Might an extra 13.5mA be enough to drain a battery in cold weather?

Calling all mechanics/auto electricians.

Our car often won’t start on cold winter mornings, mainly after being stood for a few days. Now the battery has been tested previously and was deemed to be OK, but after today’s episode I had a brainwave and disconnected the trailer lights relay that I installed straight to the battery +ve terminal block. This was drawing a steady 13.5mA, which doesn’t seem much to me (and it’s a small diesel engine, so has a reasonable capacity battery) but might this be enough to tip it over the non-starting edge over a few days in cold weather?? Thanks.

Sounds like battery is **** or the alternators no charging

Do you do short runs ? Diesel engine glow plugs take a big current to start and dont like short runs

Mine was doing this due to the stereo being incorrectly wired, was a feed to the electric aerial, could drain the battery dead within 3 days, but I hadn’t noticed straight away as car was used every day.
when did the problem start?
is it just in cold weather?

Alternator boots out plenty, (unless there’s an intermittant fault, but I think then it would happen in the summer too)

I don’t do short runs in it, I have a bike for that but, yes it does and I have already had that discussion. This may well be a contributing factor, but well you can’t go out and get your hair all messed up by your helmet and …… 🙄

You sure that the relay only takes 13.5mA? This makes it a 1000 ohm coil, which is V unusual – they’re normally around the 40-60ohms, drawing 2-400mA.

Should still not drain the battery though.

Try a few days of charging the battery overnight and see if that gives a few weeks running, or get a new battery, as yours does sound knackered

Muddoger, Yep, problem is just in cold weather. I was thinking of a similar problem you yours, which is why I disconnected the trailer relay and measured the draw though it, but 13.5mA doesn’t seen much. This is the only non-standard electrical item on the car.

Large418, I’m not quite following, I’m sure .2 to .4 amps would definitely flatten the battery overnight. The relay I’m talking about is one I fitted in the boot which gets a signal from the rear light clusters and powers the trailer electrics.

Got a multimeter ?

Static across the terminals should be aboue 12.6volts or 6×1.2volts for a healthy battery or 6x what ever the weakest cell is.

Once started should be about 14.4volts mines was 9 volts so knew alt was ****

Oh and how olds the battery ?

I wouldn’t say that would be enough on it’s own to cause non-start after a few days. It only works out at 0.3Ah every day, which for a diesel battery which will be in the 60Ah+ size, is a parasitic load at worst.

What car/engine is it?
When it doesn’t start, does it still crank, or even attempt to start?

Also, do you know what kind of tester they used on the battery?
Ye Olde drop tester (either two big leads connected to a big metal box, or ye even older style U-shaped spikes with gauge), or a new style digital tester (two smallish leads connected a box of electronic wizardry)

Yep, I used it to measure the power to the trailer relay and the alternator output. Battery voltage doesn’t tell the whole story, unless a cell’s completely gone. Our local motor factor used their proper tester to check it out properly, though that was last cold snap it was OK then.

edit – I’m hoping to convince myself there isn’t an underlying problem before splashing for a new battery.

in cold weather the voltage of your battery will drop which means it ability to dump cranking amps will be massively impaired. i forget what the reading is at zero degrees but you will only have about 85% power iirc. If your battery was abit poorer to start with then this is th eproblem
Any decent automotive facto will be able to test your battery
FWIW your battery will probably be about 75-100 amps for a normal sized car so loosing about 13.5 milliamp wont make much difference over two days
say 100 hours is still less than 1.5 amps so it wont really drain it unless you leave it for a months IMHO

mc – digital tester, but certainly wouldn’t hurt to have it checked again. It’s a Punto 1.3 turbo diesel. The starting effort varies from turning over slowly and intermittently to just the starter motor relay chattering if it’s really flat.

Thanks for the science bit, and you Junkyard, seems to confirm my suspicions. I think I will get a new battery and test the whole parasitic load afterwards. Battery is probably about 5 years old.

Those engines don’t like weak batteries (I could explain the geekery behind it, but I can’t be bothered typing that much!), so I’d get it tested again (the digital testers are highly accurate if used properly, and will pick up a suspect battery long before ye olde drop testers will).

Although you could try leaving the trailer control disconnected and see what happens, but at 5 year old, the battery is most likely starting to die.

(I could explain the geekery behind it, but I can’t be bothered typing that much!)

Fair play, you’ve helped no end. 😀

I am going to leave the trailer lights relay disconnected, ‘cos I don’t think I’ll be using it any time soon, plus I’ll get the benefit of starting the ‘why don’t my trailer lights work’ thread when I’ve forgotten in a few months.

I discovered the reverse camera screen in my van on standby would drain the battery especially when cold.

Moisture in the starter also caused similar symptons but that was vehicle specific.

Moisture in the starter also caused similar symptons but that was vehicle specific

Front wheel drive ford of some kind?

I may have missed it, but has anybody suggested disconnecting the main lead from the battery to check what the total load is on it when the car is sitting there? You might find there’s something else funny in your wiring causing a high current drain.

I’d put my money on your battery being shot though – the original one with my car went after 5-6 years.

I may have missed it, but has anybody suggested disconnecting the main lead from the battery to check what the total load is on it when the car is sitting there?

I’ve come to that conclusion myself, but I think I’ll get a new battery anyhow and still try this to make sure that the problem doesn’t reoccur over the winter.