# Float Charging



## dcb (Dec 5, 2009)

I think the concensus is that %100 charge (and discharge) is generally a bad idea, especially given the detrimental effect on cycle life.

But I don't see any reason why you can't hold them at %80 charge, basically skipping the cv portion by setting cv to the %80 voltage level, that doesn't mean it is safe to do so, just that I'm not aware of how it would affect the state of charge if you were applying pack voltage to the pack and theoretically nothing is moving anyway.

Though why bother?


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## Yabert (Feb 7, 2010)

I don't see any problem to ''float' lifepo4 at 3,4v for days since there aren't full charge at that voltage.
Because you stop at 3,4v the current will drop to 0 and the cells will not take energy anymore.

My 12v battery* in my Smart is working like that since few years now.
*4 cells 3,2v lifepo4 continuously float at 13,4v by the DC-DC.


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## Sunking (Aug 10, 2009)

dcb said:


> But I don't see any reason why you can't hold them at %80 charge, basically skipping the cv portion by setting cv to the %80 voltage level.


You are not skipping CV. just a lower voltage of say 80 to 90% SOC OCV. There is no difference between FLOAT and Constant Voltage. They are the exact same thing. 




dcb said:


> just that I'm not aware of how it would affect the state of charge if you were applying pack voltage to the pack and theoretically nothing is moving anyway.
> 
> *Though why bother?*


Then you either did not read what I said, did not understand what I said, or I did a poor job of explaining. In a Solar System, the batteries are typically charged by noon or mid afternoon. You still have lots of usable daylight. If you terminate the charge mid day, you loose all that power that could come from the panels during the day after the batteries are chargeg. You would go on batteries at noon. With the panels on line, you utilize Solar power till dark rather than battery power. You are conserving battery power and extending their life.


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## Duncan (Dec 8, 2008)

Hi Sunking

So by midday the batteries are at max and you are shorting the panels - wasting that power

But the batteries are powering your inverter - feeding the house? 
Once they have run the inverter for a bit the voltage will drop and the panels will switch back on?
Depending on the "gap" you have set between switch on/off the panels will cycle all afternoon

From my point of view that sounds fine - I would be a bit conservative and have the "off" at somewhere about 3.4v rather than 3.65v


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## Bugrug (Jun 7, 2016)

Keeping it at less than 100% seems to be the key here. There are still a few kinks to work out with this aspect of these cars (the MAIN aspect, really) but we are making headway.

Good topic.


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## Moltenmetal (Mar 20, 2014)

Sunking, there's nothing from what I understand of the chemistry that would cause damage to the cells by doing what you're proposing. But if you were to "float" them at 3.65 volts, which you are not planning to do, that would do electrolyte damage over time.

Forgive my total ignorance about solar panels: why do you need to short them rather than disconnecting them and letting them go open circuit? Where does the collected energy go when you've shorted them? They're current devices when under load, right? X photons falling on the panel per second drives Y amps of current - up to a certain maximum voltage- with peak efficiency achieved when the current is delivered at the limiting voltage. Is that more or less how they work?


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## Sunking (Aug 10, 2009)

Duncan said:


> But the batteries are powering your inverter - feeding the house?


Correct. Like any home you use power 24 x 7.



Duncan said:


> Once they have run the inverter for a bit the voltage will drop and the panels will switch back on?
> Depending on the "gap" you have set between switch on/off the panels will cycle all afternoon


No Sir Charge Controllers do not work like that. Just about every charge controller out there is for Pb batteries. They are all 3-Stage of Bulk (CC), Absorb (CV), and Float (CV). Once they cycle through to the Float stage, a timer locks them to stay in Float Modes until the next day. Just how they work because they are built for Pb and Float all day after charged. So you utilize Solar Power instead of battery power.

That is what I want to mimic. Right now I have have an additional circuit to short the panels out to stop charging. I want to lower the voltage and leave the panels on all day and work like a Pb battery.


