Power electric heating pad from batteries?

Projects General Guidance
1

Hi,

I have two heating pads (Electric Heating Pad - 10cm x 5cm : ID 1481 : $5.95 : Adafruit Industries, Unique & fun DIY electronics and kits) and I would like to power them from batteries. To get the pads hot enough I need between 9 and 12 V. I would control the temperature for each pad with PWM and a N-channel Mosfet.
My problem is that it is indoor but I don't have any wall outlets available. I would also like to power my Arduino from the same power source. And it should be easy to charge. 15000 mAh and more would be nice to have and the price should be about 70$.

I was thinking about building such a battery pack myself but the 18650 cells are quite expensive.

I would be very grateful if you have an idea for a battery power source which suits my requirements.

Kind regards,
Tom

2

You should recognise that any heating application involves substantial amounts of power. To supply that from batteries you'd need to define how long you needed to power those pads at what power level and then allow for reasonable battery capacity.
For example if you needed to supply say 11 V @ 1.0A for 6 hours, that would consume 6000 mAh from sets of 3 cells in series. To support that with 18650 cells would need at least two arrays in parallel (allowing for the fact that you cannot get anything approaching the "rated" mAH form any cell in real life). In turn, that necessitates a very sophisticated charging and discharge control environment. That's a challenge at any price for a knowledgeable technician -a US$ 60 budget isn't feasible.

What alternatives can you consider?

Why battery power?

3

The best 18650 batteries you can currently buy are Panasonics rated at 3.4 AH.
I recently bought some and the rating is real, but they are not cheap.
I paid $12 each.
Its not possible to do what you want with Lithium Ions.
You might just do it with Sealed Lead Acids, like a small motorbike battery.

4

In most cases you don't need PWM, just turn the heating pads on and off. Do you really need that fine control ? Perhaps for some special chemical process ?
For RC are battery packs of 7.2V up to 12V, and there are 12V Li-ion battery packs. But don't buy a 12V li-ion battery pack from Ebay, they are unreliable and not safe.
Some use a battery pack from power tools.

With a battery pack there is always the risk that one cell will fail. Only the best battery packs are capable to charge every cell on its own.

Do you have to carry the battery around with you ?

At 15Ah, a normal gel-lead-acid battery would do fine.
Use a normal charger for lead-acic batteries.
It is simple, safe and straightforward.

You will find a lot 12V gel-lead-acid batteries at www.elektronica-online.nl and at www.conrad.nl
Search for : onderhoudsvrije loodaccu

While I was typing this, mauried also mentioned Sealed Lead Acid batteries.

5

I think you will need to first determine how much power these are going to eat. You will probably have to run experiments to see what voltage you have to use to reach the desired temperature.

6

Koepel:
In most cases you don't need PWM, just turn the heating pads on and off. Do you really need that fine control ? Perhaps for some special chemical process ?
For RC are battery packs of 7.2V up to 12V, and there are 12V Li-ion battery packs. But don't buy a 12V li-ion battery pack from Ebay, they are unreliable and not safe.
Some use a battery pack from power tools.

With a battery pack there is always the risk that one cell will fail. Only the best battery packs are capable to charge every cell on its own.

Do you have to carry the battery around with you ?

At 15Ah, a normal gel-lead-acid battery would do fine.
Use a normal charger for lead-acic batteries.
It is simple, safe and straightforward.

You will find a lot 12V gel-lead-acid batteries at www.elektronica-online.nl and at www.conrad.nl
Search for : onderhoudsvrije loodaccu

While I was typing this, mauried also mentioned Sealed Lead Acid batteries.

Gel cell batteries are not the same as AGM batteries
Gel cell batteries require a special Lower Voltage Charger.

I recommend using 12 Volt, 18AH, AGM Deep Cycle Sealed Lead Acid Batteries.
One 12Volt, 18AH AGM Battery will run one heating pad continuously for about 9 Hours.
Maybe, 18 hours at 50% Duty Cycle.

12 Volt, 18AH, AGM Deep Cycle = $35.00 each
AGM_12V_18AH

Use one battery per heating pad?

Use a $10.00 DC-to-DC Buck Inverter to make 5 Volts for Arduino.

EDIT:
I forgot to de-rate capacity for 50% Depth-Of-Discharge.
Which is still pushing the DOD limit.
So, about 4.5 hours continuously and 9 hours at 50% Duty Cycle.
Use one Battery per heater.

7

I reckon that simple deep-cycle sealed lead acid batteries will be cheaper than AGM batteries and perfectly adequate.

