AMP-Hour Meter

Archiver

FYI,

Rather than doing this math, it would be easier to start with a known quantity
for amp-hours based on the size of the battery and using an amp meter and a
seconds time base to accumulate the current drain. Subtract the accumulated
current drain from the original amp-hour capacity for power left. Just integer
math required. Most batteries you never want to drain fully as it makes them
really hard to recharged, thus the amp-hour capacity you use should be a portion
of the total to dead condition.

Same as "who of us would regularly try to run our gasoline tanks dry", we quit
at 1/4 to 1/8 and recharge them.

Sincerely,
Ron A.




Message: 6
Date: Tue, 12 Nov 2002 21:18:30 -0000
From: "ve3crx" <ve3crx@r...>
Subject: Re: Raising to a power

Tracy (and Al)

The power is a constant within a given system (It is an amp-hour
meter for my sailboat; the available power in a battery depends on
the actual current drain raised to a power; the number is a constant
for a particular brand of battery.

A general method would of course be best; then I can just change the
constant for different batteries.

Actually, the real version of this is going to be built using a PIC
16F877 rather than a STAMP. The stamp is real nice for playing around
with concepts. The real device will probably be programmed using
PicBasic Pro.

I'll look at the web site again for your log stuff, Tracy.

Thanks

Larry



[noparse][[/noparse]Non-text portions of this message have been removed]

Archiver

Been there, done that. My first version of an amp-hour meter did just that.

The problem is that the battery capacity is dependent on the current being
drawn. The relationship is know as Peukert's Equation, and is a power function.

For example, a Trojan T105 is rated at 225 amp hours. But that is its
capacity when being discharged at 11 amps - it's "20 hour" rate.. When
being discharged at 100 amps, it's capacity is only 130 amps-hours. And at
1 amp, it's capacity is 400 amp-hours.

Thus basing an amp-hour meter on the assumption that a T105 can deliver 225
amp-hours results in overestimating its capacity for currents greater than
11 amps, and underestimating it for currents less that 11 amps.

I have two T105's in series as my house batteries - I never run them down
below 50% if I can help it, although T105s can take 80% without really
affecting their life span.

Larry

At 08:30 AM 11/13/2002 -0500, you wrote:

>FYI,
>
>Rather than doing this math, it would be easier to start with a known
>quantity for amp-hours based on the size of the battery and using an amp
>meter and a seconds time base to accumulate the current drain. Subtract
>the accumulated current drain from the original amp-hour capacity for
>power left. Just integer math required. Most batteries you never want to
>drain fully as it makes them really hard to recharged, thus the amp-hour
>capacity you use should be a portion of the total to dead condition.
>
>Same as "who of us would regularly try to run our gasoline tanks dry", we
>quit at 1/4 to 1/8 and recharge them.
>
>Sincerely,
>Ron A.

Larry Bradley
Orleans (Ottawa), Ontario, CANADA