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## major (Apr 4, 2008)

Isn't what you're talking about the same as having two cells in parallel? Done all the time. To me, seems silly to shutdown PV at noon. Who came up with that idea? What happens on an otherwise sunny morning, PV running house and charging, then dark clouds? Doesn't battery take over? Just run a couple fans continuously so PV load never goes to zero. 

Just a guess. Never done it. But 32,000 miles on Lithium battery with parallel cells. 

major


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## Sunking (Aug 10, 2009)

major said:


> Isn't what you're talking about the same as having two cells in parallel? Done all the time.


Essentially yes. Difference being it is a Charger aka Power Supply rather than a parallel cell with the voltage set to roughly 90% SOC open circuit voltage at 3.4 vpc. So on a 8S or 16S pack would be 27.2 volts, and 54.4 volts respectively.



major said:


> To me, seems silly to shutdown PV at noon. Who came up with that idea?


I think we are on the same page Major. As to the *Who* thought that up is the industry stating Lithium Batteries CANNOT BE FLOAT CHARGED. I think that is BS and the requirement assumes 3.65 vpc, not 3.4 vpc or lower. 

I understand you cannot, or I should say should not be Float Charge LFP at 3.65 volts per cell. All Float means is left on the charger with power left on at Float voltage after the battery is fully charged. The difference between PB and Lithium is Pb is left on the charger with Float Voltage applied, and Lithium is Terminated with supply voltage turned off. Well that is fine for an EV charged over night to 100% SOC and driven the next day.

Solar is a different application. There is no reason to fully charge a lithium battery. In a Solar design you want the panels on-line all day so all the Solar Power can be utilized rather than battery power. 

However Lithium batteries present a special challenge for Solar Users. There are NOT any Solar Chargers designed specifically for Lithium batteries. They are all designed for Pb batteries and use a 3-Stage Charging Algorithm of Bulk (CC), Absorb (CV), and Float (CV). In practice they are simple CCCV chargers. Once the battery cycles to FLOAT, it stays in Float Charge until next morning and the three cycles start all over again. 

In the inexpensive solar charge controllers line you can only select switch to Gel, AGM, or FLA which sets 24 volt systems for FLA Bulk = 14.2 volts, Absorb = 14.4, and Float = 13.8 volts. However some of the mid and high end controllers can be set to whatever you want via keypad or laptop. For those units you can adapt to Lithium and make them a 2-Stage charger aka CCCV. Example you can set Bulk = Absorb = Float = of say 29.2 volts or 3.65 vpc on a 8S LFP pack. Problem is when the battery reaches 29.2 volts and current taper, ther Controller does not shut off to prevent over charging a LFP battery. They were made for PB.

Most folks are stuck in a Pb battery box. They have it in their mind thy shall charge to 100% SOC no questions asked. Well that is BS with respect to Lithium batteries. There is no need to go to 100%. In fact you can double their cycle life by only going to 80/90% SOC. 

Si I scratched my head and said I think I can do that with off the shelf Solar Charge Controllers made for PB batteries. All I gotta do is select a open circuit voltage of less than 100% SOC. If I do that then I should be able to FLOAT the batteries without harming them. So I am thinking set Bulk = Absorb = Float = 27.2 volts on a 8S LFP battery. That works out to 3.4 vpc or roughly 90% SOC. I should be able to hold 3.4 vpc on a LFP with no harm. 

If I do that I do not have to use a BMS cell monitors, or any other specialized equipment to interface lithium to solar. I can set the Inverter LVD to 24 volts which is well above the Low Voltage over discharge threshold at 20 volts, so no chance of an over discharge. 

So now the question is there any harm to Float Charge the battery if the voltage is LOWERED to 3.4 VPC and hold 3.4 vpc until the sun goes down?

I do not think there is any problem doing that. I have asked a couple of manufactures who are not replying. My problem is the battery manufactures state LFP* cannot be Float Charged*, but I believe that statement is made from the POV you would hold 3.65 vpc, not 3.4 or less.

