# 12v LiFePO4 Question



## Ziggythewiz (May 16, 2010)

?: What is a typical dual battery management system?

The cells shouldn't be floated above 13.6, so while you can charge them from a 14V course, you need something to keep them from continuing to charge once full. Depending on the alternator and the lead acid battery they're paired with, that could be all the protection they need.

The max charge rate depends on the type and size of the cells.


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## ThePizza (Feb 18, 2013)

A dual battery management system essentially cuts of a secondary battery when voltage is dropping below a safe vehicle starting point. That way the second battery can operate things like air compressors and refrigerators while the lead acid battery that starts the vehicle does not go dead. When the car is turned on, the primary battery is fully charged, and then the secondary is added back in to the loop.

Okay, so I can charge the battery on 14v, but when the battery reaches 13.6v I need to hold voltage there? Is there an inexpensive controller that does this? In my research I saw something called an SSD that was linked between the terminals on the battery. Is that what I need? Is there a combine device that holds voltage at 13.6 and has a low power cutoff?

Thanks!


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## Ziggythewiz (May 16, 2010)

Something like this along with an extermal relay could do it with triggers for both over and under voltage. Similarly, a JLD404 would also do it, and along with a shunt let you know how much juice you have.

Do you know what voltage and current your system is usually putting out?


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## ThePizza (Feb 18, 2013)

Ziggythewiz said:


> Do you know what voltage and current your system is usually putting out?


Not sure what you mean by this? My vehicle has a 110 amp alternator which, depending on RPM, will go as high as 14.7 volts.

My required load is basically a constant 2-5 amp draw to power a portable refrigerator/freezer, CB radio, and cell phone and some LED lights if I feel like staying up late!

I am looking at around 30-40ah of 12v to start with. Which will allow me to run the fridge (0.7-2.3amp) overnight and packing the rig.


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## Ziggythewiz (May 16, 2010)

110A is likely the peak, but it probably isn't putting out anywhere near that when the battery is already near 14V.

Unless space or weight is an issue I don't think I'd bother with lithium for just a 40AH battery and low power usage charged by an alternator.


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## ThePizza (Feb 18, 2013)

I don't disagree with your assessment. My issue is that they will fit perfectly under the rear seat which is where my custom fuse boxes are already installed. I can get 40ah of LiFePO4 for about the same price as an Optima which is about 55ah, but LiFePO4 gives much better cycle numbers. i found some that have a built in battery management system to mimic SLA batteries, so I'm going to try them out.


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## Ziggythewiz (May 16, 2010)

I guess it's probably too large (13x6.75") but the 125AH marine DC batteries I've used a lot would do quite well in this application for $100.

There are many battery management systems out there. The ones that are built in to mimic a 12V battery are usually pretty expensive. Be sure to get something with a good reputation. Many of the half-baked systems put your cells at risk of becoming fully-baked.

I've wanted to do a lithium ACC batt for a while, but at $170 without bothering with a BMS it's not worth it till I get the rest of the lead out.


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## GoneFusion (Jan 17, 2014)

Max charge voltage on a LiFePO4 is 3.65-3.8 per cell depending on manufacturer specs.

3.8 is pretty much the norm = 15.2 V. 14 V will be just fine.

Current for charge and discharge depend on the C rating of the specific cells.


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## muffildy (Oct 11, 2011)

resurrecting this old thread....
So i was wanting a detailed responce for how to setup a lead acid replacement similar to this thread except just as a drop in for the startup battery in a typical ICE car.
My altenator currently seems to put out about 13.8v at the most, if i switched to 4x CALB lithium cells instead of the lead battery, Would just bottom balancing the cells and then connecting them without any BMS be sufficient or would doing so allow the alternator to overcharge them?

If it needs a BMS which one would protect from overcharging from the alternator?


