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Old 06-11-2014, 02:47 PM   #21 (permalink)
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As the graphs show the cells will take charge up to very nearly full at less than 3.5 Volt per cell.
The Insight powers the 12V system from the IMA DC-DC converter. It switches between two states: about 12.65 Volt and 13.90 Volt (resting and charging the LA battery) but I also see peaks and dips in that voltage.

I will only know what it does to a LiFePO4 pack if I hook that up. It can go to 14 Volt so I assume it can get it to near 100% SOC, but otherwise I have the panel to top it off

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Old 06-11-2014, 04:57 PM   #22 (permalink)
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I'm beginning to think that adding a Protection Circuit Module (PCM) or top balancing circuit to a LiFePO4 battery defeats the purpose of using LiFePO4 in the first place. The idea is that the chemistry is already ideally suited to directly replace 12v batteries.

If we're going to add voltage regulators, balance circuits, and low voltage cutoffs, we might as well run normal Li-ion batteries and benefit from double the capacity.

How feasible is it to replace lead-acid with Li-ion in a 3s 12.3 volt battery if a charging circuit is used? The alternator output of 14.4 volts would act as the source for the charge circuit. The supercap could still be used to start the vehicle.

Regarding protection circuits for the LiFePO4, I have found the following products. Which would be suitable for use with a solar charger?

All-Battery.com: Protection Circuit Module For 4 Cells LiFePO4 Battery Pack

PCM with Equilibrium function for 12.8V LFP Battery Pack @ 30A limit (PCM-L04S30-526)



http://www.batteryspace.com/pcbprote...limited-1.aspx

CMB for 12.8V LiFePO4 Battery Pack (16A limited, CE & EMC listed ) with DC charging, Fuel Gauge and full protection



Quote:
Originally Posted by Daox View Post
Depending on your alternator's output voltage, I don't think you need to worry about top balancing at all. Even at 14.5V, you have 3.625V per cell. That isn't even fully charged. This is a good thing because you don't have to worry about overcharging the cells, and it prolongs their life. The downside is you have reduced capacity because they aren't fully charged. This likely isn't an issue unless you're using tiny cells.
The reason RedDevil mentioned top balancing is that a solar panel would provide excess voltage and be capable of over-charging the battery. The solar panel is there to extend the length of time the car can sit, considering we are talking about replacing the lead-acid battery with a LiFePO4 battery that has 1/10 of the Ah capacity.

I suspect you already know this, just clarifying for others that might be following this thread.

I'm wondering if the cells would still get out of balance even though they aren't being fully charged by the alternator? How likely is it that a cell could become over-charged even though the total battery voltage is not at full charge?

Quote:
Originally Posted by RedDevil View Post
If one cell is slightly fuller than the others, it is in danger of getting overcharged when it has reached 100% SOC.
14.5 Volt minus (3.4 times 3) makes 4.3 Volt and kills a cell. Better not risk that.
I'm not following the reason for the math, please explain. My battery is 4s (4 cells in series), so that should give 3.4v times 4, or 13.6 volts total.

What are you recommending for those wanting to run a LiFePO4 battery that is charged by the alternator?
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Old 06-11-2014, 05:47 PM   #23 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
I'm not following the reason for the math, please explain. My battery is 4s (4 cells in series), so that should give 3.4v times 4, or 13.6 volts total.

What are you recommending for those wanting to run a LiFePO4 battery that is charged by the alternator?
The voltage on each cell depends on the state of charge on those cells. If those are exactly equal there is no need for balancing, obviously. But if one cell gets slightly out of line you do have a problem, because that cell will peek above 4 Volt and start destroying itself while the other cells are still around 3.4 Volt just shy of being completely full.
If your alternator or charger or solar panel or whatever provides more than (3.4 *3) + 4.0 = 14.2 Volts, then you might destroy one cell in a pack of 4 LiFePO4 cells.
Because it will not shed charge by itself like LA cells do there is no self balancing. You cannot trust them to stay balanced indefinitely without some means of control.

If you stay below 14.2 Volt then an unbalanced pack will stop being charged when one of the cells is full. If the balance deteriorates there will be less and less practical charge available. Finally you may kill a cell by depleting it too much, again while the other cells are still around 3.4 Volt.
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Old 06-11-2014, 05:49 PM   #24 (permalink)
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I like the idea Red, plus its simple. Ive read a lot about lifepo4 cells and I tend to agree with the 3.6 volts to be considered 100% vs pushing it to 3.8 or 4.0. The enginer kit stopped at 3.8 and many say that was too high. I think cut off was like 2.3 or 2.2, but that was on the low side. Either was for one cell and it triggered the system to stop charging or stop draining.

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