Car battery charger

[christopher]

I need to replace my car battery charger. Battery is on 2.5ltr diesel. So 6 amps I assume recommendations appreciated.

Parts

[Andrew Gabriel]

Would help to know why you need to charge it.

The ones often stocked by Aldi and Lidl are frequently recommended here. They're usually £12 verses £40 anywhere else. They charge at 3.8A, which is going to be enough for most purposes.

[christopher]

I need to charge it when a light or the satnav has been left on in error. at that point battery will be flat. So I need it to get me out of trouble

parts

[Andrew Gabriel]

Ah, OK. These chargers are no good for completely flat batteries. (They auto-detect them as 6V batteries, and hence stop charging at around 7V in this case, a few seconds later.)

[Adrian Brentnall]

Wouldn't one of those 'jump start' devices be more useful ? I had some of the Maplin ones (basically a sealed lead-acid battery with a pair of jump leads). Work well enough to start the Moggie Traveller, but might struggle with a larger engine. The charging 'wall-wart' is rubbish, though, and will 'cook' the sla battery if left connected too long.

Other ideas - a 'spare' car battery on a sack trolley with jump leads, or some warning device in the car to remind you to turn 'off' things like satnav & lights ??

Adrian

[christopher]

I can always jump start from wife's car in emergency.

parts

[Dave Plowman (News)]

Most modern battery chargers won't cope with a totally flat battery. It's common to have a relay driven from the battery to connect them - to prevent reverse connection.

[Andrew Gabriel]

I think you'd really struggle with a diesel, unless you are talking of one of the trolly based units used by garages.

Yep - a friend wrote one off in a few days, having plugged it in in the garage and forgotten about it.

An old non-intelligent charger in the 4-8A range would do too.

[chris French]

In message , "Dave Plowman (News)" writes

I've got a Halfords charger - bought mainly cos I needed one, and that was the handiest place to get one. I don't know how flat the batteries it has charged have been. But flat enough to barely light up any lights IIRC. Has been perfectly fine.

Like/similar to this I think.

[Dave Plowman (News)]

Yes - it only needs a few volts at near zero current to get one started. But totally flat is the problem. Of course one that flat may never recover anyway.

[thirty-six]

Took nearly two days to fully charge a 41Ah new car battery (Varta black powerframe) with one of them after the car sat there clicking the solenoid, then not even that for another day. These chargers seem too gentle IMO although with their rating one expects a better performance. I wonder if the slow charge has anything to do with Powerframe design. I'm willing to be surprised.

[Andrew Gabriel]

That would be about right if you did it on the 0.8A setting, which is the alternate charge rate they do.

[Dave Plowman (News)]

I'd guess something up with the battery. My Lidl one will charge a 70 amp.hr battery from effectively flat to starting the car overnight. I've looked at the output with an ammeter, and it maintains the same rate of charge rather better than my Halfords one which claims to be 11 amps. But falls off to lower than the Lidl one about half way through a charge. The Halfords one is huge - unlike the lidl one which will fit in a pocket.

[Bill Wright]

A proper battery charger will start off putting 30A into a flat (but good condition) battery, and taper down from that. When you've got a flat battery you want a charger that puts something like alternator output into it, not a piddling 3A.

Bill

[Dave Plowman (News)]

Depends on how long you charge it for. If you want an instant start better to use a jump start pack. The Lidl one will charge the average battery overnight ok. One which charges at a genuine 30 amps won't be cheap, and very few would need it.

[Bill Wright]

Exactly. Why wait overnight?

If you want an instant start better

The 20Ah ones aren't good enough for a big diesel engine (vehicle or boat) on a freezing morning. No chance. Won't touch it. You either need a jump start from a big battery (or a pair for 24V) or you need a good fast charge. A jump start pack capable of starting a big diesel costs a fortune and weighs a ton. It has wheels.

The Lidl one will charge the average battery

My Sealey one cost £55. 30A at 12V or 15A at 24V. It's just so much better than all the crappy low powered ones. It's red as well, and it makes an impressive drumming noise and a rather nice smell of smouldering insulation.

Incidentally the 'intelligent' charger/psus for boats are excellent. £200 but the batteries last a lot longer. I was getting 3 years from the deep discharge motorhome batteries; now I get five or sometimes six.

Bill

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[Andrew Gabriel]

Providing the car is in a good enough state that you don't have to hang on the starter for too long, you only need to get about 1-2Ah into the battery to get it to start the car. Then the alternator will bang out over 40A if it's that flat, and get it charged. You can certainly do that with a 4A charger quite quickly. You might be able to do it with one of the jumpstarter units used to charge the battery, rather than directly to turn over the engine (same as when you only have thin jump leads that can't carry the starting current).

[Bill Wright]

It doesn't work. The jumpstarter battery will be 13.2V at best when on load and that won't put significant charge into a battery after the first minute or so. The flat battery will build up surface charge in that time so the voltage difference will only be half a volt. That means that given the internal resistance of both batteries and the leads the charge rate will be perhaps 250mA.

You need a charging voltage of about 15.3V to get a reasonable charge into a flat battery.

Bill

[thirty-six]

It was bloody cold.

When the battery is initially below freezing point of water?

Well, this thing, an AutoXS is supposed to be 3.8A . Now although I'd like to use double that, one needs to watch or listen for when the electrolyte is boiling to judge the finish time and sometimes I'm just not available. As the vehicle could be left till the next day, the slower charger seemed acceptable, but even with the cold weather setting (which increases charge voltage) the battery wasn't responding as well as it should. I added a heat pad to the side (alternating after an hour or so) and this seemed to make a difference. The battery still took another day o get to full charge. This charger may not be appropriate for vehicles left outside, other than as a maintenance charger. I'm not confident of its ability to charge cold batteries, although I could discharge a good battery of different construction and leave it to chill before applying the charger, it's not a process I want to do as that is my only spare, which is kept in the boot at the mo'.

[Dave Plowman (News)]

The Lidl one is automatic. For about 13 quid. They were on offer recently, so a store may still have them.