View Single Post
Old 01-29-2018
adriaanvn99 adriaanvn99 is offline
(Adriaan) (ZS1AVN)
Premium Club Member
Join Date: Dec 2017
Location: Phoenix, AZ
Vehicle: 1998 Jeep Cherokee XJ Limited
Posts: 137
Reputation: adriaanvn99 is an unknown quantity at this point
Default Re: opinions on dual battery plans?
Originally Posted by theksmith View Post

i've been planning out a dual battery setup for the JK and wanted to put it out there for feedback before i buy all the parts...

Type of System

i'm going for the "main + backup battery" type system rather than the "starting + heavy loads battery" style.

that means starting duty and all loads (including the fridge) are usually handled by the main battery and the secondary battery exists only as a backup in case the main becomes so discharged that it can't start the rig or dies completely.


1. create a more versatile system than typical pre-packaged ones
2. ...yet not be too overly complicated! (i.e. minimal parts and ways to fail)

Main Components

1. BEP DVSR (Digital Voltage Sensitive Relay) (specs/instructions...): also known as a smart isolator, intelligent solenoid, automatic charge relay, etc. it works the same as most others in that it senses voltage. when the voltage is high enough to indicate the vehicle is running and the alternator is working correctly (13.4V), this devices combines the batteries so they both charge. when it senses the vehicle is off based on the voltage being back down (12.8V), it then separates the batteries.

i'm going with this particular one because it's a well known marine product, and has an extremely low standby current usage (1.8mA). this is also a dual-sense combiner, in other words it monitors the voltage on both sides.

its max current handling is 140A. i'm thinking that should be ok in my use case where the batteries are only being combined for the purpose of charging?

2. Marinco 4 Position Battery Selector Switch (1/2/Both/Off) (specs/instructions...): this manual switch combined with the DVSR in the correct configuration is what i'm thinking will allow me to cover a ton of scenarios while keeping the overall design fairly straightforward.

it's rated to handle 400A continuous, 600A intermittent, or 1500A temporarily during cranking. i'm thinking that should suffice for any load i'll ever have, including a winch?

the photo at the top of this post is of this switch.

3. a factory H6/Group 48 AGM battery as the main battery. the factory spec battery has a great balance of cranking amps and reserve capacity (AH) in a fairly compact size. i'm going to stick with that for the main battery for now. if i can find something with even more amp-hours that will fit in a dual battery tray along with a small backup battery, i'll go bigger the next time i need a new one.

4. the smallest battery i can find with enough cranking amps as the backup. i'm considering trying one of the over-spec'd powersports batteries that supposedly have 600+ CCA, such as the Renegade RG30L-WS, or the ThrottleX MX30L.

if those don't turn out to be all they claim, then i know the Odyssey PC1200 will work. it's a little larger and more expensive than those others, though still smaller than a typical car battery and worked to start my WJ's v8 just fine.

5. of course i'll need a JK specific dual battery tray and some other miscellaneous supplies. i already have heavy gauge wire, terminal connectors, and all the necessary tools. i wanted to focus on the electrical aspects of the design in this post.

Schematic & Intended Functionality

this is how i was planning to hook it all up:

...which i expect to result in the following behavior:

1. Marinco selector switch at OFF + Toggle switch ON or OFF: both batteries are isolated from one another and from the vehicle completely. the DVSR is completely off regardless of the toggle switch position, which means it is drawing no power. this mode would be for working on the vehicle or if i wanted to ensure a battery couldn't get drawn down from parasitic loads during long term vehicle storage.

2. Selector switch at #1 + Toggle switch ON: this is the typical mode where the main factory battery will start the vehicle and power all accessories. the backup battery will only be connected so that it can be charged when the rig is running and the alternator is working correctly.

3. Selector switch at #1 + Toggle switch OFF: main battery still handles everything but backup battery stays disconnected even when the rig is running. this mode would be used temporarily if the backup battery was not installed, or if it died completely.

4. Selector switch at #2 + Toggle switch ON: in this case the backup battery starts the vehicle and powers everything. the main battery will be connected back in to be charged only once the rig is running and the alternator is working correctly. this mode would be used temporarily if somehow the main battery became drained too low to start the rig from leaving on an accessory accidentally, or the fridge running too much, etc.

5. Selector switch at #2 + Toggle switch OFF: same as the last case except the main battery will never be connected back in. this is useful if the main battery has a cell shorted out or otherwise died in some way that wasn't just a temporary under-charge condition.

6. Selector switch at BOTH + Toggle switch ON or OFF: this manually combines both batteries immediately. the DVSR is effectively ignored so it doesn't matter the position of the toggle switch really. this mode could be used if somehow both batteries had become too drained to start the vehicle, but were still not completely dead. combining them might give enough CCA to get the rig started. this should be an unlikely scenario that might occur if the DVSR had been malfunctioning and not separating the batteries.

so basically it stays in #1 + ON unless there's a problem and i need the backup. OFF is also useful when working on the rig instead of needing to disconnect a cable from each battery.


1. is my schematic correct in terms of my expected behaviors? or do you see any issues in the schematic?

2. do i need fuses? most pre-packaged solutions do not have fuses/breakers. the schematics from BEP/Marinco do call for fuses and it seems most people in the marine world use them. if i did add fuses, would it be important to just protect the DVSR (140A fuses on its inputs), or should i use a much higher amperage fuse right off each battery?

3. general opinions/feedback on the design?

thanks for any input!
1) I can't see any specific issue with the schematic.

2) I would fuse a higher amperage fuse right off each battery. Reason being is I used to run a triple battery system in the G Wagon, and once a wire shorted coming straight off the battery hiding in the back. If the individual battery was fused, it would of prevented this. Also important to run a main circuit isolator which I see you added! I used a Hella one (

3) It looks real good to me! When I was living in Cape Town many of my friends did trans-Africa trips and would run an intelligent solenoid kit made by National Luna (, It is a South African company whom make off road electronics such as 12v fridges, battery monitors, LED's etc. Most swore by it, as it was very robust (roads are very dusty and extremely corrugated, so everything eventually shakes apart!). However, I like the idea of the marine Digital Voltage Sensor Relay as it has such a low standby amperage.

Here is the link to the instruction manual for the National Luna kit - it contains some useful info

Goodluck and look forward to following your design

Last edited by adriaanvn99; 01-29-2018 at 06:42 PM.
Reply With Quote