Battery Or Alternator Problem

[MehmetPerdeci]

Hello. I did not drive my MGF all winter long. Maybe 6 months. After 6 months of course battery was empty and took the battery and charged it at service and then replaced in the car. Everything was good since 20 days until today. Today I realized that battery is empty again. I did not drive it only 4 days. I tried to get power from another car with the cable but even indicator lights were not illuminate. Battery is not charging or charging very slow. My question is that how can I understand that battery is dead or alternator is not charging the battery. Is there a way to find it? Thank you.

[Chris Tideswell]

I would think that the battery is dead, modern batteries do not like to be completely discharged and never seem to fully recover. However it would be wise to get hold of a multimeter to measure the voltage of the battery and alternator when the car is running to make sure that the alternator is working.

[Rob Bell]

I think that there are two possibilities here:
1. The battery has been sulphonated due to the discharge (going flat) or:
2. There is a current discharge that is flattening (an already weak) battery.

The battery could be reclaimed with a battery conditioner - which has worked wonders on my MGF (N7), which the car keeping charge for weeks on end. But it's only done this once I identified the cable breaks in the wiring loom to the boot lid.

Genuinely, the boot loom is a complete menace: it causes battery discharges, false car alarms, failure to dead lock, even fires! So I'd recommend getting in there, remove the tape binding and carefully inspect the wires. The individual wires may just need binding in fresh electrical tape, or you may need to solder/crimp-connector wires back together and re-insulate, but you'll be glad you went to the trouble. I know I was!

[Stan_B]

I keep both my MGs on battery conditioners. Before you could get them the MGB was a pain. On trickle off trickle still always flat when you wanted to use it. The fractured wiring in the boot in my TF was where the boot loom Ts off from the main loom near the nearside boot hinge. I remade them with bullet connectors and didn't recover the loom. So the individual wires could flex. It's been fine since.

[Mike63]

Seems like it might be a common problem so I will post here. My latest experience has been a completely wiped flat 60Ah battery in 30 days and less than a year's use. One mistake by me, poor charge balance and some unidentified software bug.

My mistake: knocking the mirror light on. It doesn't show in daylight and I let the battery go flat before I noticed. Past damage voltage, 8V or so. The problem with going beneath about 11V is that the weakest cell has probably completely discharged and is now reverse biassed, and this causes damage. There are six cells at nominal 2V per battery = 12V.

Charge balance: this is the OEM deisgn term for a car's ability to maintain some battery charge in use. For example, it's winter, it's snowing, you are stuck in a jam, at night: you have the lights on, the wipers going, the heater on full, the ventillation fan on full on the screen and the rear demist on. How long will your battery last? It might surprise you that the answer is on idle not indefinitely, maybe as little as 30 mins. The car relies on the unlikely nature of that situation: you are probably creeping forward with higher revs sometimes. Today's smart cars will raise the idle revs when they detect a low battery.

I'm not convinced the MGF falls into this category. Measuing the Key-off current with a recharged battery, residual current is 0.36A for 10 minutes. To find the source I pulled fuses till it stopped, but it is a pattern of fuses. The key source is blade Fuse #7 under the bonnet, described as "Stop lights/horn" but also runs at least the fuel pump. Maybe someone knows what gets left on? The effect is that even if you just key-on/off, you lose 0.06Ah, much worse if you also started the car to move it back 6ft, say to open the garage. The start is draining, then my 2001 puts that up a gear with electic PAS, very high currents.

There is an additional 170mA for the ECU to do housekeeping for 30s, which is acceptable for the era. It is storing self learning on fuel trim calibration and the like. This was flowing through Fuse #2 'Engine controls' under the bonnet.

Residual current after 10mins was 35mA, about 25mA of which was my hifi. In theory a car battery can cope with this for a long time, 2 months with a fully charged 60Ah battery. But it doesn't work out like this. Your car was unlikely to be fully charged when you parked. Then a rule of thumb is lead-acid self discharge about 1% a day, AGM (advanced glass matt) better, but this means 0.6Ah first day or pretty much the same as the 35mA residual current for 24 hours. So 2 months becomes 1 month, then you had the lights on when you parked it, and moved it up the drive a couple of times, now 3 weeks later and you're flat. Now you know why.

