Testing battery chargers sending me batty!

[John CoatesParticipant @johncoates48577]

Hello again gents

I am trying to find a mains powered charger to power some lights and a laser (for the MEW 186 article about centering). I am also trying to establish whether my current motorbike charger is knackered or not. From my collection of chargers old and new I drag them out and set to measuring their outputs with my Gunson UT105 multi meter.

Charger No.1 – Black & Decker cordless drill

In use to charge up my drill so the genuine end fitting is on

Rated output 18.2v DC at 210mA

Measured output 19.6v DC and 8v AC !!!

 

Charger No.2 old Bosch cordless drill

Cable is removed from charging base unit and measurement taken across the two wires

Rated output 14.5v DC at 250mA

Measured output 18v AC !!

 

Charger No.3 for my new jump starter (bought last weekend)

In use to charge up the new jump starter so the genuine end fitting is on

Rated output 12v DC at 500mA

Measured output 21v DC

 

Charger No.4 my motorbike charger

Measured across the crocodile clips

Rated output 12v DC at 1.25mA

Measured output 0.17v DC

 

So my question is am I measuring these wrong or do the measured values usually differ so much from the ratings on their casings?

Thanks, John

Edited By John Coates on 17/02/2012 21:28:44

Edited By John Coates on 17/02/2012 21:29:10

[John CoatesParticipant @johncoates48577]

Measured voltage outputs not as stated

[confused.engParticipant @confused-eng]

Hi John,

From my experience with chargers, they don't work in the same manner as a power supply.

Batteries are usually charged with a constant current source, eg 500mA. Also to get a battery to charge the voltage needs to be higher than the battery voltage.

What this means is than if you measure the voltage from the charger without a battery connected it will most likely be on the high side. Also, some chargers are 'intelligent' and detect if a battery is connected before turning the charging output on.

To power lights etc you would be better investing in a proper power supply. You can use a battery charger as a supply but you need to make sure that the load is correct. If it is a trickle charger eg. 16hrs then the current available will be quite low whereas a fast charger pumps out a much higher current.

To test if the charger is working connect a current meter in series with the battery you are trying to charge and see what the current is. If the battery is nearly charged then the current will probably be lower than the rated current. Also if you are fortunate enough to own two meters, measure the voltage accross the battery, this should be higher that the battery voltage.

Some modern battery technologies have special charging requirements so it might be worth searching for specifics for these.

Hope this helps.

Philip

[John CoatesParticipant @johncoates48577]

Philip

Thanks for the tip about testing in series. Will try that tomorrow. I have got another 2nd hand battery for my bike but don't want to attach my current battery charger before I know it is good.

John

[Les Jones 1Participant @lesjones1]

Hi John,

There are two reasons why the meter could read a voltage from a DC power supply when set to the AC range.

1 The design of the meter may just put a bridge rectifier in front of the circuitry that reads DC voltages. You can confirm this by measuring the voltage of a battery with the meter set to the AC range. If it gives a reading then your meter is designed in this way. (This is more likely to be the case with analogue meters.)

2 Threre will be ripple in the output from most chargers which will read as an AC voltage.

Charger 1 I think is OK.

Charger 2 Is either faulty or the output IS AC (It could be rectified within the drill.) With no power to this charger measure the resistance between the output lead in both directions. It should be the same in both directions and less than about 4 ohms if it is AC output.

Charger 3 This is probably OK The regulation of these plug in chargers / power supplies is very poor so it is not unusual for it to be so much higher than the rated output with no load on it.

Charger 4 Point 1 I do not believe it is only rated at 1.25 mA output. Are you sure it is not 1.25 A ? Point 2 It may have electronic control to control the charge current and maximum charge voltage and also reverse polarity protection. If this is the case then it may only switch on its output when it sees the correct polarity from the battery.

Les.

[John CoatesParticipant @johncoates48577]

Les

You are quite right about charger no.4 it is rated at 1.25A (just been into a cold garage to check!)

Luckily I am into my second large glass of port so that hardened me up for the journey

Am going to follow it up with a glass of malt in case I have to go in there again! 

