# Float Charging - Replacing UPS Pb batteries with LiFePO4



## Ziggythewiz (May 16, 2010)

Where'd the 1 watt figure come from, if the UPS floats at up to 13.65? If you're shunting below that float voltage, the UPS won't even be in float mode, it'll sit in charge mode indefinitely, or until something burns.


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## bgeery (Oct 17, 2011)

Ziggythewiz said:


> Where'd the 1 watt figure come from, if the UPS floats at up to 13.65? If you're shunting below that float voltage, the UPS won't even be in float mode, it'll sit in charge mode indefinitely, or until something burns.


There is no "charge mode" and "float mode". This a simple CV float charge. The charger puts out up to [email protected] Volts. When switching to Lithium, we need to burn off the energy difference between what the charger wants to float at and what voltage we want the new lithium cells to float at.

The difference between the 13.65V Pb float voltage and 13.52V (3.38V/cell) LiFePO4 float voltage is 0.13V x 1A = 0.13Watts of energy to burn off when cells are fully charged and floating. If I drop the float voltage down to 13.4 Volts (3.35 V/cell) then we need to burn off 0.25 Volts x 1A = 0.25 Watts to burn off.

The Pb battery already "burns off" power in the form of self discharge, so I don't see how a under 1 watt load would endanger the charging circuitry. Then again, we'll see after I try if anything needs a heatsink added.


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## Roy Von Rogers (Mar 21, 2009)

My advice is, dont do it. As far as I know you DO NOT want to float a lithium cell, when it reaches 3.4/3.5 vpc, you want to turn the charger off, period. You can get a cheap programable voltage meter to do that, but...

The problem in a UPS is, you have to find how to turn it of within the circuit, you cant turn it off at the battery wires, for obvious reasons.


Roy


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## jeremyjs (Sep 22, 2010)

Couldn't you use a properly sized and rated resistor to get the float voltage down to something the batteries can handle. You can float lifepo4. It just needs to below the open circuit 100% charged resting voltage.


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## bgeery (Oct 17, 2011)

Roy Von Rogers said:


> My advice is, dont do it. As far as I know you DO NOT want to float a lithium cell, when it reaches 3.4/3.5 vpc, you want to turn the charger off, period. You can get a cheap programable voltage meter to do that, but...
> 
> The problem in a UPS is, you have to find how to turn it of within the circuit, you cant turn it off at the battery wires, for obvious reasons.
> 
> ...


Roy,

Floating LiFePO4 is fine, as long as you CV regulate below the 100% charged resting voltage. That would be 3.38V/cell at room temp. With no voltage differential, no current flows and charging stops naturally. I'd go a little lower just to be kinder to the cells; say about 3.35V. Still have 95% charged and can avoid the need for temperature compensation from minor environmental temperature swings.

This is all perfectly safe, and actually much easier to manage than Pb batteries that require you to ride a knife-edge balance between killing them from sulfation from undercharge or killing them from slow overcharge. With LiFePO4 I can choose to undercharge to whatever degree makes me happy without fear of ruining the cells.

The problem in my situation is I don't have direct control of the CV setpoint used by the UPS. Luckily, the UPS does not do multistage charging, instead simply applies a steady [email protected] I can burn off the extra bit of voltage without wasting much power so a kludge can make things work.


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## bgeery (Oct 17, 2011)

jeremyjs said:


> Couldn't you use a properly sized and rated resistor to get the float voltage down to something the batteries can handle. You can float lifepo4. It just needs to below the open circuit 100% charged resting voltage.


I'd have to think about that for awhile, but my first reaction would be that things might work for charging (assuming the voltage regulation in the UPS is very accurate) but the resistor would be dissipating a ton more energy during discharge. If I put a 300 Watt load on the UPS, I'd be drawing up to 120 Amps from the cells. That tiny 0.25 watt dissipation during charging turns into 30 watts of heat at discharge and 10% wasted energy. That's why I'm looking a a shunt regulator with adjustable voltage setpoint-- it won't be inline during discharge, and also allows for more fluctuation in the CV voltage regulation in the UPS float charging circuit.


