Questions and Answers for Li-ion Batteries

A list for questions:

1. Is it a good idea to mix old and new batteries to power your electronics?

2. Can I leave lithium ion batteries on the charger?

3. Can I replace a 2400mAh battery with a higher 3000mAH?

4. Can I use rechargeable a 1.2V battery instead of Alkaline 1.5V?

5. How to decide the battery charging current?

6. What decreases battery capacity the most?

7. Is fast charging bad for your battery?

8. Can I replace 3.6V Ni-MH battery pack with a 18650 battery in RC toys?

9. How does temperature affect Li-ion batteries?

Q1. Is it a good idea to mix old and new batteries to power your electronics?

A: No. As batteries discharge, the internal resistance increases. This will limit current flow from the new battery since most devices require batteries in series. If you insert one new and one (50%) used battery into the device:

1. Assuming the current drain remains the same, the old battery will run out after half the usual time. Unless you marked them or can measure them afterwards, you will have to throw out both batteries. Which means you have wasted half a perfectly good battery.

2. If the device is sensitive to voltage (say a torchlight), the combination of new & old will start at 2.85V (e.g.1.5+1.35) instead of 3V. The torchlight may be less bright even at the beginning. That is, perform less well.

3. Old batteries tend to degrade chemically faster than new batteries. If you keep them in the device unused for a long time, you may find that the old battery could leak after a few months vs a year for new unused batteries, ruining your device.

Q2. Can I leave lithium ion batteries on the charger?

A: With a properly designed charger, you should not have to take them off the charger when fully charged, the charger should disconnect and end the charging for you. Such as the XTAR chargers all can intelligently detect the fully charge and automatically stop charging.

If you have a poorly designed charger, then yes it could damage the battery. So buying a good, reputable brand charger is really necessary.

Q3. Can I replace a 2400mAh battery with a higher 3000mAH?

A: mAH is mostly a measure of charge in the battery. The larger it is , the larger the battery is, or higher the density of energy that the material battery is made of can store.

To know whether you can use 3000 mAH battery to replace your 2400 mAH battery , you would need to know:

– voltage (a 2400mAH 1v battery can most definitely not be replaced by 3000mAH 12v battery for e.g).

– internal resistance (this defines how much current needs to be drawn from it at a given instance in time.)

Q4. Can I use rechargeable a 1.2V battery instead of Alkaline 1.5V?

A: 1.5 V batteries are typically Alkaline-manganese and/or carbon zinc which have a native open-circuit voltage of 1.5 volts. But during discharge, they will drop to lower voltage levels due to becoming depleted and internal resistance to current flow. Knowing this, virtually all battery-powered devices are designed to be able to work with depleting batteries down to 1.1, 1.0 or even 0.9 volts per cell.

If you want to use rechargeable cells like NiMH or NiCD, it will work fine, even though those have a rated 1.2 volts per cell, somewhat lower than 1.5 of Alkalines.

The reason is that those two rechargeable chemistries generally have a flatter discharge curve and lower internal resistance. So as they are depleted, the voltage goes down much more slowly. They are generally regarded as depleted around 0.9 to 1.0 volts, similar to the end of life voltages of alkaline cells.

Q5. How to decide the battery charging current?

A: It depends on the battery chemistry, lithium ions are usually charged at 50% of it’s Ah rating and lead Acid batteries are usually charged at 10% of its Ah rating. For example, if the 18650 battery is 2000mAh then the charging current set to 1A.

When you choose 1A as the charging current, the charger may not charge with 1A immediately, for the chargers use three-stage charging method, only when it enter constant current stage, it will charge according to your choose. All XTAR chargers use this method.

Nowadays, there are many smart chargers that can intelligently select an optimal charging strategy by detecting the internal resistance of the charged battery, input power and battery type, makes charging faster and safer.

Q6. What decreases battery capacity the most?

A: the following decrease battery capacity dramatically:

1. High charging/discharging rates.

2. Charging the battery to 100%, discharging the battery to 0%.

3. Storing battery at low or high state of charge(SoC) for long duration.

4. Keeping device/battery plugged-in after charging has completed.

5. Storing/operating batteries in extreme temperatures, low or high.

Q7. Is fast charging bad for your battery?

A: It is not the speed of charging that degrades battery. It is the heat that may build up. For lithium ion batteries charging, XTAR chargers have mainly 3 stages:

1. Trickle charge (pre-charge to revive the over-discharged battery)

2. Bulk charge (constant current)

3. Saturation charge (constant voltage)

When you charge your battery from 0-80%, voltage rises to 4.18-4.20V with the constant current if 0.5-1C. After this, battery is set to a constant voltage where voltage is set to 4.2V as the limit, the charging current decreases as it reached from 80-100%. Here the charging process is carefully monitored by internal charging circuit and controller.

Slow or fast upper voltage is always limited to 4.2V. So fast charging is only at the constant current charging stage, and will always start to slow down at constant voltage stage. The manual selecting button on the chargers actually just switch the charging current in the constant current stage, not the whole charging process. Choosing the larger charging current can shorten your waiting time.

What’s more, the high-quality chargers have built-in temperature sensors to detect and monitor the real-time temperature of batteries. When the temperature is too high, the chargers will automatically adjust charging current to ensure safety.

That’s why we suggest to buy high quality and reputable brand battery chargers.

Q8. Can I replace 3.6V Ni-MH battery pack with a 18650 battery in RC toys?

A: Inadvisable. It will power the toy, but the problem will be the charging circuit. NiMH and Li-ion need different charging due to their different chemistry. Most likely the toy’ s charging is just a resistor voltage dropper and nothing more than that. Definitely not good scheme for lithium. If you know how to DIY, then a lithium cell charger module can be considered.

Q9. How does temperature affect Li-ion batteries?

A: As temperature affects the reaction rates of the various chemical reactions taking place when operating a battery, the balance between these reactions will be destroyed.

At high temperatures, mainly the side reactions like gassing get increased. This results in higher self-discharge, but also in reduced lifetime (by increased corrosion, increased derioration of battery components, …)

Low temperature decreases the conductivity of ionic conductors used in electrolytes, separators, or electrodes, which reduces the performance of a battery. Additionally, low temperatures also much decrease in diffusion. As diffusion is not voltage driven, there is a maximum current which can’t be topped by setting higher potentials.