Float charging dual batteries
Cross-Country
Joined: Sep 2021
Posts: 63
Likes: 30
For what it's worth: I left an LED on inside my camper shell which almost entirely drained both batteries (I have a 7.3L 2022, dual alternator, dual battery). Voltage on both batteries was around 1 or 2V. I connected a trickle charger on the passenger side battery only. Within about 24 hrs, BOTH battery voltages were normal. So they are in parallel and there is nothing shutting off the connection between them.
Cargo Master
Joined: Mar 2021
Posts: 2,044
Likes: 1,336
So I learned the other day that I can only jumpstart another battery using my passenger side battery.
when I tried with the driver side battery I wasn’t getting anything. Switched to passenger side and it worked.
I had a dead battery on a telehandler and thought that was interesting because I always thought they were both in parallel and thus would be able to jumpstart all the same.
when I tried with the driver side battery I wasn’t getting anything. Switched to passenger side and it worked.
I had a dead battery on a telehandler and thought that was interesting because I always thought they were both in parallel and thus would be able to jumpstart all the same.
Fleet Mechanic
Joined: Jul 2008
Posts: 1,937
Likes: 1,221
From: Southern Crescent
You could charge as you stated, positive on one, negative on the other, but what would be the benefit of that?
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2nd Gear
Joined: Nov 2023
Posts: 2
Likes: 0
I finally decided to experiment and put the charger on the passenger side (2022 F350 Diesel) battery and charged it until I got a green light. Next, I hooked the charger up to the driver's side battery. It was hours before I got a green light. I conclude that charging the passenger side battery does not charge the drivers side. I have yet to try this the other way around or chack with a volt meter.
Fleet Mechanic
Joined: Nov 2015
Posts: 1,675
Likes: 677
From: DeMotte, IN
Put the charger/maintainer on the passenger battery and it read lower than I expected for ac3 month old truck, 60% or so. Should have checked the driver side but didn’t. Left it on maybe a week. Checked the driver battery and it was 95%. Left in on several days. Rechecked the passenger side, it was 95%.
Hotshot
Joined: Oct 2015
Posts: 19,718
Likes: 12,858
From: Wyoming
I finally decided to experiment and put the charger on the passenger side (2022 F350 Diesel) battery and charged it until I got a green light. Next, I hooked the charger up to the driver's side battery. It was hours before I got a green light. I conclude that charging the passenger side battery does not charge the drivers side. I have yet to try this the other way around or chack with a volt meter.
The ideal connection is to put the positive lead of the charger on the positive of the passenger battery and the negative lead of the charger on the negative of the driver battery (or vice-versa). This way the resistance is the same for both batteries and they both charge, assuming that both batteries are in good shape and there isn't something wrong internally.
In theory, the overall time to charge both batteries to 100% is the same with both connection methods. In practice, there's likely some difference. If you have limited time to charge and want both batteries at the same state of charge then use the second connection method. For my truck, which has AGM batteries (aftermarket), I'm using the simple connection method. My truck sits for weeks in the garage during the winter.
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