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[Mr Skrunts]

https://www.google.co.uk/?gws_rd=ssl#q=aroma+3-fm-4

Trying to help a neighber out, she brought me a battery and charger that had come with a girls toy car for her sisters grandaughter that had been given them because it would'nt charge.

Without checking the spec I put it on charge for 24 hours  (had 6.1 v in it before chargeing) tested at 7.1 volts after charge.


Thinking it could be a faulty wire I checked the voltage on the charger and it was 12v.

Conclusion is that the toy car could be fubar.  But (thinking ou loud) would'nt a 12v charger damage a 6/7.2v battery.?

[zirk]

Your getting 12v because your measuring the charge voltage without a load (I presume).

Charging your type of battery is all about charging current, the charger your using could be in its simplest form a simple 12v supply with limited current and designed to charge the 6v battery via an in series resistor.

[Bigron]

Assuming the charger was intended for use with rhe toy, which is what you are saying, then how old is the battery? It coulkd have either gone low capacity through frequent use or sulphated up due to long periods of idleness in a discharged state.
If the former, probably not a lot to be done to save it, but it could be worth discharging it through a suitable load (even the toy itself) and then charging it again - repeat many times until the discharge time lengthens to a reasonable level. Fingers crossed!
If it's the former and it's a wet-plate battery (and not a gel type sealed one), you can do something similar but empty the electrolye ( the acid inside) out - and SAVE it - and refill the battery with plain water. charge/discharge the battery over many cycles and if you are lucky at least some of the sulphate will dissolve away, restoring more capacity for the battery.
PLEASE be careful if you attempt Plan B as the electrolyte is dilute (but still dangerous) sulphuric acid and will eat clothes, furniture - and YOU!

Ron.

[Marks DTM Calib]

Lead acid battery charging, in a nut shell, is based on constant current until the terminal voltage reaches a set value at which point its constant voltage. (smart chargers are more advanced).

Some of these cheap battery chargers are evil things, I have seen them where the current limit is based on the transformer saturating with no voltage control at all (these tend to be the reason why the batteries don't last long!) so you can measure pretty high 'V's with no load on their output.

[Mr Skrunts]

Many thanks for all the reply's