Dave Plowman (News) wrote:
In article ,
Gary Cavie wrote:
Should never connect batteries in parallel (without protection), just
choose a bigger battery in the first place.

If a cell in one should fail due to say a internal short (quite
possible in lead acid), the other 3 will dump their load through the
failed battery with quite interestingly devastating consequences. I
have seen the exploded remains of a largish (using 100Ah batteries)
UPS where some clever spark added a second battery to improve its
capacity and after a month or two one of the batteries ruptured,
exploding the UPS case and spraying suphuric acid gel everywhere. Bit
of a mess, including all the 415V switch gear and cabling it "ate"
away.

I have heard this said many times, but have done it myself several times
without any problems. My old Hiace had two batteries paralleled up
straight from the factory, so I'm not sure just how big a problem it is.
Obviously, if you happen to be unlucky, it is a problem, but just how
often does it cause an accident?

It is total ******** with lead acid batteries - but not with Ni-Cads.
And I've never known a lead acid cell to fail short circuit - it's the
reverse. And even if it did, another battery connected in parallel
wouldn't deliver enough current to blow things up. Unless several cells
failed short circuit at once.

Both Nicads and lead acids can be discharged in parallel with no special
circuitry..and lead acids can be charged in parallel - the
voltage/charge curve equalizes the charge..Nicads can be trickle charged
in parallel but not delta peak charged.,

In all cases the voltage/charge-discharge curves are such that no
especial protection is required - no diodes - nothing.