Wiring Aux Battery in Parallel?

[Homey '71]

Has anyone simply wired a second battery in parallel with the start battery to double Ah capacity? I know the common setup is to isolate the start from the Auxilliary battery and run a second fuse block for camping accessories. I'm considering the pros/cons of both setups, but when I search for it I find very little information on dual battery parallel circuits that seek only to increase capacity. It seems that if one were willing to forego the safety of having an isolated start battery, they could simply run a cable between the positive poles and ground the second battery to the chassis. Am I missing something?

[telford dorr]

This can work if the batteries are the same brand, type and age. If they're different brands or ages, one of the batteries can dominate the other, reducing total capacity. Different types is never recommended.
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[SGKent]

the main reason people use an isolator is so they don't get stranded when both batteries get pulled down after a couple days camping. Also, when a battery dies it is common for one or more cells to short, which lowers voltage in the one battery. I can see a situation like that also discharging the second battery.

Gary - Aeromech has done lots of second battery installations as I recall. You might PM him to see what parts he recommends as being the easiest solution. Telford and Gary are good friends, so is Robbie so all of them would probably be able to answer that question.
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[Homey '71]

Thanks for the replies. I figured it was just that easy. I get it about the safety factor of running down the battery, on the other hand, I can't count how many times in my younger days I've bump-started a bus when I played the stereo all night long. Nowadays, I carry a LiIon jump-starter, but have rarely needed it. Also the concern of damaging the cells in a lead acid battery is warranted, but I would probably use a set of identical AGM batteries that can handle a lot more abuse.

As far as I can tell the pros of such a setup would be dead simple wiring, no additional components needed (ACR, Isolator, Fuse block, etc). The Ah capacity of the vehicle would jump from 75ish Ah to 150ish Ah which would give me breathing room to run lights (all led these days) and heater for a few days. If I end up running the batteries down I can just use my jump starter or bump start it...

To me the logic works out...But the fact that there isn't much talk about doing it this way makes me nervous that I'm missing something.

[Shonandb]

I did something simple for connecting my Aux battery and the main starting battery via a high amp start relay. The batteries stay isolated when the ignition is off but once it is turned on, both batteries are connected and get charged by the alternator when the engine is running.

At start up, the Aux bat complements the main battery's starting amps and the alternator provides a quick fast charge of the Aux battery if I run it flat and no time to wait for the solar panel to charge it. When camping, the solar panel is only connected to the Aux battery and keeps it charged.

You could connect two reg batteries the same way or just have them on a switch or timer and relay to connect both if/when needed. I bought and tried a low cost ($25) battery isolator but it didn't work well. A better quality one might work but opted for a simple connection as I had the solar panel already.
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[aeromech]

Consider how they wire boats. Big boats.
Consider how they wire motor homes. Big motor homes.
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[aeromech]

If you really want super capacity, get four 6 volt golf cart batteries and wire them in series/parallel. Some motor home people do this for their house batteries.
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[airschooled]

You have to ask yourself what you're trying accomplish by doing this. VW buses aren't exactly full of excess horsepower, so the extra weight will really need to pull its own… weight…

I do a five month #VanLife road trip every year, and I do not have a second battery. One 100w solar panel combined with driving every other day allows me to run a 12v fridge, interior LED lighting, monster stereo, laptop charger, water pump, and USB chargers. If I wanted to stay parked for weeks at a time, or had a partner with me I would run 200w of solar before adding the weight of an extra battery.

Tech has changed a lot in the last few years, my entire solar setup was 1/3 the cost of the marine battery I use. (Just don't submerge the charge controller in muddy water for an hour…)

Robbie
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[richparker]

Aeromech posted his new battery cost like $300 and something dollars. Robbie says a solar panel, mounting hardware, roof rack, controller and wires cost less then a battery. Where are you guys buying these ridiculously expensive batteries?

