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Forum Index : Microcontroller and PC projects : Nothing to do with uC: measuring parasitic draw from car battery
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thwill Guru  Joined: 16/09/2019 Location: United KingdomPosts: 4036 |
Sorry folks, but one of you will know the answer without having to engage a single brain cell. Can I use a normal hobbyist multimeter to measure the parasitic draw on a car battery, I naively assume I can just connect it in DC current mode in series with the red probe to battery + and black probe to the (otherwise disconnected) positive connector ? Best wishes, Tom Game*Mite, CMM2 Welcome Tape, Creaky old text adventures |
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| Bleep Guru Joined: 09/01/2022 Location: United KingdomPosts: 509 |
Yes, but, the parasitic current should be very low, a few mA or less, however, on my car a Ford, if I do anything, like open a door, or something, the computer seems to power up, which takes several amps. Disconnecting and reconnecting the battery may also do this, so either your meter needs to be able to cope with say 5 to 10 Amps, without blowing up, then you have to wait for the computer to all shut down again, 5 minutes maybe, to take your reading. Or connect a thicker piece of cable between the meter probes/Croc clips, when you connect the meter in series to the battery terminal, this should take that surge current, then remove that extra cable, when things quieten down & take your reading. Obviously, whatever you do, only connect in series with one terminal, preferably the negative, because you are less likely to then accidentally short out the battery, to the chassis, assuming negative chassis, with a bang. Polarity doesn't matter, assuming it is a digital meter, just ignore the - sign, if analogue it will need to be the correct polarity. Take care. Regards Kevin Edited 2024-08-16 06:45 by Bleep |
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| thwill Guru Joined: 16/09/2019 Location: United KingdomPosts: 4036 |
Thanks Kevin, Digital meter with 10A setting. The car battery went very very very flat whilst sat for a couple of weeks ( < 0.5V ), I did charge it back up but it then drained again in 4 days - my understanding is having got to that low a voltage it is probably fundamentally broken now ? Before I buy and potentially ruin a new battery I'd like to know if there is an electric fault (high parasitic current). And if I'm making no sense then it is because I am of the generation which didn't get taught any useful mechanical skills. Best wishes, Tom Edited 2024-08-16 06:51 by thwill Game*Mite, CMM2 Welcome Tape, Creaky old text adventures |
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| Bleep Guru Joined: 09/01/2022 Location: United KingdomPosts: 509 |
10A will be fine, if the battery did drain that flat then yes it's had it, but as you say worth trying to find out why before you get a new battery and kill that too. Usual suspects, a light that's staying on, glove compartment or boot light are favourites, otherwise keep pulling fuses till you find it. Regards Kevin |
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| GrahamJ Newbie  Joined: 12/12/2016 Location: AustraliaPosts: 5 |
A DC clamp on current meter would do the trick without disturbing any wiring. I had a similar problem with a car a couple of years ago but my clamp on meter was only an AC model. The battery would need to have some charge left in it to give meaningful result |
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| GrahamJ Newbie Joined: 12/12/2016 Location: AustraliaPosts: 5 |
A DC clamp on current meter would do the trick without disturbing any wiring. I had a similar problem with a car a couple of years ago but my clamp on meter was only an AC model. The battery would need to have some charge left in it to give meaningful result |
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| GrahamJ Newbie Joined: 12/12/2016 Location: AustraliaPosts: 5 |
A DC clamp on current meter would do the trick without disturbing any wiring. I had a similar problem with a car a couple of years ago but my clamp on meter was only an AC model. The battery would need to have some charge left in it to give meaningful result |
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| DaveJacko Regular Member  Joined: 25/07/2019 Location: United KingdomPosts: 76 |
yes, Tom, interesting thread. This isn't rocket science, you'll soon be up to speed! you're quite right if your lead-acid battery spends any time below 11 volts, it will sulphate, as any old motorcyclist will tell you.. buy new battery ! Also, the 10A range on your meter is quite robust, might even have a fuse, but the milliamp range might go bang if you allow more than a few amps through it. by the way, what is your new toy? regs |
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TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6097 |
A drain of 200mA will draw 67 amp hours out of the battery in 2 weeks. You are looking for something less than that so a multimeter on 10 amp setting will be rather low resolution but still 'adequate'. Bleep is right, there will be a sizable surge current when you first connect the meter so using a low range meter setting is not advisable. This is where clamp meters are useful. Most battery sales people will have battery testers which will quickly tell if the battery is stuffed. Once a battery gets run that flat, it's life is reduced but by how much depends on the battery composition. Since getting the EV, I have to charge the battery on the gas guzzler every 4-6 weeks if it hasn't had a decent run. Jim VK7JH MMedit MMBasic Help |
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| matherp Guru Joined: 11/12/2012 Location: United KingdomPosts: 9109 |
I had a car that was routinely left at an airport car park for extended periods (annual parking permit). I had a high current isolation switch fitted to the battery so that there was no current drain, before that it was almost always flat after a month. Modern cars have so much electronics that residual current drain is a real issue. |
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| TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 6097 |
