Thread: Doesn't hold standby power if I crank the car right away

Yep got me thinking the thing I paid $90 for was a waste..
Although the solenoid I have should still work as the relay and should handle cranking and winching if I desired. And I could still use the battery monitor so it's not all a loss..
Just might skip on it's "brains"

What do you guys think of this one?
http://cgi.ebay.com/ebaymotors/ws/eB...6926&viewitem=

I am not really up on the current transfer between the batteries when charging but I am going to guess it would be ample for what you want..
I assume you are not going to be using it to jump the two batteries or use it for powering winches..
Both of those tasks can require many times the amperage rating of that isolator but even those ones should be able to take those tasks for short periods of time if required..

But I am a sucker for overkill

No only trying to provide seperate battery for standby current of computer. Currently i use the m4-atx... which says 250w. My battery is a 17ah sealed lead acid battery that is basically half of the RBC7 battery replacement pack.
http://www.americanbatterycompany.co.../apc/rbc7.html

Right now my isolator is rated at 80 amps so anything over that i am assuming should work.

If you already have an isolator why not use that?

His is a MOSFET type; presumably fully integrated (unlike yours where you can separate the relay).


antimatter - thanks for NOT linking another voltage-sensing isolator.... (so many do, and then ask my opinion - which only goes to show how too much verbiage misses its (my!) mark!)
Not only that, but you picked one for continuous use - ie, not a starter solenoid (relay) or similar which is for short-term use. Yay!


I just wonder if it is overkill - ie too big. (And that I think its a re-branded relay anyhow...).
But if you did use it, I'd suggest a buffer relay between the charge lamp (circuit) and that relay's coil - ie the charge lamp circuit powers a relay whose coil is (say) 250mA (ie, 48 Ohm, equivalent to a 3W bulb) and that relay switched +12V to the bigger relay(s)... Bigger relays (aka solenoids) can have coils (aka solenoids LOL) that take 1-2A and maybe more.... (I think my vehicle's original starter relay takes 10A - hence in part why they are not continuous (ie, 12V x 10A = 120W)


Trying to find specs for your ABC 17AH battery, the max I found was a charging current of 6.4A which is well above the specs for my battery which is the usual 10% of its Capacity (aka 0.1C), hence ~4A for my 38AH UPS AGM battery. (But as stretch knows, it can take 45A for a short while (tens of seconds). And it usually drops to under 10A within 1 minute; and some AGM specs may say 0.2C = 8A...)


But using 250W for the M4-ATX, 250/10 = 25A. Add 6A for the battery and a 30A relay might suffice.
But assume 250W is output power and it may go below 12V input (hmmm - not really if the front is charging - it should be over 13V...), and higher recharge current....
The point is - would a 60A relay suffice?
And 60A relays are reasonably common and with similar coil currents as 30A relays - whereas that 100A relay looks like a heavier coil...

Maybe have it like my circuit - a 60A relay (AUD$9) with two weatherproof 50A self resetting circuit breakers (2 x AUD$7)?

Let's see - 50A continuous means (say) 40A for the M4 and 10A recharge... (40A for M4 @ 6V - 6x40 = 240W in; assume 240x0.8 = 192W out; at 8V = 8x40 = 320W in =~256W out... that sounds ok...) Not that the battery will be recharging at many Amps at such a low voltage... (it'll be discharging, but lets not confuse the facts - this is just design parameters...)

But normally (say) 14V for the M4 => 250W/0.8/14 = 22A in; is, 25A. That leaves (50-25=) 25A for your battery to draw - and I doubt it will do that (ie, mine is also a UPS battery but is 2x as big with a lower ESR and it seems to max at 45A).


Even if you do exceed the breaker's 50A rating things should be fine. EG - at 60A they might take well over 1 minute to trip by which time the battery should have dropped high initial charge current.
And if they do trip, they will reconnect (usually with a few seconds; maybe 20 secs max for a BIG overload (ie, short circuit!) and each time that battery recharge current should reduce. Should. (It depends on its drain etc, but if the M4 is under half the breaker rating....)


You might even try a normal 30A relay but that risks contact fusing etc, and since that'd need 30A fuses which are likely to blow (even if not the relay) unless you get 2x30A breakers - but then it's better "safe" to start with and get the 50A breakers with 60A relay.
BTW - 50A breakers because that is the largest economical self-resetting breaker...
eg:
(Courtesy of NA Auto)

- that's what I have, plus this relay....


(from Jaycar - their Horn Relay - 60A SPDT. Note that despite the horny name, it is continuously rated, and it uses the ~9.4mm (3/8"?) spade terminals for the power connections; normal 6.3mm (1/4") for its coil.)

{ Verbiage! Wot verbiage? }

What a wealth of information... thanks for taking the time to break down my situation. I really appreciate it.

I originally looked at 60 amp relays because they were the most cost effective. Thanks for linking what you use.

Not that it is necessarily the solution to your problem, but I suspect it is, or is part of the solution.


BTW - both of you (antimatter & stretch4x4) should keep your existing isolators - ie, do not discard at a cheap price.
Besides, they may come in handy as a battery protector - aka Low Voltage Cut-Out (which is the same thing as a battery isolator - a high-voltage connect is a low-voltage disconnect, though the voltage settings will vary).

Originally Posted by OldSpark

Not that it is necessarily the solution to your problem, but I suspect it is, or is part of the solution.


BTW - both of you (antimatter & stretch4x4) should keep your existing isolators - ie, do not discard at a cheap price.
Besides, they may come in handy as a battery protector - aka Low Voltage Cut-Out (which is the same thing as a battery isolator - a high-voltage connect is a low-voltage disconnect, though the voltage settings will vary).

The reason I am pretty sure that it is part of the issue is because for a short period of time during crank I watched the battery voltage go down to about 7 - 8 volts. This would surely cause problems with the M4-ATX because of the 5vsb cutout setting. Stock is set to 11.24 as the lowest voltage that it will hold the standby power.

The other part of the issue I am sure is a weird thing with the M4-ATX where the standby power would drop when switched on and off rapidly on the initial start up. Since changing to the IGN instead of ACC I haven't seen that issue (though another more annoying issue has arose and I don't think it is related to switching IGN sources) Ill think I start a new thread for that.