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Auxiliary Battery Charging

A

apatti

Well Known Member
All,
I am having trouble keeping my auxillary battery charged. I would appreciate any assistance the group could provide. Here is what I have...

My main battery is a PC680. Coming off the PC680 I have a diode (low forward biased voltage drop around .3v) and a power resistor of around 5 ohms leading to the positive pole on the aux battery. Actually it is two 10 ohm 10W resistors in parallel. I expected that this circuit would keep the aux battery charged to within .3v of the main battery. This is not the case. I most often find the aux battery is around 10.7 or so even when the main battery is around 12.4 or so. If I put the Oddesy charger on the main battery overnight, the aux battery does come up to 11.1 or so. Of course, the main battery voltage increases as well. My annual condition inspection is coming due in July and I expect to replace the aux battery. I'd like to make any necessary changes to the charging circuit then. Any suggestions?
 
Tony,

I have almost the same circuit. I use a 10 ohm but may add another just to speed up the recovery.

So far mine is working really well. Mine charges when the master is on.

Are you sure about the diode you have? I used a schottky diode.

What are you measuring the voltage with? Some devices have diodes on their power input and therefore may indicate a diode drop or two lower than actual.
 
Yes, it is a schottky and is only supposed to drop about .3v. The aux battery measurement is at the input to my GRT EIS. The PC680 voltage is being sensed at the input to the GRT display units. If I let the plane sit for a week or two I will see aux battery voltage down in the 10.3 range sometime. A couple of times one of display units would not boot. Mine is not wired where the master has to be on to charge the aux battery. I come straight off the PC680 to the diode and resistor.

I am thinking I will need to take more detailed measurements with a volt meter when I have good access during my annual.
 
apatti said:
Yes, it is a schottky and is only supposed to drop about .3v. The aux battery measurement is at the input to my GRT EIS. The PC680 voltage is being sensed at the input to the GRT display units. If I let the plane sit for a week or two I will see aux battery voltage down in the 10.3 range sometime. A couple of times one of display units would not boot. Mine is not wired where the master has to be on to charge the aux battery. I come straight off the PC680 to the diode and resistor.

I am thinking I will need to take more detailed measurements with a volt meter when I have good access during my annual.
Click to expand...


I know this is an obvious question, but is the Shottky diode connected in the proper direction? Your comment about how the PC680 voltage came up after charging the aux battery led me to think that could be the problem. Either that or the aux battery has a very high rate of discharge for whatever reason.
 
It almost sounds like your not getting any charge.

I would double check the diode direction as well. The stripe or the cathode needs to be toward the aux battery.
 
12.4 volts is too low for a fully charged main battery. It should be closer to 13 volts.
A fully charged battery can not charge another battery because its voltage is too low. The charging voltage needs to be about 14 volts.
An always-on load could be discharging the aux battery. With everything off, measure the aux battery current with a milliammeter‎.
Joe Gores
 
100% charge = 12.8 volts.
50% charge = 12.2 volts.
25% charge = 11.8 volts.
0% charge = 11.2 volts.

Ideal charge rate = 14.2 volts.

The 100% charge figure is after the battery has been
removed from the charge source and fully cooled down.

Readings are taken directly from the battery posts.
Any remote volt meter must have perfect connections
for accurate readings.
 
So, if my PC680 is sitting at say 12.6v, is it wrong to assume that after considerable time (a week or more) thay my auxillary battery would charge up to around 12.3v (assuming the diode drops .3v when forward biased)? That is what I was thinking. However, I know nothing about battery chemistry.

The only always on loads I have are the clocks for the EFIS's. And, the aux battery voltage being sensed at the input to the GRT EIS would also be a little lower than actual aux battery voltage due to drop in the wiring runs. But, because of the low current draw, I wouldn't expect that to be more than .5v if even that.
 
Maybe use a DC-to-DC converter to charge the aux battery? A quick web search yielded a small unit weighing about 10.5oz that will accept an input voltage ranging from 10-15 volts and output 15v @ 2.5amps max output current.

http://www.powerstream.com/dc-2171.htm

A diode in series with the output of that would give you 14.something volts to charge the AGM-type battery with.
 
