# 12V aux battery (4x3,2LiFePo) charging



## brainzel (Jun 15, 2009)

I want to replace my old lead acid auxilary battery with 4 x 40Ah (maybe 20Ah) WB LiFePo cells or a precompiled 12V WB-LP12V40AH.

Charging should be made from th DC/DC, which would permanently be connected to the battery at 14V (12,0 to 14,4V is possible).
So the cells would be charged to 3,5V.

Would this be OK or would it causes problems like "slow baking" the cells with some mAmps at the end of the charge?


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## Yabert (Feb 7, 2010)

Hi

I'm also interesting to know how build a fault free / bulletproof auxiliary battery with four Lifepo4 cells.
I have enough spare Headway 10Ah cells and I'm really interested to build a 12v 10Ah battery that weighs 3 lbs instead of buy a 12v lead acid battery.

Maybe than dimitri can help us with what we can read here.



> This largely depends on your DC-DC connection. If its always on ( my preference ) then aux battery can be very small. I used lawn mower battery ( $25 at any hardware store ), later switched to 4 Headway 10AH cells ( under $80 and will last forever ).


For me, 80$ only imply the price of four cells and don't include BMS!.


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## dimitri (May 16, 2008)

Yes, I use 4 Headway cells of 10AH as my 12V aux battery, hardwired to DC/DC output and DC/DC is hardwired to main pack, so 12V battery is always topped off at around 14V.

When I initially built this battery I top balanced all 4 cells at 3.65V to make sure they won't get overcharged at the top, since this battery spends most of its life fully charged.

Although I have miniBMS modules attached to each cell, I do not actively manage it, I just observe green LEDs on occasion. I did not really need to do it since its just 4 cells and usage is light, so I don't expect them to stray outside of normal voltage. If I was to do it again, I would not attach BMS at all. This is relatively cheap battery and even if I lose a cell long term, its not that important.

Its been working great for many months now, absolutely no concerns.


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## dimitri (May 16, 2008)

brainzel said:


> Would this be OK or would it causes problems like "slow baking" the cells with some mAmps at the end of the charge?


There is no such thing as slow baking of LiFePO4 battery. Once it reaches same voltage as the source ( DC/DC or charger ) current drops to zero. There is no concept of "float charge" in this chemistry since they have virtually zero self discharge.


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## brainzel (Jun 15, 2009)

So I would take four Headway 16Ah, four WB 20Ah or 40Ah and strap them in series together and that's it 
Many thanks for this informations.


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## GizmoEV (Nov 28, 2009)

dimitri said:


> There is no such thing as slow baking of LiFePO4 battery. Once it reaches same voltage as the source ( DC/DC or charger ) current drops to zero. There is no concept of "float charge" in this chemistry since they have virtually zero self discharge.


Doesn't the electrolyte decay faster at higher voltages or is it just at higher temperatures?


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## dimitri (May 16, 2008)

Who said anything about "higher" voltages? typical 14.4V DC output is only 3.6V per cell, not even close to upper safe limits for LiFePO4.


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## GizmoEV (Nov 28, 2009)

dimitri said:


> Who said anything about "higher" voltages? typical 14.4V DC output is only 3.6V per cell, not even close to upper safe limits for LiFePO4.


It is possible to charge a TS LFP cell to 100% SOC with a 3.40V cutoff as long as the current continues long enough. Anything over this value can overcharge the cell. The charging procedure for a LiFePO4 cell specifies a specific ending voltage along with a matching current.

In any case, 3.5V is higher than 3.3V. Cells last longer in at lower states of charge which is why they are shipped partially charged.


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## dimitri (May 16, 2008)

GizmoEV said:


> It is possible to charge a TS LFP cell to 100% SOC with a 3.40V cutoff as long as the current continues long enough.


Just because its possible it doesn't make it practical, nor that its absolute truth.
I have LiFePO4 cells from variety of makers and charge them all to 3.8V and even to 4.0V every day and they all work like new.


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## GizmoEV (Nov 28, 2009)

dimitri said:


> Just because its possible it doesn't make it practical, nor that its absolute truth.
> I have LiFePO4 cells from variety of makers and charge them all to 3.8V and even to 4.0V every day and they all work like new.


I never said that it was practical for cycle applications. Because it is possible that is what should be seriously considered in a "float" application. CALB even lists 3.4V as the "float" voltage for their cells.

You didn't say what the current was when the charge was stopped. Try holding them at 3.8 or 4.0V for several weeks at a time and see what happens.


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## MalcolmB (Jun 10, 2008)

GizmoEV said:


> It is possible to charge a TS LFP cell to 100% SOC with a 3.40V cutoff as long as the current continues long enough.


