Re: A simple question - beginner stuff

["David Roden" <roden@ald.net>]

> what sort of problems do you
> run into if you hook up 10 more 12V batteries in parallel, so the Voltage
> is the same but the number of Amp-hours goes way up?  I have thought of
> only 1 - the charge time goes way up. 

You bet.

On the positive side, charging batteries in parallel tends to help 
balance their state of charge.  Unfortunately, for the kind of energy 
involved in most vehicles, the current needed to charge ten 12v batteries 
in parallel in time to drive the car sometime this week <g> is pretty 
frightening.

But you say yours is a small vehicle.  So let's say it sips energy, 100 
wh/mi (possible if it weighs under 1500 lb and you drive slowly and 
gently).  Suppose you want to have a 30 mile range, a modest goal.  You 
will use 3000 wh and, because of batteries' inherent inefficiency, you 
will need to replace about 3300 wh.

Suppose further you want to recharge in 8 hours.  On the average, you'll 
need to squirt 412.5 watts into those batteries for those 8 hours.  At 12 
volts, that means averageing 35 amps throughout the charge period.  

You'll probably need to start somewhat higher than that (say, 45-50 
amps), because your finish rate will be lower.  If your charger is an 
ordinary taper charger, it will probably have to start at around 70 amps 
if it's going to charge in 8 hours with a 35 amp average.

What if you used the typical 10 amp automotive taper charger?  These 
average around 6 amps, so you'd need at least 36/6 * 8, or a minimum of 
48 hours for a full charge.  In practice, it would probably be a good bit 
more.

These figures are greatly simplified and they ignore many significant 
factors.  But this will give you some idea of what you're up against in 
trying to charge ten 12v batteries in parallel.  And we haven't even 
mentioned that if you're discharging the batteries in series, you need 
some way to reconnect them in parallel when you charge.

Now, if you were to buy 10 transformer isolated 12v chargers and connect 
one to each battery, ~that~ might work pretty well ...

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