Battery capacity and Fridges

[Reallyfree]

Trying to work out how long my 75A/hr battery should last running a Waeco CR50 fridge. Does anyone have a similar setup? I know there are lots of variable factors such as ambient temperature/fridge settings etc but I am sure mine used to run for longer but now won't last overnight. its difficult to be sure how long it lasted before because we always started and ran the van within a 24hr period.

Last night I used a 5A Ctek to charge the battery in recondition mode for 15 hours and then ran fridge on max setting to see how long it would last. Voltage unloaded seemed a healthy 13.5V which soon dropped to 12.7V with fridge turned on. The fridge then lasted 5 hrs before cutting out which would have meant the battery voltage had dropped to 10.4V according to the fridge manual.

I have a few techy questions regarding electrics?

1. Fridge rated current: 5.7A at 12V. What does this mean?
2. Average power consumption: 40W - which by my reckoning is 40W/12V=3.3A. Is this correct?
3. Current Consumption (12 volts DC): 1.4 Ah/h at +25°C ambient temperature (not in manual - source waecofridges.co.uk)

So am i correct in thinking when 75A/hr battery has dropped to 10.4V it is still at 86.6% capacity or has drained it by 13.4%?

If i am correct so far this is where I am stuck with the theory...how to calculate the theoretical run time of the fridge based on the above?

Also I am interested to know if its possible to fit a larger leisure battery under the bonnet? It looks as if the tray could take a 2" longer battery.

Sorry for my ramblings but any help greatly appreciated before I rush out and buy a new battery!

[g8dhe]

Yes you should be getting a lot longer, I would have thought about 2days+.
Most likely problem is an aged LB, how old is the battery, and has it been run down many times ?

Ah the questions lets see;
1. It draws a peak current of 5.7 Amps when the compressor starts up.
2. Average current draw 3.3 Amps whilst the compressor is running, then it draws that current over the period its actually running.
3. Because the compressor doesn't run all the time the average consumption of the fridge is like a load of 1.4 Amps on the battery at all times, i.e. the compressor only runs about 40% of the time - but this will depend on climate and contents!
When a Lead Acid battery gets to 10.4 Volts its totally empty, and is being damaged, it won't last for long if this happens regularly. Normal practice is NOT to draw more than half the batteries capacity i.e. 50% of 75AmpHrs = 37.5 AmpHrs so 1.4 Amps = 26 Hours on the battery, but a couple of days if you choose to flatten the battery totally

[Reallyfree]

Thanks Geoff for the clear explanation. The battery is just over a year old and came with the conversion. Yes I guess the battery has been run down but only noticably this season since the winter, although the van was driven occasionally over that period but with no use of the LB. Must admit I didnt give it a proper charge before using the LB for the first time this year, relying on the alternator to charge on the journey. This is probably where I went wrong?? One thing I dont understand though, is that the 10.4v cutoff is supposed to protect the battery, yet you say this is totally empty?

Having just got back from a week away in Cornwall, we came unstuck with the flat battery/warm fridge scenario and didnt have a hook-up charge facility as a backup. Managed to buy a nice 80W solar panel locally which got us going during the day. They wired it up to a solar controller with croc clips so I could temporarily connect to the LB. I was very impressed with the panel with the controller LCD showing 6A in full sun. They had a look at my wiring and pointed out a melted fuse housing and the connected long wire running back to my fridge was way too thin and was probably causing additional voltage drop (maybe then its not dropping to 10.4V at the battery?) For the proper install they gave me some 40A cable to pull through and new fuse housing/connectors/roof mounting brackets etc all in for £130 which didnt seem bad to me.

One further question Geoff, is about your leisure battery. What rating is it and do you know the maximum you can fit in a Bongo?

