Charging LifePO4 batteries.

[SteveK]

Over the past two weeks at anchor I needed to charge/top up the house bank almost daily.
I re read the charging profile and came up with this questions.

If bulk charges until absorb voltage is reached 14.4v and then absorb for 800Ah bank requires 150 minutes or 2-1/2 hours does that mean I have to charge for nearly 4 hours to go thru the profile before it goes to float?

I have been charging for about two hours per day to gain enough SOC for the next 24 hours. Only ever 100% SOC and into float plugged in at the dock, or underway for 4-5 hours. Everything works, but leaves me confused as LFP is touted as not having to be fully charged.

 

[Delta Riverat]

How many amps is the charger giving at bulk?

 

[SteveK]

How many amps is the charger giving at bulk?

About 200A

 

[Delta Riverat]

Yeah, with an 800 AH bank empty to full should take about 4 hours.

 

[Mambo42]

You don't need to charge LiFePO4 to 100 %, in fact some say it is actually better to go to about 90 %, then discharge to about 50 % etc etc

Till what percentage do you drain your batteries ?

If you need to charge 2.5 hours at 200 amps you are basically putting in around 500 Amps. In reality it will be a little less since you are also running boat systems while charging. So my guess is you will be charging between 425 and 450 Amps in that time frame.
When the batteries reach 90 % you will should have enough again for a full day of operation. Just check if the batteries are balanced, that is the one important item.

 

[SteveK]

Yeah, with an 800 AH bank empty to full should take about 4 hours.

about correct, but I do not want to charge for 4 hours. I start charging on day 2 so always the bottom 1/2. That was LFP claim, no need to fully charge like FLA.

I am saying apparently to get full absorb time you need to charge until full.

 

[Delta Riverat]

You can up the bulk voltage to 14.6 to pack in the amps as fast as possible and then taper to float.

 

[READY]

If you are OK (enough capacity) with your current 2 hours charging starting day 2 then I see no problem. However if that means you run them to zero % (or close) then not the best for the battery. But there is no reason to charge them all the way to 100% during your cruise.

I wasn't clear on how many amp hours you normally need day? If it is around 400 amps then I would just recharge them back to about 70% and end of day it would probably be around 20% remaining. Then charge 2 hours to get you back to about 70%. At dock or for your 5 hour runs you will bring them back up to 100% and the BMS will balance.

 

[Delta Riverat]

I agree. You said bottom 50%, don't know if that's 10% or 30%...

Running LiFePO4 80% to 20% is no problem.

 

[Barking Sands]

Over the past two weeks at anchor I needed to charge/top up the house bank almost daily.
I re read the charging profile and came up with this questions.

If bulk charges until absorb voltage is reached 14.4v and then absorb for 800Ah bank requires 150 minutes or 2-1/2 hours does that mean I have to charge for nearly 4 hours to go thru the profile before it goes to float?

I have been charging for about two hours per day to gain enough SOC for the next 24 hours. Only ever 100% SOC and into float plugged in at the dock, or underway for 4-5 hours. Everything works, but leaves me confused as LFP is touted as not having to be fully charged.

What charger? What are the charge parameters set? What are you using to determine SOC? Are you able to set absorption time..or possibly tail current to exit absorption? How many amps are still going in at the end of your typical charge day if you hit the absorption voltage?
LFP does not have to be fully charged...every time. But it does need to spend some time in the "upper knee" where cell voltage are likely to split so that balancing can occur and be applied... on a regular basis. How regular depends on how balanced your particular batteries stay. Keep in mind looking at cell voltage delta at any voltage less than the "upper knee" voltages will not be helpful in determining balance. You need to reach probably a minimum of 13.7 or preferably 13.8 depending on your BMS balance parameters and in some case probably at least 14. 14.4 seems a tad high to me, although the additional voltage will help charge time, especially on solar. If you monitor your amps while charging along with voltages that are reached, you can determine quite a bit on what the likely charge level or capacity is likely to be. You can also determine if your absorption time is excessive.
What are you using to determine SOC? Victron shunt? Battery app SOC? SOC readings are subject to drift if you spend a bunch of time in a mid level SOC and cycling day after day. Most SOC measuring devices also require a trip to full full full to reset to 100% accurately. The parameters for detecting the 100% reset for Victron shunts can be set. For drop in battery apps and the like they are usually fixed and can get wildly out of whack. Even on my Epoch batts, floating for weeks can throw off SOC by 10% or more over time. It can take a bit of work to get the SOC to reset to 100% at the top of the charge...even though the batteries are in fact full charged by watching the voltage reach absorption and watching the amps dwindle to a very low level.

 

[tiltrider1]

I have been using 600 amps a day out of a 1040 amp bank. Charging takes about 6 hours to hit 100% SOC. What I have noticed is I get to 90% SOC at 100 amps per hour which is basically what my charger can do given house loads of 25 amps continuously. At 90% SOC the amps start to drop, not sure if this is the absorption cycle or exactly what. I just know that from 90% to 100% takes longer because amps in are dropping. Usually by the time I reach 96% I am only 30 amps from full so I shut the generator off.