What can you do? Well the easiest is to drive it an hour monthly. If you have put a battery on the bench you observe that it takes a really high current for 1min (30A), reasonable for 20min (10A) then trickle for hours. So you need at least 30 mins to put back a good state of charge. In winter, use 3rd for the first 10 mins when you'd normally use 4th, after which it won't help because you will already be at 14.4V charging.

A viable option is to charge your battery monthly with a typical charger. What I use is a float charger, only 1A but set at 13.8V. You can leave this connected indefinitely. On my classic Morris and motorbike I just plug in and move a few days later. If you don't want to splash out on an Optimate then some cheap LED or 15V supplies have a voltage adjuster and can be made to suit. The key point is the unloaded voltage is regulated.

Note: if you find you have a completely dead battery, when you charge or replace it charged, the alarm will sound. Fix this by opening the driver's door using the key, then repeatedly press the alarm remote, should be 4x, maybe more for the rolling codes to re-synchronise.

[11-GT-NH]

It all comes down to the quality of the battery, i`ve put a yellow top in my F, the purity of the lead is very high !
13.6 V is not enough to charge a battery to 100%. 14,4 or 14,5 is ideal.
Car alarm, clock, radio draw some current but with a fully charged battery it should last for a long time depending
on the temperature...

[Helsbyman]

A most interesting read. My TF has an alternator which makes 115 A/hr not the 85 as standard but it only does 5 mls on Monday, nil Tuesday, 5 Wednesday and Friday and nil on Thursday,Saturday and Sunday and I don't have any trouble I do use a Ctek charger once a Month/6weeks

[RobboMC]

A great reminder of a Nissan turbo I had a while back. Left the dome light on one night and got a flat battery. Road service came and started car, said to idle it a bit to charge battery. Idled for 1/2 hour. Flat battery again.

Road service came and started car again, said to drive it a bit to charge battery. Drove to shops, flat battery.

Road service came and started car 3rd time, said to take it to auto electrician for test, tested fine and perfect, said to drive it a long way to charge battery.

Car had running: engine management computer, high intensity ignition, electric fuel pump, headlights, radar detector ( legal here in those days ), and usually heating fan. We reckon on some trips we were actually discharging the battery with normal driving, then parking it with the alarm activated.

In the end we fitted a much larger battery.

[Mike63]

I don't think it really is down to quality of battery. It is true that some technologies and additives make the battery more robust, AGM, calcium, gel et al, if your battery goes below about 10V then at least one cell is likely reverse biassed and suffering damage. Graph attached is for classic liquid acid batteries but shows the problem. If your battery is constantly bouncing off zero SoC state of charge then it will not live as long.

My float charge at 13.6V is designed to be left attached. Currently it is on a relative's motorbike and will stay there for months. This is why it is not 100% SoC. If it were 100% at 14.4V as per most alternators, then when the ambient temperature rose by a few degrees then the excess energy would have to go somewhere: absorbed by the battery, gas emissions (liquid battery), self heating etc. Float chargers typically run at max 13.8V.

[Forrester]

I use a battery immobilisor which you can get for £6 off Ebay.
After using the car, I use the immobilisor to stop any battery drain, so it always starts ok next time.
I don't bother locking the car [on drive] as there is nothing of value inside & the bonnet can't be opened without opening the boot with key.

[Mike63]

I don't think immobiliser is the right term but we know what you mean. The nearest I find is this:
http://www.ebay.co.uk/itm/12V-5A-Lead-a ... 2023799999
which I'm going to speculate is 'inspired' by Optimate.

Not locking is an interesting topic of itself. There's an argument not to lock the doors on a soft top car based on the cost of the hood, and another to lock it because the doors can be deadlocked i.e.: you can't open them even after you can reach the inside lever. I think the engine immobiliser works independently via RFID on the keyfob but I may be wrong on this.