Dual Battery & Battery tender(s)

WY-Dave · Feb 12, 2021

We are about to get some below zero temps and the truck is going to sit through it. I want to put a/some battery tenders on it to put some charge on the batteries. I would assume that there is an isolation circuit on the battery charge system. I would rather use a tender on each battery, but would that mess something up?

fireflymedic · Feb 12, 2021

Does the battery cable on positive post post go to both batteries without a disconnect? If it does you could probably get away with a dual battery charger, I have one on my boat. But if you only have one charger you can overcharge the battery it's connected to trying to top off the other battery which would not be as big of a problem with a dual charger. Of course batteries need to be same kind, size and condition for this to work.

BWL · Feb 12, 2021

The batteries are connected in parallel so no need to run 2 separate chargers and the 2 chargers may fault working against each other. How far below zero and how long will the truck be sitting?

BWL · Feb 12, 2021

I never have battery troubles with my truck sitting for weeks in warmer than -25C/-10F range so if your batteries are in good shape it may be overkill anyways. I run battery warmers vs chargers when it's colder than that. Eliminates the overcharge potential. Could just take them out and put them inside if it's going to be sitting more than a month.

WY-Dave · Feb 12, 2021

BWL said:
The batteries are connected in parallel so no need to run 2 separate chargers and the 2 chargers may fault working against each other. How far below zero and how long will the truck be sitting?

That's true, but is there an isolator in the circuit. That is what would decide if 1 or 2 tenders are needed.

BWL · Feb 12, 2021

WY-Dave said:
That's true, but is there an isolator in the circuit. That is what would decide if 1 or 2 tenders are needed.

Should not be an isolator, but just follow the cables between the batteries and you'll know for sure. Should just be a positive connecting the 2 positives. Whether or not they have separate grounds paths I don't know, but that wouldn't matter. Also if you hook up 1 side with the truck off and they both come up to the charger voltage on their respective terminals you'd know as well.

BWL · Feb 12, 2021

It's possible they could have added fuses on either end between the 2 in case of a short in the cable between them, but on the previous generation there wasn't.

Travelin Ram · Feb 12, 2021

WY-Dave said:
That's true, but is there an isolator in the circuit. That is what would decide if 1 or 2 tenders are needed.

There’s no isolator, for electrical purposes it’s just one big battery. The connector cables are designed for cranking current of the diesel, they’ll stay equally charged just fine.

WY-Dave · Feb 12, 2021

fireflymedic said:
Does the battery cable on positive post post go to both batteries without a disconnect? If it does you could probably get away with a dual battery charger, I have one on my boat. But if you only have one charger you can overcharge the battery it's connected to trying to top off the other battery which would not be as big of a problem with a dual charger. Of course batteries need to be same kind, size and condition for this to work.

Ok guys thanks. I meant to go look at the wiring before the cold, but right now it's -8 with a 25mph wind.

fireflymedic · Feb 12, 2021

I would use a dual charger like boats use, if charger was going to be on for an extended amount of time. I can tell you from experience if you only use one battery charger you better keep an eye on battery acid level because it can overcharge the battery your charger is on trying to get current over to the other battery.

stevenP · Feb 12, 2021

I have my 3500 stored in the winter. I have the tender hard wired to one battery, and it does charge the second one just fine...I checked with my DVM.

BossHogg · Feb 14, 2021

stevenP said:
I have my 3500 stored in the winter. I have the tender hard wired to one battery, and it does charge the second one just fine...I checked with my DVM.

^^^Exactly^^^

My 3500 Cummins sits in the garage most of the winter with a battery tender on it. I have the passenger side battery connected to the battery tender and that keeps both batteries at full potential. Keep in mind extreme cold slows the battery's chemistry and heat increases the parasitic losses.

Just an FYI, if you were to jump the batteries or charge them at a higher current, then you would connect to the positive post on one battery and the negative post on the other battery. This minimizes battery cable voltage drop.

The Battery Tender website has a very nice FAQ section that answers these types of questions.
https://www.batterytender.com/faq

DOT · Feb 14, 2021

BossHogg said:
^^^Exactly^^^

My 3500 Cummins sits in the garage most of the winter with a battery tender on it. I have the passenger side battery connected to the battery tender and that keeps both batteries at full potential. Keep in mind extreme cold slows the battery's chemistry and heat increases the parasitic losses.

Just an FYI, if you were to jump the batteries or charge them at a higher current, then you would connect to the positive post on one battery and the negative post on the other battery. This minimizes battery cable voltage drop.

The Battery Tender website has a very nice FAQ section that answers these types of questions.
https://www.batterytender.com/faq

Good to know about the jump start. This truck is my first dual battery setup and I’d never have thought of connecting across both.

Sent from my iPad using Tapatalk

BWL · Feb 14, 2021

No need to connect across both. As far as I know the alternator only feeds 1 side so why would you need to boost from both or charge both sides separately with a dual charger set up if the vehicle itself only charges from 1 side? Last I recall the alternator feeds the drivers side battery so you can hook to that side if preferred, but the other side works fine too if it's easier to reach. Not that hooking up positive on 1 battery and negative on the other will hurt anything, but it does leave more room for error if you get absent minded for a moment.

Doug Johnston · Feb 18, 2021

I use one battery tender with a float mode once full charge is reached. Both batteries stay at the same level.

Dual Battery & Battery tender(s)

WY-Dave

Senior Member

fireflymedic

Senior Member

BWL

Embrace the skeptisism

BWL

Embrace the skeptisism

WY-Dave

Senior Member

BWL

Embrace the skeptisism

BWL

Embrace the skeptisism

Travelin Ram

Senior Member

WY-Dave

Senior Member

fireflymedic

Senior Member

stevenP

Senior Member

BossHogg

Senior Member

DOT

Junior Member

BWL

Embrace the skeptisism

Doug Johnston

Junior Member

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