[RE-wrenches] How many Ahs?

[Phil Undercuffler]

Mark,

The battery is marked with a 6 hour rate because if it's used in a forklift
in a warehouse, they figure on a 6 hour discharge cycle -- they hammer it
hard for most of the day, then park it for the night, plug it in and let it
recharge.  They also mark it with a 20 hour rate because that's the
"standard" discharge rate for comparisons -- as you notice, the 6 hour rate
is considerably smaller than the 20 hour rate, so it helps to make sure
you're comparing apples to apples.

If your standard discharge cycle is a ~24 hour period, then you can use the
20 hour rate to evaluate charger setpoints.

Also, in regard to your question:

Is it fair to say the following: For rapid bulk charging to bring the
voltage up from some discharged voltage of say 48VDC to a bulk charged
voltage of say 60VDC, I want a charger that can provided a constant current
of C/10 or 1320/10 = 132 A across that voltage range.

I would say that is not correct.  Use the C/20 rated capacity, not the C/10.
 That means you want a charger that can provide 169 amps if you want to push
the battery as hard as it can reasonably be charged.  Charging that bank
with anything less than ~85 amps of charging current (after you account for
daytime loads) is really more of a trickle than a charge.  Ideally, your
charging source should be somewhere between those two numbers, and when in
doubt err to the higher side.

Also, you mentioned:

And at the time that I complete this bulk charge, the battery bank will be
restored to 1320/1690 = 78% of its starting capacity.

I think you're swinging at the fences there.  You can't tell anything by
comparing the various C/numbers, and besides they really only apply to the
discharge cycle.  I think it would be more accurate to say that once your
constant current charger brought the battery voltage up to somewhere around
58.8 to 59.2 (temperature compensated), then it should begin to taper the
current to hold the battery voltage at that point while still flowing as
much current as possible. Once the bank has been held at that voltage for
some period of time (which can be confirmed by specific gravity
measurements, guesstimated by calculations based on depth of previous
discharge, or ballparked at 3-4 hours for a bank that size), then the bank
can be considered full.

Phil Undercuffler
Enjoying solar independence since a week ago last Friday


On Sun, Jun 20, 2010 at 9:59 AM, Mark Frye <markf at berkeleysolar.com> wrote:

>  Suppose I have a battery bank made up of 4 - 12V Enersys HUP flooded lead
> acid batteries. The batteries are wired in series for a nominal 48VDC
> system.
>
> The side of the battery, which is actually a forklift battery, is marked as
> 1320 Ah @ C/6. The vendor also rates the battery as 1690 Ah @ C/20.
>
> So, for the purpose of evaluating the battery back relative to the charging
> system settings, what size is the battery bank?
>
> It is interesting to think that at a normal discharge rate of C/20 I can
> get 128% more energy out of the bank (1690 Ah) than I can put back in at a
> C/6 charging rate (1320 Ah)?
>
> Is it fair to say the following: For rapid bulk charging to bring the
> voltage up from some discharged voltage of say 48VDC to a bulk charged
> voltage of say 60VDC, I want a charger that can provided a constant current
> of C/10 or 1320/10 = 132 A across that voltage range. And at the time that I
> complete this bulk charge, the battery bank will be restored to 1320/1690 =
> 78% of its starting capacity.
>
>
> Mark Frye
> Berkeley Solar Electric Systems
> 303 Redbud Way
> Nevada City,  CA 95959
> (530) 401-8024
> *www.berkeleysolar.com* <http://www.berkeleysolar.com>
>
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