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## mora (Nov 11, 2009)

Top balance them if you are not using BMS. I've had 4x 90Ah in place of 12V lead battery for two years already, though they receive only 30A from DC/DC. No BMS, top balanced. Bottom balance works best if you plan to run the battery "flat". Overcharging is more likely with bottom balance and no BMS in this case.


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## EVfun (Mar 14, 2010)

I've got to agree with Mora in this application. You don't know how long the battery will sit at 13.8 volts. That isn't a big deal if they are all at 3.45 +/- 0.05. I wouldn't want one sitting for potentially hours at a time at 3.8 volts so I wouldn't choose to bottom balance for this. 

I will point out that I only charge my LiFePO4 traction pack to about 3.45 vpc. After about 45 minutes at that voltage the current drops from 10 amps down to less than 1 amp and the next morning the cells read 3.345 volts, so the pack is very nearly fully charged at that voltage with a bit of time.


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## GoneFusion (Jan 17, 2014)

muffildy said:


> resurrecting this old thread....
> So i was wanting a detailed responce for how to setup a lead acid replacement similar to this thread except just as a drop in for the startup battery in a typical ICE car.
> My altenator currently seems to put out about 13.8v at the most, if i switched to 4x CALB lithium cells instead of the lead battery, Would just bottom balancing the cells and then connecting them without any BMS be sufficient or would doing so allow the alternator to overcharge them?
> 
> If it needs a BMS which one would protect from overcharging from the alternator?




I just read another post above, when you say a BMS system do you mean a balancing system or a complete management controller ?

I have been talking with manufacturers of these and yes management systems are available for these but it's not required if you are able to balance them as a top balance charge to set voltage. The bottom balance wont do much and I wouldn't recommend it. Maybe a storage balance but draining them to flat often is just going to lead to premature death. As long as they are top or storage voltage balanced charged they are fine. If you need some manufacturer links (pre sorted lol) for Chinese sellers I am happy to share.


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## onegreenev (May 18, 2012)

GoneFusion said:


> I just read another post above, when you say a BMS system do you mean a balancing system or a complete management controller ?
> 
> I have been talking with manufacturers of these and yes management systems are available for these but it's not required if you are able to balance them as a top balance charge to set voltage. The bottom balance wont do much and I wouldn't recommend it. Maybe a storage balance but draining them to flat often is just going to lead to premature death. As long as they are top or storage voltage balanced charged they are fine. If you need some manufacturer links (pre sorted lol) for Chinese sellers I am happy to share.


You mistake the bottom balance issue. You bottom balance ONCE. You under charge to 3.45 volts nominal for your pack and if you happen to have a cell that has such low capacity it drives it up in the 4 volt range while charging you need to replace it. Top or bottom balancing won't help. You are always limited to your lowest capacity cell. As for running them flat over and over and over, well that is just ludicrous. No one ever PLANS on driving the car to empty but IF and WHEN you do if your pack is bottom balanced you won't KILL any cells. You won't kill any cells on the top either if you charge to 3.45 volts per cell and your pack is reasonably well matched in capacity. 

But if you insist on top balancing its ok but you must have a way to guarantee you can't run your pack flat. EVER because if you have an unbalanced pack at the bottom and you go beyond the lowest capacity cell you WILL kill cells. 

Ask me how I know? 

I can show you the results of what happens. 


Pete


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## GoneFusion (Jan 17, 2014)

onegreenev said:


> You mistake the bottom balance issue. You bottom balance ONCE. You under charge to 3.45 volts nominal for your pack and if you happen to have a cell that has such low capacity it drives it up in the 4 volt range while charging you need to replace it. Top or bottom balancing won't help. You are always limited to your lowest capacity cell. As for running them flat over and over and over, well that is just ludicrous. No one ever PLANS on driving the car to empty but IF and WHEN you do if your pack is bottom balanced you won't KILL any cells. You won't kill any cells on the top either if you charge to 3.45 volts per cell and your pack is reasonably well matched in capacity.
> 
> But if you insist on top balancing its ok but you must have a way to guarantee you can't run your pack flat. EVER because if you have an unbalanced pack at the bottom and you go beyond the lowest capacity cell you WILL kill cells.
> 
> ...


