Battery Smart Isolator - Dumb Idea?

[wagohn]

Hello,
So I ordered a 140amp smart battery isolator to help charge my leisure batteries. The way the device works is that it sits between your starter battery and your leisure battery and switches the charge over to the leisure battery when your driving and the starter battery reports as "full" (13.2v I believe the docs stated) . When the starter batter drops to less than 12.8v, the charge auto-switches back to the starter battery, and the dance continues.
After ordering, a thought occurred. If my bus sits for a week, wont that smart isolator be sat "listening" for a charge to direct? It sits between two batteries with no on/off switch so Im thinking it has to be active all the time, and so using battery power while sat in my garage.
Thoughts? Anyone try one of these things?
Thanks!
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1976 VW Transporter, 2.0 FI Engine

[metahacker]

The programmable one I am using has a standby current of 0.04 A

It would take somewhere around six months to drain two batteries
Basically it is only wasting an almost meaningless amount of power

The draw for yours is probably documented somewhere (and likely similar)

One thing to consider, a traditional diode based battery isolator would drop the voltage supplied to your other battery by approx 0.6v and recharge slower (and potentially not to full capacity) - your 'smart' isolator likely does not have a noticeable voltage drop

If those voltages you listed are not adjustable they would be fine with SLA and AGM and keep the batteries separated when the engine is off

With 2 x lithium batteries (and those voltages) you would actually be drawing from the starter battery as well for the leisure loads, but it would end up isolating the starter battery with around 25% of its charge left (12.8v). the higher voltage shown by LiFePO4 makes things work a little differently with the unit you describe. a very cool byproduct IMO makes for a nice future upgrade if you are using your leisure stuff a lot - you can drop 2 lithiums in there and have it use both of them for leisure but still save enough juice to start the engine.

[mikedjames]

The voltages chosen are slightly higher than the off charge voltage of a standard lead acid battery. So once the voltages settle down a 100% charged battery is 12.7 volts.
The switch will open as the battery voltages settle and stay open until one of the batteries reaches 13.8 volts again on charge.
So trickle charging either battery (or charging faster) will close the switch and charge both batteries only once the battery being charged has reached a good state of charge.
The current needed by the switch to monitor the battery is similar to self discharge rates.
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Ancient vehicles and vessels

1974 VW T2 : Devon Eurovette camper with 1641 DP T1 engine, Progressive carb, full flow oil cooler, EDIS crank timed ignition.
Engine 1: 40k miles (rocker shaft clip fell off), Engine 2: 30k miles (rebuild, dropped valve). Engine 3: a JK Preservation Parts "new" engine, aluminium case: 26k miles: new top end.
Gearbox rebuild 2021 by Bears.

1979 Westerly GK24 24 foot racer/cruiser yacht Forethought of Gosport.
1973 wooden Pacer sailing dinghy

[Abscate]

50 mA is roughly the self discharge rate of a battery, the rate at which it will lose charge sitting disconnected. Look at your usage pattern and logistics before you throw cheap electronic crap at your setup

You might be better off isolating your coach battery and putting it on a 6 amp charger once a month overnight than spending money to have to car do it.

If you match your starter and coach batteries you can rotate them to keep them in similar life cycles too.

Keeping this stuff simple is cheaper and doesn’t introduce the fail points of crappy electronics and ( sorry) bad wiring that’s rampant here
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.ssS!

[wagohn]

[Abscate]

I would say a marine style combiner switch , 1-2-both-off would serve you well

Would let you go to Both for your winter needs , drawing from both for the heater would keep your drain at 20% for each , and then alternating between 1 and 2 weekly or monthly for driving.

Just remember not to switch with engine running.

Those switches are about $60 from defender or west marine
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.ssS!

[wagohn]

[SGKent]

imho it depends on how you camp. If you take off where you don't go home in a day you will need something to isolate the batteries from one another and charge them with the car. Aeromech (Gary) and Telford both have really nice setups, and Telford is the brightest electronics guy I've ever known. And I grew up in electronics, and worked in it off and on much of my life. What Telford has forgotten in electronics is more than what most will ever know,
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“Most people don’t know what they’re doing, and a lot of them are really good at it.” - George Carlin

[aeromech]

I found out that the Bluesea ACR has a constant miliamp draw and can run your battery down over a couple weeks. It's a boat part. Boats turn off their batteries after every trip. So no issue. I started installing a $6 relay that shuts the ACR down when the coil loses power.
_________________
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

[wagohn]

[aeromech]

The other thing I discovered about the ACR is that if one battery drops below say 10 volts, the ACR will not combine and therefore not charge the low battery. There is a good reason for this design though. If one battery get a dead internal short the ACR prevents the other battery from failing too.
_________________
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

[blue72beetle]

Do you put the relay on the small ground wire? Is that like a trigger wire?
_________________
-Andy-
-1970 Ghia-
-1971 Bus 1776 Microsquirt EFI-

[aeromech]

Yes. The ACR has a spade connector that goes to battery ground. Install a relay and the ground is removed when the key is off.

On a car you use the vehicle chassis as ground. On a boat you need to run a wire
_________________
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