Planning of a DIY LiFePO4 battery - air conditioned

[fmeili1]

Hi,
since a couple of weeks, I'm thinking and planning about building my own LiFePO4 battery to replace my two "old" lead acid 100AH AGM group 24 batteries (installed below the stairs) for my 2020 Entegra Odyssey 26D.

I would like to stay below $1,000 for the whole project and try to design the battery in a way that it would automatically heat and cool itself, depending of the ambient temperature to maximize life time and prevent damage. I don't want to move the battery to the inside of the coach because of space reasons, so it must fit in existing battery space. Later I think about adding solar, but first the new battery has to be build.

So far I've a battery design with the following components in my head:
- 4x EVE 280k, 3.2V, 280AH grade A cells (from an US distributor - about $540)
- JBD BMS 4S 12V 200A (about $125)
- TEC1-12706 peltier cooler/heater with two headsinks and two fans, max. 70 watts (about $25)
- 2x digital temperature controller XH-W3001 (about $10)
- 2x SPDT and 1x DPDT relay to control the peltier/fans with the temperature controllers
- Renogy 500A shunt mounted on the battery (display mounted inside the rig) - (I already have that - costs about $75)
- 300A circuit breaker (about $20)
- CellMeter8 battery cell monitor ($14)

Some additional design considerations:
- Everything should be mounted in/at a wooden box made from 1/4" thick plywood (<=7.5" width, <=14.5" heigth, <=22" length).
- the peltier heatsink located inside of the battery needs a small funnel underneath with a pipe to get rid of the condensate water if cooled
- the DPDT relay is required to change the polarity of the peltier to choose between cooling and heating
- the two SPDT relays are required in XOR to only active the peltier if either of the heat or cool thermostat is active, but NOT if both are active by mistake!
- the conditioned airflow of the pletier (directed via a plexiglass sheet) runs over, under and one end of the battery (too less space on the side of the cells)

Here is the planned schema:


The battery should look like this:







I would be interested in your opinions and suggestions for improvements!

Thanks in advance,
Frank

[rlw999]

Goostoff wrote:

fmeili1 wrote:
Hi,
since a couple of weeks, I'm thinking and planning about building my own LiFePO4 battery to replace my two "old" lead acid 100AH AGM group 24 batteries
Frank

Be sure of what you are replacing. lead acid and AGM are not the same. If you have lead acid batteries maybe a simple upgrade to AGM is all you need and keep it simple.

AGM batteries are a type of Lead Acid battery. They of course differ in charging and discharge characteristics, but they aren't all that different in capacity.

His 280Ah of LiIon batteries will give around 2X usable battery capacity compared to his old pair of 100Ah AGM's (or FLA)

[Goostoff]

fmeili1 wrote:
Hi,
since a couple of weeks, I'm thinking and planning about building my own LiFePO4 battery to replace my two "old" lead acid 100AH AGM group 24 batteries
Frank

Be sure of what you are replacing. lead acid and AGM are not the same. If you have lead acid batteries maybe a simple upgrade to AGM is all you need and keep it simple.

[frankwp]

As Wizard said, Peltiers are very inefficient, but more importantly, are quite limited in how much energy transfer they're capable of. On the other hand, running them in reverse will keep the batteries warm while using less energy than pads. Much like a heat pump.

[rlw999]

fmeili1 wrote:

1st stage: inside fan only
2nd stage: inside fan plus peltier and outside fan

In 1st stage the energy consumption would be very low, just the inside fan. The 70 watt peltier will only be used when 2nd stage will be active.

I'm skeptical that the Peltier device is going to make any significant difference in cooling. When you need it the most (it's hot out), it will help the least. 70W is pretty small, a 70W peltier cooler will yield around 200 BTU/Hr of cooling, or around 1.5% of a typical 13,500 BTU RV air conditioner.

[Lwiddis]

“3.2V, 280AH grade A cells…”

280 AHs seems insufficient in your area even to keep the batteries cool.

[fmeili1]

Maybe I should change the circuit in a way that I first try to use only the fan which is mounted at the peltier (inside the closed battery case) to cool the battery and add the active peltier cooling only if this pure fan cooling will not be enough to keep it cool. Like a 2 stage cooling:

1st stage: inside fan only
2nd stage: inside fan plus peltier and outside fan

In 1st stage the energy consumption would be very low, just the inside fan. The 70 watt peltier will only be used when 2nd stage will be active.

The problem with this approach is, in 1st stage I usually need an open system to let outside air in and for 2nd stage, a closed system with recirculate air would be better.

Until now, my design is a closed system with just recirculate air (for heating and cooling).

Btw. I only plan ONE peltier element with 6 amps (70 watts) - not two.

[StirCrazy]

I like the idea, I have 2 bar fridges, two water coolers and 3 coolers that use peltier coolers they work very good, but I am not sure how efective they will be if you drawing in air. not sure if thats what you are intending or not by the drawings. if they are in an insulated case and you just recirulate the air that might work, but I am not sure if it will be the most efficent way to go about it. 70 watts X 2 is a lot of power to keep a battery warm, and as for cooling are you going to be charging it fast enough to make that type of heat or using it in over 60 degree temps?

Steve

[theoldwizard1]

fmeili1 wrote:
Because I need the peltier anyway for cooling, I've decided to use it also for heating and so I don't need additional heat pads.

A properly placed fan with flow through vents is more than enough and will draw less power than a peltier cooler.

[fmeili1]

Probably a better idea, but I only have tight space (for 2 group 24 batteries). But in general a mini fridge with a compressor would be ideal.

[time2roll]

How about build the cells in a mini fridge?

[fmeili1]

Thanks for your comments. My first idea for heating was also to use heat pads. But for active cooling (beside just a fan), I don't have many options beside a peltier. The idea is, even if the peltier (in fact a heat pump) has a relatively bad efficiency, it will be more efficient as a heat pad for heating. Because I need the peltier anyway for cooling, I've decided to use it also for heating and so I don't need additional heat pads.

[valhalla360]

theoldwizard1 wrote:
Moving air does a good job of cooling if it is properly directed. Check out Noctua fans. Very quiet and last a long time

That and make sure they are shaded with an airgap.

Basically any location that is so hot the batteries have problems, you probably aren't going to want to be there without air/con for you anyway.

[time2roll]

How hot do you expect these cells to get? Top of the operating range is 120F to 140F.

[theoldwizard1]

Peltier coolers are not very efficient. You need to study them more regarding how much heat energy they can actually move. Anyone who has tried to use them in refrigeration gave up and went to a compressor solution.

Moving air does a good job of cooling if it is properly directed. Check out Noctua fans. Very quiet and last a long time

For heating, I would go with some kind of heating pad.

Your housing needs several coats of epoxy.