On 09 Dec 96 at 16:30, Roy J. Tellason of 1:270/615
wrote to Chris Harper about :
 RJT> Hmm,  I would test things to maybe see if they would operate at a 
ittle
 RJT> lower voltage.  Say,  105,  or maybe even lower.  Some switching power
 RJT> supplies have pretty wide input voltage operating limits.
The inverter I have says it'll drain them down to 10.5 volts before shutting 
itself down, and it requires anything between 11-15 VDC input.
BTW - those who were worried about J.C. Whitney, don't. They sent my inverter 
to me just fine. Sooner than I had expected, really.
It's a Statpower PROwatt 2500. It converts power in two stages. First  is 
DC-to-DC (145VDC), the second stage inverts the HVDC to 115 VAC (RMS) at 60Hz 
in a quasi or "modified" sinewave. It's a stepped waveform that they say is 
superior to the squarewaves other converters use. It's warranted for a year.
 RJT> Sounds reasonable to me.  One thing to keep in mind,  though,  is that 

 RJT> switching power supply is going to regulate its outputs properly,
It doesn't say whether or not it's a "switching" power supply.
 CH>> I was considering trying to find a less than 3 volt battery I
 RJT> I wouldn't think it'd be that beneficial to take lead-acid cells all 
he
 RJT> way down,  if you can avoid it.  Also,  you're typically looking at
 RJT> 2v/cell,  so it'd be kind of hard to find a 3v lead-acid "cell".
As you can see from my quoted statement above, I wasn't referring to a single 
cell, nor one of 3 volts, in particular. LESS than 3 volts, preferrably one 
volt, is what I would be looking for.
 RJT> Finally, you'd have a different state of charge between that "extra" 
cell
 RJT> and the rest of them.  I always figured it'd be better to have all 
ells
 RJT> more or less tracking so that they're in about the same state.
But that would depend, you see? If the one low voltage battery significantly 
extends the life of the multiple 12V batteries by allowing them to run down 
further, it'd be economicly better to replace the single low voltage cell 
more often than replacing the entire pack of 12V batteries.
 CH>> BTW - do you know the average "usable capacity" (of an average
 CH>> cycle) of an average 12 volt deep cycle battery I can obtain
 CH>> easily?
 RJT> I think that in large part this is going to depend on your inverter,  
and
 RJT> on a number of other variables,  such as what kind of loads you're
 RJT> running there.
I am asking for average deep cycle battery capacity. The load is irrelevant 
to that question in particular. I assume an average 1.5-2KW load for 
estimation purposes, however. In actuality I'll probly only have a .75-1.5KW 
load on average, with rare 2KW+ draws.
 RJT> The one thing that bothers me about connecting batteries in parallel 
like
 RJT> that is what happens if one of them has a shorted cell?  Typically 
t'll
 RJT> pull the rest of them down, for starters.
And I'll know something is wrong right away then too!  ;-)
 RJT>   You also want
 RJT> parallel-connected batteries to be all of the same type from the same
 RJT> mfr. and about the same age.
Same voltage is necessary. Type (deep cycle or auto) can be mixed is they all 
run within the same voltage range, and manufacturer is irrelevant to the 
circut, I'd think. Having all the same type, age, and mfr., makes it easier 
to predict wear, etc..., but can be difficult to maintain. Should one battery 
fail after a year's use, to maintain similar age I'd have to replace the 
entire pack, and that's just not feasible.
 RJT> I wouldn't use car batteries at all in this application.  There's no
 RJT> problem with pulling lots of power out of a deep cycle battery if you
 RJT> need to,  most of the ones I sold were "dual purpose" in their 
beling.
I suppose if I have enough in parallel, I can pull heavy loads without fear. 
But I once had a deep cycle in a truck which was used for starting it as 
well, and it didn't last six months. Waranty replaced it, the next one lasted 
about 4 months. Waranty replaced that one too, but I paired it with a normal 
auto battery in parallel without any isolator, and they both lasted over 6 
years and when I sold the truck they were both still working.
 RJT> For example,  the two that I have sitting here are "Exide Nautilus Gold
 RJT> NG-27" types,  which are rated at 670 MCA (Marine Cranking Amps,  rated
 RJT> at 32 degrees instead of zero degrees).  That's quite a bit of kick,  
and
 RJT> should take care of what you need in that respect.
670A x 12V = 8040W Yup! That's quite a kick! But for how long can it deliver 
that constantly? I'll need around 250 amps when operating at maximum loads. 
100-150 average, I expect.
 RJT> Of more concern regarding the idea of pulling large amounts of power 
ut
 RJT> of your setup is whether the inverter will deal with it or not.
It will do 2.5KW constant with 3KW surge capacity until the heat sensor shuts 
it down (they estimate 3-7 minutes).
 CH>> The car batteries would handle not only surge demends, but
 RJT> Nope,  I wouldn't do it...
You've got to remember that batteries in a DC circut are similar to a large 
(charged) capacitor with a resistor (limited current flow) hooked like this:
+R...
-....
where the "." are the conductors to the circut. The "capcitor" charge and 
discharges MUCH slower than a real capcitor.
I have seen railgun designs that use capacitors instead of batteries to power 
it, because they give up their stored power much quicker, as required for 
that purpose.
 RJT> I see no reason at all to get into using car
 RJT> batteries for this sort of stuff,  and wouldn't bother with them for 
he
 RJT> most part.
I probly won't either. At least, not to start with. However, one CAN obtain 
used car batteries a LOT cheaper than new deep cycle batteries. And I've yet 
to see an outlet for used deep cycle batteries.
RYL,
       Chris Harper
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