After we helped you to pick the correct LiPo battery for your car now it is time to learn more about how to charge and store your investment. Charging and storing heavily differs between NiMH and LiPo batteries as both show significant differences due to their chemistry.
First, let us explain the differences between the charging methods of NiMH and LiPo batteries. To determine if a NiMH is fully charged the Delta Peak method is used. During charging the overall voltage level of the battery pack rises up to a certain level (depending on how many cells are used in the pack). Then, when the cells are fully charged, the battery starts to become warm and the voltage drops a certain amount. This voltage drop is called Delta Peak (Peak as in peak and Delta as Δ meaning a change). The dropping voltage is recognized by the charger's software which then stops the charge. As the voltage drop that indicated a fully charged pack differs a bit between NiCd and NiMH packs (and also from manufacturer to manufacturer) many (mainly expensive) chargers have an adjustable Delta Peak setting making them suitable for many types of batteries and manufacturers. Also the Delta Peak setting is used to increase the voltage level of NiXX batteries during use as a slightly overcharged (deeper voltage drop) pack delivers more power. Generally spoken. This does not apply to every NiXX pack and also shortens the overall live of the battery. We strongly recommend stick with the recommendations of the batteries manufacturer in terms of the charger's Delta Peak setting.
U mean temperature?
Above we also mentioned that the cell temperature rises when charging a NiCd or NiMH battery pack. When only NiCd cells existed years ago (which are now more and more replaced by NiMH cells as they are more 'environmental safe') using the pack's temperature as an indicator to determine if a battery is fully charged was one of the most common methods. Nowadays some chargers additionally offer temperature cut-off but most deceives use Delta Peak as these chargers are much simpler in design as no additional temperature sensor is needed.
And LiPo?
Now that you know how NiCd and NiMH batteries are charged, it's time to go to the bottom of this feature: the mysteries of LiPo charging. As Lithium-Polymer (LiPo) batteries are of a different chemistry they need to be charged with a special charger to avoid damages of the battery. NEVER EVER charge a LiPo battery with a NiXX charger or with in NiXX mode! LiPo chargers or chargers with a LiPo mode use the so called Constant Current / Constant Voltage method. This means that the charge of a LiPo battery pack is separated into two different parts. First, the charger uses a constant current to bring the battery pack up to a voltage of 4.2V per cell. The charging current differs from battery to battery as is is highly recommended to use a charging current of not more then 1C. C indicated the capacity of a battery pack and means nothing more that - for example - a 5000mAh battery pack needs to be charged with 5A(mpere) of charging current. And a 4400mAh pack with not more than 4.4A. Simple as that. Most of todays LiPo batteries are approved to become charged with 1C. Exceptions are some special LiPo packs that allow for 2C (and more) charging meaning a current of 2 time the capacity of the pack (5000mAh x 2C = 10A of charging current). Again, please refer to the manual supplied with the battery pack of the information given on the manufacturers website.
Part two
Given our battery has reached a voltage of 4.2V per cell during charging, the charger now switches into the Constant Voltage mode. It now is important to hold a maximum voltage of 4.2V per (LiPo) cells as everything above means an overcharged cell and therefore potential danger (overcharged LiPo cells tend to explode …). By decreasing the charging current the charger tries to hold 4.2V per until a charging current of around 0.2A is left. In most cases this indicates that the pack is fully charged. Often the single cell voltage starts to drop below 4.2V further indicating a fully charged pack. The charger now stops charging and your pack is ready for use.
Set-up is important
Reading the above breaks again makes clear why it is so important to set-up a charger properly. Most chargers need to have the amount of cells indicated before charging begins to avoid overcharging. If you want to charge a 2S (2 LiPo cells in series) pack the charger needs to know that a 2S pack is connected. The same applies to the charging current. A 5000mAh pack needs a maximum charging current of 5A. A 3800mAh pack 3.8A. Well, but you know that already.
Storage
Opinions widely differ when it comes to storing LiPo batteries. Some say that the packs have to be stored fully charged. Other state that 40 to 50 percent of maximum charge is the key. The fact is: if you charge a pack at low ambient temperature, for example when charging packs outside in the cold, and you then store them in your warm(er) workshop, voltage can exceed 4.2V per cell as the cells warm up leading into a dangerous situation. As mentioned before: an overcharged pack (with a cell voltage >4.2V) CAN explode and start a fire. It not necessarily has to, but chances are. Therefore we would agree with those experts stating that a half charged pack is the safest. It is unlikely to increase in voltage higher that 4.2V per cell and also unlikely to drop in voltage below 3.0V (which is as destructive as overcharging) when the ambient temperatures change.
So, what to do? Some chargers offer a 'Storage' mode, which only puts a certain amount of charge into the battery. Just enough to avoid deep-discharge damage when temperatures fall and not too much if temperature rise and overcharging can become an issue. But what to do if your charger does not have a fancy Storage mode? Run your battery pack(s) until the controllers LiPo cut-off kicks in and recharge your pack until around half the capacity is put in. For example: If you have a 5000mAh pack, put around 2500mAh of charge in. This is best monitored with the LC display most of the modern chargers have. If your charger neither has a Storage mode nor sports a display then charge your pack for around 20 to 25 minutes with 1C. This basically has the same effect.
When storing LiPo batteries, always to this in a suitable and fire-resistant case or bag, away from explosive such as fuel or chemicals and . Many companies offer specific LiPo charging and storage bags made from inflammable material that avoid damage to the surroundings in the unlikely event that a packs goes 'BOOM' during charging or storage.
