As in most things, you should start with a statement of goals. What do you
need the battery to do?
What voltage do you need?
How much current do you need?
How long will the current be delivered?
At what temperature will you be operating?
Answering these questions will help you select the correct battery - neither
too small (won't do the job) or too large (too heavy or too expensive).
Once you have selected the battery, the manufacturer's literature will tell
you how to charge it optimally.
Let's take Marek's example. We know the fan runs on 12V, so he should
nominally pick a 12V battery.
The next question is how much current does it draw at 12V? The fan usually
states this. For the purposes of this example, I'll pick 150mA.
How long does the fan have to run? Let's pick 6 hours for a typical night.
But if he doesn't want to have to charge during an event like Calstar, he
might want a battery that can deliver three nights of current - 18 hours.
Case A) 6 hours X 0.15 A = 0.9Ah
Case B) 18 hours X 0.15 A = 2.7Ah
So it looks like Marek's 8Ah battery is overkill. However, that isn't the
whole story. Battery life is dependent on how deeply you discharge it.
Batteries made for deep-discharge can be discharged to 80% of their rating.
Batteries that aren't designed for deep discharge should only be discharged
to 20% of their rating. You can discharge more, but your battery won't be
able to survive as many charge-discharge cycles. Expect 200-1000 cycles for
a well-cared for battery.
So, if Marek has a deep-discharge battery,
Case A) 0.9Ah/0.8 = 1.1Ah
Case B) 2.7Ah/0.8 = 3.4Ah
Marek's battery is then a little large. However, if it isn't a
deep-discharge battery, then he should only use 20% of its rated capacity.
Case A) 0.9Ah/0.2 = 4.5Ah
Case B) 2.7Ah/0.2 = 13.5Ah
If Marek's battery is not a deep-discharge type, then it looks like it is
sized pretty well. It has plenty of capacity for one night, but he will dip
below the recommended 20% discharge if he uses it for three consecutive
nights. Not too bad.
However, that isn't all of the story. The capacity of lead-acid batteries
drops quickly with temperature. At 40F, you'll only get 75% of rated
capacity. At 0F, you only get 50% of rated capacity. During typical Winter
nights around here, the temperature is easily in the 40's, so let's assume
the battery will only deliver 75% of rated capacity.
With the deep-discharge battery discharged to 80%,
Case A) 1.1Ah/0.75 = 1.5Ah
Case B) 3.4Ah/0.75 = 4.5Ah
With a non-deep-discharge battery discharged to 20%,
Case A) 4.5Ah/0.75 = 6Ah
Case B) 13.5Ah/0.75 = 18Ah (Or, to look at this another way, in cold
weather, Marek's battery has a total capacity of 8Ah/0.75 = 10.7Ah. Only
2.1Ah are available before reaching the 20% level. A fan drawing 150mA can
be run for 14 hours. If he ran the fan for 18 hours, he will have discharged
the battery to 25% of its capacity.)
So, well done Marek! You have made a conservative choice that allows you
safe operation of a standard lead acid battery. You don't need a deep
discharge battery for your application. Alternatively, you could use a
smaller and lighter battery if it is designed for deep-discharge.
A couple of other notes:
Batteries have lower capacities if they are discharged at a higher rate.
Consult the manufacturer's literature.
Lead acid batteries should be re-charged as soon as possible. They will
degrade if left in even a partially discharged state.
Do not use chargers for NiCad or other batteries for charging lead-acid
batteries and vice-versa.
For sealed or "gel" lead-acid batteries, chargers should be able to deliver
10-20% of the rating. Charge rates can be higher for car batteries. For your
8Ah battery, look for a charger that can deliver a peak charge rate of
0.8A-1.6A. It should be a self-limiting charger, designed specifically for
sealed lead-acid batteries, that reduces its voltage as the battery becomes
fully charged.
I like the PowerSonic brand and similar sealed lead-acid batteries. You can
see typical data for these batteries at: http://www.power-sonic.com/
Albert
>---------------------------------------------------------------------------
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>This is less OT than it might sound... it's related to a mirror cooling
fan.
>I read Alan Adler's Jan 2002 S&T article the other day, and decided to
>install a fan in my XT10, to cool the mirror and break up the thermal
>boundary layer.
>I bought a 12v, 8ah sealed lead-acid battery at Fry's, and now I need to
>figure out how to charge the battery. (I know virtually nothing about
>electricity beyond Ohm's Law. I'm mildly suprised that I got the fan,
>switch, and potentiometer to work.) I bought the battery based on its size,
>basically. (It's not as big as the 12v jump-starter battery that I use to
>run my laptop while observing, and it fits nicely on my dob's base, but it
>looks bigger and burlier than a pack of D cells.) It's about 7" long by 4"
>high by 2.5" wide.
>Does anyone know what kind of charger I'd need? Maybe something used to
>charge motorcycle batteries?
>(I'd like to report on how well the fan improves the performance of the
>scope, but the seeing last night was gawdawful here in Palo Alto. Stay
>tuned... The good news is, my first shot at mounting the fan on the scope
>seems to be nicely vibration-free, even at 250x. Nothing fancier than a
>mousepad and some rubber bands.)
>Thanks,
>Marek