I agree with the consumer voltmeter comment. My cheapy digital VM appears to
under read the voltage of fully charged batteries. However about charging
voltage the distributor of the RG batteries we sell recommend an alternator
charging voltage of 14.4V. They say "Our batteries are compatible with the
standard alternators used on cars and A/C. The alternator should be set at
l4.4V (25C)". If the charging voltage is too low the battery will never
reach full charge and the operating life of the battery will be reduced.
Generally the Rotax 912 alternator and regulator appear to work well. I have
used my battery for 3.5 years with no problem and I put it on a good quality
car charger about 2 times per year when the A/C comes in for service or
permit.
The table, below, gives Open Circuit Voltages that indicate state of charge.
Figures shown represent readings after 24 hours rest.
Approx Capacity % Open V
100 13.00
75 12.70
50 12.40
25 12.10
0 11.80
Jerry
Jerry@ban-bi.com or LTS@avnet.co.uk
www.Ban-bi.com or www.avnet.co.uk/touchdown
----- Original Message -----
From: "Fred Fillinger" <fillinger@ameritech.net>
Subject: Re: Charging and Alternators
> Kevin Taylor wrote:
> > ...
> > I put a digital AVO on to get an accurate reading and I'm finding the
> > battery is reading 12.9 volts when off load and engine off. With the
engine
> > running its reading 14.2. My interpretation of this is that the
alternator
> > is permanently charging the battery and therefore overcharging.
>
> Unfortunately, even a digital AVO or "multimeter," but consumer-grade,
> can be assumed not suited to the task here. Cumulative errors in the
> thing as little as a 2%, ignoring the least-significant-digit problem,
> can cause +/-20% error in reading the actual "state of charge" on a
> lead-acid battery, and each .1V in charge volts is significant at full
> charge.
>
> Indeed, if a used-battery is reading 12.9V after sitting for some
> hours, even for a recombinant gas type, or "AGM" or "VRLA" (but not
> necessarily gel cell), that means an indicated 14.2 charge volts,
> after replenishing starting energy, might be 2-4 tenths more than
> actual, which is not over-charging. Though may be upper limit for an
> RG battery, as they say never "float" them much beyond 13.5-13.8V.
>
> Due to surface charge, reading standing volts can be irrelevant
> without following mfr's recommendation to deal with it, and regulator
> voltage doesn't say much more and is particularly dependent upon
> battery type.
>
> A measure which ignores all variables is the actual current into any
> type battery, and at any temp, using its ampere-hour rating. After
> starting power has been replenished, current at float voltage is
> typically 1% of its AH rating; "overnight" charge is 10%. Anything
> between these two should be fine. At the high end for flooded-cell
> batteries, it's overcharging a fully-charged battery, but the way
> recreational airplanes are used it will not harm the battery.
> Fully-charged RG's may not like 10% x AH.
>
> W/o any panel ammeter that can resolve down to an amp or even less,
> there are ways to check it with even a $10 multimeter and not a bad
> idea for RG's. Manually bring the batt to full charge. With a
> rear-mounted battery, 10 feet of #4 cable will show 2.5 millivolts
> across its length, per amp. Scale for shorter/longer cable. Start
> engine and read the drop. For firewall mount, exactly one foot of #20
> _aircraft_ wire, carefully stripped to not cut any strands, will show
> a drop of 10mV per amp. Securely wire it between a battery post and
> either cable, jumper-cable across or temporarily attach cable to
> start, then disconnect and similarly read the voltage drop at
> necessary RPM and time to replenish starting loss. Yank fuses or turn
> off everything in the plane not needed, since if the #20 wire is
> accidentally disconnected, things can be damaged when powered by
> alternator current w/o a battery!
>
> Regards,
> Fred F.
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