Battery Charger V/Ah

Your post doesn't make a whole lot of sense.

You need more than 12.4V to charge a 12V battery. If you want to limit the current, use an LM317 wired as a constant current source, choose the resistors accordingly to set 800mA.

On Thu, 17 Apr 2008 20:01:08 -0700, Robert LaCasse declaimed the following in alt.scooter:

HOW did you test? Just hooking a DVM across the leads will not give reasonable readings.

A 12V lead-acid battery tends to need >13V to charge, with 14V being a common value.

Put a fully charged battery on the other end -- likely the back pressure will reduce the actual current flowing to nil (if not actually drain the battery due to the low voltage of the charger)

.8A for 8Ah implies you are aiming for a 10hour charge rate... if the battery were drained...

Under voltage for charging -- even at trickle rates.

Slow charge, risking sulfation, is 2.3V per cell... or 2.3 * 6 =>

13.8V.

Fast charge is 2.45V per cell... 2.45 * 6 => 14.7...

On low-beam (55W I believe), at freeway cruise, my Aprilia only manages 13.5-13.8; on high-beam (35W) it achieves 13.8-14.1 -- according the trip computer voltage display.

Then don't rely on cheap chargers -- for a $240 battery, surely you can justify a $120 computerized charger.

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|>

|>> I tested some battery chargers last night , and found "as usual" |>> that the amps and voltages stated on the modules were not the same as what |>> I |>> found on the Volt and Amp testers.... |>>

|>> Some 12v charging modules, would say 1.5 amp, whereas the voltage |>> test was 12-->17v and/or 4-->6amps on the meters.... |>>

|>> All the "Walwart" max output 800ma (.8A) with 3 volts selected |>> showed 2amps on the amp meter, and almost 4amps at the 12v setting. The |>> inline charging modules were pretty much the same... |>>

|>> I have a 12v 8Ah MF SLA battery to charge at .8amp and I can't get |>> anything to charge as low as .8 amp from the walwarts.......when I do, |>> nothing happens for days, as far volt checks are concerned... |>>

|>> The charging volts are not higher than 12.4 volts on any of the |>> chargers.... |>>

|>> Some ppl discard this discrepancy with the volts compensate for the |>> current, expression, but does it really, if a low charging amp can't be |>> achieved? I don't want to buckle cells or dehydrate the $240 MF SLA |>> battery. |>

|>Your post doesn't make a whole lot of sense. |>

|>You need more than 12.4V to charge a 12V battery. If you want to limit the |>current, use an LM317 wired as a constant current source, choose the |>resistors accordingly to set 800mA. |>

"Walwarts" = Universal Chargers

A Universal Charger's max 800mA "Walwart" will read 16v as a charging current, when set at it's max 12v.

The current/amps will read 2--->3.5amps at 12v, which is something I never really understood, since the Universal Charger is not rated to go or handle more than .8amps maximum?

At the highest setting, the Universal Chargers won't do much unless your talking days to charge an older battery at it's rating 800mA......

Are the Universal Chargers defective, most electronics shops say no, and they are rated correctly....?

I usually put the battery on an older UPS that had the same battery and I get good results on that.....It (voltmeter) reads 14v at 1.2amps

Am I confusing the .8aH suggested charging rate of 10% for an 8amp battery with the Universal Chargers 800mA.

|>> I tested some battery chargers last night , and found "as usual" |>> that the amps and voltages stated on the modules were not the same as what I |>> found on the Volt and Amp testers.... |>>

|> HOW did you test? Just hooking a DVM across the leads will not give |>reasonable readings. |> Right, that's my problem with some of my chargers, since "you can't believe everything you read on the chargers" that was the best way of checking the "real" volts/amps, but applying a known amp load might help....

|>> Some 12v charging modules, would say 1.5 amp, whereas the voltage |>> test was 12-->17v and/or 4-->6amps on the meters.... |>>

|> A 12V lead-acid battery tends to need >13V to charge, with 14V being |>a common value. |> There seems to be no other way there, unless you wanna set the universal Charger to 6-->9v to get a trickle charge, but even trickle charges tend to sulphate more than just leaving the battery off a circuit....

|> On low-beam (55W I believe), at freeway cruise, my Aprilia only |>manages 13.5-13.8; on high-beam (35W) it achieves 13.8-14.1 -- according |>the trip computer voltage display.

That's an interesting observation.....

That's amazing, I mean the low-beam (55W), unlike the Aprilia, I have the standard 2 halogens, and they draw 120watts so your system is different.

Unless you have a HID lighting system....it's hard to beat the battery drain from the mains....

My headlights (mains X2) don't come on till after the scooter is started, so I'd have a very mission impossible problem to add there....I suppose your mains work the same way....probably easier to change than a Majesty400.

Thanx for the nfo.....

could you please delete sci.electronics.repair from this thread, as it seems to be drifting away to a mostly 'motorcycle' issue rather than an electronic one. I have no idea what an Aprilia is, nor any desire to know.

Generally speaking, crossposting is frowned upon.

jak

I almost responded to your first post but then I read the thread first and James reply was everything I would have said.

