LifePO4 GBS Amp Hour Testing 2.5v to 3.6v per cell

[dax]

The carnot cycle is not efficient, it's really only theoretical and the more I read from the suppose experts here, the more I realise they are clueless when it come to lifepo4. The biggest laugh is they continue to use li-ion as an example of how lifepo4 works and completely misunderstand them, meaning they just babble on making claims which don't represent the reality. Lifepo4 are totally different to li-ion in ever way when dealing with charge discharge.

Unless you have experienced lifepo4 systems where outside 24hr temps ranges from -3 to 35degC in winter and 25-50degC in summer, you're just whistling in the wind. It's also clear they have no idea of how to set up a lifepo4 system, relying again on last century lead acid approaches. If you set up lifepo4 in 500ah banks, using 50ah cells, there is very little increase in cell temps during charging. The only temps you have to worry about are changes to the ambient temperatures and that's when peltiers come in handy and as the majority of their usage would be in the day time, the energy lose is negligible in a decent system. Those in th USA are years behind Aus and probably decades in understanding.

[J.P.M.]

It is not so much about heat dissiption but heat distribution. My understanding is if there is nothing to transfer the heat away from the cells in the center of the battery and they have to rely on the cells around then to do this that they will end up being hotter than the cells on the ends of the battery. This difference in cell temperatures will mean that the cells in the center of the pack will age faster. It will also mean that the cell "leakage" in the hotter cells will be more so the battery will go out of balance faster.

Simon

I've done some applications involving power transformer cooling, but I've got no hard #'s on thermal design of batteries. However, my experience is such that conductive heat transfer - the type probably responsible for most of the heat transfer within a battery outward to the cover/case, combined with what's probably some convective heat transfer from the contained fluids will be an order of magnitude or so greater than the convective heat transfer that takes place between the outside of the case and the surrounding air. If so, the max. temp. difference between the interior of the battery and the cover will be a fair amount less than the temp. difference between the cover and ambient surroundings. If so, I'd suggest that if the battery is too hot to touch, there might be a need for some internal aux. cooling, otherwise, not.

My gut guess might be that the highest interior battery temp. will be no more than a few degrees warmer than the inside surface of the battery case.

As for efficiency, thermoelectric cooling is indeed only about 10 - 15 % as efficient as a Carnot cycle, with Carnot being the limit in terms of highest efficiency for a cycle operating between two temperatures. the thermoelectric effect is limited by the thermal and electrical properties of the materials being coupled to produce the effect. See the open literature for details.

Thermoelectric internal cooling of a battery is an interesting idea, but perhaps more as an academic investigation than practical. Seems more like a solution looking for a problem to me.

[karrak]

No heat is not going to be an issue for you. You will not be able to pull 200 amps from your batteries for any meaningful amount of time for them to get hot enough to worry about. Using 1% rule on battery voltage sag, your batteries are only good for 20 amps (C/5) or 1/10th what you want.

Is the 1% voltage sag rule from "Sunking's Little Red Rule Book". Which rule number is it? Maybe you could give some technical reasons for it.

[karrak]

I'm not as well versed and knowledgeable about batteries as some, but from the little I think I may know about heat transfer, that may not be a very efficient or workable method of heat dissipation.

It is not so much about heat dissiption but heat distribution. My understanding is if there is nothing to transfer the heat away from the cells in the center of the battery and they have to rely on the cells around then to do this that they will end up being hotter than the cells on the ends of the battery. This difference in cell temperatures will mean that the cells in the center of the pack will age faster. It will also mean that the cell "leakage" in the hotter cells will be more so the battery will go out of balance faster.

Simon

[karrak]

Lets do some analysis of using a peltier effect device to regulate the temperature.

From my logged data the average power efficiency of my ~ 10kWh battery is 94.5% and the average amount of power that is cycled through it on a daily basis is ~ 1.6kWh. The amount of heat generated in my battery is equal to 88Wh per day ((1-0.945)*1600). This equates to a average power of 3.7W (88/24).

My understanding from the very brief research that I have done is that the power efficiency of a peltier effect device is around 50%, the 10% quoted earlier is the Carnot cycle efficiency. This means that for every Watt of thermal power transfered it takes 2 Watts of electrical power. This means that to transfer the 3.7W of heat generated by my battery would take a huge 7.4W (177Wh/day) of draw from my power system.

