Constant 14.4V not OK for charging LiFePo4 batts with internal BMS?

[KS_Kampers]

This is for shore power charging two Li Time 12V 100AH batteries wired in parallel in my RV.

I'm trying to determine if I should upgrade my WFCO 9855 charger to a lithium compatible unit. My WFCO initially charges at 13.6V. I don't know if it folds back after bulk charging is done.

I'm considering purchasing a fixed voltage capable charger such as the Powermax PM3 55LK. I note in a thread on this forum that there was some comments indicating set-and-forget charging at 14.4V is not advised.

I assumed that in order to reach full SOC, that 14.4V is required. I also assumed that trusting the BMS to do it's job was ok... attach 14.4V forever and be on my merry way. I'm at a little bit of a loss understanding why this is not advisable. If the BMS doesn't stop charging when complete at 14.4V, how is using 13.9V better for the system? Again.. shouldnt the BMS hand this?

Can someone provide guidance for me?

Thanks!

 

[justinm001]

This is something I've never understood either, but I believe it has something to do with equalizing the cells. Hopefully someone more knowledgable will chime in.

BTW I have a couple chargers like that laying around, one might be that new in box and another is 100a and I thought it's 3 stage with LFP profile. It's been on my to do list to swap the 100a for the lower Amp I use for my generator sla charger

 

[sunshine_eggo]

This is for shore power charging two Li Time 12V 100AH batteries wired in parallel in my RV.

I'm trying to determine if I should upgrade my WFCO 9855 charger to a lithium compatible unit. My WFCO initially charges at 13.6V. I don't know if it folds back after bulk charging is done.

See link below about 13.6V...

I'm considering purchasing a fixed voltage capable charger such as the Powermax PM3 55LK. I note in a thread on this forum that there was some comments indicating set-and-forget charging at 14.4V is not advised.

I have multiple PM converters. They can operate as constant voltage or 4 phase charging (switch selectable)... 1) bulk, 2) 14.6V absorption for 15-30 minutes (and this is POT adjustable on the LK units), 3) 13.6V absorption for 12 hours and 4) 13.2V float

I assumed that in order to reach full SOC, that 14.4V is required.

Nope.

LFP full charge @ 3.4V/cell

Thought I'd try a little something on a cheap 12V I've been testing... Charge to 20A @ 13.6V (3.4V/cell), 2A tail current. When testing at cell level and dealing with testers that don't have separate sense leads, lead resistance plays a big role, and I would typically observe cells get charged...

diysolarforum.com

I also assumed that trusting the BMS to do it's job was ok... attach 14.4V forever and be on my merry way. I'm at a little bit of a loss understanding why this is not advisable. If the BMS doesn't stop charging when complete at 14.4V, how is using 13.9V better for the system? Again.. shouldnt the BMS hand this?

Nope. You never rely on the BMS for routine protection. You have your hardware work inside the limits.

The bigger issue in all of this is RV wiring is usually shit. You get a lot of voltage drop between the converter and battery.

IMHO, try the 13.6V and see how it flies. If you need generator charging, it's going to suck. If you typically charge off shore, it's cool.

 

[Robert Ibe]

I have a few of the LiTime 12v 100A TM batteries I use in my boat in a 2s2p configuration for running a mini fridge , microwave , coffer maker, etc

When I was checking their capacity before installing them the bms would shut down charging at 14v according to the bench power supply I was using to charge. I believe they were getting a full charge as they tested 100 to 104 Ah capacity.

ETA The bms was shutting the batteries down just below 10.7v during the capacity test.

 

[KS_Kampers]

Thanks... I'm rebuilding the wiring and the battery system. Relocating the batteries to behind a panel in the pass through. The batteries will be approximately 3' from the charger, connected through a bus bar and #4 pure copper wire I got from Windy Nation. There will be a MPPT solar charger tied in parallel for boondocking.

I thought that LiFePo4 batts did not like to be on a constant source maintenance charge? Doesn't this mean that we rely on the BMS to disconnect from power once charging is complete? If so, what difference does it make if it disconnects from 14.4 or 13.6? (I'll read those links when I get back home)

 

[sunshine_eggo]

Thanks... I'm rebuilding the wiring and the battery system. Relocating the batteries to behind a panel in the pass through. The batteries will be approximately 3' from the charger, connected through a bus bar and #4 pure copper wire I got from Windy Nation. There will be a MPPT solar charger tied in parallel for boondocking.

Like a lot.

I thought that LiFePo4 batts did not like to be on a constant source maintenance charge?

Depends on voltage. Lots of confusion about LFP not needing float. True only if you're putting them on a shelf and not using them. If you're using them in a power system and you want to keep them topped off while solar is available, you need a float.

Doesn't this mean that we rely on the BMS to disconnect from power once charging is complete?

Never. We rely on the BMS to protect the cells when something goes wrong.

If so, what difference does it make if it disconnects from 14.4 or 13.6? (I'll read those links when I get back home)

It only disconnects when cell voltages go out of limits. At either voltage you reference, all cells can be within limits, so the BMS won't necessarily disconnect.

