Help on how to rig up 2nd battery

Hi,

Need some advice on the best (and most economical) way to set up an aux battery. I have a Jayco offfroad pop up and need a battery to run lights/water pump. I don't have room in my engine bay for a 2nd battery and want to know the best way of having a 2nd battery in the camper that can be charged by the car. I have a 3 way fridge in the camper.

Many thanks
Johnny
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Reply By: Maîneÿ . . .- Friday, May 14, 2010 at 18:28

Friday, May 14, 2010 at 18:28
Reasonable and 'most economical'

Place an AGM battery in the Jayco, in a black battery box, in the area under a seat or bed etc.
Run thick cable from the cranking battery to the new AGM, via an Anderson plug at the tow bar and with an accessible manual connect/disconnect switch between the Cranking battery and the Anderson plug.

The new agm battery will charge when connected and the engine is running, when you disconnect the new agm battery it will run your accessories.

variations;
Cranking battery instead of 'agm'
Deep Cycle wet cell battery instead of 'agm'
Electronic solenoid, Mechanical solenoid or Electronic battery isolator switch instead of 'manual connect/disconnect' switch

Maîneÿ . . .
AnswerID: 416692

Follow Up By: JohnnyTasman - Friday, May 14, 2010 at 18:48

Friday, May 14, 2010 at 18:48
Thaks Mainey, sounds easy!
Can you recommend a brand of AGM?
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Follow Up By: JohnnyTasman - Friday, May 14, 2010 at 18:52

Friday, May 14, 2010 at 18:52
Mainey I've read your post again and forgot to ask how the fridge and lights are connected to the AGM? I want the fridge to be working while driving which it does now. Would it now be running on the AGM not the cranking battery?
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Follow Up By: Maîneÿ . . .- Friday, May 14, 2010 at 19:11

Friday, May 14, 2010 at 19:11
JT
in that case place the 'agm' battery near the fridge, maybe in a cupboard, wire the fridge direct to the 'agm' battery, BUT….. place a fuse in the positive cable between the 'agm' and the fridge adjacent to the battery, also have an inline ON/OFF switch that's easy to get to so when you turn OFF the gas, so you can turn ON the fridge with-out moving away from where you are at the time, that way you don’t forget to turn on the power.

Obviously you will need a fuse at each end of the positive (+) cable near each battery terminal, must be politically correct here :-)

The lights and pump can also be connected to the same fridge cable with a connector box, after the fuse but before the on/off switch.

I see your on the small island so would not have a clue what is available to you down there, a good warranty would be a clue.

Maîneÿ . . .
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Reply By: Member - John and Val - Friday, May 14, 2010 at 19:58

Friday, May 14, 2010 at 19:58
Johnny,

I don't usually disagree with Mainey, but....

A 3 way fridge draws a lot of current on 12 volts, usually a continuous 10-12 amps. You should, as Mainey says, use heavy wires and anderson plugs to get from the engine bay to the battery in the trailer, but the current drawn by the fridge will drop the voltage reaching the trailer, no matter how huge the cable. Because of this voltage drop, if the battery is being charged from that same line, it will never be charged anything like fully. There is also a risk that you may leave the fridge running on the auxilliary battery when the engine isn't running, which will very quickly run the battery down.

There are basicly 2 ways of handling this problem.

You could run the fridge directly of the supply wire coming from up front, then connect a 12v-12v charger (also called a battery to battery charger) between that line and the battery. These gadgets increase the charging voltage for the battery. Lights and other gear, but not the fridge, would then be connected via fuses to the battery.

My alternative is to run two cables from front to trailer, one for charging the battery, the other for running to the fridge. This way the fridge current doesn't affect the voltage reaching the battery.

In either case you need a relay (controller) in the engine bay to connect the trailer wires only when the engine is running. In the 2 cable system you need 2 relays. (If the 2 cables shared a single relay, the line running to the battery and the one running to the fridge would be joined together in the engine bay. The auxilliary battery would therefore be permanently connected to the aux battery and quickly flatten it.)

You might find our blogElectricity for Camping useful.

Hope that helps

Cheers

John
J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein

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AnswerID: 416702

Follow Up By: Maîneÿ . . .- Friday, May 14, 2010 at 21:57

Friday, May 14, 2010 at 21:57
John, I have preferenced my reply with;
" Reasonable and *MOST ECONOMICAL* " because the original post stated..... “Need some advice on the best (and most economical)” as a requirement.

John you say; “You could run the fridge directly of the supply wire coming from up front, then connect a 12v-12v charger (also called a battery to battery charger) between that line and the battery. These gadgets increase the charging voltage for the battery. Lights and other gear, *but N0T the fridge* would then be connected via fuses to the battery”

John, if the fridge was connected only to be powered off the alternator, when you turn off the vehicle you loose power to the fridge too.

