Club 80-90 Forums

Oldiebut goodie wrote:The only thing that I have found with them is you have to check the soldering - I have found poor joints on all sorts of stuff.

I must admit I'm a bit hesitant when ordering parts from China or Hong Kong - if I can source the same thing but via a UK supplier I will generally buy via them even if it's a bit more expensive so I can have a chance of a refund/replacement; such is my (lack of) faith in their build quality..................

Actually oldiebut goodie, thinking about it there won't be any current flow from the starter battery to the leisure battery when the engine's running because the alternator voltage is higher than either battery and current will be flowing into both batteries from the alternator, albeit a much larger flow into the partially discharged leisure battery. Once the engine stops the split charge relay will drop out so the connection will be severed. The terminal voltage at both batteries will be raised to the alternator output voltage.....

But the starter battery is most probably at a higher potential than the leisure so will still give that fuse blowing surge. With the split charge relay kicking in I would expect to see the alternator voltage drop momentarily, probably to the level of the starter until it has caught up with itself again. It wouldn't take a long surge to overwhelm the fuse.

It's something that needs careful and precise study to ascertain exactly what occurs in these situations. (Just stick a bigger fuse in! )

Oldiebut goodie wrote:But the starter battery is most probably at a higher potential than the leisure

Irrelevant I'm afraid; the two are not connected until the engine actually starts (assuming the split charge relay trigger wire is wired correctly) and once the engine's running the alternator output voltage will overwhelm both battery's voltages and all three will be in equilibrium regardless of their quiesent disconnected values. Your scenario only would only apply if someone by passed the split charge relay without the engine running with jump leads or similar.

I've also investigated the max rating of me cables and no matter what angle you look at it 70A is the max. If I want to fit 75A fuses and keep the same cable c.s.a I'll have to replace it with tri-rated which scrapes in at 75A

Battery to battery current surge is not really an issue in or applications and that's only if he relay connected before the alternator cut in. Taking Oldies 220A setup as an example and assuming they are starter batteries as the leisure batteries and 40% discharge which is the worst case scenario the the surge would only be approx 30A, with true leisure batteries or a totally flat battery with their higher internal resistance the surge would be even less, and that is assuming the electrolyte has reached 38 deg C when a battery can accept a bulk charge. And no you don't add this to the alternator output the higher alternator voltage cancels out any battery to battery flow. Ideally the split charge circuit should match the maximum output of the alternator. Even though the alternator can produce about 5% more power than the rated output when the alternator is below its normal operating temperature, taking away what amperage is actually used to run the engine about 10A for a petrol engine and whatever the starter battery is taking to recharge it after starting the engine, normally cancels this out but if you have a large output alternator it has to be considered.

I'm out.................................

But it is an issue as the numerous testimonies to the blowing 30A and 50A fuses show! (which is how we digressed onto this).
Your theory isn't borne out by practical experience. If you are using a voltage sensitive relay (as I and many others do) the starter can be sitting at 14v as it is being charged, if the relay is set at 14.3v and the leisures are drained to, say 11.5v, there will be a surge - no escaping the fact when the two systems are connected. 1664 saw 58A as he stated earlier.

We will have to agree to differ on this.

Can see where you're coming from with this set up but most things have the amperage on them or can soon be calculated using ohms law. I just buy low consumption stuff at the outset and in my case I kept the monitoring as simple as possible - as shown in WIKI. Its good enough as a guide to tell you how quickly your battery is depleting.

If you haven't discovered already, you'll get preoccupied with what the meter is doing instead of looking out the glass at the front - either that or induce mega paranoia. As my old man always said - keep it simple.

This is a great mod but the ebay link in the 3rd post is just for an ammeter (guess how I know!) the same seller does sell the combined ammeter/voltmeter but it's over £20 with postage http://www.ebay.co.uk/itm/DIGITAL-AMP-V ... 2768wt_975 although it is a 200A one, I'm not bothered about having to use two meters because I'm going to install them next to the leisure battery.

Oldiebut goodie wrote:"My split charge cable can take 70 amps but guess the next size fuse link up from 50 amp? 75 smeggin' amps that what!! To say I'm tempted is an understatment"

Sorry I'm a bit late with this interesting one, but I must have missed it first time around last year.

I think I can help with the 70A cable/75A fuse thing. The Amp ratings for cables are calculated on a continuous basis, that is a cable will take that load continuously, with a certain set of conditions such as temperature. The cable gradually warms up with that amount of current, and if that goes on for a long time, the cable will reach a steady state at the maximum safe temperature for the insulation material. This might take several minutes. In this application, high values of current will only persist for some seconds, until battery voltages have stabilised, so the cable will not have the time to heat up. The purpose of the large value fuse is really to protect the battery and wiring against a short ciruit, and under these conditions, even a 100A fuse would blow in a fraction of a second, long before the cable had heated up. Prolonged overloads of 70A and above are so unlikely that you could ignore the posiibility, and overloads of that magnitude ought to blow the fuse feeding the faulty equipment anyway.

I would have no hesitation in using a 75A or even 100A fuse in this application for short circuit protection only of a 70A cable, just so long as the equipment fed off the battery is properly fused. Then the rating of the relay connecting the batteries would be the limiting factor, not the fuse.

Good point.

colinthefox wrote:

Oldiebut goodie wrote:"My split charge cable can take 70 amps but guess the next size fuse link up from 50 amp? 75 smeggin' amps that what!! To say I'm tempted is an understatment"

This might take several minutes. In this application, high values of current will only persist for some seconds, until battery voltages have stabilised, so the cable will not have the time to heat up.

Actually I wrote that, not 'Oldiebut goodie'.

I did consider that a really high current would drop significantly in a relatively short space of time as the leisure battery recharged, but didn't fancy upping the fuse rating over 70A as I didn't know how long or for that matter just how high the initial current flow would be if the leisure battery was discharged even more (I have run it so low that even a set of jump leads got hot recharging it; only once mind, before I appreciated how much juice the TV/DVD drank)

As I pointed out earlier, the 50A fuse links were too quick/sensitive (and expensive) so I have replaced them with a homemade 60A fusewire link which is a much more tolerant to initial overcurrent conditions and haven't had a problem since.

I found that out of the box, these meters tend to be way off the mark and had to calibrate them using a known good accurate meter. Might be disturbance in shipping I suppose. You can usually find a small potentiometer on the back for adjustment. Once thats done however, they're spot on.

There's two potentiometers on the combined display - amps and volts. Difficult to set amps as accuracy throughout the range depends on just where you set it since it covers 0-200 amps. I set it at about the 0-5amp level to watch the camping equipment consumption. The recharge current doesn't have to be particularly accurate since being out by an amp at the 50A level is pretty academic...