*I know there are some battery Reps on this forum and hope they will chime in. * There is another market out there for LFP, *SOLAR*. Right now Lithium batteries used for solar are far and few between. Solar has different requirements than EV's. They operate in Sub-C-Rate or fractional C-rates. There is no requirement to go to 100% SOC. Time to tap the solar market and feather your nest egg. We are talking 8 and 16S batteries from 100 to 1000 AH. That is a fair chunk of change on your commissions.


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## major (Apr 4, 2008)

From a quick Google on float charge Lifepo, I don't see any "don't do". See graph on link.

http://www.batterytender.com/Battery-Basics/#answer5


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## Sunking (Aug 10, 2009)

major said:


> From a quick Google on float charge Lifepo, I don't see any "don't do". See graph on link.
> 
> http://www.batterytender.com/Battery-Basics/#answer5


Thank you for that. It does support that I can Float at reduced voltage indefinitely as I suspected. 

Realistically cannot perform all 5 steps the Battery Tender does like Recovery Mode. Nor would I go to Full Absorb Voltage 100% SOC. Just use Bulk to get to 3.4 vpc and hold Float at 3,4 vpc.


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## Duncan (Dec 8, 2008)

Sunking

What do the solar controllers DO on "float"? - they limit voltage, - what about current?
And what do they do when the sun is shining and the batteries are full and the house is not drawing any current?


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## ken will (Dec 19, 2009)

If you want to really baby your batteries, keep the SOC at the best anti-ageing state by predicting future use.
If the next few days are going to be sunny float at a lower SOC, but if dark rainy days are coming float at a higher SOC.

Bringing us back to Electric vehicles,.. if I plan on driving only a few miles I charge to 60%,.. If planing a longer adventure I charge to 80%.


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## Sunking (Aug 10, 2009)

Duncan said:


> Sunking
> 
> What do the solar controllers DO on "float"? - they limit voltage, - what about current?


Duncan a Charge controller acts like any DC battery charger or DC Power Supply. Any battery charger is just a DC Power Supply with voltage and current regulation. 

The amount of charge current a battery will take depends on the battery OCV, Battery Internal Resistance, Charger Voltage, and Charger Current Limit. 

Example let's say a 12 volt 100 AH Pb battery with an Ri of .02 Ohms a troughly 50% SOC open circuit voltage of 12.2 volts. We connect it to a 10 Amp charger set to 14.4 volts. Initially the battery will draw 10 amps from the charger continuously aka Constant Current because that is all the current the charger is capable of. When you connect the charger to the battery, the voltage will drop from *14.4 volts* to Battery OCV + (Charge Current x Ri = 12.2 volts + (10 amps x .02 Ohms) = *12.4 volts* The voltage of the charge source Folds Back as it cannot maintain 14.4 volts. The charger would have to be a 110 amp charger, an dif it were would let the magic smoke out of the battery.

The charger will supply a Constant Current until the battery OCV reaches 14.2 volts. At that point (Constant Voltage mode starts) charge current will start to taper off to Zero Amps. When the battery reaches 14.4 volts, all current stops flowing because both charger and battery voltage are equal. The battery is fully Saturated or Charged. 

A Solar Charge Controller works the same way with one difference. Sola ris not a Stiff Source of Power like a AC powered charger. A Solar Panel is a Current Source not a Voltage Source like a battery, generator, or AC supply. The amount of current a panel can generate depends on the amount of sunlight striking it surface and current is directly proportional to the amount of sunlight striking its surface. In other words current available is dynamic. Early morning very little power, as the sun gets higher, th eamount of power available goes up until solar noon. Then drops off as the sun passes overhead to sunset. So the difference is a Solar Charger works in Constant Power, and then Constant Voltage as the battery reaches full charge.