I’ve purchased 5 batteries since 2019. One for my Frontier, one for my wife’s Rogue, one for my Beetle, a start battery and a Leisure battery for my bus. Each one was $100 or less. They are all made by Johnson Controls, came with a 2 year free replacement and have pro rated replacement after that.

I’m sure a leisure battery weights about as much as a solar panel, mounting hardware, the roof rack it has to be mounted to, wires and a controller. Plus you have the wind drag you with a roof rack on your tin top. Then of you put a fork mounted bike up there, you got a fin that the wind loves to use to push you around. But, I bet that’s only with 2L strokers, no way a 100% stock 100% German 1600 SP (did they even make those, what’s the m-code for one of those) is effected by that kind of drag.

I’m very happy with my start and leisure set up, both of my batteries charge off the alternator. We’ve gone as long as 5 nights and 6 days at one campsite and didn’t kill the leisure battery and we run a lot of stuff. Sure, I lived 6 years with a single battery set up and never killed it, but I always had a jump box with me just in case. Not I don’t have to worry about carrying one of those. If my start battery dies, I can just jump it off the leisure battery.
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[aeromech]

Where are you guys buying these ridiculously expensive batteries?

Amazon

https://www.amazon.com/dp/B000TCTD3Y?ref=ppx_pop_mob_ap_share
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[nemobuscaptain]

[richparker]

[quote="nemobuscaptain"]

[metahacker]

Your idea is sound and logical.

The standard camper configuration of starter battery and house battery is typically running through an isolator.

The house charges when the engine is running but it is dropped off (and isolated) from the circuit with the main battery when the engine is off.

There are two issues with this configuration:
- Your starter battery just spins the starter and then basically does nothing. Once you turn the engine off, that battery just takes up space and doesn't contribute to your total Ah available for e.g. camping systems. So, with a 80Ah battery in each position you are still only getting 80Ah total of useful reserve out of your 160Ah bank with half of it (80Ah) just sitting there and collecting dust doing nothing except waiting to start the car.

- If the isolator is diode based this will drop the voltage to the house battery while charging. This will will be 0.5V+ (I've seen as high as ~1V). This may cause the house battery to never reach 100%, and also causes it to charge more slowly. For example, with a LiFePO4 battery, a 14.2V alternator output with a 0.7V diode drop on the isolator would not reach 100% charge.

Now, if you run the batteries in parallel your 80Ah x 2 now becomes 160Ah and you have the same voltage to both batteries. Those are obvious advantages.

Now, when your "house" flats out the engine battery you can just manually jump start it. That's one way to handle it.

Another approach is to run a digital battery combiner/isolator with programmable voltage level triggers. They will typically have no voltage drop between the two battery circuits.

Then you can run basically the same setup as you would with the isolator... but have the batteries bridged by default.

Then you can set a voltage threshold to approximate a specific state-of-charge (SOC)... for example, 12.0V exactly would be about 10% SOC on a lithium battery... 12.8V approx 20%... and your charge voltage from your alternator would typically be 14.4V....

So, basically you could set the digital isolator/combiner to have the batteries disconnect at 12.5V, which would let your starter battery drop off at 15% SOC (lithium)...and keep that juice to start your car... then the other battery can flat out a bit further.

You can set the connect voltage higher, say 13.2V .. and they will bridge at that voltage. That way, when your solar panel or alternator or AC battery charger is feeding the system (from either side, typically) the batteries will bridge together.

This is precisely how I run my camper.

I'm using two Aolithium brand 100Ah Lithium iron phosphate (LiFePO4) batteries with internal shunts / JBD BMS Bluetooth reporting and a Thunder Auto Thunder TDR15100 electronic battery isolator/combiner.

I run a 160A alternator into the starter side, and occasionally have a solar panel connected to the house side. There's also a hard-wired/permanently mounted 20A AC-to-DC LiFePO4 charger feeding the house side battery and connected to the 110VAC input from the camper "hookups". Any charging on any of those sources will generally cause the batteries to bridge after an isolation event and the charging to be shared (and batteries to auto balance).

https://www.manualslib.com/manual/1450825/Thunder-Tdr15100.html

This provides 200Ah total capacity, of which I use about 180Ah in "house" consumption.

