Guest Bill Posted February 1, 2005 Share Posted February 1, 2005 Ive just been doing some checks before heading off to France in a few weeks time and putting my volt meter on the leisure battery with the engine running gave me a reading of 14.1 volts but on switching on the fridge i notice that it dropped to 11.8volts. Is this correct i thought that the fridge worked off the engine battery when charging.Also the ammeter in the van read a discharge of 20amps. Link to comment Share on other sites More sharing options...
Guest Derek Uzzell Posted February 1, 2005 Share Posted February 1, 2005 My understanding is that 'smart' fridges (eg Dometic's AES models) should be fed from the leisure-battery rather than the engine-battery. I recall Swift installing a batch of AES fridges in the traditional way (fed from the engine-battery) and the unlucky owners being unable to start their 'vans as a consequence, even though they were on 230V hook-up. Still sounds a bit odd though. I could understand the voltmeter-reading dropping if you just switched the ignition on and the fridge was being fed, but not if the motor was running too. You haven't by any chance got a Batterymate or Batterymaster fitted. I almost ditched my leisure-battery because of one of these. I had fitted a dashboard voltmeter and couldn't work out why the leisure-battery's reading was significantly lower than the engine-battery's. Finally I twigged that I had the fridge switched to 12V and, when I turned the ignition on, the fridge was drawing power from the engine-battery that was, in turn, taking power from the leisure-battery via the Batterymate. Link to comment Share on other sites More sharing options...
Guest Clive Posted February 2, 2005 Share Posted February 2, 2005 There are too many electrical gimmicks around and my experience is to Keep It Simple. We do not have (nor want) an AES fridge. Ours is still Dometic (Electrolux) and the 12 volt side ONLY runs when the engine is running. It and the split charge supply are independantly wired through their respective relays back to the alternator B+ terminal. What does stand out is that if both your starter and leisure batteries are actually connected to the alternator then the method of connecting them is deficient or wire size inadequate. With low voltage equipment it is not the current rating of the cable that is important, it is the volt drop within it because we cannot afford to lose a couple of volts. Have you changed any of the fuses with cheap ones? Some cheap blade fuses have quite a volt drop. 2 volts lost at 240 volt mains is of little significance but 2 in 12 is a lot. Link to comment Share on other sites More sharing options...
Guest PeteC Posted February 2, 2005 Share Posted February 2, 2005 The correct way for the fridge to operate is:- 1 From the mains 2 From Gas 3 From the engine Alternator NOT from either battery When you connect the meter to the terminals with NO othr input you get the battery voltage Switching on the charger you get the voltage that the charger is delivering When the engine is running The ammeter shows a discharge because the fridge itself is drawing this current (that is why it isnt connected to either battery - Assumng a fully charged 100Ah battery - The fridge would discharge this in 5hr or less depending on the actual state of the battery) Read some of the technical articles in MMM PeteC BA (Hons) HNC CGLI Full Tech Cert Cert Ed. Fromerly Senior Lecturer in Electronics & control Engineering Link to comment Share on other sites More sharing options...
Guest Derek Uzzell Posted February 2, 2005 Share Posted February 2, 2005 PeteC: Hmmm... I'll just quote part of Dometic's Electrical Installation Instructions dealing with (Wait for it!) BATTERY CONNECTION. This says "The (12V) heating element circuit must be connected to the vehicle battery by a suitable ignition operated relay in order that the 12V supply is only live while the vehicle ignition is switched on." Now it's certainly true that Dometic's current AES models do demand an alternator-connection, but this is to provide a control-signal to the electronics regarding power-source selection NOT to provide power to the 12V heating element. If (as it seems you are suggesting) fridges are normally connected directly to the vehicle's alternator rather than to a battery, perhaps you could explain the purpose of the fridge-relay that every motorhome 12V circuit I've seen has included. (Incidentally, I was wrong (though not that wrong) originally about AES fridge battery connections but I'll return to that later.) Link to comment Share on other sites More sharing options...
