Battery problem, or...?
Discussion
My car is a gen 4 Subaru Legacy, although I think is is more a generic battery behaviour question.
I've had a bit of trouble with the engine going into limp mode over the last few months, maybe once every 2 or 3 weeks, it's been in to be looked at, but nothing pinned down. Anyway, one of the possible causes was mentioned as being voltage drops, which could confuse the ECU.
And, while at the garage being looked at, the car wouldn't start the last morning it was there, a recharge and it was fine, no previous sign of the battery struggling according to the mechanic. He also said they'd tested it and it was fine (and "quite new"... although my car is a recent JDM import with zero service history, so who knows).
I bought one of those ciggie lighter voltmeters to keep an eye... generally it's been alright, reports 12.5v before turning on, usually sitting at about 13.4 - 6v with the engine running. Unplugged it a while back as no change, no obvious voltage drops, including when the engine hit limp mode (although if that was the cause, it may well have been too brief to see).
I have a dashcam hardwired to a voltage sensing kit, which is permanently live, but cuts the current if the voltage falls too far... usually it's OK left over a weekend, but from time to time (and a little more frequently lately), I hear it come back on when I start the car, suggesting the voltage has fallen while parked over night (or during the day... between commutes, anyway).
The car has always started fine though. (Touch wood!)
The drive to work is about 30 minutes, so the battery should always recharge OK.
This morning, not only was the camera off until I started up, the radio had put itself into demo mode as well, which suggests the voltage had dropped a long way. The car started as easily as normal though! I plugged the voltmeter back in, it was showing 14v for a lot of the run to work - it hasn't gone that high before, so I guess the battery was taking more charge than usual. It had settled back to 13.7ish when I got there, the battery was only showing 11.5 once the engine was off. At home time, I had 11.3v and fine starting, less high charging voltage as well (13.7ish going down a few points).
Then we went out for a meal tonight, battery was now showing 12.3v and charging about 13.5v - that is, back to normal more or less.
So, is it a battery problem, time to sling a new one in, or do I have some sort of strange, very occasional, parasitic drain? Is there a known problem on my car? (I'm thinking anything like the hedgehog on BMWs, which I've had to do on 2 separate cars)... actually, on that tack, the ONLY thing I can think of that I've done differently of late is using the HVAC in full auto to get it to blast cold air at me... I usually have it in windscreen+feet manual mode (still with AC on)... so it may well have been left in that mode when switched off, not that it should make any difference...
Thanks in advance
I've had a bit of trouble with the engine going into limp mode over the last few months, maybe once every 2 or 3 weeks, it's been in to be looked at, but nothing pinned down. Anyway, one of the possible causes was mentioned as being voltage drops, which could confuse the ECU.
And, while at the garage being looked at, the car wouldn't start the last morning it was there, a recharge and it was fine, no previous sign of the battery struggling according to the mechanic. He also said they'd tested it and it was fine (and "quite new"... although my car is a recent JDM import with zero service history, so who knows).
I bought one of those ciggie lighter voltmeters to keep an eye... generally it's been alright, reports 12.5v before turning on, usually sitting at about 13.4 - 6v with the engine running. Unplugged it a while back as no change, no obvious voltage drops, including when the engine hit limp mode (although if that was the cause, it may well have been too brief to see).
I have a dashcam hardwired to a voltage sensing kit, which is permanently live, but cuts the current if the voltage falls too far... usually it's OK left over a weekend, but from time to time (and a little more frequently lately), I hear it come back on when I start the car, suggesting the voltage has fallen while parked over night (or during the day... between commutes, anyway).
The car has always started fine though. (Touch wood!)
The drive to work is about 30 minutes, so the battery should always recharge OK.
This morning, not only was the camera off until I started up, the radio had put itself into demo mode as well, which suggests the voltage had dropped a long way. The car started as easily as normal though! I plugged the voltmeter back in, it was showing 14v for a lot of the run to work - it hasn't gone that high before, so I guess the battery was taking more charge than usual. It had settled back to 13.7ish when I got there, the battery was only showing 11.5 once the engine was off. At home time, I had 11.3v and fine starting, less high charging voltage as well (13.7ish going down a few points).
