lord lucan Posted August 31, 2008 Share Posted August 31, 2008 hi looking for circuit details of how to make a battey link up on my motor home for charging the engine battery similar to the comercial one battery mate iam sure it just reguire just use of diodes etc Link to comment Share on other sites More sharing options...
hallii Posted August 31, 2008 Share Posted August 31, 2008 I did it once with a couple of massive diodes out of an old fork lift truck, they have to be able to carry a lot of current. A more elegant way is to use a relay switched from the alternator or manually. Again it has to be able to handle the current, I wouldn't use one that couldn't handle 50 amps or so. But the split charge relays are cheap enough? About £18 up. Geoff Link to comment Share on other sites More sharing options...
Silicongyro Posted August 31, 2008 Share Posted August 31, 2008 Lord Lucan I had a similar requirement to you and made up a simple and very low cost circuit some years ago which has worked perfectly ever since. This provides a modest (up to a couple of amps) charge to the vehicle battery whenever the leisure battery has a voltage greater than the vehicle battery ie. when on hook up and charging. This is intended to make up for the loss due to vehicle alarm (cobra), self discharge etc when stored for any time. I intercepted both the vehicle battery and leisure battery positive wires connecting to the relay which connects them together when the engine is running ie the relay energised by the alternator D output. This relay is situated conveniently under the drivers seat by the leisure battery in my Laika. Brief details I made up a simple circuit consisting of 3 components wired in series :- a diode, a 1ohm power resistor, and a fuseholder with 2 amp fuse. The diode should be a 3amp Schottky barrier type (34p from Maplin) to minimise voltage drop, and the resistor should be a 10 watt wirewound (40p from Maplin). The 3 components are housed in a suitable enclosure and one end of the series network is wired to the aforementioned leisure battery wire, and the other end to the vehicle battery wire. Do observe correct diode polarity to ensure current can only flow from leisure battery to vehicle one ! Of course I would only advise attempting this if you are conversant and competent with such modifications ! Link to comment Share on other sites More sharing options...
Guest JudgeMental Posted August 31, 2008 Share Posted August 31, 2008 I actually meet Lord Lucan recently whilst on Holiday in Carnac....... I could have made a fortune with the information of his whereabouts. BUT being an English Lord as well, I would not dream of such a thing! Welcome to the forum Lord Lucan, there are three lords on here now..... MHF top that! That is of course if you include Lord Thornber, who I believe is a charlatan..... And of all things works for the post office! Link to comment Share on other sites More sharing options...
MelvynT Posted September 1, 2008 Share Posted September 1, 2008 Linking batteries isn't as easy as that, all diodes drop voltage when working, about 0.7V, adding a resistor will add to the voltage drop, depending on the current flowing. However, you can do it, the vehicle battery will never be fully charged, due to the above voltage drops. Even with the leisure battery full, the vehicle battery will alway be at least 1V below, but for a top up, yes it'll work. Remember that when starting the van, or at periods of high discharge, parked in a cue with the lights on, the leisure battery will discharge. You should arrange to disconnect this setup before starting the engine. One of the more experienced members links the two batteries with a fuse, so if they forget, the fuse blows, at a cost of about 0.5p, this method also gets round the volt drop problems. You just need a accessible place to mount a fuseholder (about 50p), several fuses, because you will forget, and the wire. Link to comment Share on other sites More sharing options...
Silicongyro Posted September 1, 2008 Share Posted September 1, 2008 MelvynT Thanks for the comments - however at the risk of being seen as a pedant I must qualify your comments as I fel they could mislead others! Schottky diodes being metal/semiconductor devices have a lower junction voltage - and typically only drop 0.4v in this sort of application (I have also measured to check). So they are usually specified in this sort of low voltage application rather than conventional silicon diodes. Yes, of course the resistor will drop a voltage - but that is it's job in this case to limit the flow of current. The worry you have re. the battery discharging in slow traffic etc is misplaced and not correct. Even in slow queueing etc the alternator maintains 14v or so - but in any event the relay joining the leisure and vehicle batteries will be energised as soon as the vehicle starts so bypassing this circuit and fully joining the 2 batteries whatever their individual states ! I agree this arrangement will not strictly fully charge the vehicle battery - but neither does it need to. It will provide a significant charge to it which will condition it well above the requirements for eventual starting and maintaining the alarm etc. When on hook up after a few hours the leisure battery settles to about 13.9v and the vehicle battery about 13.3v in my vehicle set up. So this is a "fit and forget" solution requiring no further attention from the driver - as has actually been the case for several years for me. Link to comment Share on other sites More sharing options...
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