# Float Charging LFP Batteries.



## Sunking (Aug 10, 2009)

OK a bit of a different application for LFP batteries using solar to charge say a 48 volt 16S battery or 48 volt system. 

I know you should not Float LFP batteries at 100% or say 57.6 volts on a 48 volt system. But what about something less than 100%, say 80 to 90% of 52 to 54 volts. Basically a Solar Charge Controller can be set up as CC/CV setting Bulk = Absorb = Float = 52-54 volts. It will behave like any CC/CV charger assuming there is good sun light with at least 4 Sun Hours in a day which is about average. Once a Controller goes into float acts just like a well regulated DC power supply. 

So say by 1:00 pm the batteries reach 54 volts and current tapers to 0 amps. Any load demand after that assuming strong sun will come from the panels providing the panels have the capacity. At night, cloudy days power, or demand exceeds generation power comes from the batteries. Next day repeat. 

Anyone see an issue or problem operating a charging algorithm like that? Somewhere from 3.2 to 3.4 volts Float?


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## samborambo (Aug 27, 2008)

I've been trialling LFPs for the past 6 months at the company I work for to deploy in our next gen off-grid systems. The systems convert some of our more remote network customers to standalone. All of our systems are primarily solar with automatic diesel generator support. There's a lot of interest in this space recently because the numbers really stack up for LFP vs PbA for cyclic applications.

With our PbA systems we made a design decision early on to AC-couple generation instead of using DC charge controllers. We've based our systems around SMA Sunny Island gear. This makes the system more modular and flexible for "cookie cutter" deployments. Since exploring LFP, we've not found any DC charge control systems that are capable of working and scaling with LFP batteries.

Top end charge control is extremely important. We took this advice from another lines company in Australia who conducted similar trials with LFPs and cooked up their cells due to inadequate BMS/charge control. The charge controller needs to precisely feed small currents when balancing and must respond quickly (<5s) to voltage constraints. Secondly, SoC estimation is difficult if the battery doesn't visit the top of charge regularly. Thirdly, the environment the battery will live in should be thermally managed - for hot and cold. LFPs should be kept well inside 5-45deg.C. The BMS also needs to compute accurate temperature and load compensation to determine the true cell SOC.

The Sinopoly cells we're trialling haven't disappointed us so far. Less than 1% cell capacity imbalance over a month without top balancing. The balancing isn't really the important consideration. Temporarily losing a few percent of capacity isn't a big deal. What's important is reacting timely and appropriately to a cell voltage excursion.

The new SMA Sunny Islands (6.0H, etc) have the ability to talk to an external BMS via SMA's proprietary CAN messaging, available under NDA. The only off-the-shelf BMS we've found that supports this is REC BMS 9R (SMA version). SMA require that the BMS has isolation control of the battery so that the BMS can take unilateral action in the event of a cell behaving badly and/or the SI not responding on the CAN bus. SMA were quite cautious in their design and had to consider the more dangerous li-ion chemistries as well.

If you decide to go down the route of using SMA gear and adapting a BMS to talk to their gear, be prepared for months of testing and bug-fixing. There are several nuances about getting the BMS and the SI to play well together - the CAN messaging function is comparatively trivial. Our first LFP BMS was an adaptation that was still incomplete when it was replaced with a REC BMS - those units are worth their price.

Hope this helps.

Sam.


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## Sunking (Aug 10, 2009)

samborambo said:


> Since exploring LFP, we've not found any DC charge control systems that are capable of working and scaling with LFP batteries.


You are correct, there are no Charge Controllers made to work specifically with LFP batteries. Well there is one made by Genasun but is too small to be of any real use. Even if the Genasun were large enough it is nothing more than a Float charger and no way to scale currents.

However not having a Charge Controller made for LFP is not really a problem. You just need to use a Controller that allows you to set Bulk, Absorb and Float to any voltage you want and set it to the same voltage. Example 54 volts on a 48 volt system is below 100% and takes no BMS to operate. Just an occasional re-balance at the Top or Bottom. 