OK so we get to fully charge or Float. If no load is connected, it is noon with bright skies, and the battery is charged. Panel are not generating any current because no power is demanded. Turn on a light and we now are demanding power. Assuming the panels have enough sunlight to supply the load, all the power comes from the panels. If the load exceeds what the panels can produce, the battery make sup the difference. When the load is turned off, if power was used from the battery, the panels replace or charge the battery back up. Otherwise all power stops as the battery is fully charged and no demand for power. Electric Energy has to have a place to go. 




Duncan said:


> And what do they do when the sun is shining and the batteries are full and the house is not drawing any current?


I think I covered that already. No demand, no power or no current flowing Same thing when you open a switch, current stops flowing in an Open Circui


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## Sunking (Aug 10, 2009)

ken will said:


> If you want to really baby your batteries, keep the SOC at the best anti-ageing state by predicting future use.
> If the next few days are going to be sunny float at a lower SOC, but if dark rainy days are coming float at a higher SOC.


Solar Battery systems are designed with x amount of days autonomy. 

For PB batteries minimum is 5 days which in practice give you 3 days as you never discharge more than 50%.

Lithium minimum is 3-1/2 days to give you the same 3 days as Pb

Any Off-Grid System is required to have a Generator and AC battery charger for those long cloudy spells. Or else you go Dark and wait for 3 Sunny days to recharge before the lights go back on. 

So there is no reason to change voltage set point. Just something less than 100% OCV. Magic number is somewhere from 3.3 to 3.4 vpc


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## ken will (Dec 19, 2009)

Sunking said:


> So there is no reason to change voltage set point. Just something less than 100% OCV. Magic number is somewhere from 3.3 to 3.4 vpc


Oh, sorry, I thought Lithium cells sitting at 60% did not deteriorate as quickly as cells sitting at 80%... I guess I was worry that unwanted chemical reactions were more likely to happen when surrounded by a higher energy state.

.....It has been over 50 years since I last took a chemistry class!


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## Sunking (Aug 10, 2009)

ken will said:


> Oh, sorry, I thought Lithium cells sitting at 60% did not deteriorate as quickly as cells sitting at 80%...


Well there is some truth to that to my knowledge. 

I am also an RC pilot and have over 100 battery packs. We use what is called LiPo which to my understanding is a hybrid Lithium Cobalt battery with a gel electrolyte. Every battery manufacture if hobby LiPo batteries instruct you not to store the cells fully charged. They instruct 40 to 60% SOC for storage. 

LiPo's are a more unstable than LFP, so not so sure if that applies to LFP. I do know however when you receive nex LFP cells they are sent at 40 to 60% SOC. Thus is my uncertainty.

Really wished a manufacture rep would chime in or at least point me to a link. Major did that but I think he comes from the motor manufacturing. I have a lot of respect for Major. We may not always agree, but I think we both respect each other. 

So Hollie May jump in here, you are a battery rep.


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## Karter2 (Nov 17, 2011)

probably "Elithion" (Davide) would be the reference needed for this.


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## Duncan (Dec 8, 2008)

Thanks Sunking
So setting a "float" to 3.4v will keep the system live and operating using as much as possible of the sun until nightime while looking after the batteries

So the only question is - is 3.4v the best number? or would you be better being conservative and going even lower?


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## Moltenmetal (Mar 20, 2014)

It seems to me that the potential trouble with operating on the flat part of the voltage vs SOC curve isn't hurting the batteries per se- it's that you're on the flat part of the curve, so subtle variations in voltage can mean large differences in SOC. While it's observed that batteries charged using the normal "CC/CV then trip" regime don't drift too much, if you "float" them at any voltage on the flat part of the curve it seems likely that some drift in SOC between cells over time would be inevitable. Periodic top balancing would likely fix that.


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## Sunking (Aug 10, 2009)

Duncan said:


> So the only question is - is 3.4v the best number? or would you be better being conservative and going even lower?


Not married to 3.4 vpc, just the first even number that popped into my head. Actual number I assume would be 3.3 to 3.4. Would take some trial and error to find the sweet spot.