Hi Gizmo. I'd like to believe that, but just wondered if you have documentation or experience to back it up? I'm building a little four-cell LifeBatt pack as a 12V auxiliary battery. The cells are charged individually by small DC converters, permanently wired to the pack. I'd already decided to trim the converters for 3.4V each, to avoid long-term damage to the cells.

From reading the LifeBatt documentation it seems there could be a slight loss of capacity if cells are not cycled fully to their usual 3.65V limit. I'm not worried by such a loss of capacity for my auxiliary battery, but I've been thinking about using the same low cutoff limit for my main pack, so I'm curious to know if this has any drawbacks.


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## frodus (Apr 12, 2008)

Any voltage above their nominal 3.2V will charge them. 3.65V is for optimum charge times and energy. 3.4V will charge it, but it'l take longer and may not get it to 100% (depends on the battery, temperature, and the type of charger). 

The larger the delta in voltage between the cell voltage and what the charger is trying to charge to (lets say 3.65V), then the larger current it will pull. The closer to nominal (say 3.4V), the less current it will pull, and longer it will take. 

You can get slightly more energy in there, but it's not a huge amount. If you use these as your aux battery, you may only be slightly undercharging them. It will actually help the batteries last longer in the long run. And since you're not ever really discharging them all the way, you're not really doing much in the way of cycling these cells...... they'd be more there for redundancy.


long and the short, it'd be just fine. If possbile, I'd turn the DC-DC off when you're not running the vehicle to save on the parasitic draw.


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## GizmoEV (Nov 28, 2009)

MalcolmB said:


> Hi Gizmo. I'd like to believe that, but just wondered if you have documentation or experience to back it up? [snip]
> 
> From reading the LifeBatt documentation it seems there could be a slight loss of capacity if cells are not cycled fully to their usual 3.65V limit. I'm not worried by such a loss of capacity for my auxiliary battery, but I've been thinking about using the same low cutoff limit for my main pack, so I'm curious to know if this has any drawbacks.


To add to what Frodus said I have been trying to get data and info on this for quite some time. From what I have gleaned from reading various papers, one of which was a NASA summary document, LiFePO4 (and other lithium battery types) are essentially 100% efficient which means even small currents can charge them since there is no self-discharge to speak of. The CALB documentation actually list 3.4V as the "float" voltage. I have no idea how they arrived at that figure but it is there. Someone on the EVDL said they worked for a LiFePO4 battery manufacturer said that 3.4V was enough to fully charge a battery if left long enough. They didn't identify what company they worked for, however.

I have been trying to get some testing done on a TS-LFP40AHA cell to see what the ending current has to be to fully charge a cell if the ending voltage is lower than the TS spec of 4.00V at 0.015C. Preliminary testing shows that it is sufficient to merely stop at a lower ending current to return the same energy as was removed but the West Mountain Radio PWRcheck isn't cooperating very well with such low voltages and current from a single cell. I had to return the cell today so I will have to wait until I can get another small cell or two to start testing again. In the mean time I hope to get the issues resolved with the PWRcheck unit so I can put more value in the results.

Your statement from the LifeBatt documentation about capacity loss is the first I have heard about that being a possibility. Are you sure they weren't referring to the possibility that the cell wouldn't be fully charged if not charged to 3.65V rather than the actual cell capacity? In other words, is the documentation saying something like under charging would cause say 20% loss of total capacity in 5 years rather than 10 years if fully charged? What happens if you charge to 3.65V once each year. Does the capacity return?


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## MN Driver (Sep 29, 2009)

I think the 3.4v was the voltage where it will be fully charged if held there for a long time but also the level where it could safely be held indefinitely if used in a float application. I've thought about this a little bit considering that the voltage of the alternator of one of my cars goes up and over 15v in the winter and the service manual shows it going over 16v as part of its temperature compensation but in an EV the voltage is static and the car is likely not driven for over 2 hours anyway so I'd imagine that bringing it up to 3.6v a cell for a few hours at a low current would probably be fine. Either way 4 cells isn't going to cost too much but would provide a nice answer over time to whether or not it harms them. I'd venture that its fine.


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## MalcolmB (Jun 10, 2008)

GizmoEV said:


> What happens if you charge to 3.65V once each year. Does the capacity return?


I think you may be right. The sentence that caught my eye was this: "The final capacity test was able to recovered capacity from end of PSOC cycling measured at the 1C rate (8.61 Ah) to 8.91 Ah measured at the 0.5C1 rate. This recovery is attributed to the deep-cycle, lower rate, and full charges using an end of charge current of 0.2 A"
(This is after several thousand cycles)

You may be able to glean more from the report than I can. The full report is here: http://www.lifebatt.com/sandiareport.pdf

I'll be interested to hear the results of your capacity tests. For practical purposes it sounds as if the capacity reduction will be small if cells are only charged to 3.40. I'm especially curious though, as I'll be using a relatively small pack, to begin with at least, so every Wh matters.


