# LTC4013 Low battery Pin question

Hello, I am trying to design 7s li-ion Battery with LTC4013 charger.

In the data sheet, Low Battery Pin (page 22), there says,

A commonly used low-battery voltage for a 6 cell battery
is 10.4V which represents the voltage with one of 6 cells
shorted. This LB voltage is set with an 86.6k resistor for
1.73V/cell, or 10.4V/6 with a 6-to-1 FB divider.

I think 1.73V/cell for low voltage is for Li-acid battery.

If I use Li-Ion battery with low voltage about 3.0V, then I think 150k ohm should be used for LB Pin.

Am I right?

Parents
• If I use Li-Ion battery with low voltage about 3.0V, then I think 150k ohm should be used for LB Pin.

I think that you misinterpret the Low Battery Pin.
It is stated: "In 2-stage and Li-Ion charging a transition below the LB threshold triggers a new charge cycle if terminated".
So LB-level determines at which voltage the battery is charged again. If you use 3,0V/cell, the the battery is only charged when fully discharged, not earlier. So I suggest that you use 3,8V/cell for LB-voltage instead.

• Dear Alex.

In the datasheet 15,

I think you are meaning Li-ion Recharge level. I can choose recharge voltage level 97% or 95.6% of Vfb with selecting mode pins. If voltage goes under 97% or 95.6% of fully charged voltage, then it recharges againg with out LB setting.

LB is for detecting deeply discharged battery. Am I misunderstanding?

My question is LB threshold. Li-ion Battery is normally used in 2.8V~to 4.2V.(nominal voltage :3.7~3.8V)

3.8V is not deeply discharged battery in Li-Ion. 3.8V has over 50% remaining capacity.

Low voltage with high current charging has bad effect so normally charging current is low when Battery goes in low voltage. In this case, 3.0V for low battery setting is adequate setting for LB pin.

And I think then to make 20uA, 150k R is needed for this.

Please, could you check this again?

Am I misunderstanding with LB, and Vfc(recharge)?

And One more.

How does LTC4013 can recognize connected battery is 6-series cell? 6 to 1 FB divider is in the LTC 4013's LB Pin?

Then If I use 7series cell,

For example, If I want to set LB for 3.0V for Li-Ion 7series battery (21V-7series)

then for 6 to 1 divider 21/6 = 3.5V setting is needed in this case? (175k)

• Hello,

You should set LB to a deep discharge voltage.

The LTC4013 does not know how many cells are connected. The LB voltage that you set (based on your LB resistance and the constant 20uA on that pin) is compared against the FB battery voltage. This FB battery voltage is essentially normalized for all cell counts.

So, (1) determine your stack's full voltage, (2) set your FB voltage divider resistors, (3) find the FB voltage at which you consider your battery to be deeply discharged, (4) set your LB resistor as:

RLB = VFB_deep_discharge / 20uA

And remember, that's the voltage on the FB pin, not the battery voltage itself.

Regards,

Zack

• Hello, Jack

I understood. Thanks for the reply.