LiFePO4 heating pad for cold temperatures

[convme]

This solution would be an external one to solve the problem with charging over Solar-Power, right?

I'm afraid, I need an internal solution, that has to be placed inside the battery and handle any kind of chargers. In fact I have three different types of chargers putting load into the battery and through the same ports I also use to serve all devices. So I'll need a more general solution.

I also don't know if the heating pads would handle the higher voltage coming directly from the panels if you pass the scc.

 

[convme]

Yeah there would need to be some smarts to it, like when the sensor detects the temp is right it would still continue to heat for a little while to give time for heat to transfer to the middle of the cell. The timer could be based on the initial temperature. So if you're going from -50C then you know you need to heat them longer before you add charge.

Yeah, these kind of details also have to be solved, but I think it is a question of configuration. E.g. if your sensor does not support a delay time, you could also set the desired temperatur 1-2°C higher than necessary. This would force the pads to heat longer.

The placement of the pads and the sensor is an other question, that has to be solved. In my configuration I've got an isolated case with 3 heating pads at the walls/sides in opposite. In fact there are four pads, but the last one is a bms . The cells are heated up from outside to the inside. And so the temperatur sensor is placed far away from the pads somewhere in the edge of the case. In fact I'm not sensing the temperature of the cells but the temperature inside the case. The idea behind is that coldness enters the battery from outside and therefore the sides cool down first. They also should warm up last.

 

[SCClockDr]

The aim is to avoid interruption of the charge process then if that becomes not possible then interrupt it but attempt to recover the battery temperature of solar power is available, correct?​

So

1. At say 39°F begin diverting SCC output to the heating system.
2. At say 35°F
   1. Interrupt the PV>SCC link
   2. Switch in a buck step down supply to power the heating system.
3. Now you either revert to grid power to heat the battery or wait for conditions to improve.
4. Implement an improved insulation/heat production program for the future.

 

[FilterGuy]

If the batteries are in a very well insulated box, you don't need much power to keep them warm. A 120 W heat pad would only need to run a 10-20 minutes a day to keep things warm. Consequently, I would stay away from the complexity of trying to only use charge power. It just adds parts that might fail. A simple thermostat on a heating pad is far less likely to fail.

Ok, what if we put a relay between the PV and the SCC?
It triggers the PV relay to switch and instead pass power directly to the heating pad, bypassing the SCC.

This works, but you have to pay very close attention to the resistance of your heating pad. If you 'just' use a 12 V pad and a 12v nominal solar panel, you are likely to be so far off of the Max Power Point that you may only get 20% to 30% of your power.

I am planning on using a small (20w 24V) solar panel dedicated to directly powering a restive heater inside a well insulated battery storage box. (In this case, the batteries will be disconnected and I will not have charging power at all so I don't want to charge use power of the battery)

Here is how I calculated the resistance I need for my heat pad.
Solar panel specs:
Vmpp = 35.2v
Impp =.52 amps.

V=IR so Rmpp = 35.2/.52= 67.7

That means that to get the most out of the little panel, 67.7 ohms for my heat pad.

Unfortunately the resistance of the heat-pad is never given. Just the voltage and the wattage. Worse yet, you don't know what voltage they use for calculating wattage. If it is a 12 volt pad, do they use 12 volts or 13.6 volt?

I am considering a couple of options for my resistivity heat source.
1) Wire 22 3-ohm resisters in series and glue them to the bottom of an aluminum plate that the battery sits on. This would create a gentle heat that the plate spreads and dissipates.

2) Glue this to the bottom of an aluminum plate. https://www.amazon.com/dp/B00PM87W3I and hook the panel directly to it.
The pad is 250 W at 120V
Watts Law P=IV
Ohms Law V=IR
A little algebra and you R=V^2/P=120^2/250 = 57.6 ohms

It is close to the correct resistance and would easily handle the 20 watts I would be feeding it.

 

[Bob B]

@FilterGuy .... I think your math would work with the DC pad, but, I assume the 120V pad is AC and becomes more of an inductive load than a resistive load. I would difinitely measure the resistance of that pad to make sure your calculations are working.

 

[convme]

Ok, seems I made a mistake thinking about the stuff, sorry.

Having the same port for charging & discharging, the electrical consumers are still powered by the charging device, even if the battery itself is cut off by a relay. So there is no necessity for a DC/DC buck step down inverter to be installed. That's because they are all connected to the same pols and so the circuit is still closed.

This means, the combination of a temperatur-sensor, having attached a voltage-controller at it's signal line and a SPDT-Relay hanging on the signal of the voltage-controller on the other hand would cut off the battery and pass the current to the heating pads by switching the circuit. This would probably work and the devices attached to the battery poles are still served with power by the charger itself. (I hope, I'm think right here)

If the enclosed voltage drops under the charging voltage controlled by the voltage-controller or the temperature rises over a certain value controlled by the temperatur-sensor the relay would loose its signal and switch back to the original position. The pads would stop heating and the battery would be connected again.

