Introduction #

We’ve all probably seen videos and photos of Lithium bases batteries on fire. It is (I hope) a well-known fact that they can be dangerous if they are not handled properly. Not all Lithium-based batteries are the same so the safe operating temperatures will vary. It is usually considered safe to allow the battery to heat up to 60°C - 80°C, though, before it becomes dangerous. That being said, those high temperatures are far from ideal for the optimal performance and longevity of cells.

What makes the battery heat up (apart from keeping it close to some high-temperature source, like fire) is, of course, charging and discharging (including short-circuiting) at high currents. What we are interested the most here, is charging. In normal condition, a healthy battery charged at room temperature should not heat up more than 5°C. If, on the other hand, the battery is damaged, or becomes damaged during charging, it may heat up quickly and the results may be fatal.

For this reason, most hobby chargers have the possibility to connect the external temperature sensor that will turn off the charging if the temperature threshold is reached. While you can buy those sensors, it is also easy and cheap to do them yourself.

What do you need #

• LM35 sensor in a TO-92 package. It can be LM35CZ, LM35D, LM35DZ, it doesn’t matter. It should be around 0.3 - 0.5 EUR a piece on Aliexpress. It’s worth buying a pack of a few (and maybe making a gift to a friend).

• One of:
  • Female to Female Dupont wires, you can usually buy them from the same seller as the LM35. You need 3 but they come in larger quantities so you will get spares. They come at different lengths so choose whatever you think will be a good length for your sensor. I suggest something more than 10 cm. They shouldn’t cost you more than 1 EUR.
  • Ready-made 3pin female Dupont wires. They shouldn’t cost more than 2 EUR for a pack of 10.
  • 2.54mm pin headers. You can buy headers with 3 pins or larger ones that you will have to cut.
  • If you know how to crimp connectors, buy the kit for crimping Dupont wires yourself.
• Nice to have:
  • Heat shrink tube
  • Hot glue gun
  • Superglue
  • Soldering iron
  • Velcro cable ties

Build instructions #

All you have to do is to connect the 3 pins of LM35 sensors to the sensor connector in your charger. You can do this in one of a few ways, depending on how permanent and how handy you would like it to be. You have to attach the 3 pins of the LM35 sensor to the 3 pins in a temperature slot on the side of your charger:

This slot always looks the same - it is a rectangular hole with an additional cut on a ground side and 3 pins inside.

Don’t worry, you will not break anything if you connect the sensor the other way around. It will just not work and will show the wrong temperature.

The simplest wiring #

The simplest way of wiring the sensor is to use 3 Dupont female-female wires and plug the sensor on one side, then connect to the port on the other side.

Make it easier to plug #

To make the sensor easier to plug (if you plan on unplugging it) in the right order, you should make sure your plug pins are not loose. To do this you can either glue all of them together in the right order with a hot-glue / super-glue or use pre-made 3pin Dupont wires.

Alternatively, you can solder the 3 wires to 2.54mm pin headers that were cut to only be 3 pins (you may have to file it a little bit), like that:

After soldering, use the glue to make the connection more reliable.

Make the sensor connection permanent #

To make sure your sensor does not disconnect, you should attach it to the wires more permanently. If you do not want to solder you can use glue. If you choose on soldering, make sure you isolate the individual pins, preferably with a heat shrink tube.

If you don’t like any of that, at least use some tape to secure the sensor.

Label the top side of your plug #

The original sensor comes with a special type of plug that can only be inserted one way. We don’t have that luxury and while inserting your sensor the wrong way should not destroy it, you will not get proper results. For this reason, it is a good idea to mark the upper side of the connector somehow. Use a sharpie, a label printer, or whatever comes to your mind.

Sensor to the battery attachment #

The sensor will not work correctly if it’s not touching the battery. You can of course use tape or typical cable ties to make it stick but you can also use velcro cable ties. They come in few meters long tapes or premade short ties like this:

The end result might, for example, look like this:

or as simple as that:

You may also use a rubber band to attach the sensor to a battery.

Enabling the cut-off feature in the charger #

Specific charger models may have a slightly different way of doing that but they are all similar. The default temperature limit is sometimes as large as 80°C. This temperature is considered the critical one, below which the battery should not catch on fire but my advice would be to set it much lower, to something like 40°C (or even lower) as in normal circumstances the lipo battery should never raise its temperature more than 5°C above the initial temperature and we should charge it at around 25°C for best results. If the battery heats up more, it is worth noticing that and maybe lowering the charge current or verifying the battery’s health.

The iMaxB6 v2 / iMaxB6 mini iMax S65 have this set to 50°C by default, You can find it at:

BATT/PROGRAM SYSTEM SETTINGS -> Temp Cut-Off

Make sure it is set to ON and the temperature is set to the proper value (for example 40°C).

The iMaxB6AC and Redox Alpha v2 has this set to 80°C by default. You can find this setting at:

USER SET PROGRAM -> USB/Temp select

Make sure the Temp Cut-off and not USB Enable is chosen, and the temperature is set to proper value (for example 40°C).

It should now be much safer to charge your batteries and even leave them partially unattended as the charger should stop its operation before it has a chance to overheat.

Warning: you should not consider this protection mechanism fully secure. If your sensor or a connection breaks or if the charger starts to malfunction, you may still get into trouble.

Hint: make sure the charger can read a sensible temperature from the sensor just after starting charging. Your sensor or some cable might be broken but the charger will not warn you about it!