What they do with cheap chargers limit the output by the windings on the transformer and the physical size of the hunk of soft iron in it. There typically isn't any regulation, just retification. If you really want to see something that will tie your shorts in a knot, just charge up a high value capacitor with the output. You may see >25V DC at no load! There is some buzz word they use, something like 'magnetics regulated' which just means the current drain will lower the chargers output voltage. That is, it will put out say 16V into a 1 amp load and 11V into a 2 amp load. I think your observation is ~'This is a wacky way to do things' is right.

Really, go with a 317 set up as a constant current source if you want it done right but don't want to go the $120 charger route. In a perfect world you should set up a charger with two 317s: First one to regulate the DC voltage to ~16V max and the second one to limit current to the .8 amps you are looking for. It would only cost you a couple of bucks to do it that way.

Just a couple of BTWs, I had one of those cheap 'float' chargers and it smoked. Just no way it limited anything but its own life. I will never buy another. My 55 amp charger will boil/melt a motorcycle battery even when set on its lowest setting.

Rick

"Robert LaCasse" wrote

Your concern is paranoid and your testing method(s) are seriously flawed. Just buy a real battery charger that says it drops to "float" when fully charged and find something better to do with your time.

For more information about batteries than you thought you might ever need go to:

:-)

I'd be utterly amazed if any charger said to have a maximum output of 1.5 amps managed 4-6. The reverse I would believe...

Just how are you measuring this? It needs to be done under actual operating conditions.

Can you explain what you mean? An SLA battery needs a dedicated charger designed for a constant voltage output. The charge current will then take care of itself. A 12 volt power supply may well not charge a 12 volt battery at all - the correct figure for that constant voltage is 13.8.

Then they won't charge a 12 volt battery to anywhere near full capacity.

Sounds like you need to buy a proper charger for such an expensive battery.

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"Dave Plowman (News)" wrote

That IS somewhat of a stretch but not totally impossible. The max. current output shown is at the rated voltage. So, for instance, a device rated at 1.5A, 12V should supply 12V up to the rated current load of

1.5A (or somewhere close). If you continue to increase the load, the amperage may increase beyond 1.5A......as the supplied voltage drops below 12V.

It would be a strange power supply that allowed that. Although it might be possible with different tappings on a transformer.

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"Dave Plowman (News)" wrote

True that a "real" power supply shouldn't work like that but I believe most of what he was talking about were simple transformers, with or without diodes for rectification.

A simple transformer is capable of supplying only so much power and P=IxE so as the current goes up, the voltage goes down. That's also why the output voltage reads higher than the "rating" when it is open (supplying NO current).

If you put the ammeter straight across it, shorting the output of the charger, it would not surprise me at all if it managed 4-6A, obviously this is not a useful measurement and it wouldn't take long for it to overheat though.

I think that's the point. The OP didn't specify under what conditions he did his measurements. The ratings on any piece of equipment are only valid under the conditions specified. IOW, 1 amp @ 12 volts, 'might' supply 4 amps, but perhaps not at 12 volts...and probably not for long!

My impression is that the OP is not very technically competent and that he is being 'penny wise and pound foolish'...buying a very expensive battery and trying to cheap-out on the charger.

If he had the chops to analyze his situation, he could probably build a charger which would be relatively inexpensive and safe; but he hasn't shown that capacity.

jak

Thinking about it, it could also happen with a battery that had some or all cells shorted.

Easy test to see the quality of your charger would be to just short the leads together at a low amp setting. i.e. set it for two amp charge and watch what happens when you short the leads together. If the amp gauge goes full excursion or greater then 2 amps anyway, you don't have real current regulation on your charger.

I don't know that I have ever seen a battery charger that has real current regulation although there may be some. Typically you have to pay through the nose for a lab bench type of supply or build one yourself. I can't remember who made the kit, IIRC you used to be able to buy a dual output power supply with variable voltage and current for about $20. That is, you could set it for voltage 0-30 and current 0-1 amp, and if the drain was greater then the current set point it would limit the current. A setting of .8 amps in a dead short would produce .8 amps on the amp meter and no more. Thanks for the trip down memory lane, I think I have to find one of these or maybe hit up a surplus house to find something similar.

Rick

Or blow a fuse or the shunt in your ammeter. An ideal ammeter has zero ohms across its test leads.

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But only if the meter is not capable of measuring the absolute max current that a transformer can produce into a short circuit. If a 1A transformer manages 6A for a few moments into a short, and the ammeter is good for 10A, it won't blow a fuse or shunt in the meter.

Most basic chargers have only rectification of the secondary voltage not smoothing, so the reading with a DMM is likely to be very inaccurate. If you put an electrolytic capacitor on the output it will charge to the peak voltage (1.414 x RMS) but as others have stated battery charger transformers are impedance regulated - that is they have a higher than stated voltage off load which drops to around 13.8 - 14.4V on load, transformers for general purpose power supplies (including wall warts) are definitely not suitable for Pb battery charging unless they have electronic current limiting (like the LM317 as others have suggested) and then the secondary voltage must be higher by at least the minimum drop out voltage of the current limit chip.