[Ghey Thunder]

I am planning on building several 12V starter battery packs for the all the junk vehicles I have sitting around. Tried a 4s4p pack and it works dandy.
After reading a bit more on how to promote longevity on the Lifepo batteries I am thinking of building 7s5p packs to have a little more cranking amps and also to promote longevity.

Battery cells: A123 -ANR26650M1A --> nominal voltage 3.3V / 2.3ah ( 10 sec. discharge rate up to 120C)
On a 4s4p configuration these would sit fully charged at 13.3V and have a 9.2 Ah capacity
On a 7s5p configuration it would be 16.6V and 16.1 Ah.

The alternators of most of the cars output is 14.4V which on the 4p pack means they could be overcharged and on the 5p configuration would mean the cells would be charged to 86% max - I think.

Am I looking at everything correctly? Any different configuration or improvements I could make? How about running these packs without BMS?

 

[brian_]

Battery cells: A123 -ANR26650M1A --> nominal voltage 3.3V / 2.3ah ( 10 sec. discharge rate up to 120C)
On a 4s4p configuration these would sit fully charged at 13.3V and have a 9.2 Ah capacity
On a 7s5p configuration it would be 16.6V and 16.1 Ah.

You seem to have "s" and "p" reversed. The pack voltage is the cell voltage multiplied by the number of cells in series ("s") and the pack amp-hour capacity is the cell capacity multiplied by the number of cells in parallel ("p"). So you presumably meant:

• On a 4s4p configuration these would sit fully charged at 13.3V and have a 9.2 Ah capacity
• On a 5s7p configuration it would be 16.6V and 16.1 Ah.

No, those pack voltages (13.3 V ad 16.6 V) are nominal, not fully charged. Nominal voltage is a sort of mid-range value. Strangely, the spec sheet from A123 doesn't list fully charged or discharged voltage in the list of specs, but in the discharge graph you can see that the fully-charged cell voltage is a bit higher. Charging must also overcome some internal resistance in the battery (admittedly low in most lithium-ion cells), so charging voltage must be higher than the final charged battery voltage, especially at higher charging rate (current).

The alternators of most of the cars output is 14.4V which on the 4p pack means they could be overcharged and on the 5p configuration would mean the cells would be charged to 86% max - I think.

So presumably this meant

The alternators of most of the cars output is 14.4V which on the 4s pack means they could be overcharged and on the 5s configuration would mean the cells would be charged to 86% max - I think.

A typical automotive charging system would certainly not go high enough in voltage to properly charge a 5S configuration of these cells, but that 14.4 volts is barely enough for the 4S configuration. One reason that LiFePO4 (or "LFP") cells are popular in batteries for recreational vehicles is that a 4S configuration is a reasonable direct replacement for a 6S (nominally 12 volt) lead-acid battery; in other lithium-ion types the voltage of 3S would be too low and 4S would be too high. Even in those RVs, many (perhaps most) users replace the standard charger (intended for lead-acid) with a charger with slightly higher peak voltage (and different charging profile) to use the LiFePO4 batteries effectively.

14.4 volts would be high for charging a 4S pack at -20 C, according to the chart... but these cells probably can't withstand any charging (at any voltage) at temperatures much below freezing anyway.

14.4 volts on a 5S pack would be only 2.88 volts per cell; the discharge curve shows that to be perhaps 20% at -20 C, perhaps 97% at +25 C, and yes, maybe 86% at freezing (0 C). In any case, it's way below the normal operating voltage of these cells at reasonable temperatures. A 5S pack would provide quite a boost, but the voltage might be too high for the vehicle being boosted when fully charged, and it would not get properly charged by the running vehicle.

 

[john61ct]

You get higher Ah capacity and therefore higher peak cranking amps at the same C-rate

by increasing the "group" size, parallel first

then connect in series. You could parallel two strings if you want separate packs, but not optimal.

Stick to 4S for 12V nominal. Just use an adjustable HVC (edited) for longevity charging, down to maybe 13.8V,

but tbh without holding CV stage, you could go a bit higher.

The end result should be, after isolating from the charge source, over time the resting voltage should settle to no higher than 13.4-13.5V

That is a good charging profile for longevity.

What you don't want to do, is store the packs uncycled at that high a SoC.

12.8-13V would be a reasonable storage voltage.

Only charge to Full the day before you need to start cycling them again.

