All the discussion about batteries, their use, charging and maintenance got me to thinking and, ultimately, worried about the standard installation of ignition backup batteries.  There does seem to be important differences between power batteries and stand-by batteries with respect to discharge/recharge cycles and maintenance (avoiding the AH loss over time with no recharging). 

 

First, a description of my Light Speed Engineering electronic ignition backup battery setup.  Note, this discussion is for a 12 VDC system only.

 

 

As can be seen, the left (upper plugs) ignition can be powered by the Main bus (M) or the Backup Battery (BB).  Note that although not diagrammed here, the right ignition is powered by M thru the Essential bus (E).  In order to monitor many possibilities, the VM EPI 800 voltage/charge display is switchable between various sources.  The EPI 800 itself is powered off E. 

 

Use:

The main 28 AH B&C BC110-1 battery (hereafter P) use is to supply power to M and E (directly or indirectly) for starting, electrical noise ballast and limited aircraft power thru E should something go wrong with elements associated on M.

 

The Backup Battery (BB) use is to supply full power to the left ignition for starting (not affected by starter motor draw) and otherwise to act as a standby battery with the sole function of keeping the left ignition operating long enough to reach a landing site should the rest of the electrical system fail.  Its' selector switch is positioned back to normal after engine start.  It is a Power Sonic 5 AH SLA AMG battery with specifications described in the attached .pdf file.  Further information can be found at:

 

 

Charging:

P is directly charged off M by the B&C 70 A alternator thru the B&C LR3C voltage regulator. The charging voltage is from 14.3 to 14.6 VDC.

 

BB is also charged off M, but thru the Perihelion Power Diode that only has a voltage drop of .2 to .4 VDC, variable depending on the current being passed.  Thus, the BB charging voltage should be from 14.1 to 14.4 VDC.  This diode is used to prevent backfeeding as is the same diode connecting M to E.  This is much better than other bridge diodes that frequently see a drop of .6 to .7 VDC.  See the link below for more information:

 

 

Maintenance:

Generally, SLA batteries can be maintained at a float voltage of 13.7 to 13.8 VDC. This is common for "stand-by" batteries.

 

P is only charged during flight.  It sits idle between flights except for occasional stand-alone ground use and restoration thru a Battery Tender that can charge and maintain a float voltage.  This is sometimes hooked up if there will be 3 or more weeks between flights.  I do not know the degradation rate of the BC110-1.

 

BB is only charged during flight. The voltage is above the float voltage.  Power Sonic's pertinent charger produces voltage in the range 13.65 to 14.7 VDC depending on the sensed condition of the battery.  There is no provision to maintain it on the ground since the Battery Tender pigtail is connected directly to P's terminals.  I.E. No juice can get to BB.  The degradation rate for disuse is available in the attached .pdf.

 

Data:

 

In the hangar at 60F and after 1 week disuse, the E bus was engaged to power up the EPI 800 and the voltmeter.

E reading 12.7-12.8 VDC (E sw on, Mstr Sw off). Measured under load (EPI 800, gear lights, Turn coordinator, etc) - see Panasonic FAQs for info about measurement under load.

B reading 13.1 VDC or 2.18 v per cell and no load (ign sw off, BB sw on). 

Master switch was turned on and the following readings were taken (M load includes E, master relay, alt field, etc):

M = 12.3 VDC, -8 A (charge/discharge sensor on battery cable to master relay only)

E = 11.9 VDC (E sw off)

1200 RPM 2 minutes after start:

M = 14.3 VDC +2 A with radios, strobe, etc.

E = 14.1 (E sw off)

B = 14.3 (main bus VDC, BB not selected)

B = 13.9-14.1 with BB selected.

 

In flight at 3500 MSL (1290 D alt), -5C, 2480 RPM:

M = 14.3 - 14.4 VDC +2 A

E = 13.9 - 14.1 VDC (E sw off)

B = 13.9 (BB sw on) This is lower than expected and is near the float voltage.

 

Note, there is a blast tube directing cold air to the voltage regulator located on the engine side of the firewall.  In 20C flight conditions I usually see 14.6 VDC on the M bus.  The voltage regulator does not have a battery temp sensor attached.

 

The poser:

 

Should a voltage regulator especially for a stand-by backup battery be constructed that senses battery condition that charges when necessary and provides 13.8 VDC float voltage during stand-by times if no charge is needed?

 

The BB diode drops the charging voltage .2 currently (to 13.9 - 14.1 VDC), this is good because it is almost the float voltage but in the summer the voltage will be much higher than the float voltage.  Am I damaging the stand-by backup battery?

 

Does anyone with an electronic ignition care?