If the battery is placed in a well insulated box and there is enough thermal mass in the box and the average ambient temperature over a 24 hour period is around 20C-25C one would not have to provide any electrical power above what is needed to remove the heat generated by the battery to keep it around 20C-25C.

I must say that before doing this rough analysis I was a little skeptical about using Peltier effect devices to keep the battery temperature regulated but not any more.

Hopefully there isn't anything wrong with my logic. I have never done any design work with Peltier effect devices.

Simon

[dax]

Ahh, peltier coolers are about 10% efficient. i'd sure like to be shown otherwise.

Of course they are but in a properly set up and insulated system they use very little energy and operate less than 2% of the time.

[Sunking]

No heat is not going to be an issue for you. You will not be able to pull 200 amps from your batteries for any meaningful amount of time for them to get hot enough to worry about. Using 1% rule on battery voltage sag, your batteries are only good for 20 amps (C/5) or 1/10th what you want.

Good luck, time for me to go play

[createthis]

Are you using an Inverter with its own LVD set to 10.5 volts? What are you going to do when the bateries drop to 13 volts with lots of charge left.

Correct: http://www.xantrex.com/documents/Pow...rter%20NA).pdf

It's redundant, but I'll probably only lower the programmable LVD cutoff if I know I'm going to be drawing high loads. Otherwise I can bump it up to something more reasonable.

I don't really care if they've got lots of charge left. Nothing I can do about it, right, short of buying a different battery with lower Ri.

[createthis]

Jesse want to know how DIY EV builders deal with the Heat and Cold? They enclose their cells in a Fiber Glass or Acrylic tanks filled with Mineral Oil or Transmission Fluid. Or if they can get their hands on Transformer Oil.

Like you said before, I think I'm more worried about the cold at this point if it's going to be charging on my covered back porch in winter. Heat can probably be mitigated a number of different ways if I judge it as an issue during operation.

[Sunking]

Nah. It's programmable:

Are you using an Inverter with its own LVD set to 10.5 volts? What are you going to do when the batteries drop to 13 volts with lots of charge left in you cannot access. What about your cable losses have you accounted for their loss of 2 to 3% on top of the battery sag.

[Sunking]

Jesse want to know how DIY EV builders deal with the Heat and Cold? They enclose their cells in a Fiber Glass or Acrylic tanks filled with Mineral Oil or Transmission Fluid. Or if they can get their hands on Transformer Oil.

[createthis]

Yes you are missing something, your LVD operated at 12 volts, and you are sitting in the dark.

Nah. It's programmable:

https://www.victronenergy.com/upload/documents/Manual-Battery-Protect-12V-24V-65-A-100-A--220-A-EN-NL-FR-SE-DE-PT-ES-IT.pdf

[Sunking]

If it were me I would put the batteries and equipment in a ventilated steel cabinet

Now there is a really good way to get your arse sued. As stupid as mounting them to aluminum plates and Peltier modules. I bet you stand in water when you work with electricity with both hands tightly gripping your tools.

[J.P.M.]

I agree. The prismatic cells have plastic housings. Still, there is more surface area between cells than on the sides. Generally though, I'd say the prismatic cells are poorly designed for heat transfer.

Since (I'm guessing a bit here) the case is not usually designed primarily as a mechanism of heat transfer, part of my (incomplete) thinking involved getting the heat away from the case after it gets there. If the rate of heat transfer from the case to someplace else (that is, to the surroundings, and probably via thermal radiation) is less than the rate of heat transfer to the case (from the cells), the case temp. will increase until the two rates of heat transfer are equal. That may or may not be a tenable situation. Also, it might not be an easy thing to accomplish in a practical or economic sense. If it was, or is, someone would probably have done it by now, and for all I know, may have.

[Sunking]

The simplest method of keeping your pack at the right temperature is to insulate the housing and use a peltier unit to control temps inside. All you need is a peltier and thermostatic controls, we use them where ever a system is going to be subject to varying temps and they are set a between 18-22degC. Peltiers use very little energy when set up properly and provide heat as well as cooling.

Then all he needs is another set of batteries and panels to run those Peltier coolers .