O T - Can Anyone Help Me Please ?

[the_constructor]

Hi Everyone,

I wonder if anyone could offer me some advice please.

I have a mobility scooter and being a little suspicious that the charger was not operating at full capacity, I decided to make myself a small Voltage and Amp tester box.

My wiring diagram can be seen on the link below.

problem that I have, not being a super electronics techy is this. When I plug the output of the box into the charger socket of the mobility scooter, I get a reading on the volt meter. Then, when I plug the charger into the input of the tester box even before pluging the charger into the mains power supply, I get an even greater voltage reading. Only, when I plug the charger into the mains power supply does the Amp meter register.

This box has a 30V and a 10A meter on it.

The mobility scooter charger is for 24V @ 5A

Should I have put a diode into the circuit before the output socket so as to stop any back voltage feed and if so, what do you suggest.

What is the reason for the greater reading of the volt meter even though the charger is not connected to the mains power supply.

Your help and encouragement would be most welcome.

Kindest regards,

James

[Andrew Gabriel]

It would be useful to know what those readings are.

An intelligent charger may only provide a limited voltage at high impedance initially until it senses a battery connected with the right polarity. This prevents sparks and operation if battery is connected with wrong polarity or wrong voltage.

The voltage drop across it would upset an intelligent charger.

[The Natural Philosopher]

That will be the current (sic!) battery voltage. What does it measure?

That is curious. Possibly residual charge on the charger capacitors. If there was e.g. a big anti-reverse connection diode in the charger circutry to the scooter, that might cause something like this.

Well, that is to be expected..

No, not really.

As I said,possibly some charge left on the charger that is now (just) forward biasing an inbuilt diode..Whereas before you were just seeing leakage backwards through the diode.

[Calvin]

It never ceases to amaze me the uses to which excel gets put! Possibly a spreadsheet might not be the most obvious tool in which to draw circuit diagrams but, hey, it's worked.

I'm not sure what you intend to learn from this box. I'f the charger is producing enough open circuit voltage (easy to measure the output of the charger with a meter on its own) then it will charge the batteries over time. Depending on the discharge state of the batteries and the output impedance of the charger you may see the voltage on your meter-in-a-box fall quite dramatically during the initial charging phase but towards the end of the charge cycle it will recover as the batteries draw less current.

Fundamently if we are talking about a simple charger here (and if we're not then your box will tell you even less) then if its open circuit voltage exceeds the battery terminal voltage by a small margin it will charge the batteries to full capacity. It can't part charge them unless either it doesn't produce enough volts or you don't give it enough time.

On the examples of scooters I've seen the charger is fairly poxy, little more than a wall-wart. Being so weak it has a relatively high output impedance and the voltage collapses quite noticably during the initial charging phase so time required can be quite large.

Why not have a look at the markings on the charger and battery and tell us: The battery type and voltage The charger rated volts and amps The measured battey voltage The measured oc charger voltage

[Dave Plowman (News)]

Interesting. I'm scratching my head to think of a reason. If you leave it some time in this state do the voltages equalise?

Obviously - that's the only way to complete the circuit.

Trouble is all diodes have a forward voltage drop - which may effect the charge rate. If you do want to try this use a Schottky type - these have the lowest forward voltage drop. But I'd not bother - it's the voltage on charge that is important.

Dunno.

Generally, if the battery and connections to the charger are sound the voltage will tell you what you want. Because the combination of that with the internal resistance of the battery and voltage sets the current.

What sort of battery is fitted? If a SLA (gel) type, it should have a constant voltage charge. That should be 27.6v.

If it's a wet type, the voltage may well be higher when approaching a full charge. But with either type the actual charging current will vary throughout the cycle - starting higher and dropping as it reaches full charge.

One other thing is that modern 'calcium' wet batteries require a slightly higher voltage to reach full capacity - but this shouldn't matter if you're using the correct battery and charger. If the battery is being run near flat every day I'd expect it not to last much more than a year.