQ to OFF, or in 10 days, you fry the cells.
You want to set ABSORB time to 1 minute. no more than 5 min. Float time, who cares float is forever.

[Sunking]

No need for the BMS. All you need is a charger be it Solar or AC powered that has one adjustable Float Charge setting of 13.5 to 13.8 volts. Do not make it more complicated than it is.

[ASprinter]

Thank you for the replies. The screenshot above is just one I stole from online. As far as I can tell the only adjustments you can make with the Victron 75/15 include the settings you can see in the screenshot. There is no mention of absorb or float time, just a voltage. Do I perhaps need a more advanced solar charge controller? I know the Morningstar TriStar 60 MPPT has some adjustability, but it's huge and four times the price. I'll buy it if that's what I need though.

[Mike90250]

Morningstar TriStar 60 MPPT has some adjustability

Using 2 of them and they are completely adjustable. Voltages, times, Hi, Lo... everything is adjustable They are also very solid, and the 60A version has a web server for remote monitoring with any browser (Windows, Mac, android)

[ASprinter]

Thank you, looks like the smaller MPPT 30 may offer the same adjustability. Can anyone comment on the adjustability of the Victron 75/15? I'm wondering if it's possible to set a 13.9V bulk and set float and absorb to 0 volts, effectively topping off the batteries and then stopping charging altogether.

[Mike90250]

Thank you, looks like the smaller MPPT 30 may offer the same adjustability. Can anyone comment on the adjustability of the Victron 75/15? I'm wondering if it's possible to set a 13.9V bulk and set float and absorb to 0 volts, effectively topping off the batteries and then stopping charging altogether.

No system operates the way you just posted. I belive what's generally done with LFP, is you decide what your 24/7 FLOAT voltage is. What your "Full Charge" voltage is, and you set BULK & ABSORB to your Full Charge voltage, and set Absorb to 1 Minute.,
Then after charging up the pack to your full charge voltage, it drops back to float after a minute. I doubt you can set a shorter increment, or that 1 minute will damage batteries

With LFP you don't want to charge to 100% as that shortens battery life. 90% is good enough

[ASprinter]

What your "Full Charge" voltage is, and you set BULK & ABSORB to your Full Charge voltage, and set Absorb to 1 Minute.,

With LFP you don't want to charge to 100% as that shortens battery life. 90% is good enough

I haven't found a charger to-date that lets you set the float or absorb TIME, just voltage. There will essentially be two chargers onboard--my primary method of battery charging will be via solar through an MPPT solar charge controller and the other will be a combination inverter/charger with a programmable bulk/absorb/float voltage (but not time) that will charge when connected to 120V shorepower. A separate 600W inverter connected to the alternator will feed 120V power to the inverter/charger when the engine is running. So are there any suggestions on a work-around when you can't set the absorb time? I am totally fine with stopping the charge all-together at 13.9V to preserve longterm battery life.

[Sunking]

So are there any suggestions on a work-around when you can't set the absorb time? I am totally fine with stopping the charge all-together at 13.9V to preserve longterm battery life.

13.9 is a little high, more like 13.6 to 13.8. There are several ways to use a standard MPPT Charger, but the number of manufactures are limited. Best way I know of is to use a Controller that allows you to set via software the Bulk, Absorb, and Float voltages independently. Example set all three stages to 13.6 volts. What that does is makes the Controller a Float Charger which is exactly what you want to use for LFP batteries. Morningstar and Midnite Solar are two I know that can do that.

Another work around is set Bulk/Absorb to 13.8 volts and Float to 13.6 volts. You just got to think outside the box and know what lithium batteries need. LFP batteries are made to be drop in replacements for lead acid.

[ASprinter]

13.9 is a little high, more like 13.6 to 13.8. There are several ways to use a standard MPPT Charger, but the number of manufactures are limited. Best way I know of is to use a Controller that allows you to set via software the Bulk, Absorb, and Float voltages independently. Example set all three stages to 13.6 volts. What that does is makes the Controller a Float Charger which is exactly what you want to use for LFP batteries. Morningstar and Midnite Solar are two I know that can do that.

