Split charge relay

drewdt3

Guest
Afternoon All

I am considering fitting one of these to replace the poxy split charger supplied with our Autotrail MH: -
Voltage Sensitive Split Charge Relay 12V 140A | 12 Volt Planet

My thinking, not being an auto electrician, is to use 16mm2 cable and something like 80amp fuses at each end. Does this sound a safe way to go, or does anyone have any better suggestions?

Thanks

Drew
 
That looks very good, and your plan sounds fine.

Let us know how you get on with it, could be a good Smartcom substitute!
 
Hello Drew,

how big is your leisure battery as that unit seems totally over the top, 140 amps? If I were to use an auxiliary battery charger to charge a partially discharged 110Ah battery I would expect no more than 8 amps or so initial charge dropping off as the battery regains charge.

Alec
 
140 Amps is rather over the top for typical motorhome set ups. The alternator is capable of putting out 30A or more for a short time if the battery is very discharged, I don't think you would ever see 140A!

50 Amp cable and 30A fuse should be enough. I've seen plenty of installations with 30A cable and 15A or 20A fuses - they do tend to blow quite a bit though, if you let the battery get flat.

That said, nothing wrong with over specifying the 140 and 80, just you'll end up paying more for the installation, but it will work, and it will be robust.
 
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Hi Alec

My leisure batteries are going to be 172ah when i get them, I'm not sure whether it's over the top or not I was assuming that what will do a lot will do a little and the thicker the wire the safer it is. or am I wrong? As I said I'm not an auto electrician hence me asking the question.

Thanks for all replies so far.

Drew
 
Drew far better to be over than undersized

Alf

[:/QUOTE=drewdt3;497817]Hi Alec

My leisure batteries are going to be 172ah when i get them, I'm not sure whether it's over the top or not I was assuming that what will do a lot will do a little and the thicker the wire the safer it is. or am I wrong? As I said I'm not an auto electrician hence me asking the question.

Thanks for all replies so far.

Drew[/QUOTE]
 
Yeh Talbot B2B was the way I wanted to go until a Sterling unit, for no apparent reason, nearly burnt my van to the ground in 2013, not one fuse blew, I have been put off ever since. I also have a 140 watt solar panel although not a lot of use this time of year.

Drew
 
You won't save any significant money by downgrading it - stick with your plan, and know you'll never have a problem.
 
drewdt3 said:
Hi Alec

My leisure batteries are going to be 172ah when i get them, I'm not sure whether it's over the top or not I was assuming that what will do a lot will do a little and the thicker the wire the safer it is. or am I wrong? As I said I'm not an auto electrician hence me asking the question.

Thanks for all replies so far.

Drew
Click to expand...

It's more to do with how much the alternator can put out to a flat battery, rather than actual your battery capacity. You can get an initial current of about 30-50 amps which quickly settles down to lower values like 5-10 amps as the battery voltage builds up. Hence a 140 amp relay and comparable wiring is oversized for your application, but yes the bigger the wire the safer, and more importantly you will get less voltage drop and heating over a thicker wire, so you will charge quicker. So you should probably be thinking in those terms if you oversize this much rather than safety considerations.

As for savings, you'll probably spend about £60-70 on your parts, whereas for cheaper bits you could do it for £20-30 so the savings aren't massive either way.

There are some people on this forum who have a lot of disposable income, and others who do things on a budget. Horses for courses really. Both systems are safe, but the 140A relay and thicker wires will be more efficient and more robust at a larger initial outlay.
 
drewdt3 said:
Hi Alec

My leisure batteries are going to be 172ah when i get them, I'm not sure whether it's over the top or not I was assuming that what will do a lot will do a little and the thicker the wire the safer it is. or am I wrong? As I said I'm not an auto electrician hence me asking the question.

Thanks for all replies so far.

Drew
Click to expand...

yes you are correct about the wire size.i also would fit a numax charger 10 amp and a spliter unit as in the picture,and if fitting s/panels about 100 w use the second unit and this is what i have. The splitter unit will charge the starter batt first and the solar unit will pick out the lowest batt first ,both units are about £15 each on flee bay,the solar one is kemo m174 131402504785 and the charge splitter is kemo m102n 141107861770 ,hope this helps regards trev.
 

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Hello Firefox,

yes there is a higher initial charge from an alternator but there is little difference in voltage drop if using say a 6mm squared cable. Really high current charging of batteries is not a good thing but unavoidable when charging with the alternator but at least it is of short duration.
16mm cable is very much harder for a DIY install as who has crimp tools to attach 16mm lugs? Some jump leads are smaller than 16mm section.

Alec
 
Relay connects when voltage at either terminal reaches 13.3V
Click to expand...

This feature means that the relay could be closed when you start the engine and that means fairly high starting currents could flow from the leisure battery back to the engine battery while cranking - SO I would not be in a hurry to downsize fuses or cable sizes based on what the alternator may or may not be able to deliver.

True, the relay might open fairly soon but there are many "unexplained" instances of 40 amp fuses blowing in smaller setups so smart to keep options open by upsizing rather than downsizing.

Means that you can also use the wiring as an emergency start boost if the starter battery is flat merely by adding a manual switch in parallel to the split charge relay.

Another trick you can use is to fit a manual split charge OFF switch in the black wire to earth. This means you can allow the split charge system to do the best it can and then switch it off to allow the solar to top the system up on its own.
 
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I have fixed about 10 charge systems of wild campers in the past 4 years. Most of the ones that have broken have been fitted with a 15A fuse. I believe the smartcom relay instructions recommend 4mm2 charging cable and a 15A fuse - something like that. 15A is def too small, but recommended in instructions for a common proprietary charging relay, so figure what you like from that, cos they blow all the time.

I have not seen a 30A fuse blow yet.

I don't use a relay - prefer a heavy duty on/off switch and a 45A trip - far more reliable. I have 10mm2 cable.

Agree 16mm2 cable more difficult to fit
 
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BtoB charger

I have just fitted a Sterling Battery to Battery charger. Seems to work well. Fitted it with 25mm cable as it is a 6m run. Should be able to charge 2x11ah batteries in no time for Eberspacher D5WS heater. Will let you know how I goes in the future.
 
Any decent electrician will or should have a crimping tool and assortment of 16mm crimes

Alf


piman said:
Hello Firefox,

yes there is a higher initial charge from an alternator but there is little difference in voltage drop if using say a 6mm squared cable. Really high current charging of batteries is not a good thing but unavoidable when charging with the alternator but at least it is of short duration.
16mm cable is very much harder for a DIY install as who has crimp tools to attach 16mm lugs? Some jump leads are smaller than 16mm section.

Alec
Click to expand...
 
Smartcom instructions want launching straight in the bin. It's very useful - I have one - but only to power a decent relay.

I have various crimp tools if anyone local needs anything.
 
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