codlord

I wonder if anyone can help me with a problem I have. I recently got some info from the internet on how to wire up two batteries with a relay to get a cheap split-charging relay system.

I followed the instructions and now have a setup as detailed in the attached picture.

The only problem is one or other of the inline fuses between the batteries and the relay keeps blowing, but not consistently, and I have been unable to discover what is causing the problem. It may be o.k. for a week of driving and then blow, and even if I have not run anything off the leisure battery - obviously more than 15 amps is going between the two batteries, but how/why/when?

Does anyone know why or what I can do to fix the problem? (other than a manual switch instead of the relay). Have I just spent some time installing a circuit that is a load of rubbish?

zedex

Which fuse is blowing? The one coupled to the leasure battery?

codlord

I THINK I have had both blow at different times, but as both fuses are actually located together and I did not take note of which was which the first couple of times the fuse blow I can't be sure. I'll try and find out.

zedex

Also curious as to what relay feed you used. Its supposed to be on only AFTER the vehicle is started. Where did you take the feed from?

hilandfrog

it seems to me that if you have 2 batts and they are connected in any way even through a relay they will try to charge eachother if one is low, like when filling SCUBA bottels, cascading?. hence when anything greater than 15 amps is drawn ( just running your truck should draw 25 amps or so) both batts would attempt to share the load (12.5 amps each + any wire resistance) add headlights, your blower motor for AC or heat and your over that 15 amp thing.

if this is incorrect please help as this is how I based the wireing of my two batts.

a 4 GA.Jumper from + Batt (1) (truck batt red top) to side (A) of a constant duty solinoid.

a 4GA. Jumper from side (B) of solinoid to + batt 2 (inside camper shell yellow top)

solinoid turns on or off charging to batt #2, joins the two batts for really cold starts or using heavy power requirements.

if I had the time and $$$ the real way to do this right is up your alt. I know my Jeep had a green machine 165 amp and it kept up with everything. this will be stage two in my electrical mess.

VR
Repo

codlord

The feed I used is from a connection in the engine bay fusebox - it is only live when engine is running, not when engine is cranking (although to be honest I am not sure what it is that I have connected to, but as it was only switching a relay I figured it should not cause any problems).

I was going to connect to the battery charge warning light off the back of the alternator but found that there is a more complex circuit off the back of the alternator - not just the normal single wire so I decided against that.

zedex

The fuses are probably blowing when the vehicle starts. Because the feed your using might be triggering the relay when its starting. And the starter will be drawing alot more than 15 amps. If the relay is triggered at startup it will allow current to flow from either battery.Thats why its very important to have that feed live ONLY AFTER the vehicle is running.

codlord

I agree it is a likely reason but I can start the vehicle loads of times without either of the fuses blowing, even put all lights on, all heater/air con on full, radio, rev engine etc etc and they still do not blow - but obviously they are blowing sometime, but don't know when or if it's for a particular reason.

alanh

Did your internet source say that 15A fuses were oK?

Is there any correlation between fuses blowing and discharging one or both batteries? (Like running your accessories with the engine off and then starting it up?)

Seems that something like this might be happening if fuses aren't indeed blowing when you start...

Assume one battery is fully charged and the other is something less than charged. You then have two voltage sources (either capable of sourcing quite a bit of current) connected together. In this case, current will flow from the battery with the higher voltage to the one with the lower voltage

I think one can assume in this case the only thing limiting the amount of current that will flow is the resistance of the wire, the relay contact, any resistance of the connections, and the two fuses between the two batteries. (There is battery resistance to consider, but it isn't large enough to limit the current to less than 15A.)

I = (Vbatt1 - Vbatt2)/R.

I = current
V = voltage,
R = resistance

If you had a 2 volt difference between the two batteries, a resistance less than .133 ohms would let more than 15A flow.
With good connections, short wire runs, etc. the resistance should be low. I haven't measured something like this, but I think it could be less than that.

You could put in larger fuses if your wiring and relay will handle the current - maybe beef it all up to handle more current if necessary. There are fuses that take longer to blow than normal fuses so they can handle surges, but I'm not too sure they would help here.

Cebby

Is this what you are using?



15 amp is way too low. I think this type of relay is rated for 60 or 80 amps. If you are using 4 ga, you can safely go to 60 amp assuming you've upgraded the grounds to 4ga as well. The power and ground cable potential needs to be equal. If you still have problems, you might want to consider one of these:

codlord

Cebby - I assume you posted some pics but I cannot see them - don't know if it's just me or not.

Have not yet found any correlation between actions and fuses blowing and I have not run anything off the leisure battery so it should be at full charge.

The relay is currently just a standard 30amp automotive relay, and I am happy to re-do the wiring to handle more amps and use bigger fuses but I want to know that the wire/relay/fuse I pick will fix my problem - what should I go up to?

The original instructions I got from the internet specified 10amp fuses! (these were for a VW camper van) so I thought 15 amp would be o.k. as I did not expect a massive draw accross the two batteries but I guess I was wrong :confused:

I know you can get specific split-charging units but I just thought this was a cheap way of doing the same sort of thing.

