car battery trickle chargers

[Tom]

Hi All,

I've been aiming to buy a new car battery trickle charger for a while

- I've noticed that there are some very small trickle chargers that are 500mA output and looks like a plug in power block.

Then I found I had an old adaptor from a defunct jump start unit - the jump start was a yellow Rolson model for jump starting 12v car batteries - but the charger that came with it is, strangely enough, labelled 15v 500mA.

A subsequent jump starter (same yellow model, different branding) had a charger that was labelled 12v. Both these jump starters have gone west and I no longer have them.

Therefore could I use this little 15v/500mA charger as a trickle charger for my car battery, like the block trickle charges I've seen? If so I think all I would need is a couple of large crocodile clips to put on the lead - but for charging purposes which goes on the red positive battery terminal ?

I have a feeling that the charger's negative goes on the battery's positive terminal and the charger's positive goes on the battery's negative terminal. Is this right?

Thanks, Tom

[Andrew Mawson]

while

seen?

No! negative to negative and positive to positive or your wall wart will be history pdq

AWEM

[Andrew Gabriel]

Many of these wall warts do not contain any voltage regulation and will kill a lead acid battery if connected for significantly longer than the time to charge it. There may be a clue in the original instructions for the jump-starter, if it says something like don't charge for more than 24h (although absence of such a statement is no guarantee as the voltage regulation may be in the part you're chucked out).

A friend accidently left a jumpstarter charging for a week on one of these, and that was the end of the battery. I have the same model, but I used a proper voltage regulated charger and not the wall wart which came with it, and some 6 or 7 years later, mine still works fine.

[Ian Jackson]

In message , Andrew Gabriel writes

There's no way that a 500mA wall wart could ever deliver enough current to kill a car battery.

The important thing would be to check what the charging current actually was (flat battery and full battery), and see if there was enough / not too little to be of use / not too much so that it would kill the wall wart.

[Andy Dingley]

It can do it if it's connected in reverse to an already-flat battery.

[Andrew Gabriel]

Current, no. Charge, yes, if there's no voltage regulation. It just takes longer at 500mA.

That's another issue.

[<me9]

I recently killed a battery by substituting an AC wall wart for the genuine article. Another family member had shuffled the warts and I didn't spot it in time. I now attempt to label all warts with their original use. It may save other expensive mistakes.

[Ian Jackson]

In message , [e-mail] writes

A car battery?

[Dave Plowman (News)]

They are an expensive way of doing a simple job.

Keep an eye on Lidl and Aldi. They do a proper charger - 4 amps or so - which switches to a maintenance mode when the battery is charged. Cost about 13 quid. They are on sale every few months.

[Dave Plowman (News)]

Whether the same charger would kill a 'wet' and much larger car battery in the same time is open to debate...

[Tinkerer]

Sounds similar to my "Winfield" from Woolworths. Well over forty years old, choice of 6 or 12 volt, High or Low charge and still works as well as ever.

[Dave Plowman (News)]

Nothing like that. It is a sophisticated SMPS device which monitors the actual battery voltage, and changes from charging to a maintenance charge automatically so can be left on indefinitely. Other thing is although the quoted output seems modest, it delivers that all the time the battery is being charged (until it switches). Unlike many which claim a much higher maximum but drop to a low figure quickly.

[Andrew Gabriel]

and at the end of that period, I got a couple for a fiver. They are indeed very good.

Just 2 slight gripes...

If the battery is very flat, it assumes it's a 6V one, and because it will instantly rise to the fully charged voltage for a 6V battery, it won't get very far. You can get around this by connecting another

12V battery (or some other 12V power source) in parallel just for a few seconds. (OTOH, if a 12V battery is down to 6V, your chances of charging it again are not too high anyway.)

It would be brilliant if it had some way to make it start charging automatically after a power failure. You could then use it to build a very cheap but effective backup power unit.

[Dave Plowman (News)]

I agree - but suspect if would then draw current if left connected with no mains to it. Maybe not much - but undesirable over a long period.

[Tinkerer]

The old Woolies one has an ammeter on the front so you can momitor it. When the battery is flat it bangs out the full 4 amps but as the charge level in the battery increases this drops until it is half an amp or less. I assume that is some form of what you call a maintenance charge. I have to say that for a cheapo Woolies purchase it does seem to be very good and certainly has longevity (better than Woolies in fact).

[Dave Plowman (News)]

Typical of a basic charger with no regulation. I have a similar Halfords one which quotes 11 amps peak - whatever that means. It actually takes longer to fully charge a battery than the 4 amp Lidl one.

No. The amount of current used when a charger is connected for a long time

- like say over winter or whatever - has to be carefully controlled. Yours would be fine to leave on overnight just to make sure the battery was fresh the next day.

Chargers of this type are pretty simple devices and no real reason why they shouldn't last. However, the Lidl one is small enough to fit in a pocket. Light in weight too.

[Tinkerer]

Thanks for the explanations. It makes very good sense.

Here is my version of your tag line that I have used for years (and lived up to come to think of it): "I'm not as thunk as drinkle peep I am"

[Harry Bloomfield]

Not all all the same thing. Your chargers output drops simply because the voltage difference becomes less. The Lidl ones charge rate drops, because and only because the battery is fully charged. Your charger will eventually damage the battery, the Lidl type one will not cause damage even if left on permanently. The latest Lidle unit is really rather good - I bought two a month or two back.

[Harry Bloomfield]

Andrew Gabriel formulated the question :

I built and designed for myself a rather fancy heavy duty automatic pulse charger. That will not start to charge unless the battery voltage is within limits, to get around it I modified it by fitting an override button to force the charge to start.

Really that is its only disadvantage, but a fairly common one on this type of charger. I have another similar charger which does not need a button to be pressed - its disadvantage is that if there is a power cut, the charger will draw power from the battery and discharge it - I use this one for my bike. Fine if it just a short power outage, but there have been occasions when I have accidently tripped the socket circuit in the garage and next time I go in a week later, the bike battery is completely flat.

[Dave Plowman (News)]

Yes. The Lidl one clicks when you switch it on, so I'd guess it has a relay and diode to prevent damage by wrong connection etc.

I have one permanently installed in the old car and can confirm it draws no current whatsoever from the battery when not charging.