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## jeremyjs (Sep 22, 2010)

bgeery said:


> I'd have to think about that for awhile, but my first reaction would be that things might work for charging (assuming the voltage regulation in the UPS is very accurate) but the resistor would be dissipating a ton more energy during discharge. If I put a 300 Watt load on the UPS, I'd be drawing up to 120 Amps from the cells. That tiny 0.25 watt dissipation during charging turns into 30 watts of heat at discharge and 10% wasted energy. That's why I'm looking a a shunt regulator with adjustable voltage setpoint-- it won't be inline during discharge, and also allows for more fluctuation in the CV voltage regulation in the UPS float charging circuit.


Yeah. I didn't think about discharging. That could definitely be a problem.


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## Roy Von Rogers (Mar 21, 2009)

As I said before, you dont have, or know, how to control from within the circuit. Without that you cant use lipo4.

Believe me, I been wanting to do that same thing with mine when my batteries went bad, I just put another sealed battery in for the time being.

I will have the availability to some 100ah AGM's soon......lol

Roy


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## MN Driver (Sep 29, 2009)

If you want to drop the voltage to the cells, don't shunt them. Diodes have voltage drop, find one or a few in series with a low drop that is appropriate to where the cells will charge enough to be mostly full. Just be absolutely sure that if you decide to go this route that the diode can withstand whatever current draw will go through it during the discharge while using the UPS, which could be quite high. I'd venture that the cells would survive 3.38v/cell indefinitely and since UPS devices are a system that cuts off at a specific low voltage and don't need many cells in a string you could balance them so they are as close to the same voltage when you are fully charged. Once you hit the max CV voltage that the UPS will put out, there should be no current flowing which is opposite of lead acid which will have a constant self-discharge which is high when speaking relatively to lead-acid. You'd have a more efficient UPS in the end because of this. I've considered getting one of the UPS devices from my local computer recycler as these often get discarded when the batteries are end of life. I've strongly considered getting a powerful one that I could wire up that is powerful enough to start and run my furnace for the winter and if needed I'd run an extension cord to the refrigerator. This way if power goes out when it's sub-zero outside I don't have to scramble for the generator immediately as I have time while the battery runs the show. I'd also be able to manually charge the batteries and cut down on generator runtime.

Considering the expense of LiFePO4 and the frequency of power outages, I haven't been motivated to do this yet.


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## Roy Von Rogers (Mar 21, 2009)

MN Driver said:


> If you want to drop the voltage to the cells, don't shunt them. Diodes have voltage drop, find one or a few in series with a low drop that is appropriate to where the cells will charge enough to be mostly full. Just be absolutely sure that if you decide to go this route that the diode can withstand whatever current draw will go through it during the discharge while using the UPS, which could be quite high. I'd venture that the cells would survive 3.38v/cell indefinitely and since UPS devices are a system that cuts off at a specific low voltage and don't need many cells in a string you could balance them so they are as close to the same voltage when you are fully charged. Once you hit the max CV voltage that the UPS will put out, there should be no current flowing which is opposite of lead acid which will have a constant self-discharge which is high when speaking relatively to lead-acid. You'd have a more efficient UPS in the end because of this. I've considered getting one of the UPS devices from my local computer recycler as these often get discarded when the batteries are end of life. I've strongly considered getting a powerful one that I could wire up that is powerful enough to start and run my furnace for the winter and if needed I'd run an extension cord to the refrigerator. This way if power goes out when it's sub-zero outside I don't have to scramble for the generator immediately as I have time while the battery runs the show. I'd also be able to manually charge the batteries and cut down on generator runtime.
> 
> Considering the expense of LiFePO4 and the frequency of power outages, I haven't been motivated to do this yet.


 
A diode is a one way device.

