Battery Maintenance

Iceweasel

Strider590 wrote: »

^^ The thing with lights/heaters/etc causing battery issues is a bit of a myth, most cars charge the battery at a rate of around 4amps, most modern car alternators are capable of kicking out at least 90amps, none of your electrical equipment drains the battery once the engine is running because the alternator powers everything.

The truth is that lead acid batteries really hate cold temperatures, cold actually reduces the effective capacity, so much so that one of the specs a battery has "CCA" or cold cranking amps, refers to how much current the battery is capable of kicking out at 0 degrees celsius without the voltage dropping below 7.2volts.

This idea that having lights/heaters/wipers on causes battery failures is probably something that spread as gossip from an over simplification used by roadside assistance technicians to help make dumb drivers feel less dumb.

I agree with almost every word of that - BUT - it misses the point that the energy taken from the battery to start the engine cannot be replaced during a short journey of 15 to 20 minutes.

In my earlier post I was not suggesting that Winter journeys with lots of equipment in use would cause a battery to fail - only that it makes it more difficult for the alternator to replace the power used for starting.

coffeehound

Some modern cars don't even begin to charge the battery until the engine is up to temperature in order to reduce emissions.

tedted

coffeehound wrote: »

Some modern cars don't even begin to charge the battery until the engine is up to temperature in order to reduce emissions.

rubbish they all charge straight away

Strider590

Iceweasel wrote: »

I agree with almost every word of that - BUT - it misses the point that the energy taken from the battery to start the engine cannot be replaced during a short journey of 15 to 20 minutes.

In my earlier post I was not suggesting that Winter journeys with lots of equipment in use would cause a battery to fail - only that it makes it more difficult for the alternator to replace the power used for starting.

Well as I said the electrics make no difference because the alternator is capable of far more current than is required for charging and the manufacturer will have spent a lot of money making sure the alternator can handle the job.

Typical alternator is going to kick out 90A +
Headlights = 8A
Tail lights = 4-8A
Ignition coils = 6-12A
Heater blower = 10A
Wipers 15-20A
Electric Windows = 10A (when in use)

That's what 60-70A tops, with at least 20A in reserve for battery charging.

In truth it takes longer to recharge the battery because at low temps the batteries internal resistance will go up, which will reduce charge current and increase charge times.

So yes the battery takes longer to recharge, but no it's not because of the additional loads, it's just easier to explain it that way at the roadside to a customer that doesn't know anything about cars and much like the other common motoring myths (eg VW Golf) these things get passed on by word of mouth until they're widely believed.

I know this sounds rather pedantic and to be fair I would explain it the easiest way if someone asked me in passing, but there's a part of me that really wants to see all these motoring myths die a horrible death, it's these myths that lead to people being ripped off.

coffeehound

tedted wrote: »

rubbish they all charge straight away

I could have sworn I read that on Honestjohn's column but can't find it. Anyway apologies if I've post drivel there.

fred246

Like many things problems are often multi factorial. Ageing battery plus high drain plus low mileage. I have had a Ctek battery charger for years now. I used to use one car so infrequently that would need charging every few weeks. Our main car doesn't get used much during winter. My wife hates revving the engine and drives with the heated front screen on which I know has a 60A fuse on it. If I start the engine in winter and it just starts rather than bursting into life I put in on charge when I get home. If I know a battery is on its last legs I can get it to spring with the charger and then buy a new battery in the sales.

forgotmyname

Paradigm wrote: »

How do they make a "designed to fail sooner" battery?

Stick a maintenance free or sealed for life sticker over the water fill points and then when it runs low it kills the battery.

Where an older battery they were visible and part of the monthly check. If the level dropped slightly then you topped up with distilled water and it lasted longer than one that was not checked and run dry.

molerat

And if you do decide on a new battery price up a 065 or 075. Lucas in fact specify the 065 for a 1.8 Mondeo.

Strider590

fred246 wrote: »

Like many things problems are often multi factorial. Ageing battery plus high drain plus low mileage. I have had a Ctek battery charger for years now. I used to use one car so infrequently that would need charging every few weeks. Our main car doesn't get used much during winter. My wife hates revving the engine and drives with the heated front screen on which I know has a 60A fuse on it. If I start the engine in winter and it just starts rather than bursting into life I put in on charge when I get home. If I know a battery is on its last legs I can get it to spring with the charger and then buy a new battery in the sales.

Me too, an XS3600 which appeared to become faulty and then an MXS 5.0.

I decided to take the 3600 apart to see if I could fix it (which I did), but in the process I realised that unfortunately the clever marketing hype doesn't really match the actual product. In fact it seems to do exactly what any normal charger would do, charge to a set voltage, cut off, wait and charge again if the voltage drops below another set voltage, except ctek call this "pulse charging" as if it's a feature.

This is not really relevant to this thread, but I just thought i'd share this here and immortalise it forever on the WWW, because when I opened this thing up and saw very little in the way of "intelligent" control I felt quite annoyed.

The MXS 5.0 is a different beast and hopefully it's not just marketing hype.

Richard53

I reckon this thing about all the electrical loads draining the battery is a hangover from the days of dynamos. A dynamo can't deliver the current that an alternator can, and often a car would be driving along discharging the battery as it went (hence the +/- ammeters on older cars). A modern alternator is excited-field, which means it regulates its charge according to demand, whereas a dynamo would give a charge proportional to the engine speed. Cruising at 70 on a summer's day an alternator is tricking enough to power the ignition and keep the battery topped up; doing 20 mph on a winter's evening with every electrical load on, and it is pumping out close to its capacity. The difference will be felt in fuel consumption, not battery life.

Of course, no matter how much the alternator throws out, it still needs time to replenish the battery from the charge used to start the car. A good engine that starts quickly will need less time to recharge than one which needs churning on the starter for a time. And, all other things being equal, it's short journeys that kill the battery, not lots of things switched on.

I'm not an auto-electrician, and I'm happy to be corrected, but this is my understanding.