On Wednesday, June 27, 2012 5:31:25 AM UTC-4, dave hillstrom wrote:
On Mon, 25 Jun 2012 19:32:04 -0700, mike wrote:

On 6/25/2012 3:57 PM, wrote:
Hi

In my little world, the plan is to use a 500W modified sine wave inverter to power a couple chargers that I can't seem to find the 12V equivalent for.

One would be a regular 110V Ryobi charger and the other a 110V charger for 9 Volt Ni-MH batteries.

After getting this stuff together I read that using an transformer with a inverter is bad. What would happen? Would I fry something?

Any insight would be appreciated

TIA

Bob

The problem is the dV/dT. The rectifiers and caps are rated for the current
you get when driven with a sine wave. Stresses on those components
can be significantly higher. But maybe they can take it.

So, my experience is that it's perfectly safe to run most anything
from an inverter...EXCEPT when it isn't.

Here's an example.
Tektronix 211 miniscope. Good company. You'd think they designed good
stuff.
Plugged on into an inverter and it quit.

Gazing at the schematic showed why. They use a series cap
and rectifier as a charge pump to charge
the batteries. The cap has a different value depending on the
market, 50 or 60 Hz.
The fast rise input waveform from the inverter blew the fuse
instantly. Could have been much worse.

Compaq laptop.
They hooked the battery thru a FET to the input socket. Relied
on the current limit in the adapter to set the charge current.
If you put voltage into the port, it smokes the FET.
But the manual did say, "Use only the recommended charger."

Bottom line is that it's a crap shoot. For most consumer devices, vendors
are more concerned about saving half a cent than whether it works
in an environment not in their spec. Clever implementations
that save cost don't necessarily enhance reliability.

to follow up on this, if the battery charger is using a cheap cap-based
power circuit to produce consant current for fast (15 min to 1 hour) nicd
and nimh charging, you may very well get flames and smoke coming out of
your charger and/or batteries if used with a poor modified sine wave output
from a generator.

when i worked at Black & Decker, we started getting a lot of returns of for
commercial and industrial tool 1-hour chargers for just this reason. the
more expensive 15 minute charges fared much better, but also would have
occassional failures. and we were even using a Honda generator (supposedly
quit good) in the lab to do the tests to determine cause of failure.

i believe that B&D has made significant strides towards making their
chargers not blow up or massively overcharge the batteries under theses
conditions, but, as noted above, its something of a crap shoot unless you
check with the battery charger manufaturer first. (i would recommend
checking with both a power tools manufacturer's hotline as well with their
local repair outlet)

small trickle chargers, on the other hand, for 8-12 hour AA type charging
should be a lot safer IMO and experience, as their low levels of current
even if increased 3 times shouldnt hurt the cells much and ive never seen
them catch on fire.

--
dave hillstrom mhm15x4 meow

Thank you too Dave for your input.