 

[Barking Sands]

I am saying apparently to get full absorb time you need to charge until full.

Its really the opposite...charging to absorb voltage (provided its the correct voltage) and watching amps drop to a low level will inform you of a full charge more than a SOC meter.

 

[Delta Riverat]

Yeah. Lithium chargers only work on 2 levels, constant current and constant voltage. During constant current they output max amps until the bulk voltage is met. When that happens they keep that voltage until the amps go down enough and then set at float.

 

[luna]

Yeah. Lithium chargers only work on 2 levels, constant current and constant voltage. During constant current they output max amps until the bulk voltage is met. When that happens they keep that voltage until the amps go down enough and then set at float.

Ok, but now I’m a little confused.
Lithium (much like lead acid) chargers start to work on a depleted battery in Constant Current mode, ie running flat out.
Once the Absorption Voltage is reached the charger works to maintain that voltage by reducing the charge current.

So far we have seen 2 levels (?) and other than the actual voltages nothing is different than the lead programmed charger.
Now the charger ( based on what input, time, wind direction or?) changes to a 3rd level ( but we only had 2? ) called Float. What are the Current and or Voltage limits on this new 3rd level?

I thought after Absorb you turned the charger off.
Maybe I ‘m wrong.

 

[SteveK]

Its really the opposite...charging to absorb voltage (provided its the correct voltage) and watching amps drop to a low level will inform you of a full charge more than a SOC meter.

I know that. But it takes at least an hour to get to absorb 14.4v from around 12.8v before the 2.5 time starts. With only a 2 hour boost of the SOC I seldom get to Absorb.

 

[SteveK]

Yeah. Lithium chargers only work on 2 levels, constant current and constant voltage. During constant current they output max amps until the bulk voltage is met. When that happens they keep that voltage until the amps go down enough and then set at float.

Actuall bulk charge works the current voltage up to the 14.4v absorb start. Yes constant amps then constant volts

 

[SteveK]

Ok, but now I’m a little confused.
Lithium (much like lead acid) chargers start to work on a depleted battery in Constant Current mode, ie running flat out.
Once the Absorption Voltage is reached the charger works to maintain that voltage by reducing the charge current.

So far we have seen 2 levels (?) and other than the actual voltages nothing is different than the lead programmed charger.
Now the charger ( based on what input, time, wind direction or?) changes to a 3rd level ( but we only had 2? ) called Float. What are the Current and or Voltage limits on this new 3rd level?

I thought after Absorb you turned the charger off.
Maybe I ‘m wrong.

Float in my system maintains a constant 13.6v with no current unless say the fridge turns on and then it supplies needed amps to keep battery at full charge. This float I like.

 

[luna]

Yeah, Ok, but that is three levels right! Bulk, Absorb & Float. Just like lead acid but with different voltage set points. The Riverat guy said 2 levels, thus my confusion.


What source of power do you use to keep the batteries in Float when you are at the berth, underway and on the hook?

 

[Delta Riverat]

On my charger you only set 2 voltage levels, the high and the float. I can also set the max current on bulk, but that's it.

The charger will output the set bulk current until the high point is reached and then hold that voltage until some current level (that I can't set) is reached and then maintain the low level voltage set point.

Two voltage levels (2 break points) and two modes (constant current and constant voltage) how fast you can charge the battery is the area under the curve. If you bump the bulk voltage the battery will charge faster. Same with increasing the current.

 

[twistedtree]

What is the 150 min of absorb time that is required? Required by whom? At 14.4v, your batteries are full.

What are you using for an SOC meter? That might be the source of confusion.

 

[READY]

Without specifics on the charger (including how it is configured) and battery it is impossible to say what is really happening. The main points I think are that in general LFP batteries can charge at a fast rate up to 95% or more. Then they do slow down and to properly balance they are usually held around 14.4 volts for awhile (probably no more than 1 hour). At this point they don't need nor really should be held at full charge (float) for long periods of time. Use them and let the charger kick back in when around 80%.
I think we addressed the original "question" and informed the OP that cycling within the range of 10%-90% is fine for a few days or more. No need to fully charge every time. Then at dock or with extended solar or engine run time charge it for a more extended time 5-6 hours in his case.

As pointed out SOC can be very confusing or misleading if it is not properly set up or recalibrate.

Personally I think we all overstress on the charging of LFP batteries. In most cases I suspect people will get 10+ years if they just have a good charger and use the default LFP setting!

 

[SteveK]

Yeah, Ok, but that is three levels right! Bulk, Absorb & Float. Just like lead acid but with different voltage set points. The Riverat guy said 2 levels, thus my confusion.


What source of power do you use to keep the batteries in Float when you are at the berth, underway and on the hook?

I have Magnasine inverter/charger

 

[SteveK]

What is the 150 min of absorb time that is required? Required by whom? At 14.4v, your batteries are full.