I don't doubt you. I've had a couple early 123 cells (hobby stuff) fail on me and wasn't fun either. Controller issue but I learnt the hard way... Thanks for clarifying there - as you said running them flat is not a good idea.

Are you using Low Voltage Cutoff circuits to monitor your cells under load?

Depending on the application I guess for charge voltage. For the hobby cells we always charge them to max capacity with each cell balanced at those matching cells voltage. The cells are matched as much as possible so the drain brings down the cells evenly and when one touches the users specified LVC you get a lowered current and finish up. Solar system works pretty much the same way. At 3.45 you are definitely not charging to full capacity.

Yes I agree, abusing these things won't get you far.


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## onegreenev (May 18, 2012)

GoneFusion said:


> I don't doubt you. I've had a couple early 123 cells (hobby stuff) fail on me and wasn't fun either. Controller issue but I learnt the hard way... Thanks for clarifying there - as you said running them flat is not a good idea.
> 
> Are you using Low Voltage Cutoff circuits to monitor your cells under load?
> 
> ...



Yes, my controller has a low voltage cutback and cutoff set. If by chance I do run my cells down to nothing I will not kill a cell by reversal. Actually at 3.45 the cells are pretty damn close to being full. If you look at most charge curves you will see that 3.45 is on the rise of the curve. There is precious little room left from 3.45 to 3.65 volts. This allows for extra life by not overcharging the cells and stressing them. Same for the bottom. Even though I have my cells bottom balanced I don't want to drain them to the bitter end. But if I do they will remain viable and I won't loose a cell. 

Pete


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## muffildy (Oct 11, 2011)

i havent decided on bms or not yet; looks like top balance is better for my application though according to the above.
I was thinking of possibly using the housepower mini-bms, but if the alternator charging at 13.8 even when the battery is full will not overcharge the battery then i might not bother.
its just being used as a starter so it wont (hopefully) ever be drained much.


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## muffildy (Oct 11, 2011)

hmm actually looks like i cant use mini-bms; got a reply from them about it:

"Our BMS is not designed for replacement of engine starting battery, sorry about that."


so it seems its back to that one linked much earlier in the thread; the
http://www.lightobject.com/Programmable-Digital-AH-meter-Ideal-for-battery-monitoring-P278.aspx
or the 
http://www.lightobject.com/Programm...th-dual-control-Good-for-HHO-System-P408.aspx

But would need specific instructions on how to wire either of them up since both reviews say the documentation is awful.​


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## dougingraham (Jul 26, 2011)

muffildy said:


> i haven't decided on bms or not yet; looks like top balance is better for my application though according to the above.
> I was thinking of possibly using the housepower mini-bms, but if the alternator charging at 13.8 even when the battery is full will not overcharge the battery then i might not bother.
> its just being used as a starter so it wont (hopefully) ever be drained much.


I've been running a top balanced 4S LiFePO4 pack floated from a DC-DC converter at 13.6 volts for a year and a half now no BMS. I pulled it at 6 months to check the balance and it was still fine. I don't see any problem with doing so at 13.8 volts from a conventional alternator. There are lots of places selling exactly this arrangement as replacement batteries to the motor cycle crowd. 

The only real downside to this is that a couple of cells will most likely be ruined if you let it go dead by accident.


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## BrownBageV (Nov 10, 2011)

You can also do a poor-man's BMS. Zeva has a nice cutoff switch that disconnects the entire 4S pack at 11.5V. Unless you are way out of balance this should protect the bottom end. I think there are diode circuits similar to this floating around the forums as well (though I like their reset switch).

http://www.evwest.com/catalog/produ...ucts_id=295&osCsid=g41je4si2j9sg909kqcocb7mg7


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