When going portable with your electronics projects or diving in to RC hobby you eventually have to deal with batteries. No secret that best what is available today is lithium batteries. They come in many flavors. Why use lithium battery instead of alkaline or NiMh rechargeable? The short answer is – energy density. Lithium Ion batteries can store large amount of energy comparing to its weight and can give out large currents. This technology made it possible to enjoy long and powerful RC model (like quadcopter) flights. To achieve same results with NiMh it looks almost impossible due to weight and low current output. Lithium battery technology is constantly improving as different chemistry is used.
You can find lots of info about different chemistry of Lithium Ion batteries so we won’t be getting into this.
Rechargeable Lithium batteries require special care all the time. You must charge them with proper chargers. You cannot overcharge them or over-discharge. Batteries are specified with maximum charge and discharge currents (C rating). In most cases batteries already come with small built-in circuit that cuts-off battery if it is discharged or current is bigger that max. But in special cases batteries do not have protection circuits. For instance RC hobby stuff. You wouldn’t want your flying model to crash due to voltage cut-off. This is why here you are left with your own solutions. Lets look a little bit at different Lithium batteries.
Li-Ion batteries use liquid (GEL) electrolyte which ensures Lithium Ion exchange between electrodes. Usually those batteries come in metal cans to preserve inner structure. These batteries are more suitable for stationary electronics where size and shape doesn’t matter much. LiIon battery nominal voltage is 3.7V. They can be charged up to 4.2V (%100 charge) and discharge should not be lower that 3.0V.
One significant downside of LiIon batteries is that electrolyte is flammable and explosive. If battery is damaged, too much current is drawn or overcharged it may explode. So it is not recommended to leave it charging unattended.
Practically speaking LiPo batteries characteristics doesn’t differ much from LiIon. The only big difference is that instead of liquid electrolyte there is dry material used. It gives more options to give it different shapes. This is why this type of battery is popular among RC hobbyists. You may find info that due to dry electrolyte consistence it is harder to reach high charge/discharge rates. To overcome this problem there are LiPo hybrids where dry polymer is immersed in gel. So these have structure of dry batteries and properties of liquid. Their voltage ratings are same 4.2V max charge, 3.7V nominal and 3.0V minimum low safe discharge. These batteries are also flammable.
Lithium Iron Phosphate (LiFePo4) battery uses newer technology that overcome LiPo problems. They are much safer as they are non explosive. They can provide higher discharge current and longer life. But this battery has different voltages: 3.6V max charge, working voltage 3.2V and min discharge 2.8V. So it requires different charger than LiPo or LiIon.
In order to get more capacity or higher voltage cells are combined into battery packs. This is normally done by manufacturers as they have equipment to match internal resistance of cells and keep similar characteristics between cells in pack. So you shouldn’t build your own packs. Every battery pack is marked with “S” “P” marks indicating the number of batteries connected in series and parallel. So if you find 2S2P this means that two cells are in series and then connected in parallel. Parallel connected cells have same voltage as single cell 3.7V for LiPo but double voltage. When cells are connected in series, voltage doubles 2*3.7 = 7.4V but capacity is same as single cell. Parallel cells give double discharge current.
Lithium batteries are marked with C rating. This indicates the maximum current battery can be discharged. 1C is equal to battery capacity. So if battery capacity is 5000mAh and is rated 30C this means that it can output 5A * 30 = 150A safely. It can be also characterized with chargind C rating. If not specified, then normally it is 1C. So 5000mAh battery should be charged at 5A or less.
Balancing battery packs
When battery is combined of several cells it needs balancing. This means that all cell voltages inside pack must be kept equal or this will lead to short life or pack damage. If you charge whole pack and not look at individual cell voltages there might be situation when one cell gets overcharged and them might explode or catch on fire. Same situation with discharge – if you just monitor all pack voltage some cells might get over-discharged while rest will have enough juice. This also may lead to short life or even death of pack. So this is why most chargers come with balancers. Batteries usually have separate cable for balancer. So when you charge battery you should turn balancing one to ensure that all cells are charged with same charge. Sometimes when you battery is well balanced and you are sure that it is healthy, you may charge it several times without balancing.
We talked a bit about charging currents. It is related to battery C rating. There might be some argues about charging current – make it 1C or try to make it much lower like 0.1C or 0.5C. It may seem that charging at lower current battery is less stressed, but in reality charging process last much longer and so degrading chemical processes have more time to kick in. I think if it is allowed to charge at 1C then you should do this. Of course you should monitor battery temperature as well. High temperature is a major factor of degrading battery life. Your battery should not exceed 40 – 50ºC neither during charge, neither use or storage. Your proper charger should have optional temperature sensor that can be stuch to battery pack during charge. Charger will cut the charging process if temperature reaches dangerous level.
Especially if you are RC hobbyist you probably have more than one battery pack. You don’t use them all everyday. So what is the safest way to store batteries so they would be same characteristics even after several months. First rule is to charge batteries to its nominal voltage. If LiPo is rated 3.7V then it should be charged to 3.7V. Fully charged battery degrades significantly fast. Also don’t leave discharged batteries as they loose capacity this way. Overall it is generally recommended not to fully charge your battery to 4.2V (100%) if you want to prolong life of it. As rule of thumb is to charge LiPo batteries to 80% of its capacity. It is better to buy larger capacity battery than always fully charge smaller one. And the best way to store batteries is to keep them cool.
LiFe batteries are better at storing. LiFe batteries don’t suffer if fully charged. So they can be stored fully charged. Once stored, it is good practice to check batteries at lest once a month. Batteries slowly discharge during time and who knows you might find some cells dropped to dangerous discharge level. So you may need to recharge to keep them at optimal level.
You can imagine how high energy those batteries can store. So safety is first when working with them:
- Do not overcharge battery over safe maximum limit (4.2V for LiPo);
- Do not overcharge bellow safe minimal limit (3.0V for LiPo);
- keep charging and discharging currents within safe margins (follow C rating);
- Do not let it overheat neither internally, neither from external sources like sun;
- Do not damage mechanically. Dents and bumps are not OK;
- Use protection circuits or at least alarm circuits for monitoring;
- Do not leave charging on flammable surfaces. Use LiPo bags;
- Do not leave charging unattended.
This is really impossible to cover all info about batteries as there are lots of various cases and types. If you are dealing with lithium batteries make sure you understand all requirements, dangers and limits. If you have questions or want to add something, be sure to leave a comment or join our forum for further discussion.