John you say; "the current drawn by the fridge will drop the voltage reaching the trailer, no matter how huge the cable. Because of this voltage drop, if the battery is being charged from that same line, it will never be charged anything like fully"

John, not really correct, the alternator is capable of delivering say 50 Amps, the fridge draws say 18 Amps, the amperage supplied by the alternator/regulator is adjusted to suit various conditions, this does include the extra draw of the fridge.
The voltage drop will be associated with the cable being too thin or faulty/loose inline connections.
If, for example there is a (very exaggerated) 1.0 volt loss in the charging cable when measured at the aux battery, then the battery is still being charged @ 13.2 volts at least, even after also running the fridge.

John you say; “There is also a risk that you may leave the fridge running on the auxiliary battery when the engine isn't running, which will very quickly run the battery down”
John, not if the 12v ON/OFF switch is placed where I've specified, because you can turn gas ON and 12v power OFF from the same position, you don’t have to move away to do either of them, there is no remembering problem, often associated with aging or just simply forgetting either.

John you say: “My alternative is to run two cables from front to trailer, one for charging the battery, the other for running to the fridge. This way the fridge current doesn't affect the voltage reaching the battery”
I don’t agree with this, reason being BOTH cables are powered from the same power source, the alternator/regulator, which adjusts the current to suit the situation, as mentioned above.

Maîneÿ . . .
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Follow Up By: Mike DiD - Sunday, May 16, 2010 at 15:04

Sunday, May 16, 2010 at 15:04
John has stated it perfectly.
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Follow Up By: Maîneÿ . . .- Sunday, May 16, 2010 at 15:25

Sunday, May 16, 2010 at 15:25
Mike so you obviously agree "to run two cables from front to trailer, one for charging the battery, the other for running to the fridge.
This way the fridge current doesn't affect the voltage reaching the battery"

When both cables are coming from the exact same 12v battery connection in the vehicle, will you explain "how fridge current doesn't affect the voltage reaching the battery" please :)

Maîneÿ . . .
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Follow Up By: Member - John and Val - Sunday, May 16, 2010 at 20:19

Sunday, May 16, 2010 at 20:19
Just noticed a typing error in my last paragraph. Doesn't change any of the facts, but may have confused :

I meant to say " In either case you need a relay (controller) in the engine bay to connect the trailer wires only when the engine is running. In the 2 cable system you need 2 relays. (If the 2 cables shared a single relay, the line running to the battery and the one running to the fridge would be joined together in the engine bay. The FRIDGE would therefore be permanently connected to the aux battery and quickly flatten it.)"

I accidentally said " The auxilliary battery would therefore be permanently connected to the aux battery and quickly flatten it." which doesn't make a lot of sense!

Cheers

John
J and V
"Not everything that can be counted counts, and not everything that counts can be counted."
- Albert Einstein

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Follow Up By: Maîneÿ . . .- Sunday, May 16, 2010 at 20:25

Sunday, May 16, 2010 at 20:25
John,
it obviously made sence to Mike

Maîneÿ . . .
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Reply By: familyguy - Friday, May 14, 2010 at 22:05

Friday, May 14, 2010 at 22:05
HI johnny

My 2004 Jayco already has the 12v system is place incuding what most describe as a trickle charger. I understood most come from the factory like this but the battery was an option. The lights are 12V

Does yours have any of this set up all ready and just needs the battery? Or is yours an older beast.

I doubled my camper battery and set up system to charge from Vehicle as descibed by Mainey with assistance of some of Derek Besters posts (and products) Try post 64066 for a simpe wiring diagram

Regards
AnswerID: 416721

Reply By: JohnnyTasman - Friday, May 14, 2010 at 22:48

Friday, May 14, 2010 at 22:48
Many thanks everyone. Really helpful. Now all I need is a cheap auto electrician that can read!
AnswerID: 416727

Follow Up By: ChipPunk - Saturday, May 15, 2010 at 19:47

Saturday, May 15, 2010 at 19:47
And decide!

As above, the 3-ways are very heavy on current. I assume you would only be running it on 12V whilst mobile and hence whilst the alternator is able to charge....

When stationary, it's gas all the way, unless mains is available.

If relying on solar for the charging, it is cheaper buying an all electric fridge-freezer (though with panels hitting $5 per Watt, it's getting better).


For a simple battery isolator, tell the auto-elec to use the charge light circuit to power an isolation relay. That will connect the batteries when the alternator is charging, otherwise they will be independent.
And yes - fuses at each end near each battery. (That goes much further than mere politics.)


As to whether to run 2 lines or one thicker line, or a dc-dc converter... ah, well...
My preference is a single heavy cable - especially in this case where you are only powering a heating element and not a compressor etc.
Something like 14G should drop about 1V per 10 meters for a 10-12Amp load which is probably too large a drop to charge from the alternator....
Whereas 4G will drop 10-times less - ie, 0.1V per 10m.

dc-dc converters will overcome the voltage drop, but at the expense of a higher input current in addition to conversion losses (say 10-20%?).
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