Another work around is set Bulk/Absorb to 13.8 volts and Float to 13.6 volts. You just got to think outside the box and know what lithium batteries need. LFP batteries are made to be drop in replacements for lead acid.

That is very helpful, thank you. Let's say bulk/absorb is set to 13.8V and float to 13.6V. When the battery is charging and hits 13.8V the charging amperage is reduced (sometimes to 0?) to maintain 13.6V? What happens when you have a load, is the charge controller constantly recharging the top of the pack? If people think the $99 Victron charge controller's settings are too limited and I need to upgrade to a $478 Morningstar MPPT, please let me know as I am trying to make a decision on gear.

I am less concerned with the inverter/charger's limited settings as the solar will be doing the vast majority of the charging.

[Sunking]

That is very helpful, thank you. Let's say bulk/absorb is set to 13.8V and float to 13.6V. When the battery is charging and hits 13.8V the charging amperage is reduced (sometimes to 0?) to maintain 13.6V? What happens when you have a load, is the charge controller constantly recharging the top of the pack?

No it does not work like that. Not with Solar Charge Controllers because there are not enough daylight hours to fully complete an Absorb cycle.

If you were to buy an AC 3-Stage smart charger you program the charger telling it what AH capacity the battery is. Let's say you buy a 12 volt 40 Amp Charger to use on a 12 volt 300 AH battery. You tell the charger the battery is 300 AH and you want to terminate when charge current tapers down to 3% of C or 9 amps. You select Bulk/Absorb Voltage to 14.2 volts and Float to 13.6 volts.

You connect the battery and it starts charging at 30 amp limit of the charger. As the battery charges from say 12 volts and reaches 14.4 volts the charge current starts to taper off toward 0 amps. When it reaches 9 amps of charge current, the charger terminates the Bulk.Absorb phase and lowers the voltage to 13.6 volts holding the battery at 13.6 volts 100% fully charged. If a load turns on, the charger supplies the power, not the battery up to 30 amps. Anythin gover 30 amps and the battery has to supply it. Once th eload drops below 30 amps, the charger wil supply what was used for the battery.

OK Solar does not work that model worth a damn. The Bulk/Absorb stage can take several hours depending on how deeply the battery is discharged and what the charge rate is. Absorb phase, the point where charge current starts to taper off can take another 6 to 10 hours. There are not enough hours in a day for solar to do that if the battery is significantly discharged. A solar charger does not proved maximum charge current when it is daylight. It only provides maximum charge current for a few precious minutes around noon. So instead of using Current to switch to FLOAT, they use TIME. Piss poor way to do it. Fo rsolar you crank the voltage up as high is it will go for all three stages and pray your battery gets charged back up to before it gets dark. For lead acid batteries you use a hydrometer to tell you what the voltage should be

Lithium i snot Lead Acid and does not require to be fully charged. In fact they will last longer if only charged up to 90% SOC, twice as long. To fully charge a LFP battery you use the same 14.4 volts, and terminate the charge when the current tapers to 3% of C. That would be the same 9 amps as a Pb battery. Again a solar charger cannot do that. But what you can do is Float charge a lithium to 90% SOC of roughly 3.45 volts per cell. A 4S LFP would be 13.8 volts. Set the Absorb time to whatever you want, then switch to Float of 13.6 volts.The battery will set there and float until dark. In the meantime the solar will provide power saving the battery until after dark.

So with Solar Controllers there are a few ways to make it work. Read these Stickies and it may help a bit.

Bulk Absorb Float

Battery Tutorial

Killing your Batteries

[ASprinter]

Sunking, thank you for taking the time, I appreciate it. I read the links you provided and re-read this post several times and I think I understand. One of my concerns was the Victron solar charge controller not having an adjustable float time, but if I understand correctly, that feature is not needed on a solar charge controller due to the limited time available from sun energy. (In other words, the Victron's adjustable absorb and float voltages should be sufficient for charging my lithium batteries.)

It looks like the inverter/charger I would like to buy does not have an adjustable float current (%c), but considering that will not be my main source of charging it sounds like I can just set conservatively low voltage settings on it and be fine as well. Please correct me if I misunderstood!

[Sunking]

Sunking, thank you for taking the time, I appreciate it.