 

[KS_Kampers]

IMHO, try the 13.6V and see how it flies. If you need generator charging, it's going to suck. If you typically charge off shore, it's cool.

Hmmm. Ok. I've been scouring the web attempting to self educate on BMS and LiFePo4 batteries for a day now. I'm a little more up the learning curve. I really don't care about fastest rate of charge, as long as I can reasonably "recharge" (90%?) two 100AH batteries off shore power within 12 hours.

My existing WFCO 55A charger charges in 3 states. Bulk at 14.4v bulk, 13.6v absorption and 13.2v float.

My desire is two fold. Primarily to be safe. Somewhat distant second, to not ruin the battery while getting an "optimal" (not necessarily perfect) charge.

Two questions for you:

I'm interpreting your above statement to mean that a 13.6V constant voltage supply will not damage the battery once it achieves whatever SOC maxes out at that constant charge voltage?

If I leave my existing charger in place, and it goes through the absorption phase then dials back to 13.2V, does the BMS disconnect to prevent discharge?

Thanks.

 

[sunshine_eggo]

Hmmm. Ok. I've been scouring the web attempting to self educate on BMS and LiFePo4 batteries for a day now. I'm a little more up the learning curve. I really don't care about fastest rate of charge, as long as I can reasonably "recharge" (90%?) two 100AH batteries off shore power within 12 hours.

My existing WFCO 55A charger charges in 3 states. Bulk at 14.4v bulk, 13.6v absorption and 13.2v float.

Nice. Important to know how long the 13.6V absorption period lasts.

I'm interpreting your above statement to mean that a 13.6V constant voltage supply will not damage the battery once it achieves whatever SOC maxes out at that constant charge voltage?

If left that way all of the time for years, yes, it would degrade the battery slightly faster than decay due to calendar aging. Some manufacturers specify a float as high as 13.6V.

If I leave my existing charger in place, and it goes through the absorption phase then dials back to 13.2V, does the BMS disconnect to prevent discharge?

No. The BMS is a safety device. It only does something when:
temperature is too high/too low (not all include low temp charging protection)
cell voltage is too high/too low
charge/discharge current is too high.

The charger dropping from 13.6V to13.2V is a non-event. The charger doesn't allow a higher voltage battery to discharge into itself, so the only thing that happens is current stops flowing into the battery, the battery holds whatever its resting voltage is until the battery eventually drops to 13.2V, and starts accepting current from the charger.

 

[KS_Kampers]

Nice. Important to know how long the 13.6V absorption period lasts.


The charger dropping from 13.6V to13.2V is a non-event. The charger doesn't allow a higher voltage battery to discharge into itself, so the only thing that happens is current stops flowing into the battery, the battery holds whatever its resting voltage is until the battery eventually drops to 13.2V, and starts accepting current from the charger.

Side issue... I just talked to WFCO as the new 9855-AD converter I ordered off of Amazon turned out to be a fake Chinese copy. (Which is why I'm trying to figure out what my new options are). While I was on the phone with the customer service guy, I asked him about charging profiles.

He told me that my 9855 (non -AD) uses current measurement to determine which mode it is in, and when to transition to a different mode. This is slightly counter intuitive to me, but whatever. Thankfully, it isn't just a time based sort of profile. He did not say what the current level is when it determines that absorption is done.

As a test, I am currently charging one of my Li Time 100AH TM brand new, never used batts on my bench. After a 12 hour rest and prior to attaching the charger, I measured 13.19V. Attaching the charger showed that the charger is now charging at about 13.57V. I'm going to leave this on and check it once in awhile.

It would seem to me that if the charger switched over to 13.2 that the charger would then effectively become a load if the battery was charged to 13.6. Some corrective action must be going on in either the BMS or the charger to prevent that from happening.

Thanks again. I appreciate the attention.

 

[sunshine_eggo]

It would seem to me that if the charger switched over to 13.2 that the charger would then effectively become a load if the battery was charged to 13.6. Some corrective action must be going on in either the BMS or the charger to prevent that from happening.

You might have missed this:

The charger dropping from 13.6V to13.2V is a non-event. The charger doesn't allow a higher voltage battery to discharge into itself, so the only thing that happens is current stops flowing into the battery, the battery holds whatever its resting voltage is until the battery eventually drops to 13.2V, and starts accepting current from the charger.

In most installations, converters are ALWAYS connected, but they are not always energized. Imagine what would happen if the charger became a load to the battery - they could drain the battery to 0V.

Diodes are electronic components that allow DC current to only flow in one direction. The charger either has a diode or operates in a similar fashion that only allows current to flow OUT of the charger.

 

[pollenface]

In my early DIY solar days, I killed two 50ah lifepo4 batteries by holding them at 14.6v all day everyday for 18 months with a single-stage PWM labled as MPPT that I got for $13. They lost capacity until they could not run the standby power of the inverter overnight.

