Dual batteries / Deep Cycle Verus 2nd Starter / Controller Types

[millsy]

I like to DIY wherever possible. So could anyone suggest whether I have overlooked anything important in making up my own wiring system for the second battery.

I have just used a spot light relay switch, and another manually operated switch in series with it, to connect my second battery to the starter battery.

The relay is activated by ignition voltage, and the other switch is my manual over-ride. I only turn that one on when I want the alternator to charge the second battery.

So when I am camping the two batteries are isolated, and when I am driving I can control where the charging current is going - either to the starter battery, or to both.

And I think of the second battery as insurance for the starter battery. I run my CPAP ( anti - snoring ) machine off the second battery, so that the starter battery is not depleted overnight. And as I said, if I am out camping and the main battery dies, I just have to use my jumper leads to get going again. So the second battery is just a normal car battery.

 

[bmurray2250]

Hi Millsy

sounds good but I like more info on - I have just used a spot light relay switch, and another manually operated switch in series with it, to connect my second battery to the starter battery.

 

[millsy]

Hi Millsy

sounds good but I like more info on - I have just used a spot light relay switch, and another manually operated switch in series with it, to connect my second battery to the starter battery.

I have drawn a circuit diagram to make it clear what I have done.

I have also put an ammeter in the circuit to see that its actually charging, but unfortunately I could not find one with a very suitable scale. It is a 30A meter. Far too big. I think a 10A would have been better, since the needle does not move very far on the scale when it is charging.

 

[bmurray2250]

That makes sense, the relay between the batteries is rated 100A or greater is the only question. When starting the motor the starter motor can peck around 200A and will drop to normal amps after it spun up

 

[frosty]

I like to DIY wherever possible. So could anyone suggest whether I have overlooked anything important in making up my own wiring system for the second battery.

I have just used a spot light relay switch, and another manually operated switch in series with it, to connect my second battery to the starter battery.

The relay is activated by ignition voltage, and the other switch is my manual over-ride. I only turn that one on when I want the alternator to charge the second battery.

So when I am camping the two batteries are isolated, and when I am driving I can control where the charging current is going - either to the starter battery, or to both.

And I think of the second battery as insurance for the starter battery. I run my CPAP ( anti - snoring ) machine off the second battery, so that the starter battery is not depleted overnight. And as I said, if I am out camping and the main battery dies, I just have to use my jumper leads to get going again. So the second battery is just a normal car battery.

How many amps can a spotlight relay handle for a prolonged period?? Is it designed for this use? I use a starter solenoid that is normally open. It has the capability to be switched off manually as well.The cable could be a little heavier from battery to battery including earths. Whilst your wiring, run all of your accesory cables over to your second battery if you can.

 

[rodw]

Geez guys, why stuff around with relays and stuff? Sounds like a disaster (even a fire) waiting to happen. You can buy a 100 amp rated battery isolator from Super Cheap with all the wiring you need for $95. They work fine, I used one in my last car. You can spend a bit more around $150 and get a good one from Battery World or Redarc. In either case, they sense the voltage in the system and when they see a certain voltage, say 13.2 volts which indicates your starting battery is fully charged and then start charging to your second battery. When you stop the engine, the voltage drops and the second one is immediatley isolated. Some of these even have provision for a Link switch so you can press a button on the dash to connect both batteies together for winching or jump starting which is how my ARB isolator is set up.

I am not convinced a simple voltage based isolator is right for the modern vehicles which heaviily regulate the voltage output from the alternator down to a level that can't fully charge the battery. In this case, you need a DC-DC charging system like the ranox and I am seroiusly considering fitting it to my car one day. Olcolone could elaborate on the reasons why, but essentially it will add up to 35% more capacity to the battery in say a diesel Hilux or Prado.

 

[bmurray2250]

I have to agree with RodW it is simple and safer

 

[millsy]

Thanks for that fellas. Looks like I will need to upgrade my relay for safety reasons. Although I usually turn the isolator switch into the off position in the morning, before start up, I am sure I will forget one day!