So, if my PC680 is sitting at say 12.6v, is it wrong to assume that after considerable time (a week or more) thay my auxillary battery would charge up to around 12.3v (assuming the diode drops .3v when forward biased)? That is what I was thinking. However, I know nothing about battery chem istry
Click to expand...
Read page 3 of this pdf starting with the last paragraph of column 1.
http://www.aeroelectric.com/articles/bat_iso2.pdf

Are the alternator and voltage regulator working? What is the main bus voltage at cruise RPM?
Here is another interesting document about batteries:
http://www.aeroelectric.com/articles/Batteries/Multiple_Battery_Myths_A.pdf

This thread is about using a resistor in the aux battery charging circuit.
http://forums.matronics.com/viewtopic.php?p=316578

When battery voltage drops to 11 volts, it is almost dead.
Joe Gores
 
"When battery voltage drops to 11 volts, it is almost dead.
Joe Gores"

But if you are using it to power your Lightspeed ignition, you can use it all the way down to 5 volts before the ignition quits.
 
Electronic ignitions can operate at much lower than normal voltages. The problem is that as a battery discharges, its voltage drops at a faster and faster rate. After an alternator failure, suppose that battery voltage drops to 11 volts in 20 minutes or about 1 volt every 10 minutes. At that rate a pilot might think that his electrically dependent engine will run for another hour before it gets down to 5 volts. But that is not the case. The voltage will be down to 5 volts in another 10 or 15 minutes. Of course battery size and
load determine actual discharge time. Take a look at the battery discharge curve in figure 3 of this document:
http://www.aeroelectric.com/Mfgr_Data/Batteries/Enersys_Hawker/NPAppManual.pdf
or the discharge graph on this document:
http://www.panasonic.com/industrial/includes/pdf/Panasonic_VRLA_LC-RD1217P.pdf
Notice that the voltage curves almost straight down below 10 volts. The time between 10 volts and zero volts is very short. A battery has no useful energy remaining below about 10 volts.
Joe Gores
 
The only way to really know the endurance of your reserve battery is to do a load test with the loads that you expect during a power failure. A safe percentage say 75% of total use should be placarded near the aux. batt. switch.
 
Ray,
I have been using this for the past 150 hours and it works great for me. Email me if you need more, or giv eme a call.
Bill
 
If an aux battery has a load connected to it, then it will never recharge through a series resistor.
If the aux battery is fully charged initially and the switch is closed, then the load will draw current from the aux battery because its voltage is higher than 12 volts. A load will always draw current from the source with the highest voltage. The battery will never recharge because it needs more than 13 volts and only 12 volts is available.
Increasing the value of the series resistor will make the situation even worse. Below is an example circuit with a load that draws about 2 amps. Of course if there is no load connected, then the aux battery will recharge.
_gQ8ilqK-nkXX4ZPScEsVsZhWOqz2wxKfnnF2PqjZk9aN4MSoxWU4P4_6mCAg8623qnkXTle0NDkPuzBTFqpzKUFvKCDCTc0ovDKuUjotv8LCl4Gkk_gmTT-tEDzVpsmshkHICw8fz1zTgKC2CTqVofAZCmTuWreXPYa7PX-zgBzNh2KJvfaQE3F59Q2Cph2J-rqwe6QdrDZAyCaz9n9rFWuIXvTOcJBKhqoHQvDm3LVRppovRvLhXptto5RsmMkjIlWEC1hE6iGccqHrGykgLEEBcLQ5WOdK_17mnMszChgKYk_5-JS8lmlRgHnihrClPaaF0rk-1us2fvWEbzHRSho40CpsQ29tgqTKJEOJWVqc-7lzlm6J4d4ioI_egrWU4eX_M3nYuQfERId5ZKPEOpOR430t3n7tmd77dnQo7e4CzXqM41ki9XhFTGHlICaTlfdoHlFd322GVWSLTS06clf6Rbc7mQ-__a_m6QuNYdw6BxL7kAc3nUafd29ul7xF0VJhGYXsCkGMNXMKHnnhFFObM3gR-DXFmm_Xra-JmDrBUPIu0GA6Kq6gq5sqmo6PLFbxXqji-o_DkzwuemV1tPUEBxWy5Cy5c6pdlt1KXyuPaKKdUPm=w839-h623-no
 
You should be able to d/c the keep alive circuits on the GRT D/U's, long ago a software update enabled picking up time from the GPS serial data.

Id suggest you measure the voltage at the AUX battery while running and make sure its at least 14 volts.

I also wonder if tying the batteries together like you have (before the master) may confuse or fool the Odyssey smart charger.

BTW I have the same circuit charging a 7ah alarm battery, but without the current limiting resistors, and isolated by the master switch. I know the resistor(s) are there to limit charge current for safety, but they may be keeping you from achieving full charge in the time allowed if your typical flights are too short.
I also suspect this may be a problem with the PC680 that is overlooked when people have problems with them, they need an extended period of time charging during to get full saturation.
Tim Andres
 
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