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Larry asked...

in Message-id: ...

On 06 Nov 2004 11:10:38 GMT, Folklore killer wrote:

...snip

Connectng the batteries in parallel is perfectly safe, quite normal, will
achieve what is desired and is used in millions of installations the world
over.

If one connects a battery near the end of it's life in parallel with a
relatively new, fully-charged battery, are you saying that the old battery
will not discharge the new one?

No, that is not what I wrote.

By your own calculations, it appears that
it will.

It will, until the charges equalise.

If that's the case, then shouldn't one be careful paralleling two
batteries of different age and condition?

Yes, if they are to be paralleled while _not_ being charged.

But even then it is not so simple. The internal self drain of an old, tired is
only _slightly_ higher than a new one (unless the battery is U/S). So the
discharge is not as heavy as some people seem to believe.

Of course one could prevent this by the use of high current diodes, but
that lowers voltage by about 0.7 volts per diode.

And makes a mess of the charge voltage.

Remeber that the charge current into the battery is proportional to the square
of the voltage difference between the presented charge voltage and the open
circuit voltage at that particular charge state. So reducing the charge voltage
by 0.7 volts could actually completely prevent charging.

I think that's the reason the practice tends to be discouraged,

It is not discouraged whilst under charge. It _is_ discouraged when no charge
source is present.

Regards
Folklore Killer