Diode based isolators have large finned metal heatsinks on them. The Diode is an electrical check valve allowing electricity to flow only in one direction. from alternator to house battery. The Diode causes voltage drop, and basically wastes power as heat, thus the large heatsinks. The voltage drop slows battery recharging and makes it nearly impossible to fully charge a depleted battery no matter how long one drives. Some isolator manufacturers have workarounds for the inherent voltage drop that are quite complicated and not for the timid. it involves tricking the voltage regulator into allowing a higher voltage, and the voltage regulator could be in the engine computer, or inside the alternator, or external. NOT a good solution.
Pass.
The Solenoid is a simple switch with heavy duty contacts, that are closed by an electromagnet. When the electromagnet sees 12 volts it closes, bringing the large contacts together and allowing alternator to pass current to house battery. No 12v, and the contacts open, and house battery loads cannot feed off engine battery.
These come in latching and non latching style with the former being much more common. A latching relay only needs a momentary burst of 12v to latch the contacts, and another burst to unlatch. The non latching solenoids consume around 0.8 amps to keep the electromagnet energized and the contacts together. The latching relays need a different type of switch to latch and unlatch it, and a light to let the driver know in which position it is at. more complicated to wire and not automatic.
Much easier to have the non latching solenoid automatically controlled by a fuse live only with the engine running. One could put an illuminated switch on this for 'some' manual control, but it introduces the possibility for human error.
And Speaking of Human error, we have the 1/2/both/Off manual switch. With one of these, Say battery 1 is the engine battery, battery 2 the house. One can leave the switch on battery 1 and only the engine battery will charge via alternator. One would need to switch it to BOTH for the house and engine battery to charge, OR one could switch it to 2 and All the alternator current will goto the house battery.
One MUST NEVER turn the switch to OFF with the engine running. This can blow the diodes in the alternator and it is more likely to occur the more amperage the alternator was making at the time it the load was dumped by switching it to OFF. Some of these switches have a AFD, alternator field disconnect, where one needs to run the field wires from an external voltage regulator through the switch, and when turned off, it turns off the alternator too, saving the diodes. This complicates wiring, but can save a boat in the middle of the ocean from frying its main charging source, when the wrong turn of a switch could be life or death.
Of course One needs to remember to start the engine on battery 1, then switch to Both when driving, then switch back to battery 1 when stopping, for complete engine battery isolation. In truth one really only 'needs' to manually isolate the battery when a significant discharge of house battery is underway(ie overnight) but it is best (in my opinion) to get the engine starting battery to 100% and never use it for anything but engine starting, as starting batteries cannot be cycled without capacity loss happening. if one uses a marine battery as the engine battery this becomes less of an issue. marine batteries have lower CCA numbers compared to a starting battery of the same size so the battery might have to be physically larger and heavier to have the same CCA as what the vehicle manufacturer called for originally. Having lower CCA is likely only an issue with diesels or in the depths of winter.
There are also voltage sensitive relays that automatically parallel or disconnect batteries when it sees charging voltages. these come in single sense and dual sense. Dual sense is when either battery sees charging voltages they are paralleled. This is not ideal when one has solar and wants ALL the solar current seeking to get the house battery to absorption voltage. While a fully charged engine battery might not need much of that current to be brought upto ABSV, it will require some of it, and the engine battery might not need it, and be charging unnecessarily, while slowing down the charging of the house battery.
Perhaps not a huge issue, but less than Ideal, IF one never uses their engine battery other than to start the engine. If the engine battery is a marine battery and one does cycle it shallowly, then a dual sense solenoid can be of benefit. it really comes down to the user, and how they intend to use their electrical system.
I have chosen the manual switch route myself, and turning the switch back and forth is a bit of a pain, and I have forgotten to isolate many times, but it was never an issue other than delving into unideal. I now actually employ 3 manual switches. One to switch all stock vehicle electrics to either battery. One to switch all house loads to either battery, and one to switch my Solar or grid powered charging sources to either battery. This is great for full manual control of all aspects, but an electrical Newb would look at my electrical wall and have a nervous breakdown.
