Dual Battery Question
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Freshman User
Joined: Jan 2011
Posts: 32
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From: Mountainair
Dual Battery Question
Hey Guys;
I searched for a prior thread, but had no luck.
My first Ford, first diesel.... So I'm a bit clueless about the dual batteries and how they're set up.
Are they used equally at all times or are they wired typical of dual battery set ups with one isolated for starting priority, etc. I want to be sure I am using both batteries over their lifetime to avoid having to maintain/replace them at separate intervals.
Thanks for the great information that you all have provided in the 3 months I've had the truck...... Quite an education! Quite a truck!!
Regards,
Tom
I searched for a prior thread, but had no luck.
My first Ford, first diesel.... So I'm a bit clueless about the dual batteries and how they're set up.
Are they used equally at all times or are they wired typical of dual battery set ups with one isolated for starting priority, etc. I want to be sure I am using both batteries over their lifetime to avoid having to maintain/replace them at separate intervals.
Thanks for the great information that you all have provided in the 3 months I've had the truck...... Quite an education! Quite a truck!!
Regards,
Tom
Lead Driver
Joined: Jan 2010
Posts: 6,600
Likes: 84
The batteries are wired in parallel.
Both are used for starting.
The passenger side battery is the primary.
You want to use the primary (passenger) for jump starting.
The driver side battery is the secondary.
I've been told by other diesel owners when you need to replace one, replace them both but I'll leave this up to the experts here who have more experience.
If one load tests properly it does seem like a waste to replace it but if it's past it's expected useful life, the one that isn't replaced will fail soon anyways and the entire charging system won't be equal (as it would with two new batteries).
Both are used for starting.
The passenger side battery is the primary.
You want to use the primary (passenger) for jump starting.
The driver side battery is the secondary.
I've been told by other diesel owners when you need to replace one, replace them both but I'll leave this up to the experts here who have more experience.
If one load tests properly it does seem like a waste to replace it but if it's past it's expected useful life, the one that isn't replaced will fail soon anyways and the entire charging system won't be equal (as it would with two new batteries).
Thread Starter
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Freshman User
Joined: Jan 2011
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From: Mountainair
Thank you for your reply. It certainly seems appropriate to change out both batteries at the same time if they're wired in parallel, and both used for starting and all loads in general.
I'd also assume that if you jump into the passenger side (primary) battery, you'd pull from the driver side battery if you were looking to wire a direct to battery load, that way once again both batteries are being pulled from at the same relative time.
This group is great, the information is always timely and spot on! Thanks again.
Regards,
Tom
Corona, California
I'd also assume that if you jump into the passenger side (primary) battery, you'd pull from the driver side battery if you were looking to wire a direct to battery load, that way once again both batteries are being pulled from at the same relative time.
This group is great, the information is always timely and spot on! Thanks again.
Regards,
Tom
Corona, California
Post Fiend
Joined: Jan 2009
Posts: 6,521
Likes: 11
From: Greenwood, SC
Cool info. I did connect my audio amp power to the driver side batt, seemed to be the easiest to route cable to. And actually, I like the idea of using power from the "secondary" or "slave" batt for this, if I understand the sequence of power described.
If I ever do need to replace 1 battery, I will do both. If 1 of them is still good, it can go in the RV, dump trailer, or wherever else it can be used while it's still working. If I had a fishing boat, another trolling motor battery is always good.
If I ever do need to replace 1 battery, I will do both. If 1 of them is still good, it can go in the RV, dump trailer, or wherever else it can be used while it's still working. If I had a fishing boat, another trolling motor battery is always good.
Cargo Master
Joined: Aug 2010
Posts: 3,229
Likes: 64
Batteries should be replaced in pairs when used like this.
A 2-3 year old battery that may test good will not be in the same condition as a new battery (technology changes excluded).
Change out one and you end up with the older battery drawing from the new battery.
You also will have two different charge rates with no way to charge them separately. The alternator will charge at the rate needed. This may properly charge the older battery and overcharge the new battery.
You are best to change battery pairs as a set, just like you would your front brake pads.
A 2-3 year old battery that may test good will not be in the same condition as a new battery (technology changes excluded).
Change out one and you end up with the older battery drawing from the new battery.
You also will have two different charge rates with no way to charge them separately. The alternator will charge at the rate needed. This may properly charge the older battery and overcharge the new battery.
You are best to change battery pairs as a set, just like you would your front brake pads.
Posting Guru
Joined: Apr 2004
Posts: 1,399
Likes: 1
From: Corinth, TX
If they are wired in parallel, how can one be primary? All loads should be placed on the pair equally, as long as there are no significant wiring differences... What makes one of them primary?
Tuned
Joined: Jun 2004
Posts: 338
Likes: 2
From: Northern MA
I've had good luck with using either for jump starting, my jumper cable plug is on the driver side batt.
Porthole is spot on, unsure of your electrical familiarity but most batteries have an impedance, a resistance to current flow. A good charged battery=high impedance won't keep charging, discharged battery=low impedance and low voltage, battery gets charged by alternator.