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## Sunking (Aug 10, 2009)

Moltenmetal said:


> It seems to me that the potential trouble with operating on the flat part of the voltage vs SOC curve isn't hurting the batteries per se- it's that you're on the flat part of the curve, so subtle variations in voltage can mean large differences in SOC. While it's observed that batteries charged using the normal "CC/CV then trip" regime don't drift too much, if you "float" them at any voltage on the flat part of the curve it seems likely that some drift in SOC between cells over time would be inevitable. Periodic top balancing would likely fix that.


Yeah that is one of the questions running through my head. The batteries would pretty much live their lives between 3.2.and 3.4 volts. With the batteries being sized for 3 to 4 day autonomy, you rarely ever discharge them below 3.2.

One friend client I have has a 16S system we built about 6-months ago. He is using CALB 100 AH cells. Fairly small system with 2000 watts of panels. He is doing something rather unique, He lets his system run two days with panels disconnected. On day three reconnects and rechargers. 

His panels are a bit oversized, so in summer months he can pretty easily get recharged in a day after being offline two days. So he is letting his batteries go down to the 3.0 to 3.1 range. His line of thinking is Cycle Life will be greatly improved. While that has some merit not sure he has considered batteries have two lives, Cycle and Calendar life. In my 38 year experience professional experience with batteries, I have never seen a battery live up to its cycle life. Rare to even see one live up to its Calendar life. The old Western Electric pure lead Round Cells are the only batteries I or any telephone company have ever seen live up to their Calendar life of 30+ years. 

Although I do know some Railroad and Mining folks who claim 50 years with some original Edison NiFe batteries. But Exide quit making them back in 1977.


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## major (Apr 4, 2008)

Sunking said:


> Although I do know some Railroad and Mining folks who claim 50 years with some original Edison NiFe batteries. But Exide quit making them back in 1977.


I believe thst was Eagle Picher not Exide. I happen to have 16 of those, old stock but NIB. Any of your solar clients would want them, I'll sell. I never got around to using them and have Li now.


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## Sunking (Aug 10, 2009)

major said:


> I believe thst was Eagle Picher not Exide. I happen to have 16 of those, old stock but NIB. Any of your solar clients would want them, I'll sell. I never got around to using them and have Li now.


Perhaps Eagle Picher made NiFe too but I know Exide bought Edison Battery in 1972 and stopped production in 1975. I have worked with Exide for long time in the telecom industry You can check here for the source info. I first learned about then early in my career. They sound great up front but plagued with issues. 

I have looked at them for Solar, but they just are not a good economical fit. The initial cost (nickel silver is expensive), the high maintenance cost, steep discharge curve, high resistance, and low charge efficiency just does not make them a good fit for Renewable Energy. Some use them, but not many. 

Today there is a Ruskie and a Chi-Com manufacture or two out there making them. I even here once in a while some of the original design still being used out in a rural area RR crossing light.


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## hube (Oct 13, 2016)

To continue this thread.

I have been thinkering of making a DIY double conversion true online 4s LiFePO4 UPS. I thought that LiFePO4 can't be floated so I was thinking of some clever way of using a normally closed relay to connect the battery to the inverter when the AC is lost. However, this is not a true online UPS due to the switching time from the battery.

I have no idea how OEM's would do this, but apparently OEM's have not really adapted lithium yet in UPS's.

Now I have thought using a programmable PSU to charge the battery at 4x (3.55V) ~14.2V CV until "full" and then switch to a floating voltage of 4x3.4V ~13.6V. Top balancing could be done occasionally in this case. I have no clue if this works, and would it affect the calendar life and how much.

What happens when transitioning from 14.2V to 13.6V? Would the battery discharge for a while? Any other strange phenomenons?

