# Use 4x LiFePO4 as aux battery?



## racunniff (Jan 14, 2009)

I'm thinking of replacing my AGM aux +12V battery with a LiFePO4 battery. I also have an integrated DC-DC converter, so the battery is mostly for instantaneous surges (i.e. power steering pump turns on). I've peeked a little at using Dmitri's HousePower BMS to control this (see attached diagram). However, I have some questions:

1. Is there any hysteresis between HVC turning on and then turning off again when the cells drain a bit? I don't want high-frequency switching of my DC-DC contactor when the batteries are *close* to full...

2. Is there a way to have its HVC be 4*3.5 = 14.0v rather than 14.2v?

3. Is there a way to have its main cutoff be at 4*3.6v = 14.4v rather than at 4*3.75v = 15v?

4. Has anybody else tried this?


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## dimitri (May 16, 2008)

To be honest, in such application ( low C rate, shallow cycles ) I would just balance 4 cells and don't bother with BMS at all. I also use 4 cell LFP battery as my 12V battery and its been fine for over 2 years now, without BMS.

HousePower BMS was meant for large banks people use on boats and RVs, where they often go thru deep cycles and often reach LVC. Its an overkill for small EV battery.


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## brainzel (Jun 15, 2009)

I have four LFMP20Ah cells lying around to replace the 12V lead acid monster.
My DC/DC provides 13,8V max, so 3,45V / cell.
I'll hook them directly to the DC/DC, without BMS but will watch the behavior the first weeks.


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## Yabert (Feb 7, 2010)

I do same thing.

DC-DC supply 13.5v and the four lifepo4 cell stay balanced because I top balanced each one before.
No bms and that work perfectly since half a year.


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## racunniff (Jan 14, 2009)

Thanks, all. I also have some spare cells just lying around, so I'll give it a shot to top balance and go BMS-less (after some digging, I found my DC-DC also puts out 13.5V, which should be fine).


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## onegreenev (May 18, 2012)

I have two 4 cell 100ah packs in parallel and just charge when needed. No DC DC connected and all bottom balanced. Yes I do monitor the pack voltage of the Aux battery. Currently the large pack is being used for charge/discharge testing on some A123 cells with my PowerLab 8 and laptop. Once the car is back on the road I will put one 100ah pack back in for my Aux battery. Works great except its kinda tall. 

Pete


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## Roy Von Rogers (Mar 21, 2009)

There are a few things that have always worried me about using lithium as an aux battery.

You better be sure that when you turn off ignition that the dc-dc is off, in case of dc-dc failure.

Have an overvoltage alert on the aux.

Put some diodes in series so charge voltage is just a bit over 3.3v per cell.

Bottom balance the pack in case you leave something on, so if it does happen they all go down at the same soc, or have a low voltage cut off.

To be honest even though I like the idea of a lithium pack for aux, a lead acid is a lot more forgiving, and lot cheaper to replace in case something goes wrong.


Roy


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## JoeG (Jul 18, 2010)

I've used a 4S/4P 8AH(32AH total)Headway pack for my Aux battery for 7 months now and it has worked out great. I just charge it when I charge my main pack using a dedicated charger & have a voltage monitor in the cab to keep an eye on things, so far so good. I had the batteries laying around from an e-bike project, but even if purchase new it wound have been about the same cost as a good deep discharge gel auto battery.


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## kortas (Apr 18, 2012)

We are using 35Ah LiFePO4 battery together with a DC-DC converter having an output voltage 13,5V which is 3,375V per cell which is too low (<40% SOC). Battery is nearly discharged all the time.

DC-DC 13,8V seems to be better, but not too much - 3,45V per cell (could be 50% SOC). Ideally the DC-DC converter should have at least 14-14,4V which is 3,5-3,6V per cell (90% SOC).

There might be a possibility to "boost" the output of DC-DC, however I have not checked that yet. I have seen some 230V 12V chargers that had a trimmer to adjust output voltage from 13,5 to about 15V.


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## skooler (Mar 26, 2011)

kortas said:


> We are using 35Ah LiFePO4 battery together with a DC-DC converter having an output voltage 13,5V which is 3,375V per cell which is too low (<40% SOC). Battery is nearly discharged all the time.
> 
> DC-DC 13,8V seems to be better, but not too much - 3,45V per cell (could be 50% SOC). Ideally the DC-DC converter should have at least 14-14,4V which is 3,5-3,6V per cell (90% SOC).
> 
> There might be a possibility to "boost" the output of DC-DC, however I have not checked that yet. I have seen some 230V 12V chargers that had a trimmer to adjust output voltage from 13,5 to about 15V.


3.375v is more like80-90% charged with LiFePO4. 

You don't want to be float charging these cells at 100%SOC. That is asking for trouble.

Ideally, the 12v side of your DCDC converter should be just below the open circuit voltage of all of your LiFePO4 cells (i.e. 4*OCV for a 12v accessory battery) so that they sit at about 90% SOC and don't go above that.


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## Yabert (Feb 7, 2010)

kortas said:


> ....3,375V per cell which is too low (<40% SOC). Battery is nearly discharged all the time.


Wrong!



> 3.375v is more like80-90% charged with LiFePO4.


Generally true!


