One for the more technically amongst us!

 

I currently run a compressor fridge from the 12v in the boot.  (Very impressive pieces of kit btw).

 

The unit has a voltage sensor which means that as it draws down on the car battery, it switches off if the voltage gets too low, essentially protecting the car battery from being totally depleted (there are numerous schools of thought on whether or not to do this, personally I am in the one that thinks convenience>risk).

 

Effectively this means that the fridge will run for 8 hours or so then cut off (from a full battery).  Once you start driving, the fridge kicks in and away you go.  

 

The next step for me is to install a seperate power supply, something like this

 

EcoFlow River Pro | Portable Power Stations | International Version (hampshiregenerators.co.uk)

 

This would extend the "life" of the fridge without power to around 2-3 days (from a full charge)

 

However, if I want to leave  the setup plugged in, I would need a voltage protection switch (that is not in the power bank ).

 

This would do the job I think.

 

 

Battery Guard 12V DC: Amazon.co.uk: Electronics & Photo

 

 

However, I am a tad confused about what is needed to fit it.  Would I have to buy a 12v lead and strip it down to the wire, then solder to spade connectors etc? 

 

Same with the other side leading to the powerbank?

Edited 22 July, 20214 yr by xspartx
unfinished

Not commenting on your choice of power supply, however:

 

It might work by:

1) Convert your boot socket to switched output (easy, just needs a piggyback fuse)

2) Switched boot socket now charges the power bank thingo

3) Power bank thingo supplies power to the fridge

4) Fridge uses built in low voltage cutout to protect power bank thingo

4a) Power bank thingo has a BMS so it probably already has battery protection in it.  Lithium cells hate being over discharged.  That would probably mean you don't need the battery guard thingo.

 

Problems I foresee:

1) does the power bank thingo need a button pressed or something like that to turn on its output?  If so, you'll need to press that to start the fridge each time which would be annoying

2) Charging time 4.8 hours or more.

 - Power bank thingo does the disingenuous marketing thing of claiming watt-hours of capacity instead of amp hours.  It says 720Wh.  Assuming a 4S battery, average cell voltage of 3.2V = 12.8V.  720Wh / 12.8V = 56.2 Amp hours.  Take off 20% (you shouldn't discharge LiFePo4 below 20%) and another 10% for losses (DC-DC, cabling, connections) leaves us with 39.3Ah (round to 40Ah).  

 - Lets say your fridge has exhausted the power bank and it's turned off.  We need to replace 40Ah of power. Charging losses are around 10-20% (it's a chemical process, not 100% efficient) so we actually need to put in 48Ah.  

 - Spec sheet claims car charge input is 10A.  That's also the limit of the cigarette lighter socket in the boot, although I've seen the wiring, it's flimsy so I'd expect a large voltage drop at 10A.  We'll ignore that for now.

 - At 10A, putting that 48Ah back into the battery will take 4.8 hours.  So the engine would need to be running 4.8 hours to charge the power bank thingo back up.

- LiFePo4 charges to 80% quickly but then slows down, it's just the chemistry.  So the 4.8 hours would actually be longer.  How much longer depends on the BMS in the power bank thingo.

3) Runtime

- Not sure of the size of the fridge, but they pretty much all use Danfoss BD35 or BD55 or copies of the same.  Around 4 amps when cooling.  Depending on ambient, lets say duty cycle of 50% (assuming box contents are already down to temp and we're just maintaining temp and it's not hot outside).  That means 2Ah.  Your 40Ah battery should in theory run that for 20 hours before shutting off, but in practice it will be less, maybe 15 hours due to things like voltage drops in cabling and connectors, voltage sags when then compressor starts (even with a soft starter) and things like that.  Higher temp (e.g. car in sun), lack of ventilation, box contents not already cold will drastically reduce that time.

 

If you're happy with the above constraints, then it should work.

 

A true dual battery solution with an isolator would be better, although fitting it into the Skodiaq and running the heavy cables needed for a rapid charge would be a bit of stuffing around.

 

Other random brain dumps:

- Using a starter battery to run a fridge is not great.  These battery constructions are designed to deliver a large load for a short time (start a car) and then idle the rest of the time.  Even the modern Lead Acid Calcium ones are like this, although they're more tolerant of a deeper discharge.  Your choice of a LiFePO4 for the fridge battery is much better, but the awkward implementation makes it difficult.

- If you do progress to a true 2nd battery, make sure it taps off the starter battery negative on the car side of the hall effect sensor.  Basically there is a current sensor near the negative battery post that measures current in and out of the battery.  If you discharge the battery directly from the posts, the sensor doesn't see it.  The car uses this sensor for start-stop maintenance and emission control etc and if the sensor doesn't see the current, the system can go way our of whack and need a recalibration.  There is a ground point on the firewall, right next to the battery that is on the car side of the hall effect sensor, you can tap there.

