feedback on dual battery system design?

theksmith

Explorer
i'm cross posting this from Offroad Passport to get a wider audience... i'm genuinely open to any feedback on the proposed system - but please be constructive and limit the discussion to THIS design. in other words, i do realize there are a ton of pre-built and DIY systems available but i'm not looking for suggestions on one of those - thanks in advance!




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.


Goals

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)


continued...
 

theksmith

Explorer
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.


continued...
 

theksmith

Explorer
Schematic & Intended Functionality

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




EDIT:

WELL POO! i can't seem to paste in the rest of my original post without triggering some sort of security junk here and getting my IP banned. i'm typing this from Starbucks!

so if anyone has a moment, you can read the complete post including a few specific questions i had here: https://offroadpassport.com/forum/showthread.php?t=5331
 
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DaveInDenver

Expedition Leader
A backup philosophy in this sense might be if your main battery takes a dive afield you have one that can run the truck until you get home. A jump pack is great if all you want is a way to get you started with a dead battery but might not help if your main burned up winching or after taking a shock 'wheeling. Although your question is still good, because a backup could be significantly smaller if you presume you're going to be conscious of the loads you might run on it.
 

theksmith

Explorer
based on all the instructions i've seen on different brands of those lithium jumpers, and my own tests with one - they can't actually replace the main battery if you have a complete failure (or extremely drained state)...

i recently ended up with a shorted cell in my factory battery on my 2016 JKU and confirmed this. i turned off the rig at camp and the next time i went to move it, it wouldn't crank and my lithium jump pack wouldn't jump it. i checked the voltage and found it was sitting at 10.x volts. fortunately there was another rig camping with us, he managed to jump me with his rig, but even after charging a while my stock battery just dropped immediately back down to 10.x volts.

and yes, i was planning to run only a small high CCA motorycle battery as the backup to save on weight.
 
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Rando

Explorer
No, they won't replace the battery if you had that sort of failure. However, with a modern computer regulated alternator, you car would likely run without the battery at all as the alternator provide sufficient current to run the necessary loads. This is not really recommended, but a battery failure with a bad cell is a super rare occurrence, making this a limp to town situation.
 

Umbrarian

Observer
If it is a backup battery, why even hook it up? Just keep it charged and in the tray/trunk. Sometimes old school is the best and cheapest solution.
 

theksmith

Explorer
If it is a backup battery, why even hook it up? Just keep it charged and in the tray/trunk. Sometimes old school is the best and cheapest solution.
just convenience... before i put a dual battery setup in my previous rig, i used to just carry a small jump starter that could replace the battery completely - a JNC300XL. i found with the Phoenix heat that i pretty much had to take it out and charge it up at home every 3 months or it wouldn't have enough juice to be any use.
 

Umbrarian

Observer
However, with a modern computer regulated alternator, you car would likely run without the battery at all as the alternator provide sufficient current to run the necessary loads.
ROFL.

So MFGs go to the effort of putting batteries in cars even though "the alternator provide sufficient current to run the necessary loads."

Feel free to test your theory by pulling your battery and seeing if your car still works.
 

Rando

Explorer
ROFL.

So MFGs go to the effort of putting batteries in cars even though "the alternator provide sufficient current to run the necessary loads."

Feel free to test your theory by pulling your battery and seeing if your car still works.
The battery is to start the car, run accessories while the engine is not running and provide some amount of buffering to the alternator output. Batteries aren't energy sources, so of course the alternator must be able to provide enough current to run the necessary loads AND charge the battery otherwise your battery would go flat while you drive. As long as your alternator is well regulated, your car will run just fine without a battery, and yes I have done this when the ground lug broke off my battery cables.
 

Umbrarian

Observer
The battery is to start the car, run accessories while the engine is not running and provide some amount of buffering to the alternator output. Batteries aren't energy sources, so of course the alternator must be able to provide enough current to run the necessary loads AND charge the battery otherwise your battery would go flat while you drive. As long as your alternator is well regulated, your car will run just fine without a battery, and yes I have done this when the ground lug broke off my battery cables.
So it runs without a battery, and now it also runs without a ground. I guess it also runs without fuel.
 

theksmith

Explorer
I would be inclined to think such car less likely to run without a battery.
Thinking its ECU would recognise battery is absent then shut down. Suppose there is one way to know...
Fwiw, direct experience with oldschool cars and some ECU of 25-ish years past will run without a battery. Obviously dont shut it off or stall its motor...

Anyway, I think OP plan will work fine, but daft. Unless starting batt is regularly killed (what shortens its lifespan greatly...) & needs frequent self jumping, Its carrying around an extra battery what does nothing... ever.
If determined to use this ”just in case” battery scenario, might as well save money, forget ACR & and use a dumb relay for its charging. Using jumpercable or override switch when self jump is needed.

i do go out by myself quite often, have a fridge that eats up the juice, and have seen quite a few battery failures of differents sorts over the years - so yes, i'm quite sure that i want a "just in case".

however you do make a great point about needlessly spending money (as well as effort) - perhaps i will change the "system" to just an isolator and some quick connect battery terminals with enough slack to swap to the backup easily if the main batt eats crap completely, or use jumper cables if i just need a self-jump for a somewhat drained main batt.

thanks for the input!
 

theksmith

Explorer
for all the other comments... i don't know about every other rig out there - but i do know for sure that my JK will not start from a lithium jump pack alone - at least the one i test and a different brand that a friend tested. i'm almost certain it won't run if you start it then pull the battery - Jeeps are pretty notorious for lighting up the dash or acting very strange in general with even a somewhat low battery.

EDIT - i stand corrected on that second point - i just started it and pulled the negative off the battery afterwards to put this to bed. it did stay running. some LED lights flicker, but no crazy warning lights and the idle stayed smooth.
 
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Rando

Explorer
So it runs without a battery, and now it also runs without a ground. I guess it also runs without fuel.

It will run without a battery, of course it will run without the battery ground lug, which obviously doesn't do anything without a battery present. You really should do some thinking about how an engine works before spouting off.
 
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