Batery charging - brain fade day?

[David]

Looking further at battery charging I seem to be having brain fade. I will work it through in simplistic terms then ask!

Starting with (volts * amps = watts).

250v * 4A = 1kW

Or to put it another way, on a 240v system a kilowatt gives you 4 amps of power.

So a 1kW generator would power an appliance up to 4A. A 2kW generator would power up to 8A and so on.

However (and thinking of a 50 amp alternator):

1A * 12v = 12W 50A * 12v = 600W (or 0.6 of a kilowatt).

This Supercharger this I was looking at claims an alternator output of 50 amps so presumably the power output is equivalent to a 600 watt generator.

The proposition is that this chucks 50 amps into your batteries and thus is a very fast charger.

Now if I look at mains powered chargers they only seem to charge at around

8 amps at 12V.

Leaving aside conversion losses between 240v and 12v this seems to require (8 * 12) 96 watts of power - or slightly less than an old style light bulb.

Given that the Supercharger claims to push 50A into the batteries, why isn't the mains charger capable of pushing in a similar charge?

The 600 watts calculated above doesn't seem to be much of a load. Half that of a fan heater. Conversion the other way from 12v to 240v using an inverter is claimed to be a lot higher in some cases - easily up to 2kW. I do note that battery chargers seem to have very thin leads to the battery.

This is what is confusing me - if an alternator can chuck in 50 watts and an inverter can suck out 50 watts why can't a battery charger chuck in 50 watts?

Cheers

Dave R

[Adrian]

Because it doesn't really need to. If somebody's chucking a battery on charge, they're usually quite happy to leave it overnight.

That and cost to buy the charger.

50A mains-12v car battery charger :- (needs 230v/33A supply, btw)

This'll plug into a 13A socket, but is only 25A charging...

charger-engine-starter

Or there's > Leaving aside conversion losses between 240v and 12v

I think that might be the problem.

[Bob Minchin]

It can. I'm confused as to why you are confused?

[Adrian]

ITHM 50A.

[Dave Plowman (News)]

A car alternator isn't designed to charge at its maximum output all the way to a fully charged battery. It is designed to run the car electric system at within that operating voltage range. To charge a near fully charged battery at high amperage would require a voltage so high it would likely fry the car's electrics. Or shorten their life.

What it will do is charge a flat battery quickly to the point where it will re-start the car. The other half charge or so will take very much longer.

A basic battery charger will do much the same - high charge initially, then taper off to a lower one.

A smart charger will hopefully give the best of both worlds.

I have a large and heavy Halfords charger which says 11 amps on it.

I also have the tiny Lidl one which will just about fit in your pocket. Both take about the same time to fully charge a 'flat' battery, despite the Lidl one only claiming 3.8 amps.

You can buy fast battery chargers. They tend to be big heavy and very expensive. May include a probe to prevent overheating the battery.

[Andrew Gabriel]

*current* rather than power.

Yes for DC. AC (such as the mains) adds another issue for non-resistive loads. We say a generator can support up to 4VA or 8VA in your examples above, which will usually be less than 4A or 8A is the load is not resistive. Motors and compact fluorescents are examples of non- resistive loads.

Yes. My first car was a 1973 mini, and its standard fit Lucus alternator could supply 47A max. Modern cars can be much higher.

Also losses in the charging efficiency - Lead Acid batteries don't manage to store all the energy you put in. Some is lost as heat, and some as electrolysis particularly towards the end of charging.

(An interesting aside is that Lithium batteries are actually very efficient at storing what you put in. Many can be charged at high currents without significant heating, at least until nearing end of life - indeed increasing heating up whilst charging is one of the signs of end of life, and something that chargers have to protect against because overheating causes them to change into a lighted firework which can't be extinguished.)

You can buy ones which can. They usually come mounted on an integral sack trolley. Not so practical for keeping the the cupboard for occasional use. ;-) They require much thicker leads and chunkier crock clips (or better connectors for frequent use, such as recharging electric vehicles).

However, it's not necessary. 5 mins with a 4A charger is enough to start the car providing nothing else is wrong. You only need to get enough in to do one start, and then the car alternator will take over and charge the battery much faster if it's very low.

(I presume you mean 500W?)

It could, and they do exist. However, they aren't needed for mass-market car battery chargers, where you are just trying to get 1-start's worth of charge into a flat battery.

[Andrew Gabriel]

Can't believe I wrote that - meant 1kVA or 2kVA

[David]

Yep - told you my brain was fried.

