I think my battery isolator crapped out... suggestions?

[DiploStrat]

Not a thing if your cables are large enough. Because the cable is likely to be long, is live at each end, and will pass the length of your truck, I would install a fuse within 18" of each battery.

Just don't blame me if you forget to connect or disconnect your batteries!

 

[4RunAmok]

Even using a manual switch, like a Perko or a Blue Sea, you NEED to have fusing, or breakers... Because you have a battery at each end of the cable, you need a fuse or a breaker ("protection" from here on) at each end. If you do not, and a short occurs in the middle of the cable, having protection at only one end still leaves the other battery connected to the short, and it will continue to short out. Having protection at both ends, if the cable shorts in the middle, both fuses will blow or breakers will open, and the batteries are now protected from the short.

Protection should be within 18" of the battery at each end. I say 18" because within that distance, it is very unlikely there can be a short between the protection and the battery. My personal preference is a cooper bussmann breaker, it should be at least 175 amp if you ever intend to use the aux battery to "jump start" the main battery.

Example, using text crudely

[ALTERNATOR]-----------[MAIN BATTERY]-----[175A BREAKER]--------------[MANUAL SWITCH OR ACR]--------------[175 AMP BREAKER]-----[AUX BATTERY]

This "diagram" shows the manual switch or ACR directly in the middle of the circuit, because it can be there, or anywhere between the breakers or fuses.

The absolute beauty if this scenario is you can put a manual switch in there now, and at any time later, upgrade to an ACR, or a solenoid or a CTEK. Anything but a diode isolator .

 

[4RunAmok]

Ugh, it took me a bit longer to type mine than DiploStrat did, essentially saying the same thing.

 

[Bbasso]

I've already got the circuit breaker and large fuses. Actually came with my van.

 

[4RunAmok]

Then you are SET. Get your switch, wire as described, and you're done with the option to upgrade your switch later.

 

[Bbasso]

Oh... could be that simple. Nice.

 

[4x4junkie]

You probably understand more than I do. My only experience with a diode isolator was decades ago, long before anyone understood voltage drop. In my case, the camper battery had next to no load and ran dry quickly.

My statement was based on the inability of the alternator/regulator system to sense both legs of the diode bridge and this this guru: http://www.smartgauge.co.uk/diodes.html

.

I think the difference is in the location of voltage regulator sense points (the Smartgauge page talks about externally-sensed systems sensing at one battery (but not the other), my example was if the internal alt regulator itself was simply bumped up 0.7V to compensate for the diodes).
I would think by now (without actually having looked into it... no reason to), most diode isolators are actually using schottky-barrier type diodes, as they have a much lower v-drop (about 0.3V at low current, 0.5V at high current) which reduces the issues of non-schottky diode isolators to some extent, but of course still wouldn't eliminate them.

 

[DiploStrat]

Some things I know, and ...

some things I don't. One of the big things that I don't understand is how an alternator senses past a diode, given that the diode allows current to flow only one way. (Sorry, I majored in history, not EE. )

I know that Sterling and CTEK use diodes to show the alternator a voltage drop, thus causing it to increase its current production, which they then control in various ways. I don't know enough about things to see how an alternator feeding voltage and current equally to two batteries of differing sizes and discharges can avoid overcharging one and undercharging the other.

I do understand combining batteries physically to make one large "logical" battery. Hence my obsession with heavy duty wiring, otherwise the alternator/regulator will only "see" the nearest battery.

At this point, I am very happy with what I have on my truck and can easily meet my goal of three days autonomy without sunlight or engine start while doing all of my cooking with electrical appliances and running diesel heat. Will have to wait for the summer to get the numbers on fans and air conditioning.

As a practical matter, what with sunlight and the usual tourist driving, my batteries are typically at 100% every night and down to 75% every morning. That means that I gain some 150Ah every day. That's not bad.

 

[4x4junkie]

It's all in the voltage, not current.

A battery cannot be overcharged unless there is too much voltage going into it (greater than 14.5V typically, though some batteries can tolerate up to 14.8V). Without voltage, the battery doesn't accept current.
Basically, when one battery approaches full-charge, it's terminal voltage will rise until it matches with that of the alternator's regulated set-point voltage, and current will then taper off to that battery. Meanwhile if the other battery still is less-than-fully-charged, it will have a lower terminal voltage and will continue to draw current from the alternator (current flows from a higher voltage to a lower voltage). As the 2nd battery finally comes up to the alt's set point voltage, the current to it then tapers off as well, leaving both batteries at or near full-charge.

For a regulator to see past a diode, it needs to have an external sense wire. Having this, it simply monitors the voltage right at the battery itself. If it's less than ~14.5V (it's set-point), the regulator sees this and steps up the current (up to the alt's max ability) until the voltage reaches 14.5V. Once at 14.5V the alt simply holds it there, effecting absorptive charging of the batteries.

Where the topic of the Scangauge page comes into play is when the voltage drop across a diode affects an externally-sensed alternator's voltage regulating algorithm... If the battery that is sensed by the regulator is taking heavy current, the diode's v-drop causes a corresponding rise in the alt's output voltage (the alt ramps up it's output in order to maintain 14.5V at the battery). Ahead of a diode this voltage can be as high as 16V. Remember that both batteries (diodes) are connected to this same output... A lesser-discharged battery will draw less current, therefore the v-drop across it's diode will also be less. If this battery happens to be the one not sensed by the regulator, too much voltage will end up reaching that battery (overcharging it). If your alternator's regulator does not use an external sense wire, then there's no need to worry about this happening.

Hopefully that makes sense.

 

[DiploStrat]

Got it!

Thanks!

My first system was the Sterling Alternator to Battery Charger: http://sterling-power-usa.com/12volt-210ampalternator-to-batterycharger.aspx This worked exactly as claimed in tests, but after we actually took delivery of the truck with made a few discoveries:

-- The normal charging rate of the Chevrolet is up to 15.5v depending on temperature and battery state of charge. The truck had been standing for six months during camper construction, so as soon as it hit the road in the winter the voltage surged to 15.5v and the Sterling shut down with a loud alarm.

-- As the Chevrolet was already running at 14v, rather than 13v, there was no need for the Sterling to boost voltage.

Replacing the Sterling with a relay has worked perfectly with the added advantage of allowing both the starter and truck batteries to be charged by the solar kit.

Thank you again!