Then we went out for a meal tonight, battery was now showing 12.3v and charging about 13.5v - that is, back to normal more or less.
So, is it a battery problem, time to sling a new one in, or do I have some sort of strange, very occasional, parasitic drain? Is there a known problem on my car? (I'm thinking anything like the hedgehog on BMWs, which I've had to do on 2 separate cars)... actually, on that tack, the ONLY thing I can think of that I've done differently of late is using the HVAC in full auto to get it to blast cold air at me... I usually have it in windscreen+feet manual mode (still with AC on)... so it may well have been left in that mode when switched off, not that it should make any difference...
Thanks in advance
GreenV8S said:
It takes a long time for a small load to flatten a full battery down below 12V, and a long time to charge it back up again. I wonder whether the battery voltage is really swinging up and down like that, or whether you have an electrical fault somewhere which is causing a voltage drop. For example an earthing fault could affect multiple systems, or a corroded fuse holder could just affect a few.
I suggest attaching a volt meter directly across the battery terminals next time you see a low voltage, and see whether that reading is accurate.
If it really is discharging so quickly, you must have quite a big current drain.
The battery might not be the root cause of the problem, but being deep discharged repeatedly won't be doing it any favours. I suggest getting to the bottom of the current leak / charging problem before you replace it if possible, but start thinking about replacing it.
When it was in the garage, one of the main things they did was check all the earths they could as there's a Subaru technical bulletin which says my problem can be bad earths... reported all good.I suggest attaching a volt meter directly across the battery terminals next time you see a low voltage, and see whether that reading is accurate.
If it really is discharging so quickly, you must have quite a big current drain.
The battery might not be the root cause of the problem, but being deep discharged repeatedly won't be doing it any favours. I suggest getting to the bottom of the current leak / charging problem before you replace it if possible, but start thinking about replacing it.
But in the meantime, I'll pop my multimeter in the car and start taking direct readings, good call.
It's not quite as urgent as it might be as I already have a lithium battery booster pack in the boot from when my auto BMW played up (hedgehog, of course), so I'm unlikely to be stranded by the problem... but my HU has so many options, it's a PITA putting them all back in when it resets so I don't want that to happen too much! Also, I'm planning to install a pair of amps and new speakers in the next week or so (if the weather holds), I'd like to get this sorted one way or another before I add the extra load!
Used my multimeter a few times today, the ciggie lighter meter is under-reading by 0.5v compared to straight across the terminals, which is about what I'd expect and that seems to be constant. Battery is sitting at 14v with the engine running, dropping to 12.3v once switched off, probably to 12 or just below by morning.
I'm still perplexed as the battery seems to be dropping volts but retaining plenty of amps... I'm wondering if it has just one dodgy cell??? Maybe a loose, but not completely failed, connection to one (set of) plate(s)? Is that possible?
I'm still perplexed as the battery seems to be dropping volts but retaining plenty of amps... I'm wondering if it has just one dodgy cell??? Maybe a loose, but not completely failed, connection to one (set of) plate(s)? Is that possible?
phillpot said:
Mignon said:
I'm not. The battery is f**ked.
/\ /\ /\ /\ This.... there's nothing mystical going on, just buy a new battery. 
As the battery is only slightly f
ked so far, I've ordered a DC clamp meter to investigate possible current drains before buy a new battery (and then killing that too...)GreenV8S said:
I don't know how accurate your one is, but ones I've used aren't accurate at low currents. You may be better off putting a meter in series with the main battery cable. The way to do that is to disconnect the main battery cable, connect a temporary wire between the end of the cable and its battery post so that power is restored, connect the leads of the ammeter to the main power cable and battery terminal, then disconnect the temporary wire so that current goes via the meter. This way the meter doesn't have to carry the inrush current that will flow when you make the battery connection.
I've ordered one of these, less than 30 quid of Amazon. Looks like it should be OK within the limits of not moving it around too much and frequent re-zeroing. My car is a bit of a PITA in that there's a rather extended sequence to run through for the ECU to re-learn the engine if you want it to run properly after disconnecting the battery so I'd rather try inductive measurement first...Gassing Station | Home Mechanics | Top of Page | What's New | My Stuff