You should assume that the practical capacity of a lead-acid battery is about 50% of the sticker capacity. And make sure that the batteries are fully recharged as soon as possible after use.

A heater will need a very large battery.

...R

8

Robin2:
I reckon that simple deep-cycle sealed lead acid batteries will be cheaper than AGM batteries and perfectly adequate.

You should assume that the practical capacity of a lead-acid battery is about 50% of the sticker capacity. And make sure that the batteries are fully recharged as soon as possible after use.

A heater will need a very large battery.

...R

You are correct, in the real world,
Lead Acid batteries can not be 100% DOD and live a long life.

Heater specs:
only 1 amp at 5 volts
So, 2.4 amps at 12 volts?

The OP requested only 15AH.
Two 12V 18AH AGM's, one for each heater, at 50% DOD,
will actually supply 18 AH to the heaters and I am at budget of $70.

9

mrsummitville:
Heater specs:
only 1 amp at 5 volts
So, 2.4 amps at 12 volts?

With 2.4 amps it will get 2.4 times as hot - is that what you want?

Perhaps you need PWM to reduce the average voltage from the 12v battery.

...R

10

Robin2:
With 2.4 amps it will get 2.4 times as hot - is that what you want?

Perhaps you need PWM to reduce the average voltage from the 12v battery.

...R

Yes, that is exactly what the OP requested ...
9 Volt to 12 Volts Battery with 15AH capacity
PWM mosfet controlled temperature
And all for under $70 !

11

mrsummitville:
Yes, that is exactly what the OP requested ...

I see that now. But WHY, then, was 5v mentioned in Reply #7?

...R

12

Robin2:
Perhaps you need PWM to reduce the average voltage from the 12v battery.

Except as previously explained, for a heater you do not use PWM as such, you use s "simmerstat" function which is a very slow PWM, switching on and offer over a second or so cycle, using software.

13

Robin2:
I see that now. But WHY, then, was 5v mentioned in Reply #7?

...R

The heater has a "5 Volt at 1 amp" rating per Message #7.
We have to calculate what the amps would be at 12 Volts.

Because the OP wrote this ...

To get the pads hot enough I need between 9 and 12 V.

14

Referring to Reply #12, it sounds like the OP wants to overload his device. No bets for how long it will last.

...R

15

Robin2:
Referring to Reply #12, it sounds like the OP wants to overload his device. No bets for how long it will last.

...R

The device is rated from 5 Volts @ 1 amp to 12 Volts @ ? amps maximum.

16

mrsummitville:
The device is rated from 5 Volts @ 1 amp to 12 Volts @ ? amps maximum.

If it is a simple resistance heater then the resistance must be 5 ohms.

My understanding of heaters is that they are designed for a certain current - in this case 1 amp. If the OP wants to use 12v he should use PWM so the average voltage is 5v and the average current is 1 amp.

If that does not make the device hot enough he should probably get a different device. Pushing more amps through it will undoubtedly make it hotter - but (presumably) it was not designed for that.

...R

17

[http://www.adafruit.com/datasheets/Ultra%20Heating%20Fabric.pdf](http://"http://"http://www.adafruit.com/datasheets/Ultra Heating Fabric.pdf"")
A section of this heater fabric 10.2 cm wide will draw 1.33A at 12V (it appears the resistance goes up a bit with temperature). The device in question is described as 10 cm wide so it may draw slightly more.

The highest temperature they describe is 123°C (9.6 cm wide drawing 1.4A at 12V) so I'm guessing the fabric can stand at least that much.

18

Robin2:
If it is a simple resistance heater then the resistance must be 5 ohms.

My understanding of heaters is that they are designed for a certain current - in this case 1 amp. If the OP wants to use 12v he should use PWM so the average voltage is 5v and the average current is 1 amp.

If that does not make the device hot enough he should probably get a different device. Pushing more amps through it will undoubtedly make it hotter - but (presumably) it was not designed for that.

...R

You assumption that heater was designed for 5 Volts is flawed.
The specification for the heater is "from 5 Volts to 12 Volts"
At 5 volts it draws 1 amp.

5 Volts is not the maximum voltage.
12 volts is the maximum voltage.
5 Volts is more like the minimum voltage.

Correct, at 5 Volts the Heater Pad will be near 5 Ohms.
But at a higher temperature the Heating Element will have a higher resistance.

The Heater Pad was designed for, up to, 12 Volts.

19

That's a very weird heater then. Usually they are built for very specific voltages and power. We can PWM them to a lower power but that's not part of the specifications.