When my last flooded group 31 House battery capacity declined to an unacceptable level, and I could not afford to replace it, I removed the battery and turned all my switches to battery 2, which is my Northstar group27 AGM battery at only 90AH.
That was almost a year Ago, and this single battery is doing well for both engine starting and house load duty. It can accept huge recharge currents over its thick cabling to the alternator, and my 200 watts of solar can usually replace everything I use by late afternoon, and even depleted 65Ah of the 90 total, it has no problems starting my engine in a mild climate. This particular battery requires kind of special treatment though. After so many deeper cycles, about 4 or 5, It absolutely needs a high amp recharge via the alternator or grid powered charging source at 25+ amps, applied when the battery is most depleted. Many cycles of low and slow, solar only recharges make this battery lazy and cumulatively so.
Charge rate is *much* more important with AGM batteries when they are cycled to the 50% range, and generally the higher the better, upto 30 amps per 100Ah of capacity on lesser$ AGMs and no limits on higher$$ AGMS like Lifeline, Northstar and Odyssey. This charge rate negates many of the favorable aspects of AGM batteries, such as the Offgassing issues. Agms also really require being brought to full charge more often than their flooded counterparts, and cost twice as much, negating the 'superbattery' status that so many people assign to AGM's.
With only One battery, I have no real reason for any manual switches or battery isolation method, but no safety net either, other than the fact that I know how much I discharge the battery, and I know I can recharge it properly to keep it happy and performing as well as possible while actually using it hard and heavy. I'd not recommend this method to somebody new to living on battery power, or even someone not new to it, but who does not have an Amp hour counting battery monitor to see nearly exactly how much juice they have extracted from the battery. I am not sure I could really keep it happy without being able to plug in ocassionally, and use my grid powered 40 amp charging source. I would need to drive more than I do.
in all honesty, the simple continuous duty solenoid is the easiest simplest most effective way to have battery isolation. Use at least 8awg wire to and from solenoid for the main contacts, tap an acceptable circuit in the vehicle's fuse panel to activate the solenoid, and basically forget about it.
Check this link for how to set it up:
http://www.expeditionportal.com/for...ke-a-cheap-isolated-dual-battery-setup-for-50
Do note that if one hooks the solenoid directly to alternator(+) stud, rather than to the engine battery as shown in the link above, the charging current to depleted house battery will be significantly higher, and the vehicles voltage regulator likely will allow higher voltages for longer, also speeding battery charging, extending their useful lifespans.
But the alternator has to work harder and will wear out sooner as it gets much much hotter. If the alternator is $$$,$$ and another $$$.$$ to have a mechanic replace it, then it makes little sense to kill it trying to charge a depleted battery better/faster. In my personal case the alternator is a lifetime warranty unit, and I can replace it myself in 15 minutes, So I use fat cabling and max it out often.
When solenoids fail the main contacts usually fuse leading to No battery isolation and the driver usually remains blissfully unaware until both batteries are dead and they hear the dreaded starter 'click'. A voltmeter on each battery, with a voltage sense wire directly on the battery (+), visible from the dashboard can prevent this from occurring, and also give the driver an Idea of where the house battery is, state of charge wise, when driving.
http://www.amazon.com/Nextrox-Digit...d=1463769873&sr=8-1&keywords=3+wire+voltmeter
I've got two of these on my dashboard. But one of them now reads only 0.0v as it is not attached to any battery. Make sure the voltmeter is a 3 wire version, with one of those 3 being the dedicated voltage sense wire one hooks directly to battery (+). You can feed its other power wire (+) to the same circuit you hooked the solenoid trigger to.
Many/most vehicles have a plug under the dashboard that is live only with the engine running, finding it/testing confirming it is the issue.
Hope this was helpful and not confusing.
