Hello,

I have this waeco fridge for my camper van, equipped with a BD35F compressor.
When the fridge is plugged on 230 V, it works perfectly

When it's plugged on the battery, it does not run properly. It starts running, The voltage of the battery drops to 11,6 V in 2 to 3 seconds, and then the compressor stops working, the fan takes over... and then the same cycle repeats every 1 or 2 minutes.

This is not enough to keep the fridge cool.

The battery is less than 1 year old, barely used. It's a Deep Cycle AGM DUAL FDC 12-115, working at 118 Ah.

Do you know why the voltage of the battery drops when the compressor starts ?
Do you know why the compressor stops working at 11.6 V whereas the safety cut off voltage, according to its manual, is 10,4 V ?
The manual of the compressor says it works with a maximum current of 5 Amp (maybe 6 if it's cold, which it's not), and my battery is 118Amp.h... This should be more than enough, what do I miss ?

I barely know anything about electricity and electronics, so I may miss obvious things, so do not hesitate to explain my mistakes.
I have checked all the connexions, everything is well plugged where it's supposed to be.
The only thing I haven't checked yet is the wire diameters.

Thanks a lot

 

At the bare minimum, you must take voltage readings.
You mentioned that you are a newbie, which I interpret that you won’t have a DMM.
Well… you will have to beg, borrow or purchase one. If you go for the latter route, there are basic yet capable DMMs which can be had for about US$ 15.

 

That sized Battery will run for approx. ~20 minutes running your Refrigerator at ~6-Amps.
Then IT MUST BE completely recharged as soon as possible thereafter.

If the Battery has not been maintained at it's fully charged Voltage,
continuously for it's entire life,
then the Battery is probably ruined.

If it has been run-completely-dead,
more than ~maybe ~3-times,
and left in that condition more than a few hours each time, then it's probably ruined.
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That sized Battery will run for approx. ~20 minutes running your Refrigerator at ~6-Amps.

Can you explain to me how you do the math ? I don't understand how this works

If the Battery has not been maintained at it's fully charged Voltage,
continuously for it's entire life,

This battery is supposed to be able to perform 800 cycles at 50% discharge, according to its manual.
And I don't think it has been used much since it was bought, less than a year ago.

which I interpret that you won’t have a DMM

True. I'll get one , good idea !

 

I have a Waeco fridge in my van with a Danfoss BD35 compressor. It runs of a 100Ah 12V battery for atleast 24hrs before its about 50% discharged. What you describe is either a faulty battery, or under sized wiring or poor connections. Also check the current draw of the compressor, i would expect 4-5A max, & they dont run continously they cycle. It also helps if you post the model & picture of the fridge as theres many different Waeco models. Also which type of compressor control is used?

 

That sized Battery will run for approx. ~20 minutes running your Refrigerator at ~6-Amps.

It’s a 118Ah battery (call it 120Ah to simplify the math). The units are amp-hours. By dividing the rating by the current draw (in amps), the amperes cancel out and the result is in hours.

120 Ahrs / 6 A = 20 hours​

Not 20 minutes. Which is the minimum time given the battery is fully charged. It could be less if there are less than 120 Ah available. It could be more if the fridge is drawing less than 6 A. It’s all simple algebra.

 

My bad,
I typed minutes instead of hours.

But otherwise,
maintaining a full state of Charge,
at all times when not being used,
still applies,
as not doing so will shorten the Batteries expected Service-Life, sometimes severely.

The Battery may require a substantial amount of time to get back to a fully-Charged-state,
depending upon how it is being charged.
A weak, low-Current, or intermittent, Charging scheme,
may never get the Battery back to a fully-Charged-state.
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This is my test compressor for repairing drive units. This is what the BD35 compressor draws at 3000rpm & 2000rpm. They can be set at difererent speeds but this unit ive set up for max & min speed. This shows the current draw at each speed.

 

Hello,
So, I bought a DMM to measure the voltage, and did a few experiments.
My conclusion ; my battery is fine, my fridge is fine, so the wiring must be wrong somewhere.

There is a voltage display in my van. (I don't know exactly where it is plugged because the wiring is hired.)
Where I first turn the circuit on, the voltage display shows 12,4V initially, then decreases slowly to 11.7V. That's whene the compressor shuts down for 1 min, and the tension rapidly rise back up to 12,4V. Then, 1 min later, the compressor turns on again, but this time the voltage reading drops rapidly to 11.7V, in 3 to 4 seconds, and the compressor shuts down for 1 min.

However, I measured the voltage with a DMM at 2 points :
- directly at the battery terminal => the voltage stays at 12,4V ALL THE TIME. Even when the compressor is on, the voltage stays at 12,4V
- at the compressor 12V terminals ( + and -). The voltage here is lower. it's 1,3V less than the battery. When the compressor turns on, it drops then at 10.4V, and then shuts down. which is exactly the safety voltage it is programmed to shut down for, so this makes sense.

Then, I plugged the fridge DIRECTLY on the battery : it works perfectly fine ; the voltage of the battery remains at 12,4V and doesn't change. I let it like that for 1 hour. When I came back, the compressor was still running, the temperature has dropped 10 degrees, and the voltage was still exactly 12.4 V (measured with DMM and showed on the voltage display).

So now I wonder : why is there a voltage difference between the fridge and the battery ?
And then, why does the voltage drops so much when the compressor turns on (when it's on the van circuit) ? Is this due to a leak or shortcut somewhere ?

 

The wiring is way under sized or poor connection somewhere.

 

You have 2 possibilities,
1) A poor Wiring-Connection or Wiring-Splice.
2) Inadequately sized Wiring between the Battery and the Fridge.

Both of these problems apply equally to the Positive-Wiring, and the Ground-Wiring.

There should be ~10-gauge Wiring, ( ~5.26 sq.mm ), all the way from the Battery to the Fridge.

The Ground-Wire must also run all the way from the Fridge to the Battery.

Using the Sheet-Metal-Body, or Steel-Frame, for a Ground-Connection is completely unacceptable.
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Wires are too small for the current needs, so increase wire sizes..

 

Well, I'm gonna follow your advice and change wiring and connexions.

Using the Sheet-Metal-Body, or Steel-Frame, for a Ground-Connection is completely unacceptable.

How so ?

 

Steel is a poor Conductor when compared to Copper.

It's difficult to create a solid connection to Steel.

DIY connections to Steel are almost always made with Self-Tapping-Sheet-Metal-Screws which
do not have very much holding-power, and they strip-out easily, potentially creating a poor connection.

Steel will rust, and quite often Sheet-Metal-Screws will also rust.
Even a very slight amount of surface-rust will eventually cause a poor connection.

Quite often Low-Current-Loads are grounded through the Body of the Car by the manufacturer,
if You attempt to create a new High-Current-Circuit through the Body,
You may possibly create unacceptable Voltage-Fluctuations for
those Low-Current-Accessories, potentially affecting their operation.
You can not recreate the methods used by the manufacturer to make a reliable connection to the Body.

The more Connections that You make in a Circuit, the more potential for problems You will create.

A properly sized Copper-Wire will generally create zero problems, and last for many many years,
it's the Connections to that Wire that are likely to cause problems.
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Thanks for the details.
What should I use for connection to the ground then ?

To come back to the main subject, I found a solution based on your advice.

I plugged the 12V power supply of the fridge direclty on the battery (using a fuse), and it works perfectly well.
I disconnected the previous wiring that were going through a lot of different connections and with smaller size wires.
Since the fridge has a button to turn it off, that works well enough.

Thanks all for your help