charge voltage for lifepo4 battery

[SnakeBite]

Hi,

i recently purchase a lifepo4 battery 12v 40AH and i building a charging circuit for it based on TI BQ24630. in the datasheet of the IC you need to set by voltage divider the destination charge voltage. in the datasheet below says the nominal voltage is 12v. the minimum discharge voltage is 10v the maximum charge voltage is 16v. i want the maximize the capacity of the battery BUT still maintain the battery life.

what is the voltage destination that i need to set?

thanks
Ido

[M. András]

each lipo cell is at 4.2v max so if you charge them to 4.1 you will keep them happy for longer time

[Psi]

He's talking about LiFe, not LiPo

[ConKbot]

LiFePO4 (LFE) cells charge at 3.6 V/cell, but do not float charge them at that.  Charge to 3.6 or 3.65v/cell, balance them so each is equal (looks like your battery either has internal balancing or no balancing, if the former, then dont worry about balancing, if the latter, I'd be wary) then stop charging until drops below 3.25V/cell.  So since your battery is a 4S battery, charge to 14.4/14.6V, wait for the current to drop, to C/10 or C/30 (not a lot of extra charge gets accumulated waiting for C/30, but if every last bit counts...) It should 'rest' at 3.3v/cell, or 13.2V, and then if it drops to under 13.0V, you can top it up again.  Float charging LFE cells at the 3.6V leads to short cell life. 

Luckily, this charge regime is fairly compatible with 12V Lead acid systems, hence the availability of drop in replacements for cars.  A 13.8V float after a peak to 14.6, isnt ideal, but workable.

[amyk]

That one OP mentions appears to be yttrium-phosphate, not regular phosphate. Hence the 4V maximum charge voltage instead of 3.6.

[SnakeBite]

LiFePO4 (LFE) cells charge at 3.6 V/cell, but do not float charge them at that.  Charge to 3.6 or 3.65v/cell, balance them so each is equal (looks like your battery either has internal balancing or no balancing, if the former, then dont worry about balancing, if the latter, I'd be wary) then stop charging until drops below 3.25V/cell.  So since your battery is a 4S battery, charge to 14.4/14.6V, wait for the current to drop, to C/10 or C/30 (not a lot of extra charge gets accumulated waiting for C/30, but if every last bit counts...) It should 'rest' at 3.3v/cell, or 13.2V, and then if it drops to under 13.0V, you can top it up again.  Float charging LFE cells at the 3.6V leads to short cell life. 

Luckily, this charge regime is fairly compatible with 12V Lead acid systems, hence the availability of drop in replacements for cars.  A 13.8V float after a peak to 14.6, isnt ideal, but workable.

my battery doesn't need balancing.
all the charging is supervise by the TI BQ24630 (charge stages :CC and CV, over-voltage , over-current)
i didn't quite understand what is float charging ??

[ConKbot]

LiFePO4 (LFE) cells charge at 3.6 V/cell, but do not float charge them at that.  Charge to 3.6 or 3.65v/cell, balance them so each is equal (looks like your battery either has internal balancing or no balancing, if the former, then dont worry about balancing, if the latter, I'd be wary) then stop charging until drops below 3.25V/cell.  So since your battery is a 4S battery, charge to 14.4/14.6V, wait for the current to drop, to C/10 or C/30 (not a lot of extra charge gets accumulated waiting for C/30, but if every last bit counts...) It should 'rest' at 3.3v/cell, or 13.2V, and then if it drops to under 13.0V, you can top it up again.  Float charging LFE cells at the 3.6V leads to short cell life. 

Luckily, this charge regime is fairly compatible with 12V Lead acid systems, hence the availability of drop in replacements for cars.  A 13.8V float after a peak to 14.6, isnt ideal, but workable.

my battery doesn't need balancing.
all the charging is supervise by the TI BQ24630 (charge stages :CC and CV, over-voltage , over-current)
i didn't quite understand what is float charging ??

Float charging is CV for an extended period of time, even after current drops below a cutoff threshold, typically for standby usage so the battery is always 100% full and ready, and to compensate for self discharge. Its typically used in lead acid chargers after a 14.5 volt or so peak, and in cheap and dirty Li-Po battery chargers which are just a 4.2v voltage source with a resistor put in series to limit current. Not a problem on the former, possibly problematic on the latter especially if the 4.2v isnt very accurate.