Roy


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## MN Driver (Sep 29, 2009)

Roy Von Rogers said:


> A diode is a one way device.
> 
> Roy


That would be a huge brain fart on my part. Any other semiconductors that would do this? I know FETs drop a specific voltage but you'd have to keep it engaged the entire time that the UPS was ever powered so that might not be the best idea either.


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## jeremyjs (Sep 22, 2010)

Could you maybe figure out a way to disable the charger in the UPS and then use your own adjustable DC power supply?


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## Roy Von Rogers (Mar 21, 2009)

jeremyjs said:


> Could you maybe figure out a way to disable the charger in the UPS and then use your own adjustable DC power supply?


That was my contention all along, you have to find where in the ups the switching occurs, another words when the line power goes down its switched from charging to powering the converter. Once you find that junction, all there is left is to find a way to control it, a JLD5740 is cheap and easy to configure for that.

I've got another one sitting in my office/shop that needs a battery, maybe I'll take it apart and see if I can find that spot, but I will have availabillity to 6 - 100ah agm's soon, lot easier to hook up two (24v system), it will give plenty of power and I wont have to mess around with the ups.

Roy


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## frodus (Apr 12, 2008)

Why use lifepo4 in a UPS to begin with? It's stationary (so weight and volume is not really an issue). There's not much of a reason to go with lifepo4 in this situation. It's only used in a power-loss situation, and it was designed to use SLA's anway. Either battery is going to be "sitting" for most of its life anyway. Why put the expense into lifepo4?


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## Roy Von Rogers (Mar 21, 2009)

frodus said:


> Why use lifepo4 in a UPS to begin with? It's stationary (so weight and volume is not really an issue). There's not much of a reason to go with lifepo4 in this situation. It's only used in a power-loss situation, and it was designed to use SLA's anway. Either battery is going to be "sitting" for most of its life anyway. Why put the expense into lifepo4?


I have to agree, but the batteries in my APC1100 dont last, I can put my hand on the unit and where the battery is, its pretty warm, which is a waste of energy and not good for the batteries.

If one could find the switching point, one could cut off the charge at 3.5vpc, and not have to worry about it til the time comes when the batteries are used. And we all know that sitting for long periods wont harm lipo4, and they will last a long time.

Its no different the in an Ev, yes you can run with lead, but it will cost more in the long run.

Roy


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## bgeery (Oct 17, 2011)

Roy Von Rogers said:


> As I said before, you dont have, or know, how to control from within the circuit. Without that you cant use lipo4.
> Roy


Roy, you can't just blow smoke saying "It won't work". I explained how I believe it would work. You have to explain where you think my logic is flawed. The same method can be used with a Pb battery if the float charge voltage is higher than desired.

There are only a handful of UPS models that can have the voltages user adjustable. All are older APC models. I have a modern and efficient Cyberpower PFC model.



jeremyjs said:


> Could you maybe figure out a way to disable the charger in the UPS and then use your own adjustable DC power supply?


Thought about that, but that solution is above my pay grade. Besides, I'd need cell balances anyway, so using them to burn off the extra (under 1 watt) power ends up being the simplest solution. I just need to know a balancer that allows me to choose the voltage setpoint where balancing starts.



frodus said:


> Why use lifepo4 in a UPS to begin with? It's stationary (so weight and volume is not really an issue). There's not much of a reason to go with lifepo4 in this situation. It's only used in a power-loss situation, and it was designed to use SLA's anway. Either battery is going to be "sitting" for most of its life anyway. Why put the expense into lifepo4?


Longevity of Pb in UPS devices is legendarily bad. I either spend $480 on LiFePO4 once or spend ~$200 every 2.5 years on a Pb AGM battery. LiFePO4 is way cheaper in the long run. The break even point for LiFePO4 is 5-7 years.

Now that I think about it, I don't even think these Pb batteries are lasting the 2-3 years. People finally replace the Pb battery when the UPS lasts 10 seconds on battery power. In fact the UPS runtime has been just about useless half the time it's been sitting there, the user just didn't know.


