different question on duel battaries

Correct. The batteries would in-fact be connected in parallel.

Regards

Dennis,
The problem with adding a diode is the approx 1/2 volt loss across the diode. This means your main battery will not charge as quickly. If your secondary battery is half flat most of the alternator charge will go to this battery first.

Marty.

I am not an auto elec but the theory should be similar to what I learned as an electrician ....................

If the purpose of all this is to use the 2nd battery as the power for whatever accessories you fit (fridges, inverters etc), then it will be the one that gets drained when the vehicle is stopped and therefore it needs the most charging anyway.

Since the diode would eliminate the starting battery from providing any power to those same accesories, it should have the same charge in it when you hit the key as when you stopped (give or take whatever minor losses from the interior light, listening to the radio etc).

Therefore it shouldn't need or require urgent (or priority) charging.

If we are talking about achieving the aim (and saving a few bucks compared to the solenoid/isolating systems on the maket) then I can't see that this would be a major problem.

Any auto elecs in the forum care to pick up on this?

P.S.

A simple diode setup has no moving parts either, so in theory it should last longer and be more reliable.

I suppose to achieve the ultimate in reliability and minimise voltage drop you could put 2 diodes in parallel.

You need a diode to each battery from the alternator. As stated, there will be a voltage drop across the diode. If it's a silicon diode, the drop is actually close to 1 volt. If Shottky diode, about 0.4v. Dropping your max charge voltage from (say) 14.2 down to 13.2 may not bring your battery up to full charge.
However, all is not lost; some vehicles' alternators (but not all) have an external voltage sense for the internal regulator; ie, the battery voltage is sensed close to the battery and fed back to the regulator. Nissan do this in the GQ Patrol. This means that any voltage drop across the diode will be corrected back to normal; ie, the alternator will push out about 15.2v to give you 14.2v at the battery.
If you use only one diode to the main battery, then you will be charging the 2nd battery at a higher voltage. Also, the diode and wiring would need to be capable of carrying the start current of your starter motor, as a flat main battery will draw max current thru the diode from your 2nd battery when starting.
Another problem with diodes as isolators is the commutation noise generated by the diodes. The pulses of recified AC from the alternator cause the diodes to switch on and off, creating RF noise, affecting AM and HF radio. This can be fixed by fitting capacitors across the diodes. Commercial diode isolators probably already have these fitted.

Fully electronic isolators with FET switches exhibit quite low voltage drop, solenoid isolators exhibit virtually zero voltage drop. These are an option to consider.
Gerry

Gerry,

Quick reply, good info.

Can I press you further and ask what is his (our) cheapest/easiest option?

Easy is not necessarily cheap I understand.

But this is something anyone with a set of pliers should be able to wire up IF they know the right stuff to use.

Dennis,
I've used diodes in the past (on a GQ), but my preference for simplicity is a solenoid, which I use now on my GU. Cheap (<$50), DIY; the control wire operating the solenoid is wired back to your ignition sw.
My solenoid is controlled from the aux output of the alternator (the feed to the Ign warning light), which provides volts when charging, pulling in the solenoid, but this is a bit messier for the layman. But it means that when the alternator is not charging (ie, engine not running) the solenoid is not operated.
The solenoid can also take the full current of a winch if you want to run it with both batteries combined.
If you've got plenty of money, then the electonic isolators are good. Some of these also cut in the second battery only when the volts of the main battery are high enough, not a bad idea.
Can't remember, but there is a recommended one for a bit over the $100, which works well. Check the archives.
Gerry

joc45
thanks for the really useful info. I have used a solenoid both manually and with a Pirhana conrol unit. I was concerned about the possibility of voltage spikes which could damage ECUs etc. Could you comment on that?Bob

Hi Bob,
I've seen plenty written on this in this forum. I might get howled down, but here goes anyway.
Spikes in vehicle electrics can usually occur in a couple of instances;
1.- Switching off an inductive load; ie, anything with an electromagnet in it. When the current is switched off, the magnetic field collapses, and generates a high back emf (voltage).
2.- Changing the load on an alternator by a large magnitude can theoretically catch out the regulator, so it generates an overvoltage for a brief period before the regulator catches up.