In my experience I have found one does not need a BMS, just a good cell monitor to keep a look at things. Back to my real question

*Can LFP batteries be Floated without consequences providing it is less than 100% SOC, say 80%
*


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## dougingraham (Jul 26, 2011)

The A123 datasheet for their 26650 size cell specifically mentions a float voltage of 3.45 volts per cell.

I have been floating my 4S4P pack of these for 3 years now and it is doing great! This pack is my buffer battery. I ended up floating mine at 13.6 volts (3.4 volts per cell.)

Of course under normal circumstances this pack simply stays fully charged all the time so essentially it has been cycled only a handful of times over the last 3 three years. A couple of those cycles were to check capacity. Last summer I checked it and I still had a little over 8 AH. It started out at 8.1 AH so I have lost a little capacity. But no cell drift. This pack was top balanced once when I first assembled it and I balanced to a 12 hour rest with the variations in the cell voltage less than 0.001. This has not changed.

The daily cycle for these cells is when the car is turned off there is about a 7ma load. When the car is on the DC-DC will provide up to 40 amps. I typically see an initial inrush of several amps which tapers off to nearly nothing in just a minute. I drive the car typically twice per day for about 15 minutes each session. This appears to be enough to top up the cells because 23.5 hours of discharge at 0.007 amps is only 0.165 ah of discharge. I have run for 30 minutes at night with the DC-DC off just to make sure it would handle it.

I don't see any reason why you couldn't do a CC-CV charge up to 3.4 volts per cell. With a Solar array feeding it the CC is probably limited by the panels anyway. While my utilization is not exactly the same as a daily solar setup the full voltage portion to float is similar.

Best Wishes!


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## glyndwr1998 (Apr 27, 2013)

When I done a lot of reading into lifepo4 and charging,, some folk say do not in any circumstance float charge, and others have said they have float charged without any problems.

I am embarking on a similar project, I will be using Nissan Leaf cells, 7 in series charged to 8.1v. Discharge to 7.6v.
What I was considering doing is using a 48v meanwell psu and set up the output to my max set point, let's say 56v close to its max setting, let the psu charge till any 1 cell hits max voltage, then cut the psu and stop the charge, I don't see any use in floating at 0 amps flowing so cut psu power.
I was then going to use a 6kva ups to take the energy out of the battery till min voltage is reached on battery then cut the ups.
Repeat cycle the following day when sun is out.

Those sunny islands are really good but god damn very expensive plus you still need the pv inverter too.

6 or even 10kva ups can be purchased quite cheaply, plus they are industry spec so should last, rather than cheap Chinese pv charger inverters ( I have read that the cheap Chinese hybrid inverter chargers do not last very long, and are close to $1000, that's who I am thinkin of using meanwell psu to put charge in as they are cheap and reliable, then use server grade ups to deliver power as they are reliable and can be got cheap from industry salvage experts.

I have the batteries from a Nissan Leaf, 2 meanwell 750 watt 48 v power supplies plus I already have a 6kva ups, once I get some time maybe in September I shall start to finalise the project and test it.

Anthony.


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## dougingraham (Jul 26, 2011)

Sunking said:


> *Can LFP batteries be Floated without consequences providing it is less than 100% SOC, say 80%
> *


In my experience there is no problem floating at 3.4 volts per cell. Ive been doing it for half an hour a day for the last three years. And before I did this I put a single cell on a lab supply at 3.45 volts for 6 months to see how it would behave. That was powered 24/7. No measurable loss of capacity.


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## glyndwr1998 (Apr 27, 2013)

That's great info Doug, especially as you've done a test too, excellent, thanks.




dougingraham said:


> In my experience there is no problem floating at 3.4 volts per cell. Ive been doing it for half an hour a day for the last three years. And before I did this I put a single cell on a lab supply at 3.45 volts for 6 months to see how it would behave. That was powered 24/7. No measurable loss of capacity.


