Cons of a Solenoid Dual Battery Isolator

[-Scott-]

Hi Scott, correct me here if I’m wrong but I think it doesn’t matter if the cable is V90 but all calculations must be based on the cable having a worst case rating of V75.

That's my understanding - it doesn't matter if the manufacturer rates it as V90, it's supposed to be "treated" as V75.

Your 8Ga current rating is just about on the money but it’s surprising how many outlets are marketing 8B&S cable as 100 amp rated cable, even when it’s twin sheathed 8B&S.

I had a quick look at my collection, and found a mob in the US who rate their kapton teflon silicone insulated 8ga wire (max insulation temperature 200 deg C) to 100A - in free air at 30 deg C. Derating factor for a bundle of 2 = 0.8!

[Bluey]

either i'm getting confused or something is wrong with my solenoid/setup

i replaced my battery isolataing solenoid about 1-2 weeks ago as it didn't seem to be working. it would click on/off, but if you measured the voltage on the terminals they were different when solenoid was one. main battery reading charging volts (13.8 plus) while second was around 12 - 12.5 (not being charged). i even measured the contact resistance (leads off) and suitably low (can't remeber, will check with dmm)

last night measure both batteries with solenoid on and off, the voltage on the second battery doesn't change when the soleniod is on

possible cause:
1. i'm measuring with a small panel mount voltmeter, using the body as ground and 3 position switch to select from main batt, off and second batt. so it might be a meter problem, will check with dmm
2. solenoid is buggered
3. umm, don't know


any ideas would be appreciated, i'm sick to death of battery isolators.

[drivesafe]

Hi Bluey, it also could be a loose connection.

Anyway a quick and easy way to check if it’s the solenoid is to bridge the two battery terminals on the isolator.

If it’s the isolator, the auxiliary battery voltage will rise immediately.

If there is no rise in auxiliary battery voltage then the problem is elsewhere.

Cheers.

[Bluey]

thanks drivesafe, that was what i was thinking

measuring the secondary battery voltage with meter:
no change when apply 12v to solenoid small terminal to switch on
voltage rises straight away when i short across big terminals

so that means another dead solenoid. this solenoid is less than 2 weeks ago, only one off road trip with couple of muddy holes. how temperamental are these things to water, only thing i can think of

[Bluey]

pulled one of the solenoids apart. abr one with all terminals on the top

the terminals were corroded, very green looking. so i fully pulled apart and cleaned the contact faces with some fine wet and dry paper, now is nice and shiny. put back together and getting a contact resistance of less than 1ohm (starts at 1, then steadily drops to 0.6 - 0.7).

so i installed it (again) and now getting correct operation of the solenoid. now i have to work out how to keep it dry. mmmm

[stolper]

Actually, that's no longer the case. There's an outfit in the states that uses MOSFET arrays for high-current switching and isolating...up to 700amps continuous current with no heat-sinking and no airflow.

One more point, mosfets have been used for years before this thing came along and anybody with any real experience with automotive electronics knows only full well that they do not work well in standard battery isolators..

[bushy555]

I've got about 20 of these babies. 100 and 200 amp mosfets. Each are around the $500 to $600 brand new as listed in the 2007 Farnell components catalogue. (so, if they were new, Id have 12 odd grand sitting on the back verandah. Sheesh!)

...all coming from variable speed drives, where MOSFETs mostly live.
I'd love just to be able to use them (or flog them off just to get rid of them) as a dual or triple battery controller, or alternatively could attempt to make up a simple pulse width variable speed controller for a winch.





As far as a "safe" battery system that wont let you down when your out touring (also known as going over board), I reckon dual alternators, each charging their own batteries. One alternator charging a battery specfically for starting. Nothing else connected. The other alternator charging dual batteries which can be switched to either or both, and can then also be switched to the starting circuit for emergency starting. Triple LED voltage meter's on the dash would look pretty cool to some people and pretty wanky to others.
Throw in a modified GMC $99 generator as well. (Modified, as in throw away the 600w 240v generator and bolt on a 120 alternator)

Its the way that I'd go if I could ever be bothered to do it.
However, is this sort of a dumb concept? Or would it work ok?

[danos]

G'day people

I have finally finished this project. Thanks for your help

http://www.outerlimits4x4.com/viewtopic.php?t=137480