What are you using for an SOC meter? That might be the source of confusion.

I have two meters that show SOC. One is off the shunt, the other direct from the BMS of the batteries. These batteries are interconnected with cat5 cable and one to the monitor.
At 7am I show 95% SOC and charge was on float 13.6v with no current (this is norm after reaching 100%). Current only shows when there is a demand so bats are kept at full charge.
The manual suggests absorb time for 800Ah to be 150 minutes. But that did not happen in the past 24. It reached 14.4 and went to float. During charge from depleted I can see 0.6v higher charge than battery voltage which shunt and BMS display.

It is that 150 minute suggested Absorb that never happens that has me wondering. Usually only with shore power do I get to 100% SOC most of the time. I did get there on a 4 hour run with GEN running.

 

[SteveK]

Each dot is approx. 15 minute post.

 

[Delta Riverat]

You can see the effect of the varying loads while charging. Looks pretty good, you reach 14.4 and settle at 13.2

 

[SteveK]

My charger is set to start bulk charge if shore power is lost and go through the bulk, absorb float cycle.
At 7:08am with 95% SOC I turned off power, then on.
At 8:10am it went to 100% SOC and shows 14.66v with 3 amp charge.
At 8:25am still 100% SOC with 14.72v and 1.3 amp charge.
At 8:40am 100% SOC 14.63v and 1.0 amp charge.

Absorb, or the lack of it until 100% SOC was the reason for the thread since when cruising it never or seldom gets absorb yet we are told LFP does not need to be fully charged each time. without a full charge I am not getting to the absorb stage. That is my confusion.

 

[twistedtree]

Without specifics on the charger (including how it is configured) and battery it is impossible to say what is really happening. The main points I think are that in general LFP batteries can charge at a fast rate up to 95% or more. Then they do slow down and to properly balance they are usually held around 14.4 volts for awhile (probably no more than 1 hour). At this point they don't need nor really should be held at full charge (float) for long periods of time. Use them and let the charger kick back in when around 80%.
I think we addressed the original "question" and informed the OP that cycling within the range of 10%-90% is fine for a few days or more. No need to fully charge every time. Then at dock or with extended solar or engine run time charge it for a more extended time 5-6 hours in his case.

As pointed out SOC can be very confusing or misleading if it is not properly set up or recalibrate.

Personally I think we all overstress on the charging of LFP batteries. In most cases I suspect people will get 10+ years if they just have a good charger and use the default LFP setting!

All good points. But man, I cringe over these batteries that tell you to charge to 3.60 or even 3.65 vpc, and even more so when they want you to hold them there for hours. It just seems to be asking for trouble with zero benefit.

As for balancing, it’s only one data point but I ran a home brewed LFP system for 4-5 years, and the cells needed balancing exactly once in that time, and they were cycled every single day in an off grid solar system.

 

[dhays]

I

Yeah, Ok, but that is three levels right! Bulk, Absorb & Float. Just like lead acid but with different voltage set points. The Riverat guy said 2 levels, thus my confusion.

I have been confused before by terminology. US Batteries in their charging recommendations for example doesn't count float as a charging stage. So my "three stage" charger, they would consider a two stage charger, Bulk and Absorb. They consider float to be an optional holding function.

US Batteries considers 3 stage charging to be Bulk, Absorb, and Finish. With then an optional float holding.

 

[twistedtree]

My charger is set to start bulk charge if shore power is lost and go through the bulk, absorb float cycle.
At 7:08am with 95% SOC I turned off power, then on.
At 8:10am it went to 100% SOC and shows 14.66v with 3 amp charge.
At 8:25am still 100% SOC with 14.72v and 1.3 amp charge.
At 8:40am 100% SOC 14.63v and 1.0 amp charge.

Absorb, or the lack of it until 100% SOC was the reason for the thread since when cruising it never or seldom gets absorb yet we are told LFP does not need to be fully charged each time. without a full charge I am not getting to the absorb stage. That is my confusion.

I think the system is working correctly. With LFP there is little to no absorb cycle. Once the bulk/absorb voltage is reached, you can switch to float pretty quickly. The only reason for any absorb time is if it’s required by the battery balancers.

I’m still confused if where you are seeing the recommendation for 150 min of absorb. That sounds like charging for lead, not LFP. Are you getting it from the magazine manual, and could it be talking about lead?

 

[twistedtree]

I


I have been confused before by terminology. US Batteries in their charging recommendations for example doesn't count float as a charging stage. So my "three stage" charger, they would consider a two stage charger, Bulk and Absorb. They consider float to be an optional holding function.

US Batteries considers 3 stage charging to be Bulk, Absorb, and Finish. With then an optional float holding.

Yes, it is confusing. Different vendors use slightly different terminology. Some have different voltage settings for bulk and absorb, where others only have one setting. Balmar has their own unique set of terms and parameters. Some have a “finish” stage. Some battery manufacturers only talk about what it takes to get battery back to full charge, and ignore what it talk to maintain them at full charge, I.e. float.