This assumes that the temperature-controller will pass the original circuit to the voltage-controller without manipulating its parameters. One could also do it the other way round, but I'm not sure how the more complex temperature-controller will react to a loss of power. Also the display won't work anymore. The voltage-controller on the other hand I expect to be more robust. It's missing the display and you set the voltages by two small Potentiometers.

Here are the components I'm thinking of:
- Temperature-Controller: https://de.aliexpress.com/item/32867461365.html
- Voltage-Controller (battery protector): https://www.aliexpress.com/item/32857358407.html or https://www.aliexpress.com/item/32996091128.html (no overvoltage-protection)
- SPDT Relay: [still searching one with at least 200A permanent]
- Heating-Pads: https://www.aliexpress.com/item/4000448687939.html

And the rest (already ordered ) :
- Main battery switch: https://www.aliexpress.com/item/4000183685881.html (300A cutting plus)
- Monitoring: https://www.aliexpress.com/item/32888627400.html
- BMS: https://www.aliexpress.com/item/32921950553.html (12V 4S 200A common port)
- CELLS: https://www.aliexpress.com/item/33019962665.html (8S, always two of them in parallel)

EDIT: The case is very limited in its dimensions. Max. 350x370x210mm is everything I have, so insulation is also very limited, too. That was also the reason to buy these cells, they simply match very well in size (especially height), although it seems like I'm going to get B-cells. We'll see...

 

[FilterGuy]

@FilterGuy .... I think your math would work with the DC pad, but, I assume the 120V pad is AC and becomes more of an inductive load than a resistive load. I would difinitely measure the resistance of that pad to make sure your calculations are working.

I can't imagine where any inductance would come from. It is just a resistivity element that is heating up.

 

[Bob B]

If you connect 120V DC to a 120VAC light bulb .... it will blow the element immediately. Loads are different with AC than they are with DC.

 

[convme]

- SPDT Relay: [still searching one with at least 200A permanent]

Ok, this might be a bit harder to find than I thought. Don't know why but till now for a SPDT it stops somewhere at 100A. I found some DPDT-Relays for a fork-lift truck having 200 and 400 Amps, but my place inside the case is limited and somehow they feel a little bit overkill.

I've seen my pads don't take much power and the two controllers should be able to pass 10A to the load, so I think it should work to use the signal line directly on the pads and just use a NC SPST-Relay to cut the battery-line. These relays are easier to find.

 

[FilterGuy]

If you connect 120V DC to a 120VAC light bulb .... it will blow the element immediately. Loads are different with AC than they are with DC.

AC voltage is measured as the RMS (Root-Means-Squared) of the wave form, not the peak voltage. The RMS voltage gives you a 'DC resistive equivalence" (On a resistive load the heat generation would be the same as if it were DC). https://www.quora.com/Why-are-RMS-Root-Mean-Square-values-used-in-AC-circuits

Does a heating pad have inductive or capacitance? I would not have thought so.... but I am always learning new surprises. If I buy that pad, I will measure the resistance and post the results here. (I would have measured it anyway, but now I'll post it so we can all learn one way or another)

 

[Bob B]

The reistance in an AC circuit is impedance .... not resistance.

 

[convme]

- Temperature-Controller: https://de.aliexpress.com/item/32867461365.html

Hmm, I think it's time to rethink this too. The only reason I liked this controller over the others was the low temperature buzzer alarm installed, which could be used as a last hope for the battery if the heating pads couldn't keep the temperature any longer above the freezing point. In this case hearing the buzzer I could shut down the whole system by the main power switch. That sounds nicely in theory, but practically the battery is installed inside the car and being self in the car I'm sure it will never be cold enough to ever hear this buzzer. On the other hand it would come into critical areas when no one is inside the car to keep it warm, switch off the battery or even hear the buzzer. So, the idea is crap.

Having a relay installed and a charging-voltage detector I don't need to keep the battery any longer above the freezing point. In theory it can even cool down until -25°C. So, there's no real need anymore for the buzzer, maybe to warn the user about high temperatures that he could better understand why the BMS just shut down everything. ?

In this case it may be a better idea to find a simplier temperature-controlling module, especially because I just realized, that this one has a battery installed on the board. For what reason that?

 

[Bob B]

AC voltage is measured as the RMS (Root-Means-Squared) of the wave form, not the peak voltage. The RMS voltage gives you a 'DC resistive equivalence" (On a resistive load the heat generation would be the same as if it were DC). https://www.quora.com/Why-are-RMS-Root-Mean-Square-values-used-in-AC-circuits

Does a heating pad have inductive or capacitance? I would not have thought so.... but I am always learning new surprises. If I buy that pad, I will measure the resistance and post the results here. (I would have measured it anyway, but now I'll post it so we can all learn one way or another)

Any time you have current flow in an electric circuit a magnetic field is developed..... even on a straight wire. The constant expansion and contraction of that magnetic field in an AC curcuit causes the impedance of that circuit to be different than the resistance to DC current flow.

