Voltage sensing relay controller (electronics question)

Note that this is NOT about a tractor. It IS about electronics that I guess -might get used in one for certain uses.

Getting my 1988 Toyota Minicruiser ready for a trip (small Toyota motorhome). The OEM rectifier-based battery isolator was bad. It's purpose it to separate the engine cranking battery from the rear "camper" battery. I removed it and wired in low-draw, 150 amp relay along with some #2 copper cable that runs from the front to the back. My other RV has a relay for isolation but is wired to the ignition circuit. So when the IGN is "on" -the cranking battery and house (camper) battery are hooked together in parallel. When the key is "off" they are isolated. Works well enough but . . . I don't like all batteries charging at once when I first start.

So - I got a so-called "Smart" relay-controller from Powerstream. Several companies sell them. Main idea is - they sense the voltage of the cranking battery and keep it isolated from the "house" until it gets fully charged first. Once it gets to 13.5 volts - it closes the relay and starts charging the "house" battery. Sounds great in theory? I have the same setup on my Chinook and it works great BUT it's a different brand (Cole Hersee). I chose Powerstream this time mainly because they will sell you just the control module if wanted. Since I already had a relay - that was my choice.

Here's the problem. I installed it last week. I later discovered that once I start the engine -and it closes the relay - it won't turn the relay back off when I turn the engine off. I assumed I had a defective unit -but I was wrong. Just has bad engineering (in my opinion). I called the company to verify. Seems that unlike the other "Smart relays" on the market . . Powerstream set their's to shut off at 12.6 volts. That makes NO sense to me at all. The tech guy I spoke with seemed to agree with me. Note that a fully charged battery at rest is 12.7 volts. That means this "Smart relay" will not turn off the relay until the batteries get run down a bit. Mine was on for 4 days and still had not shut off. Keep in mind that once the relay is engaged - both batteries, front and back are tied together. So there are two fully charged batteries that must be run down a bit before they become isolated. Somewhere around a 95% charge before it shuts down. Like I said . . . mine was on for 4 days straight until I got disgusted and disconnected it.

Does this make sense to anyone??

How about a simple TIMER.... Cranking battery gets charged for say, 10 minutes @ startup, then timer times out and closes isolation relay and connects up "house battery", and disconnects at shoutdown.

Could you put a resistor in the sensing wire to "lower" the sensed battery voltage thus shutting off the relay? Jim

NOPE doesn't make a lot of sense to me, but I'm more into Electrical then Electronics gadgets...

Being a long time avid RV user I would NOT like the engine battery to remain connected to the house batteries once the engine was stopped and the alternator no longer charging. On my time delay isolation relay system the instant the engine is stopped the relay opens. I don't like the idea of the engine battery still in use until such time its voltage drops.

My RV uses a simple timer. The house battery bank sits there isolated via a normally open relay. When the RV starts (which initially drops engine battery voltage a bit) the relay remains open UNTIL THE PASSAGE OF A TIME DELAY only after which it latches in and connects the engine battery to the house battery bank so the alternator charges all batteries while driving. It gives all the alternators output to the engine battery alone for a short time to replenish it a bit prior to the engine and house batteries all being connected together.

There is one thing worth mentioning. The house battery bank consists of True 6 Volt Deep Cycle Flooded Lead Acid batteries in series/parallel, while the engine cranking battery is a regular truck/automotive lead acid starting battery. After a period of driving the battery voltage (Engine in parallel with house) stabilizes in the 13.9 to 14.2 or so volt range. However and even if you were to drive 6 hours with all the batteries at 14+ volts. IF YOU GET TO AN RV PARK AND TURN ON THE XANTREX TRUECHARGE2 SMART CHARGER it reacts almost like the deep cycle bank is somewhat discharged???? i.e. it ramps way up to near a max charge level for a time before it settles in and then maintains the lower float voltage of around 13.1 to 13.2 volts. Its like the alternator wasn't actually pumping much charge to the house battery bank even though it was connected to the alternator for 6 hours and a voltmeter on the house bank showed it from 13.9 to 14+.

Sounds like a fairly easy time delay relay fix unless you choose to just leave it as is??

John T

I see the theory, run off both batteries as long as they are safely up to charge, then drop out the truck battery with enough charge left to start the engine.

Sounds like you have some good batteries and a minimal drain. I'd give it a chance, try it at home or where you have power handy, andhave a charger ready just in case.

Motorhomes don't make any sense to me.....

