Voltage drops from 12.45v to 11.6v when fridge is running

[dvisor]

I'm running a compressor fridge at the back of the Bongo from the leisure battery. The cabling has been upgraded to reduce voltage drop. I've also got a voltmeter connected (close) to the leisure battery. When the fridge is off, I've got a reading of 12.45v. When the fridge is running, it drops to 11.60v. Turn off the fridge again, and it goes back up to 12.45v .

I've got 40A cable running from the leisure battery to a 4 way fuse box. From there, its 40A cable running to the fridge at the rear (well, its a DIN socket actually). Another, 25A, cable runs from the fuse box to the voltmeter on the dash (via a switch).

I'm not sure I understand this. Surely the fridge being on shouldn't affect the voltage reading? If I'm getting voltage drop, shouldn't it only be in the circuit to the fridge, and not to the voltmeter? In fact, the drop seems too great considering the quality of cable that I've run down to the back.

Any ideas ?

[missfixit70]

Battery iffy? Sounds like one for Geoff to answer

[g8dhe]

OK well your seeing several voltage drops;
1. The first one is an internal one within the battery, nothing much to do about this other than get a battery with a higher CURRENT rating, more along the lines of a starter battery construction, but this will impact the overall life of the battery used in leisure mode.
2. The second drop is in the cable between battery and fuse holder, even though the cable is rated at 40A it will still drop a voltage, also any fuses in the cable will also be adding to the volts drop (you do have a fuse at the battery in the 40A cable don't you?).
3. Beyond the fuse box towards the fridge, will be a further drop in the fuse, cable, DIN socket and cable to the fridge, however the voltmeter won't be seeing this drop.

The actual drops will depend on the current drawn by the fridge, can you measure this ? If you can then we can calculate the resistance at each step and if this is excessive or merely what is expected. Also the 40A cable do you know what cable this actually is, i.e. the cross section area i.e. 6mm² or makeup i.e. 84/0.3mm ?

Guessing that the fridge might draw something like 4Amps then, the volts drop in the battery and first length of cable and fuses is given as 12.45-11.6=0.85v, the resistance if 4 Amps is being drawn is then 0.85/4=0.2125Ω which isn't that bad at all, and would be the sort of level I would expect to be honest. If the current drawn is higher than 4Amps then the resistance is lower and that means its actually better than I would have guesseed, if its less than Amps then you might have a problem...

[dvisor]

OK well your seeing several voltage drops;
1. The first one is an internal one within the battery, nothing much to do about this other than get a battery with a higher CURRENT rating, more along the lines of a starter battery construction, but this will impact the overall life of the battery used in leisure mode.
2. The second drop is in the cable between battery and fuse holder, even though the cable is rated at 40A it will still drop a voltage, also any fuses in the cable will also be adding to the volts drop (you do have a fuse at the battery in the 40A cable don't you?).
3. Beyond the fuse box towards the fridge, will be a further drop in the fuse, cable, DIN socket and cable to the fridge, however the voltmeter won't be seeing this drop.

The actual drops will depend on the current drawn by the fridge, can you measure this ? If you can then we can calculate the resistance at each step and if this is excessive or merely what is expected. Also the 40A cable do you know what cable this actually is, i.e. the cross section area i.e. 6mm² or makeup i.e. 84/0.3mm ?

Guessing that the fridge might draw something like 4Amps then, the volts drop in the battery and first length of cable and fuses is given as 12.45-11.6=0.85v, the resistance if 4 Amps is being drawn is then 0.85/4=0.2125Ω which isn't that bad at all, and would be the sort of level I would expect to be honest. If the current drawn is higher than 4Amps then the resistance is lower and that means its actually better than I would have guesseed, if its less than Amps then you might have a problem...

Wow - thanks for this. Its going to take me a bit of time to digest this. I believe the current draw on the fridge is 6.8Amps, so it sounds like everything is in order. I'll read your post through again in the morning when the alcohol has cleared Thanks again.

[mister munkey]

The knowledge base on here never ceases to amaze me.

[mikexgough]

Nominal Current for Waeco Fridges are as follows - all at 12v -CF-18 is 3.1A, CF-25 is 6.8A, CF-35 is 6A,CF40 is 6A,CF-50 is 7A,CF-80 is 7.5A & CF-110 is 7.5A

I have a CF-18 and run a fused 30A cable direct to the socket just for the Fridge just for info

[dandywarhol]

All the above sounds right to me - must be a helluva big fridge to draw so much current - even an inefficient coolbox draws less

[g8dhe]

I believe the current draw on the fridge is 6.8Amps, so it sounds like everything is in order.

That is quite a high current, I would suspect that might be a Peak current, rather than the average whilst running (most fridges are only on 10-30% of the time) so even when drawing say 4 amps running, then the overall average current drawn is only going to be 1/10 to 1/3 of 4 Amps, but for the Volts drop your measuring its the running current that is important.

[mikexgough]

I think the 6.8A load is maximum for compressor start up and as been said the compressors only run 10-30% of the time dependent on abient temperature and cold setting.... the Waeco units use Danfoss BD-35F compressors on the smaller units and BD-50F for the 80 & 100 units if that's a help....

[dvisor]

Yes, 6.8A is the peak for a Waeco CF-25. Average is considerably lower. Can't remember the figures, but I calculated that I should be able to run it for two or 3 days on my LB.

[mikexgough]

Waeco Power consumption table http://www.waeco.co.uk/page.aspx?page=powercon&s=4

[g8dhe]

OK from that table then its 2.92 Amps as the running current so that's now 0.85/2.92≈0.3Ω I still wouldn't be surprised at this but I would clean all the fuse connections and battery terminals just to get the lowest possible resistance. If you want to measure the battery voltage then just run a thin wire ( but still with a fuse very close to the battery) directly from the battery terminals themselves then you will be measuring what the battery itself supplies.