 

[FilterGuy]

The reistance in an AC circuit is impedance .... not resistance.

Well, yes

Electrical impedance is the amount of opposition that a circuit presents to current or voltage change. (Wikapedia)

Impedance is a measure that includes total opposition due to resistance, inductance and capacitance. If a load is purely restive, Impedance=Resistance. If you put 120V(RMS) across a 120 ohm resister the current will be 1 amp either way. (BTW, The AC current is an RMS value as well)

Notice the definition of impedance does not specify AC or DC. The reason we don't talk about DC impedance is that at pure DC (0hz) inductance and capacitance have no effect. (Note: When you connect or disconnect DC, it is no longer pure dc. The voltage and current are changing so inductance and capacitance does matter)

1. Resistance of a circuit is the same regardless of frequency.
2. Any significant inductance in a 50 or 60 hz circuit is almost always going to come from a coil winding (Motor winding, Transformer, elector-magnet, etc)
3. Any significant capacitance in a 50 or 60 hz circuit is almost always going to come from a capacitor.

Notes and qualifiers:

• I use the term "significant" because *all* circuits have some amount of 'stray' inductance and capacitance. However, at 60 hz the capacitance or inductance needs to be fairly large to have any effect. Example: a tungsten coil in an incandescent light bulb will have a small amount of inductance. however, a small loosely wound coil in a vacuum will probably not have enough inductance to have a significant effect at 50 or 60 hz.
• I say "almost always" because there are exceptions. Example: in long distance transmission lines the inductance and capacitance of the lines between themselves and ground become significant) The key here is that the 'small' inductance and capacitance adds up over long distances.
• I specify 50 or 60 hz because, unlike resistance, frequency makes a difference to impedance. Example: The small impedance of the tungsten coil in a light bulb would have an insignificant effect at 60 hz but would have a very big effect at 6 Ghz.

In the case of a heating pad or an incandescent light bulb, it is hard to see where any significant inductance or capacitance would be involved.
I guess in the case of the heating pad, if the heat generating resistive wire snakes back and forth through the pad it would create some inductance, but I would think it would be too small to be significant. However, like I said, I learn new surprising things every day.

 

[Bob B]

@FilterGuy ..... So connect 120V DC to that heating pad and see what happens ..... or measure the AC current flow and see if it adds up to a DC resitance measurement. A heating pad is not purely resistive .... it has quite a lot of wire and the magnetic field effect will be significant with AC applied.

 

[FilterGuy]

@FilterGuy ..... So connect 120V DC to that heating pad and see what happens ..... or measure the AC current flow and see if it adds up to a DC resitance measurement. A heating pad is not purely resistive .... it has quite a lot of wire and the magnetic field effect will be significant with AC applied.

That is the heart of the debate.... Is there inductance in the pad or not? My assumption is different than your assertion. Like I said, if I buy it I will measure the resistance and post it here. 'nuf said

 

[Bob B]

That is the heart of the debate.... Is there inductance in the pad or not? My assumption is different than your assertion. Like I said, if I buy it I will measure the resistance and post it here. 'nuf said

I started this conversation by advising that you take the precaution of checking the DC resistance if you are going to use a 120VAC heating pad in a DC circuit because it may not be as expected ..... sounds like you plan to do that .... have a nice day.

 

[backwash]

Here's an option for a battery heater. it includes temperature control so it won't get nearly as hot as the silicon heating elements I've been seeing. The 7W version is 6" x 12" and just a perfect size for the EVE 280ah battery cells and will work directly off my inverter output.

 

[FilterGuy]

Here's an option for a battery heater. it includes temperature control so it won't get nearly as hot as the silicon heating elements I've been seeing. The 7W version is 6" x 12" and just a perfect size for the EVE 280ah battery cells and will work directly off my inverter output.

View attachment 5528

That should work great if your battery box is reasonably insulated. I like it because it is *simple*. ( I get leery of complicated solutions that might introduce their own failures/problems. ) The only concern might be the temperature setting. It only needs to keep the battery above 35 or so. Anything above that is wasted energy.

 

[backwash]

My install is for a 5th wheel so will only activate it when it's cold/freezing out in the desert areas. It's also way gentler on the batteries than the silicon heating pads that go up to 80deg C. (180+ Fahrenheit ! which is way too high for LFP) I'll be able to monitor the bat temp with my BMV-712 so it will be an easy manner to determine a good setting of 40-50deg F. anything above 35 or 40 should be good for charging during cold days. I've ordered two so will let everyone know how they work out.