 

[Ghey Thunder]

You seem to have "s" and "p" reversed.

Yes I did. Thank you for the correction.

Strangely, the spec sheet from A123 doesn't list fully charged or discharged voltage in the list of specs

I found this spec sheet with a reference to "Recommended pulse charge/discharge cutoff " Other spec sheet. I suppose this is close enough to fully charged and discharged voltages?

So presumably this meant

A typical automotive charging system would certainly not go high enough in voltage to properly charge a 5S configuration of these cells, but that 14.4 volts is barely enough for the 4S configuration. One reason that LiFePO4 (or "LFP") cells are popular in batteries for recreational vehicles is that a 4S configuration is a reasonable direct replacement for a 6S (nominally 12 volt) lead-acid battery; in other lithium-ion types the voltage of 3S would be too low and 4S would be too high. Even in those RVs, many (perhaps most) users replace the standard charger (intended for lead-acid) with a charger with slightly higher peak voltage (and different charging profile) to use the LiFePO4 batteries effectively.

I understand that for most applications you will want to use the full potential of each cell by charging them to capacity so that you have more energy available. Cost probably also plays a role here. Neither are concerns for my intended application however. I don't need the battery to be fully charged nor is cost really relevant. At about $2 per cell adding the 7 additional cells would be about $14.
What I am looking for is a battery that will last as long as possible and where I don't have to worry about putting it on a trickle charger when its not being used. If I am reading some of the literature correctly charging a li-ion below full capacity can not only increase its life time as far as cycling but its also much better for prolonged storage. At least that's what I gathered from here.

Most Li-ions charge to 4.20V/cell, and every reduction in peak charge voltage of 0.10V/cell is said to double the cycle life. For example, a lithium-ion cell charged to 4.20V/cell typically delivers 300–500 cycles. If charged to only 4.10V/cell, the life can be prolonged to 600–1,000 cycles; 4.0V/cell should deliver 1,200–2,000 and 3.90V/cell should provide 2,400–4,000 cycles.

On the other hand, if I can charge the cells only to 2.88 volts due to the 5S configuration, and the above rule of thumb is correct,

On the negative side, a lower peak charge voltage reduces the capacity the battery stores. As a simple guideline, every 70mV reduction in charge voltage lowers the overall capacity by 10 percent. Applying the peak charge voltage on a subsequent charge will restore the full capacity.

then I would only be able to get to about 40% capacity which on the 5s7p 16.1 Ah pack would only be around 6.4 Ah. That may not be enough for cranking anything bigger than a 4 banger.

14.4 volts would be high for charging a 4S pack at -20 C, according to the chart... but these cells probably can't withstand any charging (at any voltage) at temperatures much below freezing anyway.

Cold temperatures are not a concern. I live in sout florida and almost never see temperatures reach below about 10 deg F.

14.4 volts on a 5S pack would be only 2.88 volts per cell; the discharge curve shows that to be perhaps 20% at -20 C, perhaps 97% at +25 C, and yes, maybe 86% at freezing (0 C). In any case, it's way below the normal operating voltage of these cells at reasonable temperatures. A 5S pack would provide quite a boost, but the voltage might be too high for the vehicle being boosted when fully charged, and it would not get properly charged by the running vehicle.

If the alt gives out 14.4V and the 4s settles to lets say 14.2V. Wouldn't a 5s package also settle at that same voltage? May be I am not understanding why a 5S package store/settle at a higher voltage than 4S.

 

[Ghey Thunder]

Just use an adjustable LVC for longevity charging, down to maybe 13.8V,

but tbh without holding CV stage, you could go a bit higher.

Sorry I don't know what this means because noob. Is an adjustable LCV something like a voltage output regulator that will lower the voltage from the alternator from 14.4V to 13.8V? What is a CV stage?

 

[john61ct]

Sorry brain fart, not LVC but HVC

High Voltage Cutoff

Like a solenoid/relay, switches off (isolates) the battery from the charge source circuit when it reaches your target voltage.

Battery when depleted will pull that circuit output voltage down below the setpoint, pulls maximum amps the source allows.

That is Bulk or CC stage.

Then as SoC climbs, eventually the circuit hits the setpoint,

usually the charge source limits voltage, this is CV or Absorb stage, current starts to decline due to battery resistance climbing.

But here I am saying your HVC halts charging when its setpoint (voltage lower than setpoint of charge source) is reached

so no CV stage, not needed for charging LI.