 

To answer your actual question if you choose to use the battery guard thingo so it's all plug and play:

Make up a lead, two spade connectors to a ciggy lighter socket.  That goes from the battery guard "load" connection if you choose to use it to the power pack thingo.

Make up a lead, two spade connectors to a ciggy lighter plug.  That goes from the socket in the boot of the car to the battery guard "battery" connection.

Set the switch off voltage at something like 12.2 volts.  That would mean the switch on voltage would be 13.0 volts.  your car will run at 14.4 volts (peaking to 15 on regen) until the car feels its starter battery is charged and then turn down to about 12.8 so your battery guard will stay on at that point.  If you stop the engine, or the start-stop does so the starter battery will quickly drop to about 11.8v and the battery guard will cut out.

 

Wow thanks for the detailed and informative post.

 

Definitely food for thought there in terms of the whole system.  Looks like my goal of having a constantly running, self-charging fridge in the car will be harder than I thought.

 

I will still make up the battery guard as it is quite a useful thing to have, but more thought is needed on the setup.

 

Thanks again, I really do appreciate it.

Sorry I just read it again and realised how incoherent it is.  Please ask questions if you need clarification.  

 

I'm not a professional but I have been meaning to get around to adding a 2nd battery to run the drive camera when engine off.  The power requirements are much less than a fridge so the charging time is much lower then your solution.

Just for additional information:

 

I have Makita compresor fridge, which can work from 18V batteries (2 x 6Ah batteries = 17 hours of work, according to producer).

However I have left the fridge in the trunk for 2 days without driving the car meanwhile and the fridge still retains the given 5C and works from the mains lead, hasn`t reached the cutoff voltage yet, no switching to batteries.

Means the insulation and the compressor must be top class. 

Edited 26 July, 20214 yr by linni

3 hours ago, linni said:

Just for additional information:

 

I have Makita compresor fridge, which can work from 18V batteries (2 x 6Ah batteries = 17 hours of work, according to producer).

However I have left the fridge in the trunk for 2 days without driving the car meanwhile and the fridge still retains the given 5C and works from the mains lead, hasn`t reached the cutoff voltage yet, no switching to batteries.

Means the insulation and the compressor must be top class. 

 

They ARE top class.  I think the 17 hours you were quoting is more likely to be elapsed time rather than run time, but still impressive

 

Assume your compressor runs about 2Ah when in use.  Depending on conditions/temp/use etc it will be in use 1/3 of time.   i.e. 0.7Ah average draw (approx).

 

12/0.7= 17 hours

 

I was playing around with options over the weekend.  One interesting one (which unfortunately I now think is a non starter) would have been to put this on the parcel shelf

 

Flexible Solar Panel 100W 12V Monocrystalline Bendable 100 Watt 12Volt Semi-Flexible Mono Solar Panels Charger Off-Grid for RV Boat Cabin Van Car Uneven Surfaces : AmazonSmile: Business, Industry & Science

 

Feeding into this

EcoFlow River 370 Generator : AmazonSmile: DIY & Tools

 

This can take TWO inputs, so one from the 12v and one from the solar panel.

 

The  thought process was that this could keep the battery "topped up" without faffing around changing inputs.

 

Sadly I think the solar panel would get fried by leaving it on the rear parcel shelf. :(

 

Back to the drawing board! 

Solar panel behind automotive glass will have much lower output, anecdotally by as much as 40% through clear glass.  Your window might be tinted too, further reducing it.  

Would it not be simpler (and cheaper) to have a set-up like that when a compressor fridge is used in a campervan?  I’m thinking of a leisure battery of, say, 110 Ah, connected to the vehicle’s charging system through a split-charge relay.

Once the engine is going, the relay kicks in and allows charge to the leisure battery (potentially much more than 10A, so don’t use tbe ciggy socket circuit).  When the engine stops, the relay disconnects the leisure battery and fridge from the car’s battery.  As the fridge is used, only the leisure battery is discharged - and your fridge auto-disconnects, you say, when that battery gets low.

 

Typically, a campervan (also using lights, waterpump, TV etc) can go for a couple of days on the 110 Ah battery.

Edited 27 July, 20214 yr by DaveMiller

On 27/07/2021 at 04:34, DaveMiller said:

Would it not be simpler (and cheaper) to have a set-up like that when a compressor fridge is used in a campervan?  I’m thinking of a leisure battery of, say, 110 Ah, connected to the vehicle’s charging system through a split-charge relay.

Once the engine is going, the relay kicks in and allows charge to the leisure battery (potentially much more than 10A, so don’t use tbe ciggy socket circuit).  When the engine stops, the relay disconnects the leisure battery and fridge from the car’s battery.  As the fridge is used, only the leisure battery is discharged - and your fridge auto-disconnects, you say, when that battery gets low.