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## Roy Von Rogers (Mar 21, 2009)

bgeery said:


> Roy, you can't just blow smoke saying "It won't work". I explained how I believe it would work. You have to explain where you think my logic is flawed. The same method can be used with a Pb battery if the float charge voltage is higher than desired.
> 
> There are only a handful of UPS models that can have the voltages user adjustable. All are older APC models. I have a modern and efficient Cyberpower PFC model.


Put some lipo4 cells on a UPS, and when it gets dont charging, put a amp meter in line and see if there is no current going in, that will be the tell tale. All I can tell you if there is any current flowing, it will kill the cell eventually.

You can put some diodes in line to adjust whatever voltage you claim will float.


Roy


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## jeremyjs (Sep 22, 2010)

I've also heard, never tested, that you can float a123 cells at up to 4.45 volts without damage. I think A123 even recommends that float voltage for their RC packs. I wouldn't assume this translated to other manufacturers though.

found a .pdf http://liionbms.com/pdf/a123/charging.pdf


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## Roy Von Rogers (Mar 21, 2009)

jeremyjs said:


> I've also heard, never tested, that you can float a123 cells at up to 4.45 volts without damage. I think A123 even recommends that float voltage for their RC packs. I wouldn't assume this translated to other manufacturers though.


ANR26650M1

Nominal capacity and voltage 2.3 Ah, 3.3 V
Internal impedance (1kHz AC) 8 m1 typical
Internal resistance (10A, 1s DC) 10 m1 typical
Recommended standard charge method 3A to 3.6V CCCV, 45 min
Recommended fast charge current 10A to 3.6V CCCV, 15 min
Maximum continuous discharge 70A
Pulse discharge at 10 sec 120A
Cycle life at 10C discharge, 100% DOD Over 1,000 cycles
Recommended pulse charge/discharge cutoff 3.8V to 1.6V
Operating temperature range -30°C to +60°C
Storage temperature range -50°C to +60°C​Core cell weight 70 grams


Roy


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## bgeery (Oct 17, 2011)

Roy Von Rogers said:


> Put some lipo4 cells on a UPS, and when it gets dont charging, put a amp meter in line and see if there is no current going in, that will be the tell tale. All I can tell you if there is any current flowing, it will kill the cell eventually.
> 
> You can put some diodes in line to adjust whatever voltage you claim will float.


The charger sends up to 1 Amp at 13.65 volts. The LiFePO4 cells want to float at no more than 13.52. Without any action on my part, of course current would flow and overcharge the LiFePO4 cells. It will overcharge the cells by .25 watts per hour. 180 watts a month. It's ohms law.

I need cell balancing that I can turn down to at least 3.38 in any case, instead of the more common 3.6V most are set at by default. I have to top balance the cells in float service. Letting them burn off that 0.25 watt of extra energy would not allow the battery voltage to ever rise above the max float voltage of 3.38V and no overcharge can occur. The only time this method would not work is if your shunt load can't handle dissipating the required energy. For example, if I had a 100 Amp charging circuit in the UPS, I'd be dealing with 25watts of energy I'd have to shunt away to the keep the batteries at 3.38V/cell. Current is always going to be flowing, it's just a matter of if I allow it to be forced into the battery, or do I shunt off the .25 watts energy as heat.

I'll also like to point out that they already commercially manufacture small drop-in LiFePo4 battery replacements for UPS Pb batteries that do exactly what I'm proposing. It's not a question of "can it work", it's only a question of regulating the voltage via shunting at the proper voltage. I want 100Ah cells, not the commercially available <10Ah drop-in replacements (that they sell for like $400 BTW!)

So I'll pick everyone's mind again with my original question: *Does anyone know of a BMS that allows the user to set/adjust the shunting voltage? * Not everyone is happy with 3.6, so I know they must be out there.