The house will isolate when there is about 20% SOC and then the starter battery will just remain at 20%... the house can go to close to 0% (at which point the loads should be cut before the battery is totally flat, e.g. using a bluetooth Victron BatteryProtect).

Once input power shows up on either side, they can bridge and things charge.

In a bus, using both sides of your engine bay to hold batteries that you use to power your whole bus is a perfect solution because every square inch is precious. It's a great idea. You just need to figure out how to not get stuck... if you solve for that, having them run in parallel together is better.

It could just be two cheap batteries wired together and carrying a jumper pack. That would be fine if it works for you.

On even the most simple configuration, however, I would strongly consider a Victron BatteryProtect required equipment for controlling any zombie loads like a refrigerator. You do not want to physically flat your battery out. It will destroy it. Even a lithium with a BMS hitting it's safety cut off threshold (which is also a hassle to "wake"). You want it to flat out to a voltage YOU choose, that you know is safe. They are very cheap...and if you set it right you probably won't ever need to jumpstart, too

[aeromech]

Not many Rich. I’m almost 65 and have many miles under my belt driving VW’s. I’ve done many trips camping and touring in my bus. Just not this one which I’ve owned for about 5 years.
_________________
Lead Mechanic: San Diego Air and Space Museum
Licensed Airframe and Powerplant Mechanic
Licensed Pilot (Single engine Land)
Boeing 727,737-200-300-400,757,767
Airbus A319,320,321
DC9/MD80
BAe146
Fokker F28/F100
VW type 1 1962,63,65,69,72
VW Type 2 1971 (3 ea.) 1978, 1969
VW Jetta
VW Passat
Capable of leaping tall buildings in a single bound

[airschooled]

Lowes? Yikes prices have gone up. $179 as of posting.

https://www.lowes.com/pd/Renogy-1-Module-42-2-in-x-19-6-in-100-Watt-Solar-Panel/1001277092

--

If you enjoy tinkering, metahacker's setup is fucking cool. When he started his engine last weekend and got the main battery charging, he hit a button on his phone and I heard the alternator whir a little higher as he coupled the second battery in remotely.

If I had unlimited time and funds, I would do it that way. I don't, so I just use the sun to power my campsite for now.

Robbie
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Learn how your vintage VW works. And why it doesn't!
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[metahacker]

for simple i would just wire 2 batteries together and run this thing for $40.
put your "house" + wire on that. done.

https://www.amazon.com/Victron-Energy-Battery-Prot...U&th=1

(or the gucci one with bluetooth for $60)

https://www.amazon.com/Victron-Smart-BatteryProtec...amp;sr=8-5

+ cheap solar panel you are in fat city 200Ah easy AGM or LiFePO4

[Homey '71]

High battery cost is for AGM batteries (absorbed Glass Mat) which can take more abuse and tend to have higher Ah capacity than your typical lead acid battery. Plus most if not all "dual purpose" batteries are AGM. This basic setup would probably work fine with lead acid too.

Aeromech - Boats! Yes, I've done a lot of electrical systems on several sail and power boats. All to ABYC standards, with high capacity house battery banks and powerful starting batteries to spin up the diesels (and a generator with it's own battery). It was a lot of fun and I learned a ton. My first inclination is always to open the sailboat electrical manual (and the latest Defender catalog) and design a state of the art system that accounts for every possible scenario. But this time I'm inclined to shoot for simplicity. Probably a combination of feeling lazy and learning to really appreciate the analog nature of an old car. I also toyed with the idea of 2 T105's in series, but I don't really need the capacity or want the weight, though I think your suggestion is great advice for someone who spends a lot of time off the grid.


