battery dimensioning basics....

[vasant]

Hi all
I am wondering whether i miss something here in my calculations to understand what battery size would be proper.
So i have this famous BD35 based fridge and a 80AH NEW EXIDE battery.
I assume and I actually MEASURED that the compressor is consuming as per specs so lets say 3AMPS, lets assume 5 for simplicity....
With a 80 AH battery i think it shouldnt be too much expectation to let the fridge run for a couple of hrs even maybe more before it drops to 12.4 or 12.3 .
This isnt the case however even the fresh batetry will serve maybe an hour or so...
Do i miss smth?
Thanks all

 

[PaulRainbow]

Hi all
I am wondering whether i miss something here in my calculations to understand what battery size would be proper.
So i have this famous BD35 based fridge and a 80AH NEW EXIDE battery.
I assume and I actually MEASURED that the compressor is consuming as per specs so lets say 3AMPS, lets assume 5 for simplicity....
With a 80 AH battery i think it shouldnt be too much expectation to let the fridge run for a couple of hrs even maybe more before it drops to 12.4 or 12.3 .
This isnt the case however even the fresh batetry will serve maybe an hour or so...
Do i miss smth?
Thanks all

What happens after an hour or so ?

 

[vasant]

batterry dropbelow 12.4 and fridge stops.
This happened with previous battery obviously and now after replacing happens the same maybe a bit later

 

[lustyd]

before it drops to 12.4 or 12.3

The voltages people quote are for a battery at rest. Disconnect and wait a while and see what the voltage returns to

 

[PaulRainbow]

batterry dropbelow 12.4 and fridge stops.
This happened with previous battery obviously and now after replacing happens the same maybe a bit later

In addition to post #4.....

Is this a new to you boat, or a new problem ?

12.4v should not cause the fridge to stop. After checking the voltage at rest (post #4), measure the voltage at the compressor when the fridge turns on. When it stops, measure it again, at the compressor, as soon as you can get it to try starting again.

What size is the wiring to the fridge and how long are the wires going to the fridge ?

If the wiring is to small/wire length too long it will do as you describe.

 

[vasant]

In addition to post #4.....

Is this a new to you boat, or a new problem ?

12.4v should not cause the fridge to stop. After checking the voltage at rest (post #4), measure the voltage at the compressor when the fridge turns on. When it stops, measure it again, at the compressor, as soon as you can get it to try starting again.

What size is the wiring to the fridge and how long are the wires going to the fridge ?

If the wiring is to small/wire length too long it will do as you describe.

Indeed i guess the key question is the shutoff voltage of the fridge. I suppose its too high. Wires are new 6mm i think, so shouldnt be the root cause.

 

[VicS]

You may find the diagnostic testing described here https://penguinfrigo.co.uk/wp-conte...e/pdf/3-Support-BD35-BD50-Fault-Diagnosis.pdf useful.

.

 

[Alex_Blackwood]

In addition to post #4.....

Is this a new to you boat, or a new problem ?

12.4v should not cause the fridge to stop. After checking the voltage at rest (post #4), measure the voltage at the compressor when the fridge turns on. When it stops, measure it again, at the compressor, as soon as you can get it to try starting again.

What size is the wiring to the fridge and how long are the wires going to the fridge ?

If the wiring is to small/wire length too long it will do as you describe.

Is the OP sure that it is cutting out on low voltage? Could it be that the compressor thermal trip is operating? Does the fridge restart after a short interval, when the trip has reset. or does he have to get his battery voltage back up to a level where it will restart? Does it always run for an hour before cutting out. If so that would suggest that it is not the battery as that would never be at the same condition every time, would it? I do note he does say "An hour or so"
What would happen if he was to run the fridge with a charger connected and operating as a power supply?
Just thought I would ask!

 

[B27]

You could usefully monitor the actual volts and current over time.
There could be a fault with the fridge.
Are you sure you are actually starting with a fully charged battery?

