Flat battery

it will likely be ok, if it went absolutely dead then you have probably shortened its life, but short of changing it on spec there is nothing that will help it except being used as designed, ie drive it regularly. A charge with a modern multi stage charger can do no harm.

Don't leave your keys in the ignition with the radio on. Or if you do, don't leave it as long as two days. That'll help.

It probably wasn't. prolonged small discharges like this build up a coating on the battery plates which impede its efficiency without actually discharging it. A quick charge knocks the coating off and, as you have found, you're as good as new.

Ian

If you've got convenient parking then buy a cheap charger & give it a long charge. But if it wasn't seriously frosty in the meantime then it'll probably make it to next winter.

You'll have done minimal damage but half an hour of driving will have put very little charge back into a flat battery. Enough to start the car easily but the battery is still very much discharged rather than fully charged. It takes about 8 hours of driving to fully charge a flat battery. You can't exceed a certain charge rate or the battery gets damaged so the alternator system is designed to limit the amount of current the battery sees. The longer it stays discharged the more damage will occur so put it on a charger asap and charge it overnight or find an excuse to visit someone who lives a very long way away and drive for 8 hours. 10 to 15 hours of charge on a battery charger at 4 amps should replenish it.

What a load of bollocks! the alternator produces at least 45amps, the battery will take as much of that as it can.

Fred >> is 5 years old, but the battery 2. How much damage have I done? Will

Oh dear.....

I suspect the battery would get very hot and soon go pop if you tried to put 45 amps into it.

The true answer is somewhere between the two. The alternator will partially charge the battery very quickly - certainly enough for another start. To fully charge it does take a much longer time. But in practice it doesn't matter if the battery isn't fully charged anyway.

It will yes and boil the electrolyte. A flooded wet cell can tke safely no more than about 1/4 of the AH capacity in amps and only then if you monitor temperatire. A modern low maintenance or gelled cell/AGM will take less than that before gassing.

The preivous posters were correct - you need at least 8, more like double that hours driving to get it close to fully charged or a decent overnight charge at the very least on a 4amp or better rated charger with autoshutoff

The good news is a 2 yr old battery which recovers from a deep discharge will have experienced very little damage

On Mon, 22 Dec 2008 00:42:54 -0000, Fred >> is 5 years old, but the battery 2. How much damage have I done? Will

Which is limited by the voltage at the alternator after the first few seconds to about a 10th of its capacity at 14V.

Well the battery doesn't get particularly hot, provided you don't carry on putting 45A into it once it's charged. But car charging systems raen't that cunning. & if they where then everything else would get upset with the 17V.

"Fred" > very little charge back into a flat battery. Enough to start the car

Well thank you for those profound words of wisdom. A nearly fully charged battery in good condition that has lost only a small amount of charge, say when starting the car that morning, can indeed accept a large current for a short time to replenish what was lost. What you probably don't realise, amongst many other things I suspect, is that a heavily discharged battery has a very high internal resistance and either requires a very high voltage, sometimes over 16 volts, or a lower voltage for a long period of time, before it will even start to accept a decent amount of charge. Anyone who's watched a battery charger trying to recharge a very flat battery will have seen this happening as the charging current starts out very low and then rises as the battery becomes able to accept a higher charge rate.

A constant voltage alternator system producing about 14V is not very good at dealing with a situation like this. In the first half hour of driving the alternator might well have put only a minimal amount of charge back into the battery because the battery is not yet ready to accept a decent charge rate. Some alternator systems won't recharge a very heavily discharged battery at all. Even if the battery does accept a decent charge the current flow will taper off as the battery voltage rises towards the charging voltage and it takes a long time to get the last 10% to 20% of capacity restored compared to the first 50% to 60%.

In general, with the bulk charging stage not rising above a current of 20% to 25% of the battery's ampere hour rating it takes at least 4 hours of bulk charge at constant current plus another couple of hours of absorption charge at constant higher voltage to fully recharge a flat battery. A car alternator system will take longer still because it can't increase the voltage above its preset level in the absorption phase. That's why I said 8 hours as a decent guesstimate of what an average alternator system will take to fully recharge an average flat battery, assuming it will start charging it at all.

Alternators are not designed for recharging flat batteries. They are designed for keeping good batteries topped up. That's why you should always fully charge a battery before fitting it to your car or recharge a flat battery on a charger before driving again. Trying to recharge a flat battery with the alternator can overheat and damage both the alternator and the battery.

Finally, no matter how much current you chuck at a discharged battery it still takes time for the chemical process to take place which recharges it. If the charging current is too high the electrolyte can't permeate the plates fast enough to accept that level of charge. Even if the plates don't overheat and buckle, which they probably will, the electrolyte needs time to mix with the plates and turn back into higher concentration sulphuric acid. You appear to think (I use the term loosely of course) that if you divide the ampere hour capacity of the battery by the maximum possible current output of the alternator you arrive at the time it takes to recharge it. That misses out so many factors I'm not even going to try and explain it to you.

Now do have a happy Christmas of course and try to remember not to run with scissors again. You know what happened last year!

Bought a car that had been standing for months with a completely flat battery. A booster got it started and drove 50 miles back home but was still completely flat when I tried restart. Noticed the alternator didn't even try to charge the batt if it's too flat. My 3 phase charger also didn't want to know. An old basic 4amp charger worked but went off the scale and blew the bridge rectifier, but put enough juice into it for the modern charger to recognise it and start charging.

It hasn't been deep discharged, or anywhere near it. It will recover after a very short charge.

Ian

Any car battery allowed to go completly flat has been deep discharged

There is every chance it will recover depending how long it was allowed to stay in that condition

Not sure how much a generlisation this is, but worth posting for comments. Can't find the url I gor it from. Might have been buried in

somewhere. There is a table here which seems to tie in close to these voltages State of Charge Voltage (measure after standing 12 hours or so)

12.7+ = 100% 12.45 = 75% - And as low as you ever want to go with a starter battery! 12.25 = 50% - Deep Cycle batteries only, or damage! 12.10 = 25% damage... 11.95 = less than 5% remaining - Discharged and not well.

At 10% discharge you get thousands of cycles.

At 50% discharge you get hundreds of cycles from a deep cycle battery and tens of cycles from starter batteries.

At 80% discharge, you get 300 cycles from GOOD deep cycle batteries but 20 or so from starter batteries.

At 95% discharge, You get only get a tens of cycles good deep cycles. You get one or two if you are lucky with starter batteries.

This car battery - the OP's - has NOT been allowed to go completely flat. See my previous posting on the subject: a car battery subjected to a small drain over time may appear to be flat when it is, in fact, still almost fully charged.

Ian

[...]

The latest Ford Smartcharge system is pretty cunning...

Amongst other things, it monitors the battery temperature.

Chris

I collected a micra with a very recent but completely flat battery, I jumped it and drove it home, but it still had no go at all in the battery, despite the alternator working fine (which surprised me), I drove it for another half an hour, still nothing (I was beginning to think the new battery had been ruined), went and did some more running about for anothe hour or so and suddenly it was fine, starting just as it should, gave it an overnight, multi stage charge and it tested fine and AFAIK is still going well some months later.