Allan

Yes, it makes sense. The relay delays connection of the house battery until the vehicle battery has recovered from the engine start. This reduces the load on the alternator. At the shut off point of 12.6 volts the vehicle battery should start the engine, no problem. This protects the vehicle battery from becoming deep cycled (discharged), therefore the engine will always start even though the house battery has been discharged.

If you want the vehicle battery disconnected at engine shut off you should be able to "trick" the relay by turning on the head lights or jogging the starter.

Note: my chart shows 100% charge at 13 volts for maintenance free, 12.7 volts for CX & YuMicron and 12.6 volts for convention batterys. From this your relay can expected to remain connected longer for some battery types.

Would it not be simpler to just put another relay in series with you"re fancy new one that in on only when the ignition is on, I would think when you start it, it would close and allow you"re fancy relay to charge the second battery after the first one charges, when you shut off the ignition the voltage would be removed from the fancy relay and disconnect the second battery.

Or just put another straight plain old fashion relay in line with your fancy Wiz Bang Deluxe hi-tech voltage sensing time delay on relay that cuts off the house circuit from the vehicle circuit when the ignition key is off. That would give you your time delay at start up and instant isolation at shut down. Personally I don't see much wrong with it as it sets. As others have eluded to you use all your batteries until the vehicle circuit feels a little discharge and gets isolated. You're still isolating the vehicle battery with plenty of power to start the truck. I suspect with the nominal voltage drop to isolate by the time you want to restart the vehicle battery has recovered a lot of what it lost anyway.

I kind of agree unless it has alternative uses. Or if a person lives full time in one. We got this Toyota RV as an experiment to see if it's good enough on gas to use more. We live in three different places all far apart. So we use some sort of camper to stay in at night when traveling. Me, wife, 10 year old kid, large dog, cat . . and this time 5 chickens. Last trip we used my 91 Dodge diesel truck with a slide-on camper. Works nice and gets 17 MPG. But diesel fuel is expensive (often a lot more then gas) and . . some states do not allow riders in the camper. So we are experimenting.

I don't agree and still makes no sense to me. I
have another relay like this on another RV and it
works great. Just a different brand. Made by
Surepower and turns "off" at 12.8 volts. That is
1/10th a volt above nomimal battery voltage and
works "just right." After driving and turning the
key "off" the batteries disconnect within a minute
or two.

This Powerstream unit is factory set at 12.6
volts. That is a when the batteries are discharged
by 5%. When the relay is "on" there are multiple
large batteries all tied together in parallel.
Not just one. Takes quite a bit to run them all
down 5%. Maybe it makes sense to you - but not to
me. My isolation relay was "on" day and night
for 4 days drawing a steady 1 amp the whole time.
That's 96 hours drawing 1 amp to keep that relay
closed and voltage was still 12.72 volts.

"The relay delays connection of the house battery until the vehicle battery has recovered from the engine start. This reduces the load on the alternator"

That's what my system does by the use of a simple time delay relay (NOT fancy schmancy voltage sensing device) that doesn't close the normally open isolation relay UNTIL TIME PASSES which allows the alternators full undivided output to charge the engine battery so it recovers to some extent BEFORE it closes so the alternator charges BOTH the engine and house batteries. HOWEVER as soon as the engine is shut off the relay is open so there's no drain whatsoever on the engine battery when I use 12 VDC appliances in the RV.

Also, my house batteries are true deep cycle units while the engine battery is a conventional type and I (like John it appears) don't like them to be connected in parallel until the engine battery drops voltage. I prefer my deep cycle batteries for RV use ONLY and my engine battery for the engine ONLY. My system accomplishes that plus what you were talking about and is cheap easy and simple. IM KEEPING IT AS IS AND NOTTTTTTTTTT GOING TO USE ANY VOLTAGE SENSE DEVICE THAT KEEP HOUSE AND ENGINE BATTERIES TOGETHER.

HOWEVER, to each their own, your money your call.

John T

Regarding the length of time to disconnect, Yes, I would agree it should not take four days. Using your numbers, 1 amp for 96 hours is 96 amp-hours. I bought a Marine deep cycle battery this spring that is rated at 105 amp-hours. After a discharge of 96 amp-hours this battery would be in the mid to high 10 volt range - two batterys of similar size would be no where near 12.6 volts. If 12.6 volts represents a 5% discharge, then your total battery capacity would need to be in the range of 1920 amp-hours to maintain this voltage after a 96 amp-hour discharge. 1920 amp-hours would be 18, 105 amp-hour batterys.

You are running two batterys and after a 96 amp-hour discharge the voltage is 12.72 - can something be connected and is charging the batterys....?