[Rick Law]

.... .... .... Float charging is CV for an extended period of time, even after current drops below a cutoff threshold, typically for standby usage so the battery is always 100% full and ready, and to compensate for self discharge. Its typically used in lead acid chargers after a 14.5 volt or so peak ... ... ... ...

I know some off the shelf one sits at 14.5V.  I think that is cutting it a bit too close.  Even at below room temperature of 20C (68F), lead acid battery starts gassing at 14.49volt at 20C.

I usually soak SLA at 14V max (expecting a drift of 13.8 to 14.2).

[M. András]

He's talking about LiFe, not LiPo

my bad misread it.

[Rick Law]

...
my battery doesn't need balancing.
all the charging is supervise by the TI BQ24630 (charge stages :CC and CV, over-voltage , over-current)
i didn't quite understand what is float charging ??

re:my battery doesn't need balancing...

Battery balancing is balancing individual cells of a battery pack.  Good examples are car battery, a single car battery (approx 12V) consist of six 2V cells.  Balancing is to ensure each individual cell is in good health.  CC/CV... are good, but is it doing CV on each cell or the pack?  If you are monitoring only the pack, CV means nothing.  You could have one cell being 3.7volt and another being 4.3volt while the pack saids, yeah, CV at 8V.

If your pack consists of cells in parallel, then CC means nothing.  One cell could be drawing 1.5Amp while the other is drawing 0.5Amp, so CC of 2Amp doesn't mean you are charging the cells at 1Amp each.

Balancing does exactly what the word implies.  Making sure that each battery is about the same charge and about the same voltage.

re:i didn't quite understand what is float charging ??

Some batteries (specifically lead acid) cannot be overcharged if the current and voltage are within specific limits.  Float charging is to keep it within those limit to slowly refresh the charge (topping it up).   So, for an SLA, stick it with 13.8volt'ish and at below 1/10C, it will never be over charged while it keeps on topping up.   (1/10C means 1/10 capacity: for example, for a 20Ah battery, that would be 1/10 would be 2A)

SLA and float charge gets along well.  NiMH can float charge for a bit.  LiIon doesn't like float charge at all.  SLA should be stored at full charge where as, LiIon actually prefer to be stored at 50% charged.

[SnakeBite]

...
my battery doesn't need balancing.
all the charging is supervise by the TI BQ24630 (charge stages :CC and CV, over-voltage , over-current)
i didn't quite understand what is float charging ??

re:my battery doesn't need balancing...

Battery balancing is balancing individual cells of a battery pack.  Good examples are car battery, a single car battery (approx 12V) consist of six 2V cells.  Balancing is to ensure each individual cell is in good health.  CC/CV... are good, but is it doing CV on each cell or the pack?  If you are monitoring only the pack, CV means nothing.  You could have one cell being 3.7volt and another being 4.3volt while the pack saids, yeah, CV at 8V.

If your pack consists of cells in parallel, then CC means nothing.  One cell could be drawing 1.5Amp while the other is drawing 0.5Amp, so CC of 2Amp doesn't mean you are charging the cells at 1Amp each.

Balancing does exactly what the word implies.  Making sure that each battery is about the same charge and about the same voltage.

re:i didn't quite understand what is float charging ??

Some batteries (specifically lead acid) cannot be overcharged if the current and voltage are within specific limits.  Float charging is to keep it within those limit to slowly refresh the charge (topping it up).   So, for an SLA, stick it with 13.8volt'ish and at below 1/10C, it will never be over charged while it keeps on topping up.   (1/10C means 1/10 capacity: for example, for a 20Ah battery, that would be 1/10 would be 2A)

SLA and float charge gets along well.  NiMH can float charge for a bit.  LiIon doesn't like float charge at all.  SLA should be stored at full charge where as, LiIon actually prefer to be stored at 50% charged.

i know what balancing is but i don't have a way to do that in my case because the battery only has two terminal (minus and plus like a car battery) and in the datasheet says to behave to this battery like it's a single cell SO i think it has one built in.

what charger IC is managing all that CC and CV stage during the charge process so i don't need to do much. it also have a topping off option but i'm not using that since my product is portable (not in a place that has AC power available). i just use the charger that i'm building to charge is between uses - much like a cellphone battery.

Ido