Guest Clive Posted February 2, 2005 Share Posted February 2, 2005 Derek, If the split charging wiring and fridge supply is wired correctly they SHOULD be connected via there respective relays to the alternator B+ terminal. The control relay coils are connected to the alternator D+ terminal. It is wrong for the relay contacts to take their supply from the starter battery because the volt drop in that battery positive lead alone will be detremental to the proper operation of the split charging of the secondary lesiure battery. The only direct battery supply to a fridge will be that for an igniter. But I am not fully up to speed with the AES system. Its just more to go wrong in my eyes! Manufacturers cannot always be relied upon to get it right! Clive plus lots of letters, teacher, examiner etc. Link to comment Share on other sites More sharing options...
Guest Derek Uzzell Posted February 3, 2005 Share Posted February 3, 2005 Well Clive, I've now got a bit of a problem about who I believe! 1. The section I quoted from Dometic's Electrical Installation Instructions relates to all their current fridges and not to AES-type models specifically. 2. My Herald's fridge's 12V 'heater' supply certainly came from the engine-battery and not directly from the alternator. (I take your point about motorhome manufacturers' incompetence, but I would assume that, if Compass connect up a fridge in this way, it's common practice.) 3. As far as I recall, none of the technical articles I've read about fridge connection have referred to the 12V heating element's power coming directly from the alternator's B+ terminal: all have mentioned it coming from the engine battery. 4. John Wickersham (whose technical competence is generally acknowledged) includes a fridge-wiring schematic in his "Motorcaravan Manual" This shows the primary 12V supply coming from the vehicle battery with an ignition-controlled feed energising a fridge-relay to ensure that the primary supply is only available when the vehicle's motor is running. Within the text is a section referring to the relay and saying "this is an electric switch which operates as soon as the engine is running. Current can then flow from the vehicle battery to the refigerator, recognising that the alternator will be charging the battery to compensate for the considerable drain." Perhaps there are 2 distinct ways that the UK motorhome industry wires up fridges? Over to you. Derek Uzzell (O-level Physics - failed) Link to comment Share on other sites More sharing options...
Guest Derek Uzzell Posted February 3, 2005 Share Posted February 3, 2005 Bill: As Clive, PeteC and I debate over how motorhome fridges SHOULD or (more significantly) ARE wired up, it might be useful if you could say which motorcaravan you own (make, model, age) and which fridge (make, model, age) it has. So far, I think we are assuming you have a UK-built vehicle and a 3-way (12V/230V/gas) fridge, but if you've got something different, like a 'foreign' motorhome or a compressor-fridge, some rethinking will be needed. It might also be valuable if you could say whether you think something is actually wrong (like the fridge no longer works!) Big fridges can certainly use a lot of 12V juice - I note that Dometic's 150 litres models draw close to 15A. Regarding my comment about AES fridges and Swift - this was wrong. According to Dometic it's the engine-battery that should provide 12V power to the heating element circuit of AES (and non-AES fridges too as you've suggested in your posting). However, AES fridges have an electronics module that handles energy-source selection and it's this that must be connected to the leisure-battery, bypassing any habitation isolation relay. (Dometic's latest non-AES fridges also demand an uninterrupted 12V supply to power the interior light, freezer-frame heating, LEDs, auto-ignition, etc. and must be wired similarly.) I read about the Swift debacle in their Owners Club magazine and, with hindsight, I think what Swift actually did was connect the electronics module part of the AES fridges to the engine-battery rather than to the leisure-battery. (An odd thing to do, but there you are!) The effect was that owners who spent extended periods with their 'vans static on campsites had their engine-batteries eventually go flat due to their fridges' electronics module's (small but constant) 12V current demand. Motorcaravanners who use campsites for long stays normally rely on a 230V hook-up to recharge the batteries, but it's usually only the leisure-battery that gets topped up when the motorhome's on-board charger is running. In this instance the unfortunate owners wouldn't have discovered that their engine-battery had become discharged until it was too late. (Incidentally, Clive, I'm no fan of 'smart' fridges either, for exactly the same reason as you put forward - unnecessary complexity. It's interesting that the latest Dometic AES models now have 4 possible discrete energy-sources - solar (12V DC), 230V AC, 12V DC and LPG - in that order of priority. However, they also have the ability to select any of the last 3 sources manually, which seems to negate the automation principle behind AES. Apparently it's possible to retro-fit AES to some non-AES versions, though I really can't think who would want to.) Link to comment Share on other sites More sharing options...
