Lithium battery chargers #

Everybody knows that, unlike with nickel batteries, when charging lithium based battery packs, we have to use specialized chargers, that monitor the charging process carefully. There are different variations of such products but they can usually be put into one of two categories:

• simple maintenance-free chargers that you just plug your battery to and wait for it to finish its job,
• advanced chargers, letting (and requiring you) to setup the charging parameters, monitor the charging process closely and do more than just normal charging.

The most popular chargers used in airsoft community seems to be the iMAX B3 and similar constructions / clones (for the simple category) and iMAX B6 and similar constructions / clones (for advanced category) and those will be the ones I will focus on in this article. The arguments presented in this article should apply for other products, though.

Simple vs advanced chargers #

Simple chargers pros (compared to advanced chargers):

• you do not need to understand anything about the batteries you charge, or read any instruction / manual - just plug the battery and wait for the LED it indicate the process was finished
• they are cheaper and smaller
• they are simpler to build so might be easier to fix if something breaks

Simple chargers cons (compared to advanced chargers):

• they do not have to many protection mechanisms - they cannot sense the battery temperature and turn the charging off when they overheat, they will not disable charging after some timeout, if anything goes wrong (they will try to charge the batteries forever),
• they use small charging current and there is no way to adjust it; this means you have to wait for battery to be charged longer - this isn’t ideal as you should not leave the battery that is being charged unsupervised (though, we probably all do sometimes),
• they will not show you the mAh that went into the battery so you have no way to check your battery real capacity which might be useful especially to verify the new battery as they sometimes are of much smaller real capacity than advertised - if you can detect this, you can return the battery,
• each cell is charged separately by a separate charging controller and the difference of their parameters will cause the cells to not be perfectly balanced and age at different speed,
• if they get damaged (and they are easier to break than the advanced chargers), it’s sometimes hard to notice that as the only indicator (the diode) may just show “charging state” all the time, without progress. It may also show the “charged” state to early. Since each channel is independent, some of the cells might be fully chargers while others might not and you may end up having very unevenly charged battery which might be very dangerous to use! The advanced chargers may also break but they have a microcontroller which monitors the charging is is very likely to notice the problem and will not allow the charging to continue.
• they do not let you discharge batteries which come in handy sometimes,
• they do not let you charge the batteries just to storage voltage which reduces the battery aging,
• there is now way to track your battery aging,
• you cannot see how much the battery was already charged, you can only see when its done, you also cannot check if it requires recharging,
• it can only handle lithium batteries,
• they may have worse electric parameters of the output voltage,
• they do not let you stop charging at voltage different than 4.2V

In addition to that, advanced chargers might have following features:

• the possibility of being powered from the DC voltage so, well, you can power them from an airsoft battery, so you can have a large BOX 11.1V battery that can be used to recharge the one that fits in your stock, even without the access to AC mains
• battery diagnosis possibilities - measuring how well the battery pack is balanced or the internal resistance of each cell

That being said, a lot of owners of the advanced chargers do not use most or even all of the additional features they have.

Why you might be interested in certain features #

Temperature monitoring / charging timeout #

The lithium based battery packs can really cause a lot of damage, if they break. The most dangerous time for them is when they are charged. It is crucial not to overcharge them and not to let them overheat.

The advanced chargers will usually have a timeout value after which the charging process will stop, if the battery was not charged by this time. This is a safety feature as the damaged cell might never reach the proper voltage and could just heat instead of charge.

You can also usually connect an external temperature sensor which you wrap around your battery to measure the temperature increase and disable charging if some temperature threshold is reached (usually 80°C by default). A healthy battery should not increase its temperature more than 5°C during charging in room temperature so you could lower this value.

Charging current adjustment #

To big charge current will damage your battery or at least make it age faster. While some batteries could be charged at high currents (for example some Turnigy batteries allow up to 5C), a 1C is considered always the safe current.

If you lower the current, that may make even less stress to the battery but it it will make it charge longer. Since it is advised to never leave the charged battery unattended, this may pose some additional risk.

By being able to choose the current, you can make the tradeoff between charging time and battery stress yourself.

Capacity meter #

The advanced chargers will show you the amount of mAh sucked or pushed to the battery. If you do a full charging or discharging cycle, you can check your real battery capacity. This is useful thing to check just after you buy your battery to make sure you didn’t get a lemon and you could recheck it every half a year to monitor battery aging.

Discharging / storage voltage #

As explained above, discharging the battery might be useful to check its real capacitance. But if you want to storage your battery for a longer period, doing that fully charged (or to deeply discharge) will age it faster. For this reason, advanced chargers can discharge/charge the battery to so called storage voltage (which is around 60-70% of the max voltage). Use this feature for all the batteries you do not plan on using in the upcoming months and also on all your spare/emergency batteries you take with you to the field (it don’t make sense to fully charge the spare batteries).

Even cell charging #

The simple chargers, like the iMax B3, contain 3 completely separate charger circuits that will charge each cell separately. Due to the manufacturing tolerances, each of them will stop the charge at slightly different voltage and will use slightly different charging current. This is not a problem if it is done once, but if one of your cells in the pack is charged differently each time you do it, over time, its parameters will start to differ more and more from the other cells, which will have a negative impact on the performance of the whole battery pack.

The advanced chargers operate differently and allow the more even cell charging.

Charging to less than 4.2V #

The research (external link) shows that the battery lifetime measured in the charge cycles before death increases if you lower the voltage to which the battery is charged. The 4.2V is usually considered fully charged battery but if you will only ever charge it till 4.1V, the number of charge cycles it survives may double (this is considered to be true for each 0.1V drop down to 3.9V.

Why do we charge to 4.2V, then? Because that way you can use the most of the battery capacity. If you reduce the charge voltage to 4.1V, the battery will be charged only to about 90%. I find it a nice trade-off, though.

How can you do this? Most chargers will not let you modify the max voltage directly (or will only let you change it very slightly) but they often have a separate charge setting for a Lipo battery (charge up to 4.2V) and for Li-ion (charge up to 4.1V). Just trick your charger and tell it it’s a Li-ion! Don’t do this when discharging though. If your charger do not let you change the cut of discharge voltage, it will use the predefined one. For Lipo it usually is 3.2V but for Li-ion it can be as low as 2.9V which would damage the Li-ion!

Note: you have to be a little careful with this feature. The deeper you discharge your batteries, the faster they age. If you are going to charge your battery only partially and you will shoot the same number of shots, it will be discharged to a lower voltage which reduces the effect we want to achieve. This is the reason I don’t suggest going below 4.1V, and I suggest to either use slightly bigger batteries or have a spare one!

Conclusions #

There are many things you can do to prolong your battery life and advanced chargers may help you with that. Since the cost difference is about the price of one lower end lipo battery, this investment may pay off quite soon especially if you use the additional features you have properly. If you don’t, you still gain a better balanced battery, that was charged with a cleaner, more precise voltage. While this will also decrease the battery aging, the differences might not be easy to notice. If all you want is to charge the battery without thinking about it much and you don’t care about battery aging or safety that much, the simple charger might be suitable for you.

The probability that something goes wrong with the battery during charging and it turns into flames is low, but if it does, the consequences might be terrible. While no charger gives you guarantee that nothing goes wrong, the advanced charger does have means of lowering the risk. Most of those means, however, only works if you configure/use them in the first place! If you don’t, your advanced charger is only slightly more safe than the cheap one.