My 2019 Subaru has a drain of 30-40mA. That measurement is at the limit of my meter accuracy. I estimate that 2 months without recharge and it will be a no-go. The alarm system on my 1998 MG was a lot worse. My tractor can sit in the padock for 12 months... Jim VK7JH MMedit MMBasic Help |
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Rickard5 Guru Joined: 31/03/2022 Location: United StatesPosts: 463 |
Jim my -1976 MG Midget was Dead Reliable, I could always count on the Lucas 3 Position Switch to go from Off to Smoke to Fire. and One Thing you could always count on was it might get you where you're going, but you're Towing it Home! But you Know the SUPER COOL thing about an MG MIDGET and an Austin Healey Sprite is they Fit in the Bed of a 1979 Chevrolet Silverado Pickup :) I may be Vulgar, but , while I'm poor, I'm Industrious, Honest, and trustworthy! I Know my Place |
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| Volhout Guru Joined: 05/03/2018 Location: NetherlandsPosts: 4223 |
Tom, I had the same problem with out 35 year old camper van. The car battery would drain in the few month it was winter storage. After replacing the battery I used this method. I used a simple multimeter with 10A range. Then went to the fuse box, and one-for one took out the fuses and measured with the multimeter the current. When the engine is not running, almost all of them would be 0. But there are few fuses that carry constant voltage. I found that the car radio used 10mA...150mA depending I took the radio front panel off. 10mA is rather harmless, 150mA drains the car battery in 300 hours. To be sure, every time I park for winter time, I take out that fuse. And every spring I put it back in again. For the 10mA, you may have to select another range on the multimeter (200mA typical). No problems since... Volhout Edited 2024-08-16 16:00 by Volhout PicomiteVGA PETSCII ROBOTS |
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| thwill Guru Joined: 16/09/2019 Location: United KingdomPosts: 4036 |
Thanks for the input folks, given my excellent record of cocking up any practical activity other than cooking and decorating (paint, not wallpaper) and the fact that my time is at such a premium I suspect I'll be taking it around the corner to the mechanics but at least I am not going in a complete ignoramus now. New toy ? Do you mean the car? That's not new, it's my wife's old Hyundai i10 which she uses for commuting. Because she is a teacher it is mostly idle during the kids summer holidays. It's not been a problem before, but it's also possible nobody has ever changed its battery which would be 10 years old ... the battery doesn't have a date sticker. Or do you mean the multimeter? This is again not new, it's just that I've had enough sense not to poke it anywhere > 5 V and a few mA before. It's a Dynatool DO3144, the sort that I suspect is rebranded and sold by many different vendors. Best wishes, Tom Edited 2024-08-16 17:29 by thwill Game*Mite, CMM2 Welcome Tape, Creaky old text adventures |
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Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6779 |
I would suggest following Bleep's advice. Use the highest current range that you can *without* having to swap the meter lead to select a high current range (usually 10A or 20A). Then also link across the meter during the initial boot. Then remove the link. You can then switch the meter range down if necessary. If you pop meter fuses they can be inconvenient and expensive to replace. :( Another way, which is very safe, is to simply introduce a 0.1R resistor in the battery lead. Measure the current by measuring the voltage across it. Your meter is safe against all surges then. 1V measured will be 100mA. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| thwill Guru Joined: 16/09/2019 Location: United KingdomPosts: 4036 |
Wouldn't that require a 10+ Watt resistor to avoid melting down during what has been described here as a potential 10A startup surge? P = (I^2)R = (10 * 10) * 0.1 = 10 W Best wishes, Tom Game*Mite, CMM2 Welcome Tape, Creaky old text adventures |
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phil99 Guru Joined: 11/02/2018 Location: AustraliaPosts: 2135 |
The initial surge doesn't last long enough to be an issue. Just charging caps then settles down to something in the 1 or 2 amp range before dropping to standby mA. You can check with the meter on the highest current range first. Your car may not have a big enough surge to matter. If your car is drawing excess idle current you will get an adequate reading. You don't need to know to the nearest µA. |
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| JohnS Guru Joined: 18/11/2011 Location: United KingdomPosts: 3800 |
Tom, Many (most?) cars nowadays shut down over a period of time (when working right), but that time will restart if you disconnect the power. You can instead measure the tiny voltage drop across each fuse. You wait for the shut down, having defeated any door/boot/bonnet locks so you can get at fuses with your meter without waking the modules up. Look at the top of a fuse - you can probe those tiny metal bits. If current is flowing there will be a tiny (mV) voltage drop across the fuse because a fuse is a small value resistor (it wouldn't blow otherwise). There are videos etc about this online if you like them. You can try it with the car awake, so you're bound to have current flowing, to get used to probing. John Edited 2024-08-16 18:29 by JohnS |
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| Mixtel90 Guru Joined: 05/10/2019 Location: United KingdomPosts: 6779 |
If the resistor burns out you've lost a resistor, that's all. The meter is safe because it's on a voltage range. You won't need a physically large one, as Phil says. Mick Zilog Inside! nascom.info for Nascom & Gemini Preliminary MMBasic docs & my PCB designs |
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| JohnS Guru Joined: 18/11/2011 Location: United KingdomPosts: 3800 |
Tom, In case the car has sensitive ultrasonic sensors you may have trouble doing anything in the cabin. John |
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