Which is failing - my car's battery or the charging system?

Using a DVM on voltage range...

1 - Engine running at 2000rpm. Battery voltage should be 14.3V re alternator voltage regulator. 2 - Engine off. Battery voltage should be 12.65-12.45V, 12.24V means 50% charge level & nearing end of life.

Many alarm systems & immobilisers fail progressively over years drawing more current. This is particularly true of factory alarms which interlink with the door locks, you get a drain of 180-420mA which is far above the ideal limit of 30mA (0.03A).

Using a DVM on 2A current range (fuse protected)...

1 - Engine off. Remove negative battery terminal. Clamp one DVM lead to the negative battery terminal. Touch the battery terminal to the battery negative terminal. Press any keyfob to disable the alarm which auto-arms on power up. Clamp the other DVM lead to the negative battery terminal. Remove the battery terminal BUT keep one DVM lead connected to it AND one DVM lead connected to the battery terminal. This slightly long winded process is required to avoid the "alarm auto- arm" and bonnet-open and ECU/Radio powerup initial current surge blowing the 2A fuse. 2 - You can now read the car battery parasitic drain - which is typically 30mA or less. If the figure is higher, such as 123mA that will take most car batteries to 50% capacity within a weak, and in very low temperatures starting may difficult or even impossible, and over time the battery aging will result in "weak battery" at about 2yrs.

One unfortunate problem is the drain may be 30mA, but after some time go higher. A logging DVM ammeter capability can be handy to spot this, suddenly spiking up to 200mA after a while. Door interlock links are notorious, it can be necessary to disconnect any link (usually a plug into a box under the passenger footwell) between the door-lock module and alarm module. The alarm & powerlock remain operational, but the link is removed.

Some alarms on battery reconnect auto-powerlock the doors, flash hazard lights & turn on interior lights so even the 10A DVM current range will not be sufficient hence the need to do step #1 as above. Otherwise you can fry a meter quickly. For this reason the OP may want to pick up a cheap =A35-10 DVM from HK-UK off Ebay. They are quite adequate, just never use them with Mains because the Category rating is usually low or rubbish (=A35-10 meters can and do explode if they carry a mains fault despite HRC fuses, posh ones just explode your wallet).

If the battery is an all-black exide unit the 2009 & 2010 winters have probably stuffed it, it was very brutal on car batteries. You can pick up Bosch Silver (S3 S4 S5?) units on Ebay or online quite cheaply. The generic car shop batteries are mean to last the warranty and a day. A high current drain will kill any battery, as will a defective diode in the alternator or defective alternator, likewise a courtesy door switch in the glove box, boot, door, or moisture condensation causing a low drain in same. Worth giving the door switches a good wiggle every so often, even remove & electrical silicone grease.

For the most basic battery charger, time (hours) is capacity (Ah) divided by charge current (Amps). So a 90Ah battery charged at say 4A needs about a day on charge. You want to get up to about 75%. Of course, the battery may be too damaged. Ctek XS4003 or MXS4.3 or whatever the current model designation is a good smart charger with LEDs to tell you what it is doing and hilariously when it is blindingly obvious the battery is utterly stuffed (12.08V after a charge, stinks of hydrogen sulphide and an on car voltage of 0.00V when asked to light the door lights).

Beware shorting positive to body when working on the battery, things will get hot & spark. Remove watches, watch spanners.

Oh, another cause - bad battery terminal & car-body ground corrosion. A corroded battery negative wire & terminal will reduce charging over time, even creating occasional no-start. The battery terminal clamps with two pozi drive screws are crap, use a proper stud with crimped lug and nut tightened down on it. Very good way of killing a battery... every 1-2yrs and having a variable poor start based on temperature & vibration.

Reply to
js.b1
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Your post reminds me of something I've often wondered about when using a battery charger. Is it possible to damage a battery by leaving it on charge for too long? Should I always do the Ah calculation to determine how long to give it?

Reply to
Gib Bogle

Even the most crude charger will taper the charge rate down as the battery reaches full charge. A recent half decent one will switch off when the battery is fully charged. But they are tolerant devices, and few chargers found in the home will do any harm by leaving on a few hours after the battery is fully charged. So basically an overnight charge should be enough to start a car even with a fairly low battery, but won't do any harm to a near fully charged one.

Lidl and Aldi often have a good little charger on sale for about 13 quid. Big enough to charge a battery overnight, but fully automatic and switches off when it's charged.

Reply to
Dave Plowman (News)

Ah, OK, thanks. I mistakenly thought some completely different battery chemistry was being referred to at first, rather than modified lead-acid.

Reply to
Andy Wade

Ohh, you shouldn't do that to a battery. Normal lead-acids aren't designed to be left in a deep discharged state. Get one of those little photovoltaics and stand it in the car facing south.

Bill

Reply to
Bill Wright

Do the calc first to give you an idea of how long max it should take. Leaving it on charge for days on end with a crude basic charger is what can stuff it. Having said that, the basic crude traditional charger is still bloody useful to pump some charge into a battery when the modern electronic chargers will give up on it.

Reply to
grimly4

Yes. Someone mentioned the Aldi ones, and they are indeed excellent.

One fault I've found is that if the battery is very flat, the charger will auto-detect it as a 6V battery, and think it's fully charged in about 5 seconds when it gets to around 7V, and it's impossible to get past that point. What I did was just connect up a low powered wall- wart PSU to the battery for about a minute which got the battery up to 8 or 9 volts, which is then correctly detected as a 12V battery by the charger, and proper charging commences.

Reply to
Andrew Gabriel

In message , snipped-for-privacy@gmail.com writes

Plus a modern charger may well not charge an absolutely flat battery.

Reply to
hugh

My 2003 Focus 1.6 with no load was 14.8v with low ambient temps.

David

Reply to
David

Mine is a crude basic one, and I fear that I may have caused damage on occasion.

Reply to
Gib Bogle

Along with a dob of vaseline to keep them corrosion free. Check the earth bonding straps from the engine block to the chassis as well.

The OP doesn't say what the actual problem is but assuming poor starting it doesn't take much resistance any where in the high current starter circuit to make things "difficult".

Reply to
Dave Liquorice

Well, if you know you're laying the battery up for a while, you can still utilise a basic charger - just have it timed to come on for

30mins/day or for a longer period limit the charging current to half an amp or so. I found a headlamp filament is a readily available resistor. I've been using both of these techniques for years before I bought some Lidl chargers and the batteries survived well enough.
Reply to
grimly4

What is the quoted output in amps? But these basic units use a simple series resistor to limit the output current to the maximum the charger can supply, so will reduce the output as the battery charges. So if the normal 5 amp or so unit, leaving it on for a few extra hours won't have done any harm. Several days, possibly.

Reply to
Dave Plowman (News)

I presume you have sorted it by now, thanks for informing us BTW.

Someone mentioned checking the terminals for slack, nobody mentioned checking them for salt.

If you have corrosion on the terminals, take them off sand them clean wax them over and refit after a recharge.

A jump start and a brief run out in low gear should sort it. It should get a bang charge from the system enough to get it going after that.

If you ever find yourself stranded in the middle of nowhere with salted battery terminals, a flush with lemonade or urine or whatever you can find, will wash it off. It might then just start itself.

Salted over terminals will insulate the battery and stop it discharging; that is to say: stop it working. Cleaning them usually sorts it.

I don't have to remind you, do I, to take the battery out when you get home after pissing on it?

Or why?

Reply to
Weatherlawyer

Easier to just wipe the crystals off with a file.

With no feed back from the OP on terminal voltage under charge, all we can do is guess the cause.

NT

Reply to
NT

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