Battery Isolator

[MarcS]

There are a lot of different ways to control relays, and some of them work really well. But they still aren't isolators. Relays only have 2 modes connected and disconnected. That's just symantecs though, no big deal.

I still like to use a continious duty solenoid, but that's my opinion. There are many ways to do this. Disconnected is isolated. I'm wiring mine with a 3 position switch (DPDT). 1st position, just the main battery (Stock setup). 2nd position, batteries are combined when the ignition is turned on. Both batteries charge while running, one isolated when ignition is off (Auto-Mode). 3rd is a manual jump start. The voltage to control the manual jump is the only connection to the isolated battery (Other than the battery cable). The voltage to control it in Auto-Mode comes from a switched source. This is pretty much how the Painless battery combiner kit works.

No matter how you do it, or what system you use, leave a couple extra inches of battery cable. This way if you have to by-pass your combining system, you can just move the cable to the lug on the other side of the combiner.

 

[Dallin]

I completely agree with you MarcS. Disconnected is isolated, but isolated isn't necessarily disconnected. This is why a solenoid is a connector or combiner but not an isolator. Solenoids can only flip between parallel and isolated/disconnected. A diode isolator always isolates both batteries.

I have used and like the solenoid setup, but like every option they aren't perfect. If you leave the key in the run position with the solenoid set to auto-mode you can completely drain both batteries. A diode isolator would protect against that. If one battery has an internal short or a bad cell the other battery will try to drain into it when they are combined. Again an isolator would protect against that. These are both uncommon situations and often a risk worth taking. Good diode isolators can be expensive, solenoids are pretty cheap. There is always a trade off. No matter what you choose the extra battery cable is a good suggestion. I'll remember to add that into my next build.

 

[MarcS]

Yeah Dallin, your right. The diodes would block the current from flowing backwards if there is a internal short or dead cell, the solenoid wouldn't. I had problems with the diode setup on my boat, but that is a single wire alternator, so I had no voltage regulator compensation. That's where I got my preference for the solenoid. Still thinking boat I guess.

 

[ntsqd]

snip.......
3rd is a manual jump start.

Caution here, most of the constant duty soleniods used for this application can not pass a lot of current. 100-150 amps max. is what I recall.

When we did the dual battery system on MISF's Scout we used a diode trio for isolation and a Ford type starter solenoid for combining for winching or self jump-start uses. Ideally the starter relay would have a PWM circuit to maintain it's closed position, but we're not that sophisticated.

 

[CyberSniper]

It'd cost a lot of money to do PWM because of the EMP that comes back along the signal wire.

Most of the starting solenoids use high coil currents in order to snap the contacts closed fast enough to avoid erosion. The continuous duty solenoids usually have a much lower coil current but the contacts close slow (and not very much of a gap) so contact erosion is much higher.

 

[ntsqd]

KiloVac supplied a PWM circuit with their high voltage/current relay (500 VDC+, 1000 amps+). Don't know how much it cost, but it was purchased on a college student project's budget.

Still, it's not really a common option.

 

[dontoe]

I've used the same isolator for over ten years and never a dead battery or a problem. It's great for forgetful people like me!

 

[MarcS]

Caution here, most of the constant duty soleniods used for this application can not pass a lot of current. 100-150 amps max. is what I recall.

When we did the dual battery system on MISF's Scout we used a diode trio for isolation and a Ford type starter solenoid for combining for winching or self jump-start uses. Ideally the starter relay would have a PWM circuit to maintain it's closed position, but we're not that sophisticated.

No worry there. It's a 400 Amp solenoid I got at work. The EMP that's created when the coil is de-energized is be controlled with a diode. Again, at work, I have a huge diode selection on current and PIV ratings. You might be able to use a Varistor/MOV instead of a diode. We use them all the time on contactors I work with, but they use 120 VAC on their coils. Not sure with DC. Anybody know ???