Leisure battery ammeter question

[bongojoe]

Update on this (and more questions)...

I decided to rewire the split charger earlier this week. Took out several lengths of domestic two-core flex, the type you might plug a portable telly into the wall with, which was connecting the two batteries together. Replaced all that with 40A automotive cable and the weird dual-relay pictured earlier in this thread with one of these 40A versions from Maplin. Also installed a 30A fuse at each battery end.

Tested everything as I went along and all working well. However yesterday I noticed (care of my trusty Chinese ammeter) that the alternator wasn't charging the L/B when the engine was running. Took a look and found one of the 30A fuses blown.

Replaced it and all working OK again, but then noticed that the draw on the L/B when I turn the ignition to 'ON' shoots up to well over 20A before dropping down to 12-14A, a massive leap on what I reported when I started this thread and getting very close to the fuse rating which probably explains why it blew.

I've also replaced the L/B with a new 110Ah version since starting this thread.

So my question... why would the load have increased so much? Is it because my L/B is now much larger than my S/B, or is it because I've installed a more chunky cable? Or is something else going on?

[g8dhe]

I seem to remember that you had a Solar Panel ? If so I suspect that you are charging the L/B to a higher level during the day and hence when you switch the ignition ON, then it is pushing the excess charge back into the S/B - quite a good thing actually! - so provided you aren't exceeding the fuse /relay ratings don't get worried by it. It will have increased due to the extra cable thickness, less resistance, and probably the new battery as well.

Oh and if one fuse blew then replace BOTH, they are in series and both will have heated up, one fuse popped a millisecond or so before the other which then cooled down but is most likely damaged it will of course blow at the next most inconvenient time!

[bongojoe]

That makes perfect sense - it was at the end of a particularly sunny day I noticed the surge in power, so the L/B will have been full of juice. I made a few short trips in the van this evening (to Halfords for some new fuses) and the surge had dropped down to where it was previously.

Interesting that the heavier cable lets more power flow; makes me worried what strain the old stuff was under.

I did replace both fuses, and got some 35A ones too which I'll drop in if these blow again.

Thanks!