Any ideas? Viable or not? 

ps. I don't think drop in LiFePO4 batteries really work in lead-acid UPS


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## dcb (Dec 5, 2009)

fwiw, my old laptop stopped charging about a month ago, and I tapped in a meanwell with some bell wire shoved in the battery terminals and adjusted the voltage so that it stabilized at about a 50% charge reading, been running like that for about a month, hope it doesn't blow up (had lots of work stuff on it I needed to unload, and I just left it on for convenience in case I missed something)... I don't know if I would try it with a higher charge(voltage) level, at least not without better controls.


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## dougingraham (Jul 26, 2011)

Sunking said:


> So now the question is there any harm to Float Charge the battery if the voltage is LOWERED to 3.4 VPC and hold 3.4 vpc until the sun goes down?


I've been doing this with my 4S4P aux battery for over 4 years now. I set the Vicor (DC-DC converter) output to 13.6 volts and it naturally limits to 40 amps which is a hair over 4C charge rate. But even when completely dead (<2 VPC) it only holds that 40 amps for 15 to 20 seconds and then drops off to around 20 amps.

Before I tried this I put one of these cells on a bench supply set to 3.4 volts and left it connected for 6 months. There was no measurable current flow after a month and I have a meter that can measure nanoamps. A float charge in the traditional sense is not a charge when the current is zero.

You could be conservative and drop the vpc to 3.38 which will be less than 100% SOC.


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## Sunking (Aug 10, 2009)

dougingraham said:


> I've been doing this with my 4S4P aux battery for over 4 years now.


Thanks Doug, gives me an excuse to update my experience and progress. So IMO the MYTH is BUSTED, you can float charge at least LiFeP04 and LiCo cells with a couple of caveats.

Fist Clue and confirmation is from A123 Systems data sheets themselves The Specify a Float Voltage of 3.4 vpc. 1 note to that is they say is charge to 3.6 vpc until current starts to taper, then reduce voltage to 3.4 vpc indefinitely.

The only problem I have encountered because I do not charge to 3.6 then reduce is finding the right voltage to Float at. 3.4 is right on the knee. Just a little under optimum and you may only get to 75% on a 32S pack, just a little over and in the high 90%. So you have to experiment a little. On paper I should be at 108.8 volts. In practice turns out to be 109.7

This all started as a means to use LFP batteries on a Solar System with off-the shelf Charge Controllers. Additionally figure out how to use a Lithium Battery in Float Service like a Telephone Office battery plant. 

In a Solar System last thing you want to do is get your batteries charged up by say noon, and then have the Solar System Shut down forcing you on battery power when there is still a half day of sun available. On the other hand last thing you want to do is fully charge a lithium battery to 3.6 volts and hold aka Float as that will damage the cells.

Anyway made it work, but it has to be tuned in. First you have to have a Charge Controller that allows you to make Bulk = Absorb = Float, and set the voltage in 0.1 volts increments. Catch here is the only controllers that can do that are higher end controllers which I only use. MorningStar, and Midnite Solar. Outback might, not sure. When you set Bulk = Absorb = Float you have turned a 3-stage Battery Charger for Pb batteries into a Simple 1-Stage CC/CV of whatever voltage you program. 

Doug one problem you might run into is low voltage packs like 4S 0.1 volt resolution may not be fine enough as 0.1 volts is 0.8% of 12 volts whereas .1 volts at 96 volts is much finer adjustment of 0.1%. May not be enough resolution at .1 volts

Bottom line here is you ABSOLUTELY can Float Charge Lithium batteries. Even found a Lithium battery charger that actually does this using A123 Algorithm I spoke of earlier. So if A123 says you can is good enough for me, and I proved it. As far as using for EV's, not practical as you really have no need to Float the battery, even if stored for a few months.

Almost forgot. A123 Algorithm is Chargge at 3.6 vpc until current tapers to C/10, then reduce voltage and float at 3.4 vpc. That will get you to about 90 to 95% SOC


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## Frank (Dec 6, 2008)

I thought the quiescent voltage was 3.34? Charge some up, let sit then set float voltage using the same VOM. Maybe just set everything on the charge controller to 3.33 and leave it.


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