Other than that I was quite happy to save a few dollars. I don't mind manually flicking a switch every now and then. But thanks for the safety tip. I would doubt that a spot light relay would be rated at 200A, and I suppose the next question would be how much will I need to pay for a 200A relay? Could be $90 anyway!

 

[rodw]

How many amps can a spotlight relay handle for a prolonged period?? Is it designed for this use? I use a starter solenoid that is normally open. It has the capability to be switched off manually as well.The cable could be a little heavier from battery to battery including earths. Whilst your wiring, run all of your accesory cables over to your second battery if you can.

Exactly, a normal relay is usually rated at 30 amps, but I think they come in 60 amp ones which is still too low. Oh and by the way, here is my dual battery bible. Follow the links on this page:
Technical Information

Particularly this one:
Dual Battery Systems & Alternator charging

This is how I set my last car up but this one I left to ARB and their cable selection leaves a bit to be desired.... but plenty to run my fridge.

 

[rodw]

I suppose the next question would be how much will I need to pay for a 200A relay? Could be $90 anyway!

Millsy, sorry, we cross posted. Work your way through the links I posted and you won't go wrong! 100 amps will be fine, that is a continuous rating, peak will be a lot higher so the odd 200 amp spike won't heurt them.

 

[millsy]

Thanks Frosty, for the hint about running the accessories to the second battery.

I do not have many accessories, such as fridge, compressors, driving lights . . .

But my first thought was to put the UHF on the second battery, until I remembered that I don't always have the second battery in the car. I only take a second battery if I am heading out on isolated tracks by myself. It is set up in the passenger footwell up against the firewall, in a plastic sealed toolbox, with ventilation tubes, and trapped under the passenger seat that I push full forward.

And in my case the second battery is a normal heavy duty ( 11 or 13 plate ) car battery, rather than a big four wheel drive battery, like the main battery. So it really only gets used for my CPAP which I think draws about 2A for the 8 - 9 hours each night.

I like to drive for half an hour or so before flicking the switch and letting the alternator charge up the second battery. This also protects the original old alternator from having to push out too much current trying to charge up both batteries at the same time.

And, talking about old alternators, I see another good reason for having the two batteries. If the alternator does die, at least I have two lots of stored energy to run the ignition to get me back to the closest town.

 

[millsy]

I have not had the second battery in the car for a while now, but I think I remember that whenever I looked at the ammeter when I had the two batteries linked together ( with the manual switch turned on ) and started the engine, I thought the ammeter only showed a reverse current of about 3 to 5 Amps. Not a very large deflection on the 30A scale.

I have added in the ammeter on the diagram on the link below.

So how this relates to the very large starter motor draw that you have been suggesting is the next question. Maybe the 'thin' 8mm wires provide enough resistance so that most of this large current just gets sucked from the starter battery?

Will check it out when I fit the second battery next time.

 

[skipper60]

thanks frosty i think i might go the deep cycle for second battery. what are your thoughts on caple size from second battery to fridge in back of wagon. Iv just run 6mm twin to my fridge and didnt have any voltage drop but when i put a 30amp blade fuse in nere the battery im getting voltage drop. would you run a bigger fuse or none at all? any thoughts would be great.

I reckon Yobbo's had to many Zaps! We've had this arguement a few times now. His theory is if you get a flat battery, the 2nd bat will start it. My theory is if ya get a flat battery, the deep cycle will also start it. And it will run all your accesories for much longer and the battery life is longer, providing you don't run it to flat too often.

My main battery died yonks ago. I started off the deep cycle for several months untill I could afford a new starter bat. The deep cycle battery is now 7 or so years old.

 

[rodw]

I have not had the second battery in the car for a while now, but I think I remember that whenever I looked at the ammeter when I had the two batteries linked together ( with the manual switch turned on ) and started the engine, I thought the ammeter only showed a reverse current of about 3 to 5 Amps. Not a very large deflection on the 30A scale.

I have added in the ammeter on the diagram on the link below.