I thought I'd been through and found all the places where I'd swapped watts and amps, but missed that one.

[David]

This started with the Supercharger (linked in another thread) which is basically a 50A car alternator driven by a small 4 stroke engine. This puts the output directly into the 12v battery system.

The application is to recharge habitation batteries in caravans, motor homes and boats - claimed to be much faster than traditional suitcase generators so you don't have to run the generator overnight.

If 240v is available the motor home has a built in charger.

This generator/alternator is to maintain habitation batteries when the vehicle is static for several days away from the mains.

The publicity claims are that this alternator outputs 50 amps at 12v (calculated in the OP to be roughly 600 watts) and a 2 kilowatt Honda generator only puts out 8.3 amps at 12v so is much slower to charge.

So the selling point is that this charger is about six times more effective at fast charging batteries than the Honda.

The point that was puzzling me was why a 2 kilowatt generator was only putting out 8.3 amps at 12v.

The obvious route was to look at 240v battery chargers to chuck more of the 2 kW at the battery. This, however, doesn't look compact or cheap.

Ignoring the fact that a charge controlled alternator doesn't chuck out full power all the time, this solution does seem to potentially offer a faster charge time. However, how much faster is not clear.

Cheers

Dave R

[Dave Plowman (News)]

I'm surprised it was that big. The one fitted to my '74 Rover P6 3500S was pathetic at about 40 amps. Smaller than the 45 amp one on an earlier version of the same car. It couldn't actually balance the full load of main beam headlights, heater, heated rear window and wipers.

[Dave Plowman (News)]

It wouldn't be difficult to fit a specially designed regulator to a basic car alternator if its purpose is to charge a battery. But might be tricky if it's also designed to run car electrics.

[tabbypurr]

If PF=1, yes

Gennies are rated in kVA rather than kW

Car alternators charge at around 14.4v, so nearer 720W.

No, much of that goes to run the car. The battery only takes some of it when charging.

high cost, shortens battery life, no need.

NT

[Andrew Gabriel]

So was I - indeed I didn't believe it and bought a 30-0-30A ammeter to add to the car, thinking a 60-0-60 one would be rather low resolution. It could hammer the 30A one hard on the end-stop as soon as the engine started. Even at tickover, it was 27A output, and ISTR you only needed increase the revs by a few hundred RPM to get to the max output.

It did surprise me. Dad had Citroen GS of the same age which we regarded as a more luxury car, but had some much older Dubilier device which was a crude alternator but with external dynamo regulator relay circuit which needed careful mechanical adjustment rather than a proper alternator semiconductor controller, and produced nowhere near the same output as the Lucus ones.

[Adrian]

Standard alternator on my '80 Landy was 36A. But, being Lucas (bless 'em), there's only about two fitments - so a 45A from a Jag or later Landy dropped straight in. IIRC 65A was a similar drop-in.

A quick google suggests the Mini only ditched the dynamo finally in '72, so I rather suspect somebody had already done that swap-over on yours.

Indeed. G alternators were 30A. That was not unusual at all in that era.

OTOH, the alternator in my mid '90s Shogun was 100A.

[Adrian]

And 90A in my late '80s VW camper.

[Bill Wright]

Yes but that's just from the 'auxilliary' 12V output on the genny. At the same time the genny is producing 1.6kW at 240VAC. The reason these gennys have this 12V aux supply is because they use a 12V DC alternator which powers a built-in true sine 240VAC inverter. The 12V aux supply is just a tap from that. In other words, it costs nowt to include it, as long as it's limited to around 100W.

See above.

Intelligent chargers aren't cheap, but they are the only way you will charge a battery fully in a reasonably short time.

Bill

[Bill Wright]

I agree with all of this.

Bill

[Bill Wright]

My Bedford van had a 22A dynamo. In winter when the trip to work was with lights and wipers on the battery had to be trickle charged overnight.

Bill

[Cursitor Doom]

And of course most people don't know the difference. I'm a bit rusty myself, to be honest. Let's see... IIRC, KVA only equals KW at DC or in the case of AC it's supplying a non- reactive load. In practice, most real-world loads will be at least partly reactive causing the power factor to vary between zero and 1. This is the difference between true power and apparent power. Your true power is stated as KW and your apparent power is KVA. Did I get that right? Sure I've probably missed something important out, but it's been a long time since I studied this subject..

[Andrew Gabriel]

Well, that would tally given it was a 1973 model (PKX 886M).