Batteries pt.2 — Li-Ion and NiMH batteries charging guide

In the previous article I described the most popular battery types that are commonly used in robots — Li-Ion, Li-Poly and NiMH batteries. I also explained how to choose the right battery model for different robotic applications. So the only remaining question is how to charge them effectively. A convenient method of charging can be the determining factor during the battery selection process.

Consumer Li-Ion batteries and chargers

The word “consumer” is used in relation to foolproof batteries and chargers that are commonly used by average, often non-technical, users. The Li-Ion batteries are not available in “AA” form factor to avoid confusion (single Li-Ion cell works under 3.7V instead of 1.5V in regular AAs), but you can find them in 18650 and other cylindrical forms. Most of the consumer Li-Ion cells have a built-in protection circuit,and in my opinion they should bebe named as “consumer” only when protection is in place.

To charge them, you need a charger that is only slightly bigger than a battery holder and looks very similar to the holder itself., In fact it is a battery holder with a mini charging circuit ☺

Chargers for one or two 18650 Li-Ion cells

Fun fact: if you would like to find a charger for three 18650 Li-Ion batteries, you would have a hard time… it’s almost impossible. I only found one (ever)and it was in a set with the high-power LED flashlight (why would somebody put a flashlight in a charger?). I hope somebody will eventually start manufacturing chargers for three 18650s in the near future. The chargers for four batteries are available on the market.

If you would like to have a better control over the charging process, use one of the more intelligent chargers:

An intelligent charger for two cells

You will not only get more control but also better quality of the charging process, and therefore longer battery life. The model on the picture has adjustable contacts to fit different sizes of cells.

The drawback of using consumer, cylindrical batteries is the electrical contact which sometimes causes problems, when the quality of a holder is poor, or if your robot is exposed to vibrations.

Another drawbacks are the necessity of removing the batteries for charging and occupying an additional space by the battery holder.

To use the cylindrical cells in your robot, you only need to find an appropriate battery holder with couple springs, like this one:

A battery holder for three 18650 cells

Consumer NiMH batteries and chargers

When it comes to consumer NiMH batteries, the general rules are practically the same as for Li-Ions — you can buy easy to use batteries and chargers. The only difference is that there are “AA” or “AAA” forms of NiMHs rather than “18650” forms in Li-Ion. There are also C, D or 6F22 sizes available.

The chargers are similar:

A charger for 4 AA or AAA cells

Some models of the chargers work with both NiMH and Li-Ion cells.

Chargers and batteries for RC hobbyists

RC hobbyists should have enough knowledge and experience to deal with different types of connectors, battery chemistries, settings, modes of charging and microprocessor-powered chargers. Batteries which require more caution from the user side are available only in shops dedicated for professionals and makers, like HobbyKing.

RC (Radio-Controlled) cars and boats, less often drones, can be also powered by popular, consumer cylindrical batteries, but due to the drawbacks mentioned before, the RC hobbyists prefer the battery packs with wires and dedicated connectors. You can see examples below:

A battery packs: NiMH (left) and Li-Poly (right)

These battery packs can’t be put into the charger slots but need to be connected by cable. I will not get into very much detail but you should know that there are generally two groups of battery packs connectors:

  1. Power connectors — they have two pins. Their size depends on the maximum current. The most popular ones are XT30, XT60, XT90, T-connector, Tamiya and JST. The chargers are often sold with adapters for different connector types.
  2. Balancer connectors — only for battery packs with two or more cells. These are more standardized and JST-XH are the most popular. There are also smaller versions like JST-SH (Nano JST), JST-ZH (Micro JST). JST is a name of a leading manufacturer of these connectors. The balancer connector has two to eight pins (number of cells + 1).

You can charge these batteries using appropriate chargers that can be found in the same stores that the batteries you’re using:

Different chargers available in RC hobby stores

The black chargers from a photo above are the simplest, and they are capable of charging only Li-Ion batteries in the 1S, 2S or 3S configuration. The bottom left one is similar but has 3 modes — for LiPo, LiFe and NiMH chemistries. The biggest one is an example of advanced charger that is able to charge NiCd, NiMH, Pb and Li-Ion batteries and has many connectors:

Universal, programmable charger for different batteries

I would recommend such a programmable charger (not necessarily this brand ;) ) if you experiment a lot with different projects. The picture above shows the power connectors (on the left), and the balancer connectors (on the right) that are used to charge battery packs from 2S to 6S.

The cost effective solution

Many readers probably know that 18650 Li-Ion batteries have the best efficiency to price ratio. Thy are actually quite cheap however there are some drawback of using them:

  • They don’t have a “button” on the positive terminal and are shorter than consumer ones (no protection board), therefore it’s not easy to provide a good contact in the battery holder with the springs. You need to modify the holder, adding a flat, metallic pad, made from a good conductor.
  • Even if you provide a good contact, you don’t have the balancer connector in your battery pack. You need to solder something to springs… but sometimes it may be impossible because the plastic holder will begin to melt. Still not an ideal solution.

The solution I discovered is a battery holder for a single cell, with the different springs that can be adjusted for shorter cells:

Single-cell battery holder

Buy the needed amount of single-battery holders, connect the power and balancer wires and you have the cheapest power source, that is still easy to charge and keeps balance between the cells!

Here’s an example of a ready solution which we have developed at Husarion: https://husarion.com/manuals/husarion-add-ons/#husarion-add-ons-hbatterypack-2

Protection is not always good

In the previous article I mentioned that protection circuits, built into some of the batteries, are not too good for drones, because of the possibility of activating the protection when the drone is 10 meters above ground.

The other issue may occur when your charger is dedicated for Li-Poly batteries and you would like to use it to charge the consumer cells with the protection, using the custom-made holder with balancing wires.* The protection can be activated when, for example, battery is discharged below the lower limit to avoid further discharging and permanent damage. In effect, your charger may not even start charging because it thinks that one or more cells are damaged. So what you end up with is a discharged battery that you can’t recharge (yup, that’s bad). You can’t read about it in the manual and then decide if it’s possible to charge a cell with activated protection. You have to check that yourself.

The above comment doesn’t refer to consumer chargers, where you insert every cell separately — they are adapted to such scenarios and can charge even the cells with the initial voltage equal to 0V.

* - Li-Poly chargers, in general, can be used also for charging Li-Ion batteries, because of the same voltage characteristic, unless the chemistry is similar (there may be the exceptions that I don’t know yet).

As always, I hope it helps. Thanks for reading.