Guest Bill Posted February 3, 2005 Share Posted February 3, 2005 It is interesting to read the replies to my question. My vehicle is a hymer swing of 2022 and the fridge is an electrolux rm4280 which is electronically lit and takes 120watt on 12v running. Interesting to note that yesterday after a short drive to check that everything was functioning ok (the fridge switched on 12volts running)the display ammeter again showed a discharge,when we got home and put the vehicle on mains charge the charging voltage was 14.1 and the display ammeter showed a charge of 18amps.What is concerning me is that when we go wild camping in France and stop to pitch up for the night we will have a partially discharged battery.So for every day we are on the move the battery will be discharging which could lead to a fully dischargrd battery until we connect to a mains hook up.I was under the impression that the charging system would compensate for the load of the fridge while running the engine. Link to comment Share on other sites More sharing options...
Guest Clive Posted February 3, 2005 Share Posted February 3, 2005 Gents, The reasoning is this. The alternator B+ terminal IS connected to the starter battery positive with a relatively stout length of wire which will normally be incorporated into the vehicle wiring harness. So from a circuit diagram viewpoint these connections are one and the same. BUT This length of wire will have a small but finite resistance. At high charging currents this wire will drop a fair proportion of a volt. Now, if you then consider the circuit you have the prime source - the alternator, then this resistance, then the starter battery, then two more wires that feed the fridge and split charging relays and then from the other side of the relay contacts to the fridge and leisure battery. Any current taken by any of these appliances or the batteries will cause a volt drop in each and every wire. This first piece of wire between B+ and starter battery + handles all the current. With this clasic arrangement the starter battery will always have first priority on the alternator output. BUT If you wire the supply for the split charging and fridge relays direct to the B+ terminal then the volt drop incurred in the first piece of wire by the starter battery charging current is not going to effect the charging current to the leisure battery of the fridge. Its important that all these charging cables have low volt drop, so short and as thick as possible is best. Then, when you hang your 2 KW inverter on your leisure battery to run your microwave with the engine running most of the energy will be supplied by the alternator! Lastly, the more modern methods of running fridges from the leaiure battery when the engine is running can give the symptoms described by Bill. But if the fridge relay supply is taken directly to B+ this will not happen. Good luck Link to comment Share on other sites More sharing options...
Guest Derek Uzzell Posted February 4, 2005 Share Posted February 4, 2005 Clive: I'm not disputing the logic behind obtaining the 12V supply to a 3-way fridge's heater element directly from the vehicle's alternator rather than via the vehicle's engine-battery. I acknowledge the efficiency gains of using the former method and (as far as I can see) it also removes the need for a fridge-relay as 12V power from the alternator will only flow to the fridge when the alternator is turning. However, I now note that you have employed the phrase "with this classic arrangement" to describe the 'obtaining fridge power from the engine-battery' connection technique. This makes me wonder whether you have been recommending the B+-connection method as best practice or whether you have been claiming this is the way that the UK motorhome industry normally do it. I've rechecked a Compass handbook (about 4 years old) that contains wiring diagrams for Compass motorhomes on Ford, Mercedes, SEVEL and VW chassis - all diagrams show the fridge being fed from the engine-battery. I've consulted a French leisure publication detailing 3-way fridge connection - again the advice is to connect the fridge to the engine-battery. It seems that Dometic and the illustrious John Wickersham both advise connecting to the engine-battery. So I'm now asking myself how many motorhome manufacturers actually do connect the fridge directly to the alternator's B+ terminal? I'd like confirmation from you (and PeteC) that direct-to-alternator fridge connection is how your Auto-Trail's (and PeteC's Devon's) fridge was originally provided with primary 12V power. (I'd also be interested in reading "some of the technical articles in MMM" PeteC mentioned earlier, because I don't remember 'B+' connection being advertised as the motorhome industry norm.) I note your penultimate paragraph - as far as I'm aware modern methods of running the cooling function of 3-way fridges still don't