[g8dhe]

Right lets see;
I use a 95AmpHr and it fits in nicely on the tray thats fitted, but you need to measure up the tray and see what will fit, some people have managed 120AmpHr batteries but its a tight squeeze and sits on top of the tray rather than in it. We also have a 100W solar panel these days and the fridge has never been off since fitting it last summer, 24/7, with a similar rated fridge.

Yes do ensure that the wiring is adequate, many converters just use mains flex which is a BAD idea the insulation isn't up to the abrasion it gets in a car and also smokes badly in a fire, which in a car is very BAD whilst in a house if it attracts attention isn't such a bad idea!

Lead Acid batteries take some understanding, bear in mind that it is a chemical reaction that generates the voltage/current and for it to happen there has to be a minimum amount of free energy in the molecules/atoms available hence the 10.4 volts, drop below the that and the reaction changes to one that cannot so easily be reversed.

If your interested then resources like http://www.batteryuniversity.com/index.htm and http://en.wikipedia.org/wiki/Lead-acid_battery are a good starting point before going onto http://www.tms.org/pubs/journals/jom/01 ... -0101.html and similar

[Reallyfree]

Thanks Geoff for the advice on battery sizes and wiring.

I tried my son's leisure battery today, also 75 A/hr but known to be good and had some interesting results.

After a full charge with no load was just shy of 13V
With fridge on dropped to 12.5V
after 4.5hrs dropped to 12.3V
after 7.0hrs dropped to 11.98V
after 7.5hrs increase to 12.14V !!

Curious as to why I checked the fridge and yes the red low (10.4V) warning light had come on. The above were all measured at the battery terminals under the bonnet, so I turned my attention to the rear at the fridge. The voltage here still measured around 12.15V until the fridge compressor tried to cut in and then saw the voltage momentarily drop to 9.77V. Went back to the battery to check again and could not get a reading under 12.1V even when the compressor tried to cut in. So by my reckoning I am seeing a 2.4V momentary voltage drop under peak load when compressor starts, enough to trigger the low voltage cutoff.

So I obviously need to replace the thin wire and replace with the 40A I have been given. A job for next weekend.

Also while talking to my son, knowing the wiring setup in the van is dodgy, said a simple split charge relay does not limit the alternator charge current to the LB and after a period of time can damage it. Is this correct? and if so what setup do I need to rectify this?

Richard

[g8dhe]

There is no limiting in the SCR itself, but nor is there any limiting to the SB either other than the alternator itself! However the LB is always at the end of the wiring run and so has more resistance which will limit the current a trifle. However this isn't anything to worry about LB can be charged reasonably rapidly and a charging current of 20-30Amps for a few minutes doesn't pose a problem to the battery, but might cause a problem to any under-rated wiring, fuses or the relay itself if its not be chosen adequately (a common problem seen more and more recently). The relay is best rated at a minimum of 40Amps, 60 to 80 Amps would be a good choice and over that is overkill. Wiring should be used matched to the relay, but as its quite short, overheating would never be a problem, then of course TWO fuses each close to its battery rated to handle wiring, so either match the wire current rating or just under.


The wiring to the fridge needs to be over-rated simply because of the switch on surge and the distance of the cabling, also make sure that the earth connection to the chassis is good - I personally also use a -ve cable of similar rating rather than rely on the chassis connections which can be know to cause grief!

The voltages you measured are all reasonable even the increased one towards the end - battery voltage levels will vary considerably depending on the load and rest time allowed, the natural "relaxed" voltage is always around 12.6 to 12.8 depending on the precise metals and additives used, the initial 13+ volts will be because it was just taken off charge, given 30-60 minutes to relax it would show 12.6-12.8 instead of the float charge voltage of around 13.6 volts.

[Dodgey]

A new 75ah battery will run a compressor fridge on a little below its mid setting for 2 days.

I tired 75ah battery won't make it through a day.

I know it was for testing but avoid running the fridge on full. It consumes a LOT more power.

I'm in a hot climate right now where it is over 37 outside and I have it on just over 1/4 otherwise its on pretty much all the time.