Edited By John Coates on 17/02/2012 22:51:43

Edited By John Coates on 17/02/2012 22:52:17

[Les Jones 1Participant @lesjones1]

Hi John,

Thats what I like to see – forward planning. (It's a good excuse for having a glass of malt even if you don,t have to go to the garage again.)

Les.

[John ShepherdParticipant @johnshepherd38883]

John

Enough said about testing the chargers but they are unsuitable for the project anyway.

Over voltage will easily damage the laser module and if the voltage is outside specification it may not be eye safe.

Two AA or AAA cells in series (3V) will be ideal and will last for ages.

3V gives about the right brightness for this project – too bright and the beam 'spreads' on the target at the close distances involved.

PS Be good to know how you get on with the project.

Regards

John Shepherd

[John CoatesParticipant @johncoates48577]

John

I think I confused things as it wasn't these specific chargers I was thinking of using for the laser alignment project but for the low voltage lighting. I was trying to understand why the voltage I measured was different to the rating printed on each charger. This woud be so I could test other units to find one suitable for the laser.

I think the answers so far have helped me a lot to understand why I am getting the results I am

John

[Peter G. ShawParticipant @peterg-shaw75338]

As has been intimated, there are a 1001 reasons for the various discrepancies. Ok, I exaggerate somewhat, but nevertheless, this really is something of a minefield.

For a start, the meter may well show both an AC voltage reading and a DC voltage reading. On the AC scale, it may even show different AC voltages depending which way round the meter leads are connected. It could even show no AC voltage one way, but an AC voltage the other. Probably the best advice is to measure DC voltage if you are expecting DC voltage, or AC voltage if expecting AC voltage. Provided you always start from the highest range and work down, it should not damage the meter, and the internal resistance of the meter will not harm a power unit.

In respect of the power units, there are:

Basic transformer. AC output only.

Basic transformer with rectifier. DC output, but your meter is not likely to show the rated voltage output since the rated voltage output is dependant on the current loading.

Basic transformer with rectifier and capacitor. DC output. Again, the actual rated output is dependant on the current loading.

Transformer-rectifier-capacitor with electronic stabilisation. These will give a specific voltage regardless of the load current up to rated load current. Above this, the unit may current limit by reducing the output voltage or simply shut down until the overload is removed.

And then there are the SMPS (Switched Mode Power Supply) types. These are the ones which reside in a small box, and can give a high current output and a high (relatively) voltage output. The power that these units can supply is out of all proportion to their size. A good example is the power supply for a laptop which may be something like 19 Volts and 4.6 Amps out of a box 4" x 1.6" x 1". These are actually highly efficent which is why they are so small. Also, because of their design, they hold their rated voltage very well from no load up to their maximum loading. I don't know what happens if overloaded, but I would not be at all surprised if they simply shut down until the overloading is removed. They are, after all, a highly complex electronic circuit.

Finally, there are the specialist power units used for charging batteries. These are designed to limit over charging, and over heating in the case of NiCd and NiMh. I don't know anything about these other than they are capable of proving sufficient voltage at a suitable current for the number of cells being charged; eg, I have a rapid charger for NiMh cells which can detect how many cells are connected (2 or 4), and when they are charged (by measuring the voltage across the cells whilst charging). I also have another NiCd/NiMh charger which simply charges at a rate sufficiently low to avoid overcharging.

There are also power supplies used in desktop computers etc which can detect if they are connected to a mother board and will not supply power unless so connected.

As you can see, there is a wide variety of power supplies and really the only basic way to test them is to test for the expected voltage, eg 9V DC, 15V AC and see what comes out. If higher than expected, and not grossly higher, then it is possible that the unit is satisfactory, eg a unit rated at 9V DC, could well be ok if the meter indicates say 11V DC, but 34V DC would be somewhat worrying. If you think it is ok, then a cheap second test would be connect up a torch bulb, or a car bulb and see what happens.

Regards,

Peter G. Shaw

[Author]

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