EE Looking for existing battery and temperature sw

[JimPghPA]

Hi everyone,

I would like to avoid re-inventing and also keep cost low.

I am designing a resistive heating circuit and require a battery with about 12 to 24 Watt hours capacity, something like the smaller size 6 volt lantern battery.

The battery will be discharge over 18 hours of use.

The voltage can be anything because I will chose the resistor to match it. 6 to 12 volts is preferred.

It should have an existing indicator of how much charge is left on it, and it should make electrical contact when it is placed into a space of proper size for containing it.

It would be nice if there are existing chargers that run on 120 AC, and 12 VDC.

So the question is, is there an existing rechargeable battery out there? And if so what is the part no. for it, and/or what is it used in.

I am also looking for a low cost temperature activated switch that will close if the temperature falls below 61 Fahrenheit. I would prefer solid state. I can design one using an opp amp, but again would like to avoid re-inventing if there is something all-ready out there.

Thanks in advance.

Sincerely;



JimPghPa

 

[eljefino]

Originally Posted By: JimPghPA

I am also looking for a low cost temperature activated switch that will close if the temperature falls below 61 Fahrenheit. I would prefer solid state. I can design one using an opp amp, but again would like to avoid re-inventing if there is something all-ready out there.


Can it tip or will it be level? Would a mercury thermostat work?

12 watt hours is very little; you could probably find a cordless drill battery that will click in and fit. There are universal setups for drill/sawzall/flashlight and depending on what brand you get you might get "pro options" like state-of-charge indicator.

 

[PandaBear]

Do you need it in a particular size or packaging?

I'd imagine buying some used mechanical T-stat for home AC/heating would be sufficient.

 

[JHZR2]

I think youre going to have to do some custom work. Id look at Military 5590 type batteries myself. Some newer models have a state of charge indicator.

Why not just use a voltmeter to provide SOC, built into your IC?

I think Id start with the guts from something like a fios or cable phone unit, as they have 12VDC backups. You can modify the interface to connect with whatever you wish.

How often do you want to replace the battery? You may need to oversize and go with a marine deep cycle VRLA if youre intending to use the system a lot.

The switch I'd use is called a winter watchman. Very simple device that closes a contact at a certain temperature to energize some equipment.

 

[JHZR2]

Originally Posted By: eljefino


12 watt hours is very little; you could probably find a cordless drill battery that will click in and fit.


+1. I was thinking that most 18650 and 26650 cells are roughly 2-4Ah at 3.6V, so its pretty close. And the flashlight guys are generally starting to use these so stuff exists.

The OP would need to be sure to have protected cells if using Li-ion, so it doesnt overcharge or more importantly in his application, over discharge.

 

[JimPghPA]

The product it will be in will be portable and will be transported. Therefore a mercury switch would not work.

The unit will be shipped with two battery packs, so one can be on a slow charge and the other used. This way a freshly charged battery can be put in at the beginning of each day.

As for size, I was thinking about something the size of a your standard check book with ledger, but maybe an inch and a quarter, to an inch and a half thick. It could be smaller than that if it has enough Watt hours.

The amount of current is small enough that the switch could be a mosfet, and the forward voltage drop X the current would not throw off the temperature reading significantly, or the sensor could be separate.

 

[PandaBear]

How big is the target volume?

If anything near 1k units or more, I'd say call up Maxim and National Semiconductor and see if they have something already works. If you are flexible enough some of their application engineers may do the simple back of the envelope design for you.

 

[JimPghPA]

The number of units will start off low. Probably 50 or 100. We hope the item will sell fast enough that the volume will be in the thousands in the future.

 

[JHZR2]

Originally Posted By: JimPghPA
The product it will be in will be portable and will be transported. Therefore a mercury switch would not work.

The unit will be shipped with two battery packs, so one can be on a slow charge and the other used. This way a freshly charged battery can be put in at the beginning of each day.

As for size, I was thinking about something the size of a your standard check book with ledger, but maybe an inch and a quarter, to an inch and a half thick. It could be smaller than that if it has enough Watt hours.

The amount of current is small enough that the switch could be a mosfet, and the forward voltage drop X the current would not throw off the temperature reading significantly, or the sensor could be separate.


What about bimettalic strip?

If youre talking that kind of form factor, IMO you should look at lithium polymer cells used in larger cellphones and smartphones. Nothing else is going to make that kind of form factor, you need prismatic cells.

And your connectivity is going to be funny, youll need to diode-protect and series them (youll need a few I bet), then maybe parallel charge them at a low rate off a commercial charger.

 

[JimPghPA]

Bi-metal might work, but it also might change set point over time because of g-shock from being moved around.

 

[doitmyself]

A small thermistor attached to a solid state switch seems like it might be the most "portable"/small.

I deal with temp alarms and started thinking along their product lines.

I wonder if this company has already invented your wheel:
http://www.thermalogic.com/pdf/TempGuardMicro.pdf

 

[JimPghPA]

Thanks everyone.

Doitmyself, I looked at that web site and will call them tomorrow.

I have designed something but by the time you add in the cost of making boards, it really is better to use something that all ready exist, if there is anything out there.

It never pays to re-invent the wheel, unless you are going to sell a heck of a lot of wheels.

Thanks again.