 

[sunshine_eggo]

In my early DIY solar days, I killed two 50ah lifepo4 batteries by holding them at 14.6v all day everyday for 18 months with a single-stage PWM labled as MPPT that I got for $13. They lost capacity until they could not run the standby power of the inverter overnight.

It doesn't help that some companies actually specify a high float voltage as well. Big Battery did this for a long time. Hopefully, they've stopped.

 

[wopachop]

He told me that my 9855 (non -AD) uses current measurement to determine which mode it is in, and when to transition to a different mode. This is slightly counter intuitive to me, but whatever. Thankfully, it isn't just a time based sort of profile. He did not say what the current level is when it determines that absorption is done.

I think it might be time based. When you first turn the non-Ad model on it attempts 14.4v constant voltage charge. If the amps exceed the maximum rating it drops down to 13.6v and holds that for something like 12 or 20 hours. That could be what happened on your bench test. It happens to me all the time using 200ah of lead acid batteries. If they are low and i turn on the converter it will try to do a 14.4v charge in the first couple seconds. The amp draw will go over the 55a or so rating and cause it to back down to 13.6v. When i get lucky and it does hold a 14.4v charge a clamp meter shows around 43a going into the battery. The manual says it holds that for 4 hours. Im surprised the guy on the phone didnt mentioned the timed part. I cant see a label on my converter but its in a 2017 trailer. I know its not the ad model.

 

[G8trwood]

I had a lot of problems with my AD charger. It would switch modes and was a pain. Got a new PD and set it up.

Just got bigger batteries and was wondering about the voltages. The PD tapers off current nicely in CV at 13.6. With no charging my batteries taper quickly to 3.35 per cell. Does charging at 14.4 to a pack voltage of 13.8-14 worth it?

Tanks

 

[sunshine_eggo]

You saw my 13.6V thread. Basically, it comes down to how fast you need to charge. If you charge with genny, yeah... you want to push to 14.4. If you're just casually charging from grid/shore, then 13.6V is fine.

 

[time2roll]

It would seem to me that if the charger switched over to 13.2 that the charger would then effectively become a load if the battery was charged to 13.6. Some corrective action must be going on in either the BMS or the charger to prevent that from happening.

Power goes one direction from the converter to the battery. There is no load from the converter that has no input power. The battery just sits. Although Most RVs have a .5 to 2 amp parasitic load even when all accessories are off.

If the converter drops to 13.2 volts during little to no use the 13.2 volts is an excellent long term holding (storage) voltage for lithium.

 

[time2roll]

My existing WFCO 55A charger charges in 3 states. Bulk at 14.4v bulk, 13.6v absorption and 13.2v float.

Most are lucky to see 14.4 volts Boost Mode. Have you actually seen the 14.4 on a meter? Boost Mode is limited to four hours according to the WFCO manual and will not harm the lithium battery.

 

[time2roll]

He told me that my 9855 (non -AD) uses current measurement to determine which mode it is in, and when to transition to a different mode. This is slightly counter intuitive to me, but whatever. Thankfully, it isn't just a time based sort of profile. He did not say what the current level is when it determines that absorption is done.

Not really that sophisticated. The current is measured by voltage sag when near the converter amp rating. If enough power is demanded by a low battery the voltage will sag. When voltage at the WFCO converter (not the battery) sages below about 13.2 volts it will switch to Boost Mode. I have done extensive testing and this is the effect of how they work. Most batteries will not pull the voltage down far enough to trigger boost mode. Even lithium may need to be drawn down pretty low to see Boost Mode. I could hit mine with the inverter powering the microwave for just a few seconds and WFCO goes to boost every time.

 

[wopachop]

Not really that sophisticated. The current is measured by voltage sag when near the converter amp rating. If enough power is demanded by a low battery the voltage will sag. When voltage at the WFCO converter (not the battery) sages below about 13.2 volts it will switch to Boost Mode. I have done extensive testing and this is the effect of how they work. Most batteries will not pull the voltage down far enough to trigger boost mode. Even lithium may need to be drawn down pretty low to see Boost Mode. I could hit mine with the inverter powering the microwave for just a few seconds and WFCO goes to boost every time.

What a great post. It might have been your 5555th!!
When you do the microwave trick, has the converter been powered up for several hours and been sitting in 13.6v mode?

 

[time2roll]

When you do the microwave trick, has the converter been powered up for several hours and been sitting in 13.6v mode?

OK this worked with my 4x GC2 lead batteries and the voltage would sag a bit more. Generally if I started the generator and voltage was looking like steady 13.6 I would hit it with the microwave about 10 seconds to get boost mode for a few hours of decent charging to 14.4 volts. Yes generally the battery voltage even at close to full charge would drop into the zone to trigger boost voltage.

Hard to get the lithium to have the same voltage drop and I have stopped carrying the old generator now with 1200 Ah of lithium. The best alternative I know would be the Progressive Dynamics converter with the Wizard controller (not the Li version) as this would give full control of the Boost, Normal and Storage voltages.