Electrics

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Xray Nick

Xray Nick

New Member
Posts
9
Location
Hull
well im very much a newbie to all of this.....expecting a bit of a roasting as I'm probably doing something very obviously wrong but here goes.....

I've fitted a second battery using a manually switched split charge setup....added a cable from the positive of the second battery to a fuse box. All appears well, no flames or melting wires so far......wire USB socket with connection to chassis earth and to the positive out of the fuse box. Now the problems begin.....every fuse up to 30a blows.
It's an older second battery and not sure what the charge on it was so may need to run for a bit to charge it up so wondering if that's causing the problem, i.e. High current draw to get it up to charge.....and me being impatient to get things wired up. otherwise I'm a bit stumped. Should I run the earth from the second battery? (Not sure why as the earth is common to both batteries)
Any thoughts, and ridicule for missing the obvious, will be greatly received!!
 
Can you sketch up a circuit diagram of how you wired up the split charge, second battery and the USB socket?

Even if the second battery is flat is still shouldn't blow fuses, I am thinking that there is something amiss in the split charge wiring.
 
Thank you for the reply!!!
This is the way I ran the split charge
Photo%2002-04-2017%2C%2013%2055%2038.jpg
(Hope the image displays!). If not I shall upload when I'm at home.
Simply ran a cable straight from the positive on the second battery to the fuse box and took earth from the chassis.
 
Been down and double checked the wiring with my wife and both agree it's as in the diagram.....we're both a tad flummoxed!
 
If it's the fuses in your diagram that keep blowing, then yes they will. As soon as you throw the switch the two batteries will instantly try to balance with the potential difference between them being the only thing limiting the current flow. If for example you've been running for a bit meaning your starter battery is fairly full but you've discharged the leisure battery then as soon as you throw the switch a significant amount of your alternator's capacity will try to flow through to your leisure battery (or if the alternator is switched off, the starter battery will discharge into the leisure battery).

So if your leisure battery is say at 11.8v, starter at 13.6v and alternator kicking out 14.2v then an initial surge of the (near) maximum output of your alternator (45, 70, 120 amp?) will try to go to the leisure battery blowing the fuses with a possible surge from the starter to the leisure also happening so without your fuses you'll potential damage the starter battery.

Personally I'd have the switch between the alternator and batteries with appropriate diodes to prevent the starter from feeding the leisure battery and no fuses (in the same way there aren't any fuses in a normal alternator to battery setup).

On the earths, yes they should be linked (usually through the body) unless for some reason you want to have isolated earths for whatever you're running off your leisure battery.
 
The fuses on the battery are fine and running ok.....both batteries charging well......I was worried about them blowing when I first hooked up as the second battery was knocking around for a while unused. It's the fuses in the auxiliary fuse box that I've run off the positive terminal of the that blow if less than a 30 amp fuse is in them......included a better diagram (although it looks like its been done by a 5 year old...sorry!) with the aux fuse box on......that's where the fuses are blowing

upload_2017-4-2_20-53-54.png
 
Looking at the diagram now I have got home from work, yes....I concur with the above, that is not the correct way to run a split charge, the charge current from the alt will melt the fuses quite quickly like that.
 
Xray Nick said:
The fuses on the battery are fine and running ok.....both batteries charging well......I was worried about them blowing when I first hooked up as the second battery was knocking around for a while unused. It's the fuses in the auxiliary fuse box that I've run off the positive terminal of the that blow if less than a 30 amp fuse is in them......included a better diagram (although it looks like its been done by a 5 year old...sorry!) with the aux fuse box on......that's where the fuses are blowing

View attachment 120674
Click to expand...
If it is the second fusebox fuses blowing, I can only deduce there is a massive potential difference in the two batteries as they are connected to a common earth. Even then it shouldnt blow the fuses....
 
If the drawing is accurate (meaning there's no other connections you haven't noticed) then the USB socket must be shorting somehow. Have you checked with for a short between the socket side of the fuses and the earth?
 
Muddy Vicky said:
If the drawing is accurate (meaning there's no other connections you haven't noticed) then the USB socket must be shorting somehow. Have you checked with for a short between the socket side of the fuses and the earth?
Click to expand...
I was going to suggest disconnecting the USB and using a 12v test lamp or a spare 21W indicator bulb in its place and see if the fuses still blow.
 
Thank you both.....that's my job for wednesday! I'm also going to move the earth directly to the battery to remove any other shorting risks as well.
I'll let you know how it goes.
Also just checking......is the split charge wired up ok (without the extra fuse box)? I'm more used to high voltage circuits!!!
 
I'd still be concerned for high surge currents between batteries that could shorten their lives or worse. In theory the fuses should protect you from that but I'd prefer some diodes to prevent flow from one battery to the other.
 
Thank you for that!
Didn't go with the diode due to the voltage drop and wanted to go with switch rather than relay so I have a "booster" if my other battery runs down......haven't got round to putting an "alarm" on the lights in case i leave them on and get a flat!

After reading the tutorial I'm not using a specific leisure battery.......this is the first one that's actually defined it as a specific type.....most sites just say using a second battery......I'll try the bulb test and see where that leads me!
 
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