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## ElectriCar (Jun 15, 2008)

To lower the charge voltage and limit max voltage to this pack, you could use this diode on the charger lead. It would need to be on the charger lead only, not the lead from battery back to the vehicle. It's two versions are rated at 100A or 50A with a forward voltage drop of around 1.2V. Assuming a typical charging voltage of 14.4V, you would drop that to around 13.2V or 3.3V per cell. You may with a little research find one with a lower forward voltage drop to get the voltage up a little more.


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## frodus (Apr 12, 2008)

ElectriCar said:


> To lower the charge voltage and limit max voltage to this pack, you could use this diode on the charger lead. It would need to be on the charger lead only, not the lead from battery back to the vehicle. It's two versions are rated at 100A or 50A with a forward voltage drop of around 1.2V. Assuming a typical charging voltage of 14.4V, you would drop that to around 13.2V or 3.3V per cell. You may with a little research find one with a lower forward voltage drop to get the voltage up a little more.


he's not using a charger.... it's an adjustable DC-DC, no need.



brainzel said:


> Charging should be made from th DC/DC, which would permanently be connected to the battery at 14V (12,0 to 14,4V is possible).
> So the cells would be charged to 3,5V.


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## ElectriCar (Jun 15, 2008)

frodus said:


> he's not using a charger.... it's an adjustable DC-DC, no need.


Due to the discussion about overcharging I was speaking in general terms where the charging mechanism is fixed, ala my Harley or alternator in a gasser.


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## ElectriCar (Jun 15, 2008)

Here's something to ponder. I had 232Ah 6V batteries in my truck before switching to lithium. To equal that in lithium I would need 4 200Ah cells at about $240 each. That's roughly $1000 vs. $300 for lead. 

I was thinking about making a pack for tailgating. It would draw about 15A accounting for losses while running an LCD TV and satellite box. Over about 4 hours that's about 800watts. To have that and some reserve I'd need 4 100Ah cells, or about $480 versus a $100 lead battery. Ouch!


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## MN Driver (Sep 29, 2009)

ElectriCar said:


> Here's something to ponder. I had 232Ah 6V batteries in my truck before switching to lithium. To equal that in lithium I would need 4 200Ah cells at about $240 each. That's roughly $1000 vs. $300 for lead.
> 
> I was thinking about making a pack for tailgating. It would draw about 15A accounting for losses while running an LCD TV and satellite box. Over about 4 hours that's about 800watts. To have that and some reserve I'd need 4 100Ah cells, or about $480 versus a $100 lead battery. Ouch!


Well, not quite. If 12v and 232Ah of lead is $300 and you'll lose cycle life rapidly if you aren't using less than 50% of that pack you are down to 116Ah, then you've got peukert for the 4 hour draw and the reserve.

Sounds like its $300 versus $480. ...or more frequent battery replacements of a smaller lead-acid battery which even the 232Ah probably won't last more than 5 years anyway even if is well fed or even rarely used. Then the other benefits of lighter weight, better efficiency, smaller size, and not adding water to a floody being cycled come to mind.


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## MalcolmB (Jun 10, 2008)

OK, here's my version: four 10Ah Lifebatt cells, each with its own DC converter trimmed to 3.4V.

These little Vicor DC converters are fully isolated and take an input of 36-72V. The output voltage can be trimmed anywhere from 3.3 to 3.6V. Output current is limited to 20A. I'm going to connect two of the converters to one half of the main pack, and other two to the other half, so this will work with a pack voltage anywhere between 72 and 144V. Total weight is 2.6 kg, which is a nice weight saving over a 12V lead acid battery and regular DC converter.

This will be mounted in the same position as the old radiator to keep it cool. It could easily be scaled up for more capacity, as the DC converters can be paralleled to share output, but this should be adequate for my little Mini.


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## frodus (Apr 12, 2008)

What is the model number on those Vicors? Are they the one's off ebay that are 48V - 3.3V?

Be careful, they may not work well for a 144V nominal pack, because it'll likely go way over 144V after a charge. Double check.

Download the design guide:
http://cdn.vicorpower.com/documents/applications_manual/fas_trak_apps_manual.pdf


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## MalcolmB (Jun 10, 2008)

Hi Travis. The model number is V48C3V3C75A002. My pack will be well under 144V, more likely 115V, so no problem there. Yes, I've got the design guide. Lots of really useful information in there, though I must admit I've ignored some of it. It advises the use of bypassing capacitors across the input and output, and between the inputs/outputs and baseplate. I'm not sure how important those are in this application. Any idea? I suspect they're mainly intended to protect against AC components.

I think it might also be a good idea to put some form of surge protection in series with the input, but again not sure. Any advice is very welcome.


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