cheap temp monitor for batts? - RC Groups
Shop our Airplanes Products Drone Products Sales
Thread Tools
Dec 03, 2002, 03:42 AM
Registered User

cheap temp monitor for batts?

we all have thermistors and protection circuts left over from building 'naked' packs.

Are there any electronic smarties that can build a cheap project that uses these thermistors, and some way to:

1. stop all voltage to the pack


2. sound an audible alarm using a cheapo speaker from radioshack, or a PC speaker, etc.

is this feasable?

I wonder how much people would pay for them, if one offered such a device.
Sign up now
to remove ads between posts
Dec 03, 2002, 07:37 AM
Former bpp dude
That is an excellent idea notoatmeal! I will be making thermal probes for out SC2 charger that will shut it down and sound an alarm but did not think of making "stand alone" probes. The stand alone will be more expensive due to the relay needed to disconnect the pack from the charger. The circuit will be pretty simple also. I would imagine that it could sell in the 14-19 dollar range. It would be cheaper but decent relays that can handle the current & that are not huge automotive relays are not cheap. Again, great Idea. I am going to look into it (and I am sure others will also) & I will post the plans for those who would like to build one themselves. Thanks

Last edited by ls77; Dec 03, 2002 at 08:02 AM.
Dec 03, 2002, 07:40 AM
Registered User
jimsp's Avatar
Such a circuit would be trivial. Power to the battery would go through a relay controlled by a thermistor and voltage comparator circuit.. To initially close the relay you would have to push a switch to latch the relay on. If the comparator circuit turned the relay off because of a temperature problem you would have to manually push the button again to turn the relay on. This would only require a few parts certainly costing less than $10.
Dec 03, 2002, 03:24 PM
Registered User
I'd buy one for $20 USD!

I don't like worrying about going upstairs to read the paper, or to the bathroom, or etc, etc, etc and leaving batteries alone to get into trouble.

I think there is money to be made on this project, if some smartie can keep the costs per unit down!

furthermore, if someone posted a circut diagram showing what parts are needed, I can solder well enough to assemble it and throw it into a project box.

A simple kit would be ideal, as you would get more hobbyists building them!

how would you power the circut? I don't think you can use the charging leads, that means either batteries, or power it off 12V.

I'd prefer to see it running off the 12V powersupply... we all know that people get lazy about replacing dead batteries.
Last edited by notoatmeal; Dec 03, 2002 at 03:32 PM.
Dec 03, 2002, 04:05 PM
Registered User
jimsp's Avatar
Originally posted by notoatmeal
I'd buy one for $20 USD!

how would you power the circut? I don't think you can use the charging leads, that means either batteries, or power it off 12V.

I'd prefer to see it running off the 12V powersupply... we all know that people get lazy about replacing dead batteries.
Attached is a rough idea of the circuit I was thinking of. I would definately power it off the power supply. People using an AC charger could use a wall plug DC supply to power the circuit.

The variable resistor sets the temperature for which the relay would turn off. To start charging you would have to push the push button to latch on the relay. If power to the relay was cut because of temperature problem or there was a power interuption, charging could only begin again by pushing the push button.
Dec 03, 2002, 05:16 PM
Registered User
jimsp's Avatar
A simpler solution might be to use a normally closed thermo switch. The feed from the charger would go through this switch and you would set your pack on top of the switch which is only about .2" thick. If the pack overheats the switch would open and with most chargers this would stop the charging cycle even if the switch closes again.

Digikey has these ranging in temperatures from 15C to 115C with contacts rated at 3A at 120V so I think they should work be fine. Cost is $6-$7.
Dec 03, 2002, 05:35 PM
Registered User
fumblethumbs's Avatar
Remember all, you saw it here first, on the E-zone. Shoot, I'll buy one! -thumbs
Dec 03, 2002, 05:58 PM
Former bpp dude
There are many different ways to do this circuit. I will come out with my own as I am sure others will also. Actually when you think of it, its hard to believe that something like this is not out on the market yet. I think major recognition needs to go out to "notoatmeal" for the idea/inspiring us to make it reality.

Dec 03, 2002, 06:10 PM
Registered User
jimsp's Avatar
The more I think about it, the more I like the simplicity of using a thermo switch. You could cut a recess in a small piece of Styrofoam for the switch so the pack would have a flat surface to sit on the switch would more closely follow the temperature of the pack. For those concerned that their charger may resume charging after the switch closes, they could employ a relay that must be latched on using a push button switch.
Dec 03, 2002, 06:14 PM
Senior Moment Member
The thermal switch sounds like the simplest way to go, and it's an inexpensive solution. Overvoltage, overcurrent, etc. all lead to cell heating so monitoring temp rise seems to be a viable "check" on cell status, and a lot simpler to implement.

I do think there needs to be some kind of safeguard while charging. I've noticed that even with all the use of the surplus Li-Ion cells over the past year I don't recall a single problem with cell failure and venting. But I believe most were using the protection circuits provided by the pack assembler. At least during the charge cycle, so no failures, no problems. If anything went wrong (incorrect cell count, high amps, etc.) the protection circuit stopped the charge current before the cell failed.

Now we have all these new cells, making up packs and charging with no protection circuit and in just a couple of months we have already seen a number of failure incidents, even though the Li-Poly cells will tolerate more abuse than the Li-Ions.

Looks like those protection circuits were doing their job.
We need something for the Li-Polys.

Dec 03, 2002, 08:24 PM
registered user
DNA's Avatar
So, how hot does a Kokam cell get before it starts ballooning?
Last edited by DNA; Dec 03, 2002 at 08:43 PM.
Dec 04, 2002, 12:05 AM
Former bpp dude
Thats the big question. I will be finding out soon
Dec 04, 2002, 01:03 AM
Registered User
Danny Troy's Avatar
This thermal shut-off device is actually not new and has been around for at least 10+ years. I actually purchased several and sold most of them over the years. I think I have two left.
I'll post a photo within the next 24 hours, and hopefully I'll be able to find the manufacturer info. The small device simply strapped on a nicad pack while charging, and disconnected the pack when a certain temperature was reached. Off hand I don't remember what temp it was set for, but I know it was made for nicads, which would make it unreliable for nimh or lithium batteries, although they would probably work as a "last resort" before a pack self-destructed.
There were two versions. I had both. One had a reset button that would pop up when the peak temp was reached. The other model had no button and reset once the temp lowered.
The device looked ultra simple and was foolproof.

Other than that, the new Hobbico Triton charger has the thermal probe option for 8.95, but you need a Trition to be able to use it.
Dec 04, 2002, 03:32 AM
Registered User
a refined product would have 2 probes, I guess.

a "sensitive" one that was good for lithium cells, where temp rise is very unusual. the "not so sensitive" one would be better for nimh and nicads. those suckers can get hot when charging normally.

the tempature range would have to be set by concensus here. "how hot do cells normally get"

I definitely think an audible alarm is a must. I think this is more important than the voltage cutoff.

what good is voltage cutoff if the pack is already a molten, oozing mess?

ok, ok... it's still a great idea to kill the charging input.
Dec 04, 2002, 10:54 AM
Registered User
Danny Troy's Avatar
Here's the info on the thermal chargers I have. I thought I had both types but I only have two "automatic" left. The instructions show the type with the reset button on it, which is the better type, since it won't begin charging again after the pack cools.

I couldn't find any reference to the manufacturer, but maybe someone can find it in an exhaustive search. Also, maybe some enterprising person can start making these's gotta be since there's probably only one internal part.

Quick Reply

Thread Tools