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## Sunking (Aug 10, 2009)

dougingraham said:


> The A123 datasheet for their 26650 size cell specifically mentions a float voltage of 3.45 volts per cell.


I have seen that myself. I want to Float at 80 to 90% SOC, and that voltage varies slightly depending on manufacture. 



dougingraham said:


> This pack was top balanced once when I first assembled it and I balanced to a 12 hour rest with the variations in the cell voltage less than 0.001. This has not changed.


Want to make sure I understand you. Are you saying you initially TB the cells and have not since that time? The reason I want to clarify is because with my NEV Bottom Balanced and after about 8 months now no cell drift. So I am experiencing the same from the bottom. I do use a cell monitor and coulomb counter in the Orion Jr. I just do not use the Balance function. Back to solar application.


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## Sunking (Aug 10, 2009)

glyndwr1998 said:


> When I done a lot of reading into lifepo4 and charging,, some folk say do not in any circumstance float charge, and others have said they have float charged without any problems.


Just an educated guess but I think some of that conflict comes from the POV of assuming to Float at 100% SOC, vs say 80 to 90% SOC. Taking LFP to 100% is stressful enough and Floating them at 100% SOC comes with consequences of at least shorter cycle life, and increasing the risk of over heating. 

However I am asking at something less than 100% so as not to have to use a Balance Charge every cycle and only when needed. Now some would argue why not terminate the charge at say 80% SOC and call it good. Answer is it is solar application, and if there is still plenty of Sun to run my gizmos and lights I would rather use Sun Power rather than battery power until it gets dark or cloudy.


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## dougingraham (Jul 26, 2011)

Sunking said:


> I have seen that myself. I want to Float at 80 to 90% SOC, and that voltage varies slightly depending on manufacture.


80 to 90% is a large enough range that you should have no problem picking a voltage that will give it to you as a float. 




Sunking;663137Want to make sure I understand you. Are you saying you initially TB the cells and [U said:


> have not[/U] since that time?


That is correct. Three years sitting under the hood of a car. No environmental control except for being in the garage at night. So in the winter the temps seldom go below 0F inside the garage. And in the summer it is hot. I was seeing 115F in the garage last Saturday. And the Traction battery is treated the same. Only it is bottom balanced. I did redo the bottom balance once because I decided to add 18 more cells. But the 34 in the car didn't need it. It was just to match the new to the old.

Good luck with your solar project.


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## Karter2 (Nov 17, 2011)

Sunking said:


> I have seen that myself. I want to Float at 80 to 90% SOC,
> .... with my NEV Bottom Balanced and after about 8 months now no cell drift..


 What SOC are you checking drift/balance ?
Voltage variations don't show significantly unless the pack is near 100% or < 5% SOC .


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## dougingraham (Jul 26, 2011)

Karter2 said:


> What SOC are you checking drift/balance ?
> Voltage variations don't show significantly unless the pack is near 100% or < 5% SOC .


In my buffer battery case it is at the float voltage of 13.6/4 = 3.4 volts. Let it rest overnight and read out the voltages. If I leave it at the float voltage for several weeks the current drops to microamps and the cells are essentially at rest while on float. In the case of my traction pack I measure it at the bottom (some resting voltage less than 3 volts) since that is where I balanced them. You can't actually balance them while charging at any significant current because of the difference in diffusion delay. The voltage of the cells wanders around relative to each other. So trying to balance under this scenario is kind of like chasing a wagging tail. You get to a point and then never any closer. It is this effect that makes it look like the cells need balancing every day.

Near the middle of the charge (around 3.30 volts) a 24 hour rest voltage difference of 0.001 volts is more than a 1% difference in SOC. Near the ends 0.1 volts is barely significant.


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## Sunking (Aug 10, 2009)

Karter2 said:


> What SOC are you checking drift/balance?


