Battery Fuel Gauge [RE-wrenches]

[Todd Cory, Mt. Shasta Energy Services]

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Self discharge equates to inefficiency, which is calculated by the meter 
every time the batteries reach a full SOC. In an ideal world, this is 
compensated for by the meter the next time the batteries are charged.

If the displayed amp hours is a positive number when the batteries reach 
the charged parameters, the meter recalculates the battery efficiency 
lower. If the displayed amp hours is a negative number when the 
batteries reach the charged parameters, the meter recalculates the 
battery efficiency higher. So if the meter has calculated the batteries 
at, say 90% efficient, the shunt must pass 11 amp hours to count up 10. 
This should adequately compensate for self discharge.

The only problem I see with this is in the winter, when the batteries 
can go many charge/discharge cycles without ever getting to the charged 
parameters. In this scenario any meter error gets compounded and 
inaccurate. I commonly get calls from customers in the winter that say 
"my meter says my batteries are discharged, but the generator/solar will 
not charge them". This is one of the rare instances where looking at 
battery voltage can reveal the problem, where the batteries are actually 
more charged than the meter says. If the charge controller or inverter's 
charger talked to the meter, this issue could be avoided. So far no one 
makes a amp hour meter that will communicate with the system charge 
controller or inverter charger.

Another system that might perform better would be an electronic, 
temperature compensated specific gravity meter with a sensor/probe in 
the battery electrolyte. It would undoubtedly be a messy (yet accurate) 
way to measure battery SOC, but could not be used on AGM/gel batteries.

Todd


Bob-O Schultze, Electron Connection wrote:

>
> Of course, the problem with all shunt based metering, Link 10,  
> TriMetric, etc., is that they can't see self-discharge as it doesn't  
> pass over the shunt.
> That's probably as much a concern in a standby system as anything else.
> Bob-O
> On Nov 2, 2006, at 5:29 AM, Todd Cory, Mt. Shasta Energy Services wrote:


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