As batteries age, the baseline impedance keeps dropping over time. If you pair a brand new battery with say a 3 year old battery, you may find the new battery never getting fully charged. Also, an older battery with a lower overall voltage, even though it might only be .1 volt lower, will keep discharging a newer battery.
So best practice is to once a year disconnect your batteries one at a time and test them, write the CCA on top of the batteries in crayon or keep the info in the glove box. When they start to diverge in capacity replace the set with a new pair as close to the same production date as possible.
With good maintenance, cleaning terminal posts, grease, testing and water service where applicable, a set should last 5 years.
Porthole is spot on, unsure of your electrical familiarity but most batteries have an impedance, a resistance to current flow. A good charged battery=high impedance won't keep charging, discharged battery=low impedance and low voltage, battery gets charged by alternator.
As batteries age, the baseline impedance keeps dropping over time. If you pair a brand new battery with say a 3 year old battery, you may find the new battery never getting fully charged. Also, an older battery with a lower overall voltage, even though it might only be .1 volt lower, will keep discharging a newer battery.
So best practice is to once a year disconnect your batteries one at a time and test them, write the CCA on top of the batteries in crayon or keep the info in the glove box. When they start to diverge in capacity replace the set with a new pair as close to the same production date as possible.
With good maintenance, cleaning terminal posts, grease, testing and water service where applicable, a set should last 5 years.
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Lead Driver
Joined: Jan 2010
Posts: 6,600
Likes: 84
In our Ford 2011 manual, Ford documentation writers call the passenger battery the primary.
That's where I got that information from.
I'll have to look up the page number.
I've always understood these batteries are wired in parallel instead of series.
Where you have me is our manual's don't say that our batteries are in parallel so that is out of my head.
Parallel allows for a 12v system but more amps with the two batteries.
In a series, it doubles the voltage.
Using my understanding (whether correct or incorrect), the driver side battery is connected to the passenger battery (both positive and negative) then connected to whatever is involved in turning over our engines and running the electronics.
I'm not a diesel tech or an electrical engineer so am I way off base here?
If I'm wrong I don't need to understand bad information.
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Posting Guru
Joined: Apr 2004
Posts: 1,399
Likes: 1
From: Corinth, TX
let me be clear - I wasn't accusing you of misinformation or trying to start an argument. I just want to understand what you meant 
I do recognize the difference between series and parallel. Obviously, they can't be in series since we have a 12V system. They *could* be in parallel as long as there aren't any actual differences in each battery's responsibility. I think it's more likely that there is a controller between the two that distributes power to the various comsumers as necessary. Perhaps with biases like using one battery for starting and one for accessory systems but using the opposite battery as a failover. I don't have any facts to support that, it just seems likely to me.
I do recognize the difference between series and parallel. Obviously, they can't be in series since we have a 12V system. They *could* be in parallel as long as there aren't any actual differences in each battery's responsibility. I think it's more likely that there is a controller between the two that distributes power to the various comsumers as necessary. Perhaps with biases like using one battery for starting and one for accessory systems but using the opposite battery as a failover. I don't have any facts to support that, it just seems likely to me.
Tuned
Joined: Jun 2004
Posts: 338
Likes: 2
From: Northern MA
Naah, the only bias/distribution limitation is the 0.003 ohms in the crossover cable. Both batteries work in tandem like a good team of horses for their entire lives, strapped together till the end.
Things like your camper would have a set of 'house' batteries behind an isolator/switcher that separates responsibilities from the 'engine' batteries but not under hood in the trucks.
Things like your camper would have a set of 'house' batteries behind an isolator/switcher that separates responsibilities from the 'engine' batteries but not under hood in the trucks.
Junior User
Joined: Jan 2011
Posts: 99
Likes: 0
From: Hamilton, Ontario Canada
Ok, I'm confused. I didn't see the option to order dual batteries.
I DID order dual alternators but would have liked to have gotten two batteries also.
Did my salesman do me a dis-service by not telling me all the options I had to choose from??
I DID order dual alternators but would have liked to have gotten two batteries also.
Did my salesman do me a dis-service by not telling me all the options I had to choose from??
Tuned
Joined: Jun 2004
Posts: 338
Likes: 2
From: Northern MA
You could retrofit with the diesel parts, battery trays and such.
Lead Driver
Joined: Jan 2010
Posts: 6,600
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According to your signature, you ordered a 6.7L diesel so you will have two batteries.
Junior User
Joined: Jan 2011
Posts: 99
Likes: 0
From: Hamilton, Ontario Canada
I kinda wish the salesman went through all my choices a little better. I was pretty well informed going into the place but now am wondering if I missed anything. I may already be getting things that I want in a package or maybe I was suppose to add those. For example, I'm not sure if I automatically get *Optional* "Hill Decent" and "Locking differential" or if I was suppose to add it.....
I wont know until I see the specs on it. I've asked for the DORA but he's yet to reply.