Just some extra 'background information. Most (but not all) 'modern'
inverters are about 90% efficient (on average). So, if you take
99Watts out of them, they will need to take 110W (99W divided by 90%)
from the battery.
As you take power from the battery, the surface voltage of the
battery falls (as it discharges) so that a 'fully charged' battery
(at 13.6V, just off the charger) only needs to supply about 8A to
keep an inverter load of 100W happy. But, pretty soon, the surface
volts will drop back to 12V, and the required current will increase
to 9.25A, and when the battery is reaching end of charge, at around
11V (if you are trying NOT to 'kill' the battery altogether), that
current will have risen to just on 10A.
Becuase your load cureents are quite 'high', I would suggest that
you 'derate' the indicated Ah capacity to about 75% the suggested
value. In other words a 'rated 75Ah' capacity battery might only
really be able to sustain, say, 50Ah in 'real use'.
So, your 100W inverter load, needing - on average - something like
10A from the battery, would end up with about 5 hours 'life' from a
75Ah (12V) battery.
Due to recharging inefficiencies in the chemistry of the battery, it
would take somewhat MORE than the 'rated capacity' of energy to be
returned to the battery in order to get it fully recharged. A
reasonable 'rule of thumb' is to allow for an 'extra' 50% charging
So, if you are 'float charging' a battery that had 'lost' 50Ah of
charge, and you are staying within typical recommendations of "0.1 x
C, in Amps" as a maximum float charge current (to avoid overheating
the battery, hence causing the electrolyte to produce hydrogen faster
than it can be recombined - and hence 'gas off', shortening the
overall life of the battery, and possibly leading to vented hydrogen
and the significant possibility of an explosion), then, for a 75Ah
battery, you need a charger capable of producing 7.5A (0.1 x 75) at a
fixed voltage of 13.62V (for ambient temps of 25'C). And you would
have to charge the battery for 10 hours (50Ah x 150% / 7.5A) to get
the battery back up to a useable, useful, charge level. And a few
extra hours wouldn't charge be a bad idea.
So, your 75Ah battery can really only support a 100W load on
something like a 1:4 (6 hours in 24) duty ratio, assuming you
diligently recharged it after EVERY session, right up to the point
that you next used it.
And, with these batteries typically only giving you 300 - 500
charge/discharge cycles, you HAVE to expect the POSSIBILITY that you
may need to replace it every 12 to 18 months. That is why most
commercial battery suppliers only consider a warranty period for
batteries to be SIX MONTHS (and many that we deal with only offer a
THREE MONTH warranty period.
Commercially, we offer NO support on any battery installations that
have been operating for more than five years. Our recommendation for
these installations is that ALL batteries be replaced before we will
inspect the remainder of the installation. And THAT restriction only
applies if a regular service and inspection record is available -
without that history, our recommendation is immediate replacement,
otherwise our inspection and maintenance carries no warranty
Batteries are NOT 'robust' devices - like a combustion engine for
example. Many things can contribute to their downfall, and even
trivial deviations from 'best practice' can destroy the life of a
battery (the worst being storage in a discharged state AND at
temperatures less than 10C/50F), the second worst being over-charging
(at more than 13.62V at 25C/75F).
Keep them warm, not too hot, and nicely charged, and don't discharge
them (ideally) below about 11V, if possible, and try to keep the
discharge current less than 0.2C (or 'C/5', 1/5th of the indicated
capacity, in Amps - 75Ah/5 = 15A) if at all possible, if you are
looking to get the 'most' during a discharge.
--- In firstname.lastname@example.org, "denebola1973" <denebola1973@...> wrote:
> More on inverters I have learned. For every 100 Watts the inverter
> uses for devices, it equals ~10 Amps drained from the battery
> (sometimes a bit lower ~9.25). So if you had a laptop (Dell) that
> pulled 65W on average plus another 35 Watts from something like a
> USB2 hub, that means that a 75 AH battery would last ~7.5 hours. A
> 90 AH battery would last ~9 hours. That is not counting the
> amps for the LX90 if you have that running off the same source,
> possibly through its own DC-DC outlet from battery.
> --- In email@example.com, AstronomyW4WMM@ wrote:
> > Jeff,
> > My personal opinion is that pulsed dew heaters have the
> to cause
> > problems, however mine never has. Am I lucky? Blessed? Go
> figure. However,
> > I do agree with Dr. Clay that separating it (pulsing) by using a
> > power source buys a lot of insurance.
> > Popping a hair drying On and Off on the same circuit as the
> Autostar is just
> > as bad as the pulsing heater, if not worse, IMHO.
> > FYI: I use the original Kendrick's Dew Zapper. I hear their
> > controller is noise free.
> > ====================
> > Looking Up,
> > Alan, W4WMM
> > Mobile Astronomical Society
> > 30.69° N 88.24° W
> > In a message dated 05-Feb-09 8:06:10 A.M. Central Standard Time,
> > Jeffrey.Weiss@ writes:
> > Dr. Clay-
> > Is there any distinction between pulsed dew heater controllers
> > the always-on type (that I have)? I've just gotten a second
> > in order to isolate my items (scope, dew heater, powered hub for
> > Pro's, laptop computer). I had heard that only the pulsed dew
> > interfered and was going to put my non-pulsed dew heater on the
> > battery for the telescope along with the powered hub, while
> > the computer only on the second battery (both Optima yellow top
> > 38AH). I've occasionally used a small (2A) DC hair dryer to
> > and was thinking I'd put that also on the telescope battery,
> > turning it generates noise that has made the DSI's get dropped by
> > Envisage.
> > Your advice is greatly appreciated.
> > Jeff
> > **************Who's never won? Biggest Grammy Award surprises of
> all time on
> > AOL Music.
> > (http://music.aol.com/grammys/pictures/never-won-a-grammy?
> > [Non-text portions of this message have been removed]