One theoretical alternative is to simply put a ~0.25 watt resistive load across (not inline) with the pack. That would have the same effect as regulating shunts, but would require manual control. As a practical matter you would have to make the resistive load was actually a tiny bit bigger than needed, so overcharge could never happen even with less than perfect voltage regulation from the UPS charging circuit, then you would have to periodically disconnect the shunt load to allow the charger to catch up with the excess drain of the load. Bad idea, as you might forget to reconnect or disconnect the shunt load as scheduled.



jeremyjs said:


> I've also heard, never tested, that you can float a123 cells at up to 4.45 volts without damage. I think A123 even recommends that float voltage for their RC packs. I wouldn't assume this translated to other manufacturers though.


In a perfect world, the float voltage would be equal to the open circuit 100% charged voltage. For LiFePO4 prismatic cells that's ~3.38V/cell. Other lithium chemistry have different voltages. Pb has a higher float voltage in an attempt to compensate for high self dischage and sulfation issues.


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## bgeery (Oct 17, 2011)

I thought of one other potential complication. I've never used a BMS (I bottom balance my GEM neighborhood electric vehicle) so I don't know if this is a concern. Do BMSes have both a turn on point (say 3.6V) and another turn off point (say 3.5V) or simply a single set point? The reason I ask is ideally, I'd not want the shunt to be constantly creating micro-charge/discharge cycles. Something like a switching PWM load would be perfect. It would adjust the apparent load to precisely match the voltage setpoint.

Edit: I also forgot about http://liionbms.com/php/bms_options.php that show just about every BMS out there, including several open-source projects that I may be able to use or modify for this purpose.


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## frodus (Apr 12, 2008)

3.41v (i.e 13.65V) isn't going to do anything negative to your cells. You won't overcharge them. Overcharging is when you Overvolt them, and 3.41V is far from overvolting the cell. Once you get to 3.41V, the cell just sits there, no current flows. LiFePO4 is fine sitting at a voltage as long as it's within it's specified range of voltage, which 3.41V is. There's just not a ton of useable "charge" put in above 3.4V or so, so most times it's not really neccessary to go that high. From what I can see, most lifepo4 chargers go to ~3.6V a cell CC/CV, unless otherwise programmed. Does that 3.6V hurt it? why would it, you're not going over it's HVC. Lifepo4 batteries, by nature, are able to operate within their entire range.

I'm unclear where you're getting this 3.38V from... is it on a spec sheet somewhere?

You can let a lifepo4 battery sit on a power supply at 3.4V or 3.6V, but at 3.4V it'll take much longer to get to 100% charge than it would at 3.6V. Either way, you're below the spec'd range of the cell, and you're doing no harm. once at their set CV voltage, they draw no current, and thus won't become overvolted (i.e overcharged).


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## Siwastaja (Aug 1, 2012)

bgeery said:


> *Does anyone know of a BMS that allows the user to set/adjust the shunting voltage?*


I agree this is exactly the right way to do it.

Any "analog" simple BMS will probably do... Something like MiniBMS. Just change the voltage divide resistors. I think MiniBMS schematic has been posted at this forum, so it would help. Some experimentation needed (playing around with parallel/series resistors to make the exact values you need), and some soldering, but it shouldn't be that bad.


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## dougingraham (Jul 26, 2011)

bgeery said:


> In a perfect world, the float voltage would be equal to the open circuit 100% charged voltage. For LiFePO4 prismatic cells that's ~3.38V/cell. Other lithium chemistry have different voltages. Pb has a higher float voltage in an attempt to compensate for high self dischage and sulfation issues.


The A123 26650 cell data sheet indicates a float voltage of 3.45 volts per cell. I capacity tested four cells and then made a 4S pack and floated them at 13.8V for 6 months after top balancing them. No change in capacity. I've also done the same with one of my GBS 100AH prismatics. No change in capacity. If you float it at 3.45 volts for a couple of weeks and then remove the charger it stays at 3.45 volts. Common thinking is that this is overcharged but it doesn't seem to affect it. There was only a few microamps flowing during months of the time this cell was floating.

I suspect you are concerned about something that seems like it might matter but doesn't in practice. Once you get them balanced at 3.45V they just stay put at that voltage. I don't know at what voltage it would actually become a problem to float a cell. But 3.45v does not appear to be an issue.