-In the first instance, a significant inductive culprit would be a starter motor or electric winch (switched by a solenoid), yet no precautions are taken by the auto mfr or winch mfrs. I haven't seen any postings on taking precautions for the above.
-In the second instance, switching off a barrage of spotties or a hefty inverter could theoretically result in a spike, but again, I haven't seen anyone recommending precautions for this.

With the connection of a solenoid between the two batteries, the switching on or off can change the current by a large magnitude, but probably no more than switching off that array of spotties - the most I've seen my 2nd battery charge at is 25A, not a problem as I see it. The load created by the 2nd battery is ostensibly resistive (ie, not inductive), so I see no problem.
I should add that a battery in reasonable condition has an extremely low internal resistance, and will resist any large change of voltage from its nominal charged voltage.

However, I, like many, have that lingering doubt that something unpredictable may happen, and I have installed a hefty 16v zener diode across the main battery side of the solenoid (+ve to earth). This will absorb any voltage spike higher than about 16 or so volts. If anything, it is probably protecting the system from spike sources other than my solenoid, it sits there benignly, and will probably never see a spike in its life.
Hope this helps
Gerry

One thing I was worried about is drawing starting from one battery to the other via whatever parallel circuit I had made in the vehicle with the dual battery setup. In the end I used a LARGE lead I made up myself to join both batteries via a Redarc to carry the starting current if I wanted to use the second battery to start if the primary one carked. I bought and still have a 100a relay that I didnt end up using as the Redarc took its place.

I was trying to source a feed that was switched off during starting (like the wiper motor feed) to operate the relay.Laterally Literal
Seriously Cerebral

Just to be a bit pedantic, all the references about a 'solenoid' in this thread really mean a "relay".
A relay is an electrically operated switch.
A solenoid is an electrically operated mechanical device such as a fuel valve, etc.
You use a relay to switch between batteries, turn on your spotties, etc. etc.
The starter motor uses a relay to connect it to the battery, some starter motors also may have a solenoid/ relay combination to 'throw' the pinion gear into meshing with the ring gear and connect the battery to the starter at the same time.
Klaus

David,
could you please elaborate on your set-up; how do you monitor the batteries to know when to switch over etc.

It's all too easy, really. No "monitoring" as such required.
My spare battery is in a battery box in the back. Less vibration and a lot less heat than under the bonnet. (Heat and vibration are battery killers)
It's connected via a heavy duty relay AND a 45 amp circuit breaker (cost the princely sum of about $30) which I then turn on after starting and off after stopping.Your alternator regulator will limit voltage to about 14v and will do a reasonable job of charging both batteries- not as good as a proper three stage charger, but reasonably good never the less.
If you forget to turn it off before re-starting it doesn't matter as it just means your deep cycle battery will briefly "help" your starting battery turn over the donk- but is limited by both the length of cable and the circuit breaker.
The only real "danger" is that you forget to isolate the deep cycle battery and therefore flatten both batteries on an extended stopover- I've never done that.
I would not even consider a setup using a diode, as you lose enough voltage thru the diode to ensure you will ever charge your second battery very well at all- even on an extended drive.
This above setup has worked very well for me for many years- it's simple, reliable, cheap and effective.

If you do that to the starting battery the isolator switch must be extra heavy duty if the engine is a diesel. Also, it requires extra heavy wiring to the isolator switch to carry the starting current. And its another potentional not starting problem. I would prefer an isolating switch just for the aux. battery, if its only meant to run a fridge one gets away with much less beefy gear. (I use a 60A relay for that purpose).

Another point, it is not a good idea to switch the main battery earth off while the engine is running, could fry the alternator diodes. Might also do funny things if its a computer car.
Klaus

Hey V8troopie, i am a Caterpillar diesel mechanic, so getting heavy duty battery isolator switches is no problem. They work fine on my truck when im away and i know in my head that when the main battery is switched off, my truck will start! If not ill get out my outback battery charger!!!!

I basically agree with you 100% Eric.
However, I personally prefer a totally manual setup- manually switched relay which just gives me more options eg: letting my deep cycle battery "help" my starting battery if required.
It all depends on personal preference, but do keep it simple, do protect it using an appropriate circuit breaker, and don't disconnect your starting battery from the car- you'll probably fry your alternator or other electronics if you do.
I use 6mm wire, 50 amp relay and 45 amp circuit breaker (or is it 35 amp breaker -can't remember but either would be fine.) Works a treat.
Cheers