Right on the knee of the cliff at 2.8 volts per cell the few times I have done it to calibrate coulomb counter. Otherwise I charge to around 3.4 vpc. I wait to recharge until I get down around 30%, and that can be a week apart. So I know I can do that with an EV, a small one. 

Back to solar charging. Solar is a real soft charge source with extremely limited window of opportunity. In a solar application C/10 to C/6 charge current PEAK. So with say a average 4 Sun Hours Day and a 10 amp charge current you have generated 40 AH. Batteries are sized to last 3 days. Point I am trying to make solar power is so limited with slow charging to FLOAT at 80 to 90% SOC pack voltage should work. Then say; once a month bump up the controller voltage high enough to activate Balance Boards to and Top Up batteries. Or Bottom if you choose...


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## kineticpolarity (Jul 6, 2015)

For solar charging a 48 volt lithium battery packs with a BMS I used a
48V DC to12V DC converter to power the BMS a normally closed relay should be used that would open when batteries are topped off. If a normally open relay is used the converter will not provide enough source current to close relay when battery pack voltage is low, .cutting off charge to battery pack. If BMS fails with a normally closed relay the solar charge controller of course will float batteries and the gentle charging from solar will not damage batteries. unless left unchecked for long period of time.
Regards KP


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## Sunking (Aug 10, 2009)

kineticpolarity said:


> a normally closed relay should be used that would open when batteries are topped off.


Two issues with that:

1. The most important issue is if your batteries are topped off by say noon and you disconnect the charger, you are on Battery Power, not solar. You want to save battery power when demand exceeds solar power, night, and cloudy days.

2. Related to #1 if you float at say 80% SOC, when the batteries top out at 80% charging stops, so there should be no stress on the batteries. Say 3.3 to 3.35 vpc and cannot go any higher. Panels and charger are still on-line to supply power to any loads so you do not have to use Battery Power when the sun is out.

So that is the real question: *Can you Float LFP at 80 to 90% with no stress on the battery?* I know you cannot float at 100% without stress, but the question is about 80 to 90%.


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## kineticpolarity (Jul 6, 2015)

Leaving the BMS on at nite does not draw that much current and is replenished in the morning when the sun returns unless many cloudy days. 
Finding a way to avoid constant manual control and monitoring lithium charging


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## skooler (Mar 26, 2011)

Experience tells me that float charging below the open circuit voltage (3.34v on the sinopolys)is fine, above is bad.


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## jonescg (Nov 3, 2010)

Seems to be the problem with LiFePO4 chemistry - most battery management systems use voltage as their only indicator of a cell's well being. When you have a cell chemistry that is 3.30 V when 10% charged, and 3.35 V when 95% charged, give or take 0.10 V in variation, you have a very troublesome battery to manage.

I suspect that a charge controller (battery charger, PV controller, whatever) which takes them up to 3.55 V per cell would be the best option. It's not up there for very long, and if it does exceed it, the BMS can trim it down to 3.55 if your household load doesn't do it first. But if it does sit at this voltage for 6 hours, it's not the end of the world.


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## cts_casemod (Aug 23, 2012)

3.4 is a safe bet, but make sure the BMS is set to cut at this voltage. Most commercial BMS's start to balance at 3.65 and stop charge when the first cell reaches 3.90, which creates unnecessary stresses on the cell.

For my car I initially floated all the cells to 3.5VDC for 48H, after which they have been charged at 3.40V (No balancing, charge stops at 3.40 and resumes at 3.35V per cell). My pack has about 500 cycles done within 3 years and still healthy. 

The cells have a very similar voltage and when fully charged the voltage shoots up very quickly. However, at 3.40V, they are only allowed to shot 0.05V from their 3.35V rest voltage, which is a reliable end of charge indicator.

I charge at 0.20C, so pretty much a stand-by charge for what LiFEPO4 is concerned. 3.65V is only needed if you want to charge at 1C or similar.


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