Switching large currents

I have a current leak on my car that I can't trace. I've tried taking all the fuses out one by one to identify the circuit causing the problem but whatever I do, the vehicle is draining 50mA 24/7 when it's sat parked up. This wouldn't be a problem if I used it daily (or even every few days) but I don't. Sometimes a week or more goes by between trips out in it and then I often find the battery's been too drained to start it up. I first thought about an isolation switch, but the cold cranking current is 700A or so and finding a well-made and long-lasting switch that can handle that is going to be very expensive. So, would it be feasible to use a triac or diac instead? The 700A is only for a few seconds during start-up and the regular quiescent current during normal operation will be *much* less, obviously. Thoughts?

Reply to
Chris
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Bad regulator or alternator. ...Jim Thompson

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| James E.Thompson                                 |    mens     | 
| Analog Innovations                               |     et      | 
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Reply to
Jim Thompson

There may be more than one fuse block. (sometimes in the engine compartment) Or inline fusible links.. do you have a schematic for the car? That will help.

If you can't find it other solutions. Disconnect the battery, buy a solar battery charger to keep it topped up. (I know band aids and not fixes.)

George H.

Reply to
George Herold

Den tirsdag den 27. september 2016 kl. 21.50.01 UTC+2 skrev Chris:

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Reply to
Lasse Langwadt Christensen

First thought is to get a float charger. Harbor Freight often has them on sale. I use one with my lawn tractor. A float charger will give you a lo t longer battery life. My lawn tractor battery is 4 years old and it still starts the engine easily. With your intermittent use , a float battery ch arger will save you money and solve your starting problem

Dan.

P.S. I have some humungous SCR's. Will send you one if you wont one, but not sure how you are going to shut off the SCR.

Reply to
dcaster

I'd have thought the OP would have eliminated those possibilities before posting here. Anyway, there's one thing (at least) he's forgotten. Is it really desirable to *fully* isolate the battery? Presumably one would not want to have to keep re-setting the clock every time you take the car out! And what about the alarm system? Seems to me, the solution is not quite as simple as it may appear.

Reply to
Cursitor Doom

I was going to suggest something like that. You should be able to waltz into just about any auto parts store and ask for one.

Your clock will lose time, your radio will forget its channels, and your engine control module may need to reinitialize. So unless it's an _old_ car you may prefer to find the root cause.

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Tim Wescott 
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Reply to
Tim Wescott

They have a minimum current. A humongous one probably has a higher minimum current than a teeny one. If it's reliably higher than 50mA, he'll be made in the shade.

Or, he can have a momentary switch that shorts around the SCR when things are supposed to be off.

Or, he can go to an auto parts store and buy a battery cutoff switch.

Or, he can just take the cable off when he parks the car.

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Tim Wescott 
Control systems, embedded software and circuit design 
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Reply to
Tim Wescott

or something like this:

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A new car might just have so much junk on standby that 50mA is the limit though it sounds like a lot

-Lasse

Reply to
Lasse Langwadt Christensen

Typically the starter motor has an always-energized wire running right to it, that is switched by contacts operated by the solenoid that engages the pinion with the ring gear. It's unlikely your leak is in the starter motor, so you could just interrupt everything but the wire to the starter. That would still leave the window heaters, power steering (if electric), power roof etc. which might add up, but nothing like the starter.

If you want a mechanical switch, check out RV/marine battery master switches. They are designed to do what you need and more- carry large current continously, and huge current for maybe 10 seconds while cranking, but not to interrupt a lot of current.

Eg. Marinco 701, which is $18.xx at Amazon:

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275A continous/1250A cranking.

There are also devices that mount right on the battery post and are probably at least suitable for a car with a smaller engine. $5-10, most likely a larger one too.

Another solution is to buy a battery trickle charger which is good if you're leaving any car over a few months (and you don't want to disconnect the batter). Something like $23.

HOWEVER- 50mA really doesn't sound like enough though- maybe your battery is shot. For example, a Bosch S5 has 77Ah capacity so your

50mA would take a couple months to drain it. A week shouldn't even be noticable.

--sp

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Spehro Pefhany 
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Reply to
Spehro Pefhany

One point, 50mA for two weeks is only 17Ah. That's not enough to drain a good battery. Maybe you do need a new battery.