Not sure what 4ga is - I am assuming it is a wire of a certain rating - here in the UK as far as I know it's just sold as Xamp wire, so 17.5amp wire or 27amp wire etc.

alanh

The rating of 4GA wire would vary depending on the application and the various regulatory agencies that might be involved if you were doing something like wiring a house, but it looks good for 100 amps or more. Here's one place with some info. Lots of things like temperature, insulation, if the wire is bundled with other wires, etc. influence the rating. A lot of people use heavier wire just to keep the resistance of the wire lower when connecting batteries, but with a short run and if you aren't starting the vehicle or winching with it that shouldn't be a problem. There are different standards for wire gauges, but one table I found on the net shows 4GA as .225 inches/5.72mm in dia if that helps any.

I did a bit of searching for "dual battery" and saw at least a couple of sites with writeups that are using 200A solenoids in that application. Since I haven't experimented with something like this and haven't studied batteries to know just how much current one can take if it is partially discharged and the source can supply it, I can't add a whole lot more.

The original site you found wouldn't have been describing a setup with a battery charger of some sort would it? Unless its a big charger, it would probably limit the current itself to a value low enough that a 15A fuse would work.

Cebby

My pics are on Monty Pics and their server is down until Monday. Anyone know of any other free hosting sites? I'll get them posted if there are any ideas.

The dimension of 5.72mm would be the copper dimension I'm assuming... I've never measured it, but I always thought it was a little bigger.

Dual Battery Relay

Dual Battery Isolator

Firefyter-Emt

IMHO what you need is the isolator shown in the photo above.. I use these for camping when I tow my camper.. These are made to take in the load of the alternator, and give it out to each battery depending on how much it needs.. A solinoid will only give the same amount to each battery... Look towards the end of my electrical page Here you will see more detail of the isolator..

PS, if you move the coil a little it mounts perfect on the apron right behind the coil.. Also, the alternator wire runs almost right past it!!


PS.. read about "sudden violent transfer of energy" and I think this may be why your fuses blow..

I think you may find it good reading!!

Yoda

We wired lots of campers with dual batteries in my younger days (at an auto electric shop)

Use a large solenoid like shown in the above pix (rated for continuous duty, not momentary like a starter solenoid). Wire it to an turn it on with an accessory source like you already did. The wires connecting the two batteries together should be about 10 Ga (I think like 60 amp rated) and use a 40 amp circuit breaker inline for the protection. The circuit breaker opens when there is an overload, and resets by itself. If your aux battery is low, the breaker may reset several times while it is charging up

codlord

I've done some more searching on the net and read various articles including the ones posted here (thanks people). However I came accross this article (see below) which (if correct) could explain why my fuses are blowing. But is this right?

And what if there is a large draw on the main battery from lights/air-con etc - will this in effect draw current from the second battery to feed it or will it just discharge the main battery (and then as a side effect, the main battery will draw from the alternator or the second battery to make up the discharge)???

Article from VW camper forum:
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First, I'm really surprised that among all of the list members, there is not one single Electrical Engineer who stepped forward to explain this relay dilemma. So I'll give it to you in a nut shell: Guess what folks, if your aux.
_batt. is deeply discharged, and you're running 2ga. wire and a 75amp relay, the batt. will most likely draw 75 amps (or whatever your alt. can put out) at initial surge. But if you're running, lets say, a Hella relay, rated at
_15 amps, with the wiring provided, and with the same deeply discharged aux. batt., you are most likely gonna draw about 12 amps at initial surge, and much
_lower after a short period of time. It's all related to the Gauge of the Wire. Really. Don't buy it? Try this: Take that same deeply discharged batt. and run a really, really tiny gauge wire directly from the positive post of
_your cranker batt., to the positive post of the afore mentioned discharged batt. Now start the engine. Did the wire smoke? Have the flames reached the cockpit? Are you searching for that fire extinguisher (that is also probably deeply discharged!)? NOPE. Your dead batt. will draw what the wire can provide. Still don't believe me? Okay, lets see what VW did from the factory: On older models equipped with dual batts, the relay was rated at 15 amps( I'm holding the original one out of my ' 79 Deluxe in my hand as we speak) and the wire from said relay to the aux. batt is 12ga. Believe me folks, nothing can suck down a batt like the old style Dometic 12v/120v fridges found in the pre-Vanagon Type 2's, and even when these batts are totally discharged, starting your eng. does not cause a pyrotechnic display. If it did, VW would have been litigated out of existence by now. If that still doesn't convince you, well just flip through the pages of_ Brightons Studies in Electron Flow ( My God, how I loathed that damn book when I was in college...) to get a clearer picture. Your dead batt will only suck up as much juice as a given wire will flow. The batt. is not an active device, like an electric motor, but a passive one, and will not suck your cranking batt dry at start-up (Unless you are running Massive gauge wire to the aux. batt. That is exactly why VW went with smaller relays and wire gauge.) Think of the set-up as a "trickle charger". Okay, enough already.
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web page:
http://www.globalserve.net/~jrivers/aux-batt.htm