On Sun, 7 Dec 2003 10:42:47 -0000, "John" wrote:

Hi Guys.
I have the following problem, could you please help provide a little insight
into the calculation.

A piece of refrigeration equipment has a 50hp engine with a 12v control
system, an alternator for battery recharge and of course a 12v (100Ah)
battery for starting.
The control system has a remote temperature and standby/ready control. To
use the remote standby function the main controller must be on.
The problem is this.
When in standby mode the current draw from the battery is about 500mA, if
the system is left in this state for a few days then the battery does not
have enough capacity left for the starter to crank the engine.

Question.
What is the lowest voltage/capacity that the battery can be allowed to drop
to but still be able to start the engine (12.2v seems to be the industry
standard voltage).
Finally how does the result equate to time e.g. do we have 20 hours at 500mA
or 50 hours at 500 mA before the battery capacity is to low to crank.

Battery standard lead-acid deep cycle type 100Ah.

With a good battery during crank the voltage drops to 9.5v and rises to
10.5v.

Same battery, initial engine crank current (cold) is 135A dropping to 36A.

The engine is a direct injection diesel so starts in around 2 seconds.
Thanks.
John.


Too many variables to answer that question. The actual performance is
concerned with total power supplied to the cranking motor not the
voltage.

You would have to do a plot of voltages under all environmental
conditions, like temperature of the battery and engine being started,
state of the battery charge, wear on bearings, brushes, mechanical
factors etc..

As batteries age they build up sulfate deposits on the plates which
insulate the plates from the electrolyte. Your load of 1/2 amp is
added to the normal self-discharge of the battery. "A few days"
four days with a 1/2 amp load and self discharge would eat into your
100 amp hour capacity.

That type of service (discharging over a period of days with a low
state of charge) is bound to increase the sulfate problem. Put a
charger on it if at all possible - something that can replenish the
half amp and goose the plates enough to prevent or reverse sulfation.

Failing that solution - rig a means of turning on the engine
periodically and running it for an hour without regard for the
thermostat. That will help warm the battery and engine while charging
the battery.


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