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## corbin (Apr 6, 2010)

bgeery said:


> I thought of one other potential complication. I've never used a BMS (I bottom balance my GEM neighborhood electric vehicle) so I don't know if this is a concern. Do BMSes have both a turn on point (say 3.6V) and another turn off point (say 3.5V) or simply a single set point? The reason I ask is ideally, I'd not want the shunt to be constantly creating micro-charge/discharge cycles. Something like a switching PWM load would be perfect. It would adjust the apparent load to precisely match the voltage setpoint.
> 
> Edit: I also forgot about http://liionbms.com/php/bms_options.php that show just about every BMS out there, including several open-source projects that I may be able to use or modify for this purpose.


The Elithion BMS allows you to set the high point (to stop charging), the shunting point, and the low-voltage point.

corbin


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## palmer_md (Jul 22, 2011)

Use Lithium Manganese or Lithium Cobalt as they both have a higher OCV. Then you'll be ok with 4S pack at 13.65v = 3.4125/cell

Like others have said, best solution is just to keep the lead since it is a stationary application and the lead is cheap. Trying to use LFP is trouble at that voltage. It's close, but I'd not do it if above 13.2v. Shunting BMS is not going to do anything for you. It can keep one cell lower than the others, but if you have a potential of 13.65, it has to divide up among the 4 cells. 

A resistive divider could work, but then all the current has to pass through one of the resistors both into and out of the pack. That means losing voltage to whatever the batteries are powering. 

I vote keep the lead.


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## bgeery (Oct 17, 2011)

dougingraham said:


> The A123 26650 cell data sheet indicates a float voltage of 3.45 volts per cell. ... I've also done the same with one of my GBS 100AH prismatics. No change in capacity. ... Common thinking is that this is overcharged but it doesn't seem to affect it. ... But 3.45v does not appear to be an issue.


Agreed that I may be worrying about nothing, but If I can assure I'm not over charging by application of a shunt controller in the cells, I'd sleep better a night. Why overcharge at all if it can be avoided simply. However, yours is a good datapoint regarding the effects of float charging. I don't have any love for BMS systems, so I'd love to believe I could avoid them even in this float service.

I guess the real question is what happens to that tiny amount of extra energy being pumped into the battery, that in theory the chemistry can't store. In Pb, the battery converts the energy into heat, electrolysis of the electrolyte, and breakdown of the positive plate. In LiFePO4, it's at least converting it into heat, if not also breakdown of the matrix.

If the cells can store the higher voltage without losing it to heat conversion, then I can see how a small overcharge would have little long-term effect. If on the other hand they do lose the extra voltage to heat or other conversion process, then I'd think it would progressively degrade the battery. In either case, the effect appears to be immeasurable in your testing.



corbin said:


> The Elithion BMS allows you to set the high point (to stop charging), the shunting point, and the low-voltage point.


Thanks, Corbin. I'll try to research the different BMS options over the weekend.



palmer_md said:


> Like others have said, best solution is just to keep the lead since it is a stationary application and the lead is cheap.


Lead is only cheap up front. Having to replace a $200 100Ah AGM battery every couple of years is ridiculous. And the price of Pb batteries keeps going up, while lithium keeps coming down. I just read that someone purchased 24 100Ah LiFePO4 cells for $80 each. A year ago, they would have cost 50% more than that. Nobody knows exactly what the calender life of LiFePO4 is (or if they even have a calender life), but I'm betting it's long enough to beat out several AGM batteries and beat them economically long-term. If I could get Jack of EV.TV to read my emails, I'd like to hear what he has to say about LiFeOP4 in float service.


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## JoeG (Jul 18, 2010)

Brent,
One of the great things about LiFePo4 batterys is that they don't self discharge like lead so they don't need a float charge at all. You could just disable the float charger and then just recharge after use either manually or with a Cellog setup to cycle the charger on/off.
Joe


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