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## kortas (Apr 18, 2012)

skooler said:


> 3.375v is more like80-90% charged with LiFePO4.


Have you checked that in real life? There is a typical charging curve for 0.5C:
http://www.batteryspace.com/prod-specs/6064_1.pdf
for currents <0.5C the curve can go a bit down, but not that much down so that the cell would be 80% charged having the voltage <3.4V.

We had a caravan which was charging to 13.5V and I can confirm that the battery was nearly empty.



skooler said:


> You don't want to be float charging these cells at 100%SOC. That is asking for trouble.


This is true. However it is nice to know that you have some reasonable SOC percent


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## skooler (Mar 26, 2011)

kortas said:


> Have you checked that in real life? There is a typical charging curve for 0.5C:
> http://www.batteryspace.com/prod-specs/6064_1.pdf
> for currents <0.5C the curve can go a bit down, but not that much down so that the cell would be 80% charged having the voltage <3.4V.
> 
> ...



Yes, I have checked in real life. With a real cell that I have touched with my own hands.

Have a look at the discharge curve I posted in the below thread:

http://www.diyelectriccar.com/forums/showthread.php/bottom-balancing-85458.html

It shows that above ~3.3v there is virtually no capacity. 

Above ~3.4v you are just getting a surface charge which will drop rapidly as soon as you put a load on the cell. I wouldn't risk overcharging the cell to get at those couple of AH above 3.3-3.4v.

If any single cell is above 3.6v with a constant voltage / float still being applied to it (i.e. DCDC converter, charger etc.) I would start to get worried, above 3.8v and I'd step back!

Finally, its worth mentioning that top balancing works really well in this scenario - unless you are going to let the battery drain to zero (don't ask how I know!)


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## kortas (Apr 18, 2012)

skooler said:


> Have a look at the discharge curve I posted in the below thread:
> 
> http://www.diyelectriccar.com/forums/showthread.php/bottom-balancing-85458.html
> 
> ...


That is a good point, but I am afraid there is a "little" difference between charging and discharging curve...


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## skooler (Mar 26, 2011)

kortas said:


> That is a good point, but I am afraid there is a "little" difference between charging and discharging curve...


I disagree, there is quite a large difference! 

Not only in the lab but a real world example - A charge curve tapers down to 0.05c but a discharge curve is hit by several hundred amps in the first few seconds.

Even in a lab there is quite a large difference. It's two completely separate processes.


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## Yabert (Feb 7, 2010)

I know what you try to mean Kortas. For you, 3.375v per cell isn't enough to create imbalance able to charge a lifepo4 cell.
I don't know because I can see charge curves at very low rate (0.05C as example) to see if a cell can be charge with this voltage.
BUT the thing I know, is I've charged lifepo4 cells to 3.6v last year before build my auxiliary battery and 5-6 month later they was still full charge despite my DC-DC is set to 13.5v (3.375v per cell).

My auxiliairy battery isn't much solicitate, but I think than Lifepo4 cell can be charge at very low rate with only 3.375v (not to 100% of course).


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## skooler (Mar 26, 2011)

Yabert said:


> I know what you try to mean Kortas. For you, 3.375v per cell isn't enough to create imbalance able to charge a lifepo4 cell.
> I don't know because I can see charge curves at very low rate (0.05C as example) to see if a cell can be charge with this voltage.
> BUT the thing I know, is I've charged lifepo4 cells to 3.6v last year before build my auxiliary battery and 5-6 month later they was still full charge despite my DC-DC is set to 13.5v (3.375v per cell).
> 
> My auxiliairy battery isn't much solicitate, but I think than Lifepo4 cell can be charge at very low rate with only 3.375v (not to 100% of course).


Spot on - Good advice


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## dougingraham (Jul 26, 2011)

If you float a LiFePO4 cells at 3.375 volts it is somewhere over 90% SOC. I have pretty good resting voltage SOC numbers near the middle of the range but my highest number is for 85% SOC at 3.3429 volts. You can look at my scatter plot at LiFe voltage vs SOC I am planning on filling out the empty areas soon. Also trying a different brand of LiFePO4 to verify that different formulations are similar.

A 13.5V float on a 4 cell pack of top balanced cells should be fine and last a very long time. I have been floating my 8AH 4S LiFePO4 at 13.58v for about 7 months now and this is pretty much a full charge.


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## Ziggythewiz (May 16, 2010)

kortas said:


> Have you checked that in real life? There is a typical charging curve for 0.5C:
> http://www.batteryspace.com/prod-specs/6064_1.pdf
> for currents <0.5C the curve can go a bit down, but not that much down so that the cell would be 80% charged having the voltage <3.4V.


3.4 resting is more than 100%, so pretty sure you can get to 90% charging under that. I do it every day. There's a huge difference between .5C @ 100A and the smaller currents that most people use.


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## pigpan (Aug 7, 2011)

Hi there,
I used 4 Winston 40AH with the DC-DC set to 13,5V. But they died due to a not yet identified drain once the car shuts off most systems after 2 days just standing.








Now the battery is replaced with a 20AH Lead Gel motorcycle battery. The point is, that you may experience such failures and a LiFePo is dead (mine was), standard batteries are more forgiving...


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