involve the leisure-battery, so I think you are winging it a bit there! Bill: Like you, I don't know whether you have a problem or not. I'm unfamiliar with German practices regarding how they normally wire up motorhomes and shall remain fairly clueless until I get my own German 'van in a few months time. My understanding is that there are significant differences between the Continental and the UK way of doing things. Given Hymer's experience and expertise, whichever way they wire up the fridges in their vehicles is likely to be rational and properly engineered. When your motorhome's motor is running it is absolutely certain that its engine-powered charging system (if functioning correctly) will be able to compensate for the electrical load of the fridge. With technical Help Desk-type questions like yours, either a lot of educated guessing occurs that tends to skate around the inquiry (as is happening here) or someone really knows the answer. Unless a Hymer expert responds, I don't think you are going to get advice that will put your mind at ease. So I suggest you phone Hambleton Engineering (01772 315078) who are very familiar with all things Hymer and who have been very helpful when I've contacted them. Alternatively, try Hymer(UK). If these measures prove unproductive you'll need to get someone competent to hands-on check what's going on with your Hymer's system. Should you discover what's going on with your electrical system please let us know. I'm planning to visit the NEC Show on 18 Feb and (assuming you haven't tracked down what's happening by then), if I can find out anything useful about this at the show I'll post it to the forum. Link to comment Share on other sites More sharing options...
Guest Derek Uzzell Posted February 4, 2005 Share Posted February 4, 2005 Postscript: I searched the forum using "ammeter" and picked up a fridge-related posting dated 27/11/2004 that sounded a bit similar to this one and also related to a Hymer. Don't know if it has any actual bearing on Bill's query but it might be worth looking at. Link to comment Share on other sites More sharing options...
Guest Clive Posted February 4, 2005 Share Posted February 4, 2005 Derek, You said "I acknowledge the efficiency gains of using the former method and (as far as I can see) it also removes the need for a fridge-relay as 12V power from the alternator will only flow to the fridge when the alternator is turning." Connecting the fridge supply to the B+ terminal still requires that a relay is incorporated. Without the relay the fridge would run continuously from the starter battery as this is directly connected to the B+ terminal. Its not just about efficiency. connecting the split charge relay supply to the B+ terminal means that the leisure and starter batteries do not compromise each others charging rate. Link to comment Share on other sites More sharing options...
Guest Derek Uzzell Posted February 4, 2005 Share Posted February 4, 2005 Thanks for the correction, Clive. In fact I realised not long after my postings that I hadn't been thinking straight and a relay would still be needed in the B+ to fridge 12V supply. I restrict my weekday internet usage to pre-8am and post-6pm (it limits the phone bill and my wife's 'more time wasted on that ****** MMM forum' comments) and I had hoped you wouldn't notice until I had been able to retract the statement. What caused me to recognise my error was that I had dug out Ford's service literature for wiring up an additional battery and split charge relay for a pre-2000 Transit and, as soon as I glanced at the circuit schematic, I knew a relay would be needed using 'your' method. Ford's diagram shows the split charge relay between engine- and additional-battery in the 'classic' way. As Ford pre-wired Transit motorhome chassis that were supplied to Compass and Auto-Sleepers, I guess that neither of these makes will have employed the B+ connection method for their Ford-based designs. As I said previously, I'm doubtful how widespread the B+ connection method is in the UK motorhome industry compared to the connection-to-engine-battery technique. It seems to me easier for Colin Converter to connect split charge relay and fridge-plus-relay to the engine-battery than to delve around by the alternator trying to connect to its B+ terminal. (This will be particularly true with current Fords where the engine-battery is in the cab area.) My understanding of French motorhome systems is that the split charge relay is replaced by a 'coupler/separator' that sits between engine- and leisure-battery and only passes current from the former to the latter when the engine-battery has reached a certain charge state. This makes sense to me as I'm far more concerned about a flat engine-battery than a flat leisure-battery. Link to comment Share on other sites More sharing options...
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