So how this relates to the very large starter motor draw that you have been suggesting is the next question. Maybe the 'thin' 8mm wires provide enough resistance so that most of this large current just gets sucked from the starter battery?

Will check it out when I fit the second battery next time.

Millsy, I am no expert on this, but I suspect the ammeter may not be doing the right thing becasue there are two positive terminals in your circuit. You'd probably need to put a diode between the ammeter and the Aux battery to prevent current flow from it from distorting the situation. However, this would cause a voltage loss (0.7 volts) and that is not desirable (not to mention finding one rated high enough). A flat or near flat battery can draw an enormous amount of power, basically as much as it can get.

Remember that resistance generates heat. Too much heat generates fires.

If you have a 100 amp rated alternator and the car only needs 20-30 amps to run its internals, there is potentially 70 odd amps available to melt wires and start fires if a fault occurs.

In my experience, it only takes a few seconds for the starter battery to be topped up after you start your car before an auto voltage sensing regulator cuts in so you are wasting a lot of charging potential.

 

[millsy]

Thanks again for all this new info. You have obviously a lot a of experience in this area!

Am pushed for time at the moment. Have to rush back to work. And am busy finishing preps for my next 'little' trip to the deserts on either Sunday or Monday morning. But am looking forward to thinking through all of those issues you have raised. And I need to do some more research.

Will get back to you ASAP, if I have any further questions or useful input. Might be in two weeks time, after this trip. Not taking the second battery this time!

 

[rodw]

thanks frosty i think i might go the deep cycle for second battery. what are your thoughts on caple size from second battery to fridge in back of wagon. Iv just run 6mm twin to my fridge and didnt have any voltage drop but when i put a 30amp blade fuse in nere the battery im getting voltage drop. would you run a bigger fuse or none at all? any thoughts would be great.

Skipper, I would say, you probably have a loose connection if you get a voltage drop by simply adding a fuse. It adds negligible resistance to the circuit.

6mm cable is fine to power your fridge. Most compressor fridges draw only about 2-4 amps. However, you need to go a lot bigger to run a charging circuit back to the extra battery regardles of which type it is. The link I posted (this one):
Dual Battery Systems & Alternator charging
says to use minimum 6B&S cable. this is not 6mm auto cable, it is 13.5mm2. Six mm auto cable is only 4.59mm2. There is a huge difference.

Most Auto leccies I have spoken too use 2B&S (33mm2) cable which is similar to the size cable on your existing battery cables for charging situations. A few extra $ now is well worth the trouble.

 

[bmurray2250]

The amp gauge is between the battery and rest of the circuit. I normal use the amp gauge should show 0 amps going into a full charged battery. If is showing 3-5 amp charge then there is a device on the battery terminal drawing current. To recharge a battery after starting takes roughly 5 minutes in normal driving. This device should on a alternator side of the gauge.

The starter motor is directly attached to the main battery with a large earth and power cables that can handle 200 amps maybe more. If you are going to link both batteries together for cranking then you need a relay that can handle the max amp pull. I generally use a jumper cable to do that when and if required.

Both batteries are earthed with large cables to the motor and frame

I added my diagram for my isolator unit as a example

 

[skipper60]

thanks rodw i have run the larger 6b&s cable to the battery from charging system this is all working well, and have sorted out voltage drop. thanks mate the post you put up is a good one very similar to what iv done. cheers
any idea were to get good muilty plug cig lighter outlets(good quality ones)

Skipper,6mm cable is fine to power your fridge. Most compressor fridges draw only about 2-4 amps. However, you need to go a lot bigger to run a charging circuit back to the extra battery regardles of which type it is. The link I posted (this one):
Dual Battery Systems & Alternator charging
says to use minimum 6B&S cable. this is not 6mm auto cable, it is 13.5mm2. Six mm auto cable is only 4.59mm2. There is a huge difference.

Most Auto leccies I have spoken too use 2B&S (33mm2) cable which is similar to the size cable on your existing battery cables for charging situations. A few extra $ now is well worth the trouble.