You are more than welcome and my pleasure

I read the links you provided and re-read this post several times and I think I understand. One of my concerns was the Victron solar charge controller not having an adjustable float time, but if I understand correctly, that feature is not needed on a solar charge controller due to the limited time available from sun energy. (In other words, the Victron's adjustable absorb and float voltages should be sufficient for charging my lithium batteries.)

It looks like the inverter/charger I would like to buy does not have an adjustable float current (%c), but considering that will not be my main source of charging it sounds like I can just set conservatively low voltage settings on it and be fine as well. Please correct me if I misunderstood!

You are starting to get it, just need some redirection.

Float does not, nor do you want to adjust the time. In fact you want the charger to float 24 hours per day. And that can be dome with many off the shelf controllers by setting Bulk = Absorb = Float = 13.6 volts. It turns the controller into a basic Constant Current - Constant Voltage power supply and battery charger.

All utilities, telecom, data centers, military, 911 cal centers and any mission critical system use only Float Charging. Float charging is the kindest and gentlest charging there is and provides the longest possible battery life. With me so far?

That does not work well with solar because Float charging can take up to 24 hours to recharge a Lead Acid Battery. If you go buy say a 3-stage Golf Cart Battery charger it works just like I have said. It goes through all three cycle sand ends with Float, but does not terminate the Float. So if you only use that cart once a year, the battery wil always be 100% charged up in top shape.

Emergency backup like telcos and all critical missiomn use float, but the batteries are only used when power goes out. Otherwise everything runs off the chargers. When power goes out, the batteries are already on-line and take over. When power is restored the chargers run the equipment and recharge.

That model does not work in solar using Lead Acid Batteries, or really any application that daily cycles the batteries. With solar there is just not enough time to Float Charge, so to speed things up you use 3-stage. Bu there is the catch. Lead acids start to deteriate as soon as they discharge even 1%. They must remain n at 100%, and immediately recharged when discharged.. That is why you float them so they hold 100%. Pb batteries are not great in Partial State of Charge or PSOC.

That is not the case with Lithium batteries. They work best in a PSOC meaning they do not need to be fully charged. .You can double their cycle life by limiting the charge to work between 90/10% SOC. Super easy to do with solar and simple FLOAT Charge. You set the charger to 13.6 to 13.8 volts. and life is easy. But th etrick is is properly balance the cells initially, orr else you can do significant damage because LFP batteries do not self equalize like lead acid and most other batteries. Thus they need a good initial Bulk Balance and checked from time to time to make sure they are in balance.

I have a racing golf cart and have helped may build golf carts using lithium batteries in the past two years. In those two years have never had to reblance. We just bottom Balamnce them and charge to 3.45 vpc and go about our biz worry free. Set your Inverter to cut off at 12 volts and it is impossible to over discharge the batteries. 12 volts is roughly 10% SOC left in them, 8 volts is death so you have a 4 volt cushion.

[LETitROLL]

Sunking, thank you for taking the time, I appreciate it. I read the links you provided and re-read this post several times and I think I understand. One of my concerns was the Victron solar charge controller not having an adjustable float time, but if I understand correctly, that feature is not needed on a solar charge controller due to the limited time available from sun energy. (In other words, the Victron's adjustable absorb and float voltages should be sufficient for charging my lithium batteries.)!

The Victron has more adjustment(s) then shown in your previous post, I use mine seasonally and just hooked it up to play with it the other day and victrons system updated the software version (to 1.9 i think) they are adding more adjustments and features all the time, but it has full control of all voltage stages (in .01v increments) and it does have some time control options as well, as SK has said though if you keep it simple and just set all voltages correctly the rest will take care of itself.

[ASprinter]

The Victron has more adjustment(s) then shown in your previous post, I use mine seasonally and just hooked it up to play with it the other day and victrons system updated the software version (to 1.9 i think) they are adding more adjustments and features all the time, but it has full control of all voltage stages (in .01v increments) and it does have some time control options as well, as SK has said though if you keep it simple and just set all voltages correctly the rest will take care of itself.

Great to know, thank you.

Thank you all; this has really helped clarify some misconceptions I had and you likely saved me a bunch of money and frustration!! I will check in again once I am putting the system together next month.