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Science Project time again! #477 01/28/01 02:24 PM
Joined: Oct 2000
Posts: 3,961
Bill Addiss Offline OP
I've got a question.

Part of a Science project involves connection to a 6 volt Battery. It is one from those little Ride-around Jeeps. We have to charge the Battery (hasn't been charged in Years so I'm crossing my fingers)
The problem is that the plugin charger had the wires chewed up by some squirrels that I let squat in the Garage out back. Which way (polarity) should it be connected up?

Now, I know the polarities on the Battery + & - is printed on the top. Can I tell the polarities on the charger with my Fluke meter? When I connect it one way it says "-".
I assume that means the probes are reversed. When it is correct I should be able to tell the polarities of the charger output from the leads, Right? Black would be - ? Red = + ?

Last question, if I know these things, how should it be connected to charge the battery? I thought I remembered from somewhere that they should be connected opposite (+ of charger to - of Battery) Is this correct?


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Re: Science Project time again! #478 01/28/01 10:36 PM
Joined: Nov 2000
Posts: 2,236
sparky66wv Offline
Connect so your meter reads positive.
Black is -
Red is +

When charging, + to + (red to red)
and - to - (black to black)

Voltage can vary 10% or more, but the current will determine the charge rate and time to full charge.

For Nicads, if the capacity of the battery is say, 1200 mAh, then the 14 hour charge should be 120mAh and trickle charge would be 1.2 mAh.

I'm going more on my Model Airplane experience here than for anything I've done at work, so I'm hoping the same rules apply...

Anyone feel free to correct me here...

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Re: Science Project time again! #479 01/29/01 07:11 PM
Joined: Nov 2000
Posts: 2,236
sparky66wv Offline
May I ramble some more?

When you charge a battery, you create a parallel circuit (hot+ to hot+, ground- to ground-) and the charger will tend to "balance" the current capacity and voltage with the battery... if you hook together two 6 volt batteries in parallel, you double the capacity, but the voltage remains the same.
The capacities of the batteries will tend to balance.

If you hook the charger in series, (hot+ to ground-, ground- to hot+)without a resistor or load some where between the connections,
you very may well blow up the battery.

With or without a resistor (or load), the voltage is added together, and the current capacity would balance, or try to...

A pack of nicad cells like the ones for a drill have the same mAh capacity as one cell. The voltage is just multiplied by the number of cells. One cell would be 1.25V 1200 mAh and the whole pack of ten cells would be 12.5V at 1200 mAh...

For a 14 hour charge, the charger would connect to this pack in parallel and would have to provide 125 mAh. The voltage can be as low as 10V and as high as 15V and still work. They don't seem to be voltage dependant.

As far as lead-acid gel cells, the type in the buggy, they're about the same as nicads but with generally larger capacity in the 2 to even 100 or so Amp-hour range.

I shall step down from my soap box now [Linked Image]

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Re: Science Project time again! #480 01/29/01 08:51 PM
Joined: Oct 2000
Posts: 3,961
Bill Addiss Offline OP

Makes sense, just like jumpstarting a car. I am wonderinfg why I was thinking opposite polarity. ????


Re: Science Project time again! #481 01/31/01 07:12 PM
Joined: Oct 2000
Posts: 2,721
Scott35 Offline
Broom Pusher and

'66 covered it great. If your meter shows a Negative "-" sign in front of the DC voltage, that would indicate the leads are connected in reverse polarity. Switch the leads around and there should only be the DC voltage shown on the display. Observing the marked polarity on the meter's plugs will show which polarity is on the Battery [or even a cell]. "+" on the meter's probe plug will equal up with the "+" polarity on the Battery, or Cell. Same goes with the "-" polarity.

Definitely charge the Battery - or Cell - in Parallel, meaning the "+" polarity from the charger connects to the "+" polarity on the Battery / Cell. Same follows for the "-" polarity.

If the Battery you have is a Liquid Battery, the normal Voltage at full charge and no-load should be no less than 6.6 VDC. Try to charge it with a charger that has a loaded output of at least 7.0 VDC or maybe even 8.0 VDC. Since it has been fully discharged, you should trickle charge it for a few hours, then charge it at a higher level until it is near full charge, then let it trickle charge over night [I will re-check this procedure for lead-acid Batteries being recharged from a dead or 1.0 spec gravity state].

If it's a NiCAD, it might have reversed it's own polarity from being fully discharged and left at that state for an extended period [they are funny that way [Linked Image]..]. If the Battery is NiCAD, the voltage under load, upto the point where it dies, will remain a steady 6.0 VDC [5 - 1.2 VDC cells]. Open circuit, or no-load voltage might be as high as 10 VDC, which is why chargers for NiCADs have such a high charging voltage.

If you can charge the NiCAD, it should be done with a fast charge [prevents "memory" oxidation]. Since it is dead, the battery will draw an excessive amount of current for the first 10 to 15 minutes, before tapering off to a somewhat steady value. This might overload the charger, so keep this in mind.

On the subject of reversed polarity charger connections, hooking up a dead battery to a charger with reversed polarity will most likely fry the charger, since it will eventually become a series additive circuit, doubling the voltage across a low resistance load, which now will draw twice the current as before [once again, I need to check my books on this one, as it's been a few years since I have refreshed the subject].

Please excuse the late "Jump In" [Linked Image]

Scott "S.E.T."

Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!


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