Charging lithium batteries

[slepax]

Hopefully my question would still fit here, it's about the process of charging lithium batteries. My question are quite specific.

Following stage 1 (constant current) and moving onto stage 2 (saturation), is it the responsibility of the charger to reduce the current gradually? Or will it happen naturally because the battery reached it's peak voltage?

Thanks!

[Audioguru]

The charger must limit the maximum peak voltage. The lithium will explode or catch on fire if its charging voltage becomes too high. When the battery is 70% to 80% fully charged then it will reduce the changing current gradually. The charger must monitor the charging current and disconnect charging when the charging current drops to about 3% of its rated mAh current.
Go to www.batteryuniversity.com and read all about it. Use a charger IC designed to charge a lithium battery.

[Maxlor]

Following stage 1 (constant current) and moving onto stage 2 (saturation), is it the responsibility of the charger to reduce the current gradually? Or will it happen naturally because the battery reached it's peak voltage?

It will happen naturally if you use a constant charging voltage for stage 2. It will not go down to 0 however, a trickle will always remain. Since lithium batteries shouldn't be trickle charged when full, the charging process should be stopped manually (or by some controller) once the current becomes small.

[amyk]

Following stage 1 (constant current) and moving onto stage 2 (saturation), is it the responsibility of the charger to reduce the current gradually? Or will it happen naturally because the battery reached it's peak voltage?

It will happen naturally if you use a constant charging voltage for stage 2. It will not go down to 0 however, a trickle will always remain. Since lithium batteries shouldn't be trickle charged when full, the charging process should be stopped manually (or by some controller) once the current becomes small.

The prohibition against trickle-charging lithium cells is often misunderstood; it refers to the type of trickle-charging common for nickel-based chemistries, where a small constant current is applied regardless of the voltage required to reach that current. That is harmful to lithium cells, and very different from just keeping the cells at fully-charged voltage, where any charging current is miniscule and only compensates for the self-discharge.

It is better to think of the charging process as limited voltage and current; when the voltage is below the maximum, current into the cell is limited to below the maximum charging current, while the voltage gradually rises. Once the voltage reaches its limit, it will stay there while the current gradually becomes almost 0. The current and voltage need not be truly constant, but only limited to below the maximum values.

[edpalmer42]

I've heard that you can 'float' lithium batteries at their resting voltages without a penalty.  So that would be ~3V7 for lithium-ion or lithium-poly(?) or ~3V3 for lithium-iron.  But I've never heard whether this is an officially sanctioned idea.  It seems like any use of lithium batteries in a solar-powered system would be used like this.

Anyone have any more info on this idea, preferably from an official source rather than a DIY source?

Ed

[slepax]

Thanks for the answers, so if I understand this correctly then:

- A good lithium charger should have a setting for battery capacity so it knows when to stop?

- One cannot replace a battery with another one of higher capacity (single cell, so same voltage) without adjusting the charger?

[IanB]

Thanks for the answers, so if I understand this correctly then:

- A good lithium charger should have a setting for battery capacity so it knows when to stop?

- One cannot replace a battery with another one of higher capacity (single cell, so same voltage) without adjusting the charger?

Neither is really true. You can usually change a larger battery than the charger is designed for, but not a smaller one.

[slepax]

Neither is really true. You can usually change a larger battery than the charger is designed for, but not a smaller one.

So I'm missing something here. Charging should terminate when the current reaches 0.1C, how would the charger know what 0.1C is if we don't define capacity first?

[IanB]

So I'm missing something here. Charging should terminate when the current reaches 0.1C, how would the charger know what 0.1C is if we don't define capacity first?

The requirements for charging are that:

1) You must not exceed I_max

The value of I_max depends on the battery, and usually I_max is proportional to the capacity of the battery. So if you use a smaller battery than the charger was intended for then I_max may be exceeded. This is not good as it could lead to overheating and battery damage.

2) You must not exceed V_max

The value of V_max depends on the chemistry and is usually 4.2 V for older designs of lithium ion. However it could be 4.3 V for some newer products on the market.

3) Charging should stop completely when the battery is full

The definition of "full" is quite flexible, and is most readily detected when the charging current falls to a "small" value during the CV phase. Setting the stop point at 0.1C is just a rule of thumb. Whether you stop charging at 100 mA, or 10 mA or 1 mA doesn't really affect the battery at all, but it might take a really long time for a large battery to get down to 1 mA. So chargers tend to stop charging at a relatively high current to avoid wasting time. It's a law of diminishing returns anyway. There's no big difference in available charge between a battery that is 99% full and a battery that is 99.9% full.