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 Thanks, 
    - Win
Reply to
Winfield Hill

If it's a late model (especially post 2010) car, there's a constant drain keeping all the electronics alive. The procedure for replacing the battery recommends a "battery tender" (basically another battery or charger) so the electronics won't lose power. If you don't use such a device, you will often need a reset code for the radio and navigation system (presets clear, drops into anti-theft non-working mode). You may also need a code to enable the drive train (anti-theft again) or the transmission won't shift to drive and the throttle won't raise the engine speed.

Get a trickle charger - there's often an OEM branded one that the dealer will happily install for customers what leave their cars off for extended periods. Or you can install one yourself.

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Grizzly H.
Reply to
mixed nuts

...

50 mA is fairly normal for a modern car with key loss locking, OnStar or (equivalent) and a security system.

What car is it?

kevin

Reply to
kevin93

I had that problem once. Turned out to be the lamp in the trunk lid wasn't turning off when it was closed. I found it when when I unplugged the wiring harness under the dash that went to the rear lights and stuff. I was monitoring the leakage current while checking things out. The thought just now occurred to me that that measuring the leakage resistance might have been instructive. Also connecting some 120vac would have cleared up the problem real fast. It was my MIL's car.

Reply to
gray_wolf

On 9/27/2016 6:15 PM, Spehro Pefhany wrote: [snip]

A lamp in a trunk lid that wasn't turning off when it was closed was draining A new battery over night. I seem to recall that the current drain was about

50ma but my memory maybe may be incorrect from 20 years ago.
Reply to
gray_wolf

The alternator is the most common source in a situation like this and it's usually on a fusible link not readily recognized as a fuse and bolted in place.

Reply to
bloggs.fredbloggs.fred

An incandescent trunk light bulb would be something like 5-10W, so

0.5-1A, at least an order of magnitude higher.

--sp

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Spehro Pefhany 
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Reply to
Spehro Pefhany

How are you "identify(ing) the circuit causing the problem"? When you remove a fuse, do you reinsert your ammeter INTO the fuse socket/receptacle to verify that there is *NO* load on that circuit while the fuse is removed? Vs. a 50mA load??

(I.e., LEAVE THAT FUSE OUT for the balance of the test -- there is nothing ACTIVE on that circuit so you risk nothing being "unpowered"!)

Eventually, you will be left with *one* fuse. If removing *it* still leaves a drain on the battery, then there's (obviously) another path of which you are unaware.

Are you sure its a 50mA *DC* load? (i.e., how are you coming up with that figure)

How old is the vehicle?

How large (AHr) is the battery? Does it make sense for it to be "discharged" when taxed with a 50mA load for 168 hours (1 wk)?

Find the problem. Who's to say you won't have a *500* mA drain next month?? (Newer cars tend to have various amounts of kit that is "necessary" for the vehicle to "sit still")

If you just want to ignore it for now, put a cheap float charger on it (a dinky little "12V" supply behind a resistor)

Reply to
Don Y

50mA over a week is still only about 8 amp hours. If that's enough to stop the car starting, I'd suggest you need a new battery, or that the battery isn't being fully charged when you drive (alternator issue).

Sylvia.

Reply to
Sylvia Else

I have had a similar problem in my 1989 Toyota 4WD pickup. I use it only rarely, and sometimes a couple months go by without using it. I tried to read the current draw of the system and it blew the 200 mA fuse in my cheap DMM. There are probably some fairly large capacitors in the system that may draw a large surge current. I should try it with the 10.00 amp range, which is unfused, and it should detect 50-100 mA accurately enough.

My solution was to leave a battery charger hooked up to it, and that has done the job. The battery is probably somewhat damaged from having gone flat a few times, but it seems to be OK driving it and starting up again after sitting a few hours - probably a few days would also be OK. Just to be safe I usually carry my jump starter box with me. I have had a battery fail (in my car) so completely that after getting a jump start and allowing the engine to run a few minutes, when I touched the brakes the voltage drop killed the engine. I got a new battery in September 2010 and it has been fine since then - probably time for a new one - 6 years is pretty good.

You can get a 600 amp IGBT for $20-$30 or so. 12 VDC should be enough for the gate. But if you put it on the (+) side of the battery to the rest of the 12V circuit, you will need an isolated 12V gate supply.

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There is usually a separate (+) lead from the battery to the fuse block and another heavier cable to the starter. So you can probably get by with a switch rated at 60 amps or so. If it is leaky diodes in the alternator, you might be able to put a switch in that line.

Paul

Reply to
P E Schoen

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