Car battery during lockdown

forgotmyname

Lots of people buying battery chargers when its more likely the battery needs replacing. Just commented on a couple of forums
that even the best charger is unlikely to recover a battery that only shows 11volts after driving for 1 hour.

If the voltage drops after a decent drive then spend the money on a new battery not a jump pack or charger.

TychoTMA

This issue will vary from car to car. Most cars continuously drain the battery because the security system takes a small amount of power. Some cars take power for other functions, and in any case batteries self-discharge to some degree. Starting  a car takes a good deal of power out the battery. The amount of power the alternator produces depends quite a lot on the speed of the engine, and isn't that great at tick-over speed. I attached my charger to my Toyota yesterday and was quite surprised at how long it took to get the battery up to full charge (about 4 hours). I just cannot believe that anyone would be "too scared to even try it".
If you go in to Halfords, they may be pricier than eBay, but they should be able to walk you out to your car and show you what to do. Bonnet up, and 30 seconds later, you will be charging.

forgotmyname

Are you thinking of dynamo's?  They produced very little if any charge at tickover. Modern alternators put out plenty
of charge at tickover.

Mine puts out enough charge at tickover to run over 100amps of electrical items and still puts a charge into the battery
and that is from a cold start on a frosty morning.  Heated front and rear screens, heater blower, DRL's, heated seats
and the NAV/AC unit.

AdrianC

TychoTMA said:

The amount of power the alternator produces depends quite a lot on the speed of the engine, and isn't that great at tick-over speed. I attached my charger to my Toyota yesterday and was quite surprised at how long it took to get the battery up to full charge (about 4 hours).

Most domestic battery chargers are something like 5-6A output. A typical modern alternator is 100A output.
A car battery is often 70-80+Ah. 

So if you have a totally flat battery (you won't - it'd be irretrievably internally damaged if it was), it'll take a charger ~16hrs to charge it, but the alternator less than an hour.

TychoTMA

One of the issues here is mentioned in the Table above regarding "the battery having a significantly higher voltage until the surface charge decays". Can you really charge a "flat" battery  to full charge from an alternator in an hour - I suspect that may not happen, not at idle. Greater the charge rate, greater the surface charge. For a 60AH battery battery to be charged in an hour from 50% discharge would take an average current of 30amps. What voltage do you have apply to a battery to achieve 30amps over an hour? Most conventional batteries would not like that. The "silver" batteries being used by Ford (mainly) might. Why is it that the bench charging rate typically recommended by manufacturers for a 60AH battery is a measly 4A?. That's not much compared with an alternator, even at idle, but that's the bench rate. I'm not sure about the "car idling on the drive for long periods" rate; don't think I've ever seen that quoted.

Of course, using the alternator to charge the battery makes the significant assumption that the car will start in the first place, especially while the weather remains rather quite cool. If it doesn't start, this discussion becomes completely academic and you will wish you had a charger! 

AdrianC

TychoTMA said:

Of course, using the alternator to charge the battery makes the significant assumption that the car will start in the first place, especially while the weather remains rather quite cool. If it doesn't start, this discussion becomes completely academic and you will wish you had a charger! 

Jump leads - or jump box.

Superscrooge

Last year I was working away from home for three weeks at a time, so frequently my car wasn't used for three weeks.

I never had a problem with a flat battery, even though my car is six years old and still has the original battery.

Unless your car already has a dodgy battery, it shouldn't go flat if unused for a few weeks. 
  

REJP

Leaving your engine running for a while on the drive does not charge your battery, but will annoy the heck out of your neighbours as well as adding to pollution.
I use a small battery charger as suggested by other forumites, also have a solar powered panel which keeps the battery topped up on sunny days.  Make sure you connect positive to positive and negative to negative though.  If you are not technically minded, positive is red terminal or marked with a "+" sign, negative with a "-" sign.
my neighbour who is a garage mechanic told me that to replace the power used by starting the car I should drive at least 10 miles.  May or may not be true, I don't know.
 So use a charger if you can connect to the car, otherwise if you are strong enough bring the battery in to a place with a power point and recharge.   Do this in a well ventilated space though.  And when you reconnect the battery, make sure you connect it correctly or you will cause damage to electrics or battery.   Some car batteries are designed with terminals that will only accept the correct size connectors to avoid this problem.  Remember always + to +, - to - and be safe.

REJP

chrisw said:

AdrianC said:

ripplyuk said:

twhitehousescat said:

get a small car battery charger , inteligent sort (in lidl last week for £11) , and on a sunny day connect it , if your car is garaged you can leave comnnected indefinatly 

That’s far too technical for me. I posted before about battery issues and that was recommended but I’d be too scared to even try it.

Literally the only thing you need to know is that red = +ve, black = -ve. That's it.

You don't even need to know that. Red clip to red wire, black clip to an unpainted bolt or bit of metal.

I thought that was for jump starting the car, not charging the battery?

AdrianC

REJP said:

chrisw said:

You don't even need to know that. Red clip to red wire, black clip to an unpainted bolt or bit of metal.

I thought that was for jump starting the car, not charging the battery?

Either. -ve terminal and body/mechanicals are all electrically connected anyway. Connecting -ve to block rather than battery -ve may help jump-start if the block-body earth strap is poor and high-resistance, but apart from that it makes no difference. The internal resistance in the jump leads is going to be far higher, especially cheap thin ones. Just go with whatever's most convenient.