 

Typically, a campervan (also using lights, waterpump, TV etc) can go for a couple of days on the 110 Ah battery.

 

 

Certainly cheaper.  Not sure about the simpler bit as you would need to do a fair bit of wiring.  

Either of these would help.

 

https://www.amazon.co.uk/Kemo-M148A-Battery-Guard-12V-Black/dp/B004AUJ002/ref=dp_prsubs_1?pd_rd_i=B004AUJ002&psc=1

 

https://circuits-diy.com/low-battery-cutoff-for-12v-batteries-using-555-ic/

 

Otherwise as somebody else mentioned, a 12V leisure battery, which only charges when the engine is on.

You could move the 12V in the boot to a switched live, then put the battery charger onto that. The old 12V socket could then connect to the 12V leisure in the boot and to the fridge.

 

 

Ok I've made the purchase.

 

Wired one of these Battery Guard 12V DC: Amazon.co.uk: Electronics & Photo up to a standard 12v extension lead.  

 

Will attempt to use it to charge a power bank PowerHouse II 400 - Anker  which in turn will charge my compressor fridge.

 

With my driving habits I expect the powerstation to be empty perhaps 65% of the time (which may/may not wreck the battery), but hopefully I can charge it at approx 100w or so from the DC input AND Usb C (while driving), so around 3-4 hours for a full charge.

 

I have a splitter on the 12v that allegedly can handle 150w, but will probably play have to play around with inputs to see what I can get.

 

Will report back on the success/failure!  

 

 

Thanks to all who have contributed to the thread.  

 

Sounds interesting.  If the cut-off voltage is set to about 13V, it will automatically allow charge through to your power pack when the engine is running, and shut off when it's not.

 

1 hour ago, DaveMiller said:

Sounds interesting.  If the cut-off voltage is set to about 13V, it will automatically allow charge through to your power pack when the engine is running, and shut off when it's not.

 

Except that the Kodiaq has a micro hybrid/ smart alternator and tries to only really charge on the overrun to save fuel and emissions.

4 hours ago, DaveMiller said:

Sounds interesting.  If the cut-off voltage is set to about 13V, it will automatically allow charge through to your power pack when the engine is running, and shut off when it's not.

 

Tested tonight and that definitely works.

 

I've set it around 11.8v to eek out a bit more charge after the power is cut, but may well raise it later once I have seen the effect.   So far so good.  

 

Next stage will be the charge life of the battery/fridge.

Well a few weeks on and I am happy.

 

It's not perfect as the battery will lose power and cut off if the car isn't used for a couple of days, but generally speaking, for 90% of the time I have drinks in the car at 1c.  

 

Whether this is worth around £900 or so is up to the individual, but for me, a £900 ice cold drinks option that can be migrated to future cars is money well spent.

Update:

 

For those interested (those who aren't can always skip!)

 

After lot's of testing/fiddling/swapping, I have finished on the "fridge in the boot" experiment.

 

Fundamentally if you want a fridge in the boot, then it's highly unlikely you are going to charge it full time, automatically from the 12v socket.  It just takes too much driving to achieve this.

 

So you are then left with the option of using some form of battery, charged externally to power the fridge.  

 

A reasonably efficient 12v compressor fridge will take around 200wh of power each day, depending on how hot it is and how well ventilated the fridge is.  To be honest the boot of a car is close to the worse place to store one!

 

So there you go.   You could buy an external power supply with around 400wh for £300-£400 and, you would need to recharge it every 2 days or so to enjoy ice cold drinks on a hot day from your car.

 

Or spend around £1000-£1500 or so for a 2000wh power supply and recharge the battery every 10 days or so.   Add in the cost of the fridge and you are looking at £2k for the privilege.

 

Is that worth it?   Over the lifetime of a car probably not.  Over the lifetime of 3 or 4 cars......maybe.

1 hour ago, xspartx said:

A reasonably efficient 12v compressor fridge will take around 200wh of power each day, depending on how hot it is and how well ventilated the fridge is.  To be honest the boot of a car is close to the worse place to store one!

 

Interesting that you say that, the new fridge freezer I have in the UK draws practically no energy whenever I check the quiescent consumption on the smart meter, the compressor probably cuts in when my back is turned but even when it was running the consumption was very low.

 

I am currently living on site in a touring caravan with a 3 way ammonia absorption type fridge freezer, I was surprised at how much mains current was being drawn, with all the lights, hot water heater, 24v circuits and battery charging circuit etc switched off it was still drawing 200 watts which turned out to be the fridge freezer, it is a constant load 24/7 and given that the caravan heats up to 50°+ in the sunlight in this current heatwave it is working overtime.

 

I left a week ago to collect more stuff, return tomorrow, I did not want to defrost the unit because the frozen food would have defrosted on the 12 hour journey but it will have cost me a lot of money to leave it running, 4.8kw per day,  7 days, that has cost €5.60!!!