It may be worth checking the voltage right at the fridge connection, cable drops matter.
I think some of these fridges check the supply volts during a start-up, which is quite high current and can shut off long before the battery is low, let alone flat.
I've seen boats where the fridge basically only runs when the engine is running, due to cable sizing and the cut-off voltage. Some people are happy with that, or were in the past.

 

[vasant]

Is the OP sure that it is cutting out on low voltage? Could it be that the compressor thermal trip is operating? Does the fridge restart after a short interval, when the trip has reset. or does he have to get his battery voltage back up to a level where it will restart? Does it always run for an hour before cutting out. If so that would suggest that it is not the battery as that would never be at the same condition every time, would it? I do note he does say "An hour or so"
What would happen if he was to run the fridge with a charger connected and operating as a power supply?
Just thought I would ask!

hi, When engine is running it will run forever. Itstops and leds are flashing pointing to low voltage

 

[lustyd]

It could be that the startup current of the compressor when it cycles is much higher, causing deeper voltage drop

 

[Refueler]

It could be that the startup current of the compressor when it cycles is much higher, causing deeper voltage drop

But if the compressor runs for an hour or so - the initial high power demand to start would not be an issue surely. The voltage would have recovered from the initial start ... and as OP says - when engine is running the compressor is running continuously .... so it points to a battery - compressor matter.

 

[lustyd]

But if the compressor runs for an hour or so - the initial high power demand to start would not be an issue surely. The voltage would have recovered from the initial start ... and as OP says - when engine is running the compressor is running continuously .... so it points to a battery - compressor matter.

Compressors very rarely run continuously, they cycle on and off to move gas about but once all the gas is on the liquid side they shut off and wait until some gas moves back through the system. At least that's what they should do, otherwise they're not set up efficiently

 

[KompetentKrew]

It's a little expensive to buy a Victron SmartShunt to address only one problem, but if this is the house bank of your boat and you plan to keep the boat for any length of time then a SmartShunt would be a very good investment.

This would allow you to be absolutely certain of how much current is being drawn and the battery's voltage. It allows you to be confident that it's being fully charged overnight.

You say that you "assume and have measured" that the fridge is drawing 3A but, from what you've told us here, maybe something else is drawing current.

You haven't told us how the battery is being charged - is it getting a proper bulk / absorption / float cycle, or are you just charging it for a couple of hours' motoring with your alternator?

You're looking at the fridge with its believed 3A draw and the brand new battery because these things are obvious to you - sod's Law says it's something else that you're overlooking because it's not where you're looking.

Measuring the voltage and total current draw at the battery is a good place to start.

 

[Refueler]

Compressors very rarely run continuously, they cycle on and off to move gas about but once all the gas is on the liquid side they shut off and wait until some gas moves back through the system. At least that's what they should do, otherwise they're not set up efficiently

I agree .. but OP's and also my fridge on board seem to share same characteristic ... so much so in fact - that I rarely have it on now.
When I bought the boat - it was not too bad but fridge was more demanding than expected and now partly that my domestic bank is pretty bad as found on last trip ...

Will see what I get when I swap out both batts for new ..

 

[MontyMariner]

Try setting the cutout voltage to 12.3 and see what happens.

 

[VicS]

Try setting the cutout voltage to 12.3 and see what happens.

How would you do that?

.

 

[MontyMariner]

How would you do that?

.

Instructions
Electronic Unit for BD35/50F, BD35K Compressors,
101N0212 and 101N0650, 12-24V DC

 

[VicS]

But 12.3volts is way outside that range.

ITYWF the normal cut out voltage is 10.5

 

[ChromeDome]

It's a little expensive to buy a Victron SmartShunt to address only one problem, but if this is the house bank of your boat and you plan to keep the boat for any length of time then a SmartShunt would be a very good investment.

Agree, but a clamp meter is much cheaper and universally usable for checking amp draw. Also "mysterious" battery drains. Most have multimeter features as well.

Many versions available, e.g.:

6000 Counts Auto-Range Multimeter 600A AC/DC Current, 600V AC/DC Voltage with LCD Backlit, Capacitance, Continuity, Resistance, Duty, Diodes, Frecruncy, Temperature, NCV​