Isolator Question

Greggbear67

Greggbear67

Forum Member
I have 2x 110ah leisure batteries which are charged via the original power supply cable for the now removed tail lift. Currently the cable runs from the starter battery & through a heavy duty isolator as used on rally cars etc. When the isolator is open the batteries don't draw off the main battery. Just before the leisure batteries I fitted a midi fuse holder & 80amp fuse. Yesterday the bus wouldn't start (been stood a while & not run)
I locked the isolator in to try & slave power forward but I think I blew the fuse as when I finally jump started the bus the leisure batteries don't seem to be getting any charge.
I have ordered a heavier fuse (150amp) but wonder if
A. Will this bigger fuse be better?
B. Do I need a fuse at all in the line?
Thinking if I wire straight thru it should then work as a slave starter system if needed, but will the lack of a fuse be a safety issue?
Advice please.....
 

wildebus

Forum Member
Greggbear67 said:
I have 2x 110ah leisure batteries which are charged via the original power supply cable for the now removed tail lift. Currently the cable runs from the starter battery & through a heavy duty isolator as used on rally cars etc. When the isolator is open the batteries don't draw off the main battery. Just before the leisure batteries I fitted a midi fuse holder & 80amp fuse. Yesterday the bus wouldn't start (been stood a while & not run)
I locked the isolator in to try & slave power forward but I think I blew the fuse as when I finally jump started the bus the leisure batteries don't seem to be getting any charge.
I have ordered a heavier fuse (150amp) but wonder if
A. Will this bigger fuse be better?
B. Do I need a fuse at all in the line?
Thinking if I wire straight thru it should then work as a slave starter system if needed, but will the lack of a fuse be a safety issue?
Advice please.....
Click to expand...
in reverse order, absolutely DO NOT connect a cable from the starter battery to the leisure battery without a fuse. In fact there should be a fuse at both ends of the cable (and this is not optional if you want a safe installation)

the cable that you have for the tail lift is probably 16mm at the most? This cable is rated at 110A in free air with a relatively short run. A 150A Fuse is oversized for that gauge of cable. 100A is the maximum you should use for 16mm cable

If the starter battery is properly flat, then the kind of current that it could be trying to pull off the leisure battery when turning over is way beyond the ampacity of a 16mm cable plus will cause a big voltage drop that will make the situation even worse.

It is possible to use a leisure battery (of the right type) to assist the starter battery, but not as a replacement using the type of split charge cabling that is typically fitted.
 
Greggbear67

Greggbear67

Forum Member
Was hoping for your input mate thanks. Back to a fresh 80amp fuse then. If I get a flat battery again, sounds like I should isolate the LBs first, then jump start as normal. Would that sound reasonable? Thanks again.
 
SquirrellCook

SquirrellCook

Forum Member
After the embarrassment some years ago on a ferry, we were unable to start. The fridge had been left on the engine start batteries via an inverter.
So out with the generator to charge the batteries enough to get it started.
Now we are able to charge each way depending on the problem without anyone knowing what we are doing.
So each battery type has an inverter and a battery charger ready to go.
 

wildebus

Forum Member
Greggbear67 said:
Was hoping for your input mate thanks. Back to a fresh 80amp fuse then. If I get a flat battery again, sounds like I should isolate the LBs first, then jump start as normal. Would that sound reasonable? Thanks again.
Click to expand...
If the starter battery is flat, then connecting the two together WITHOUT attempting to start will get some energy back into the starter battery by the simple process of the batteries trying to equalize themselves. So if you are not too much of a hurry, leaving them like that for maybe a hour could well be enough to get sufficient charge back to the starter battery?
The situation you got with the blown fuse is not that unusual at all when the starter battery is dead on a relay-based Split Charge setup . Usually there is not a connection between the LB and SB anyway unless there is charging going on but sometimnes they engage and then spike - what kind of split charge system have you got installed in the van?
Turning the isolation switch to disconnect is not a bad idea to make sure you don't have a unwanted circuit there when jumping.

When my Starter Battery was rubbish, I bought one of those small lithium jump-packs for around £60. Worked very well and started my 2.8L Diesel no problem. Also used on the car and at one of the Wild Camping meets to help someone else with a dead starter. Not used for ages now, but it is a nice precaution to carry :)
 
Greggbear67

Greggbear67

Forum Member
My start battery is wired as normal, then a thick red cable goes from there to the leisure batteries at the back. The leisure batteries are then earthed to the chassis under the bed at the rear. Looks to be an inline fuse under the bonnet same brand as the tail lift (ricon) then the midi 80amp fuse next to the leisure batteries. I have fitted an isolator switch on the dash in the positive cable that goes back to the leisure batteries. When off, this breaks the connection between LB/SB. Theoretically, after I start the engine I engage the isolator & all 3 batteries begin to charge. When I stop the engine I remove the isolator key & only the LBs are run down while parked. Very basic & low tech but it seems to work ok. A voltage meter connected to the leisure batteries reads 12.4-12.8v when disconnected going up to 14.4 when running & with isolator locked in.
 

wildebus

Forum Member
Ah ok, so as you say a very basic 'split-charge'.
When you say a "thick cable", how thick? If you have repurposed a taillift cable, I would be surprised if it that is anything beyond 16mm, which when it comes to the type of currents seen on engine starting is really pretty thin.
Put a photo of various cables and connectors below which might help confirm if the exact size cannot be determined by connector or cable labels
Cable&Conns by David, on Flickr

FWIW, I replaced the 16mm cable that was used for a taillift on my own van with 35mm cable for my 'split-charge' setup and use 70mm cable between the batteries and the inverter/charger as it can draw 240A (Victron recommend 2x50mm (100mm) in fact but I stuck with 70mm as the distance between the two is so small).
Bearing those cables sizes in mind, a starter motor can draw easily in excess of 300A the cables used for that is around 50mm minimum and is very short.
 
Greggbear67

Greggbear67

Forum Member
Reckon the cable is similar to the red one in the centre of the picture. Probably not quite as thick as my jumper cables...
 
trevskoda

trevskoda

Forum Member
The simple way is fit a 200ah relay bypassing the fuse with a switch so you can connect all the batts together for starting,thats what i have,just remember to knock the switch of when camping as not to drain starter battery.
 

wildebus

Forum Member
trevskoda said:
The simple way is fit a 200ah relay bypassing the fuse with a switch so you can connect all the batts together for starting,thats what i have,just remember to knock the switch of when camping as not to drain starter battery.
Click to expand...
Bypassing the fuse? You mean bypassing the FUSES plural as there needs to be a fuse either end as there are two sources of power. How would you bypass both fuses with a single relay.
Then you will be pulling significantly more current then the cables are rated for, which is a hazard. Then a 200A relay will be undersized anyway if all/significant amount of the starting power comes from the Leisure battery. And finally, that could damage the relay in such a way that it melts ON, and then you have the starter and leisure batteries connected together permenantly through an unprotected (unfused) cable run.

Sorry Trev but your suggestion is not good and actually dangerous!
 
Greggbear67

Greggbear67

Forum Member
That's all my problem was, been stood a while & taken some pretty hard frosts too. Usually cracks straight up especially if used regularly.
Cold weather is not your friend....!😕
 
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