Meta Hacker - You're picking up what I'm putting down. With the reduced capacity of a dual purpose battery as opposed to a real deep cycle battery I figured it would be a shame to preserve all 75Ah of the starting battery for the sake of safety. Of course I could spec a deep cycle 100ah battery and separate the two, but as far as I understand it, I'd still be limited to the same chemistry if I wanted to charge both batteries with the same charger or solar controller. I like your idea of a low voltage cutoff and will look into that.

Airschooled, I'm with you. I just don't think I need the complexity of a "do it all in every scenario" system. I'm still enamored with the idea of adding another battery worth of capacity in spite of the weight though. I'll be running a heater and it's not uncommon for me to find a spot and hang for a long weekend without starting the bus. I feel like the extra Ah will satisfy my desire for convenience.

For now the simplest system with a margin of safety seems to be 2 identical AGM batteries wired in parallel through a 1-2-off switch so I can isolate if needed. Low voltage can be indicated by an led in the cabin coupled with a voltmeter for monitoring. If I get nervous, I can isolate one of the batteries. If I screw it up totally, I can jump it with my jumper.

From what I've read all systems have drawbacks in certain scenarios. For starters, an electrically isolated setup is limited to the capacity of the house battery. The isolating methods have drawbacks too, for instance a solenoid can still leave you stranded if one battery is deeply depleted and sucks all of the juice from the other battery when you are trying to run the starter (I assume this goes for jumper packs too). An ACR is timed to avoid this, but it has a vampiric draw that can potentially deplete both batteries if the car sits for months. From here, means to mitigate the drawbacks start going up in cost and complexity. A "perfect" system will be both $$ and complex. Plus imagine trouble shooting all this on the side of the highway when your ACR fails in 10 years!

[metahacker]

IMO, battery isolators are NOT ideal in a bus.

just cramming as much battery will fit, wiring it all together and adding a <$100 voltage cut off (see above...Smart BatteryProtect is your friend)
for your expected loads (e.g. ones that will just keep running and could leave you stranded - sink, fridge, heater, inverter) is all that's needed.

if you want to be a hot-shot put one on your primary power wire for the main vehicle fuse box, too (e.g. in addition to your house specific panel). then you can just toggle that cut off temporarily on, separately, to start your engine and let the house one come back on at a different point if desired.
your starter is the only one that really needs a gigantor unfettered power source. everyone else can run thru a voltage sensitive cut off.

just generally speaking, for a "properly designed system," you need a battery cut off to operate "safely," anyway... once you add shit like a fridge, or you will 100% for sure end up trashing your batteries IF you are not constantly monitoring and staying on top of things.

it only takes 1 run-away load one time to make your battery "never the same again."

even experimenting with these fancy lithium batteries that have a Battery Management System (BMS) computer on them.... the ones that actually got drained enough to hit BMS cut off and drop down to like 8V are kinda shitty performance wise after that one event. don't even get me started on a SLA or AGM with a load left on it.... taking those just under 50% SOC fucks them up.. and letting something run totally rampant on them makes them garbage as a "house" bank

but, you know, if you have a way to monitor at least voltage level (ideally SOC% with the proper coulomb/temp calculations, like a Victron Smart Shunt..another cheap amazing device)...and you take responsibility, then you could theoretically just go free ballin' it and have 2 batteries raw dog... modern fridge controllers like the Danfoss Secop BD-series have their own voltage cut offs, as do quality inverters like Victron........you might be OK, if they're set at reasonable values, but can be pretty risky.

either way i'd pick a raw dog 2 battery system over an isolated-by-default house + starter if those were my two choices....
unless someone using it is going to be irresponsible and do shit like run a cheap inverter until the battery is obscenely flat

also running everything bridged all the time will alleviate the system of any potential issues between bridging two batteries in a state of massively disparate SOC, which you have mentioned, and can be a real issue.. especially if you use fancy shit that will happily charge at 1C rates like a single 100Ah that pulls 100A continuous draw if you let it. i don't think connecting a bank of 2-3 of those at <50% SOC to a single 100Ah starter battery at 99% would make for a good time.