Charging EI backup battery ?
 

I purchased duel ignition units for SDS CPI system. Ross sent me a schematic for a 7AH battery to power the second unit as backup. My question is, can I use the main buss alternator voltage to charge TWO Earthex batteries. The second to power one ignition as backup if needed.

Larry,

I asked Kathy at EarthX about this and she pointed out that their batteries are limited in maximum allowable charging current. For the ETX12A for example this limit is 20 amps. I noted that in a backup capacity the battery would normally be at full charge and thus would likely never pull anywhere close to 20 amps. Kathy nonetheless felt that the backup battery should have a charging current capacity equal to or greater than output capacity of alternator that was charging it. So, the backup battery would either need to be relatively large (and expensive), or else it would need to be charged on the ground rather than via the main aircraft bus. The latter is probably not a bad solution since these batteries are supposed to hold their charge well?

I've been using this Sterling battery maintainer unit for about a year now to charge my avionics back up battery (EarthX 12A). Operation has been flawless to date. I previously ran a diode resistor setup but I never really liked that due to the charge curve. The Sterling unit is more of a constant current 3A unit and charges the backup battery much quicker than the old diode/resistor setup. Great service and communication from the company as well.

http://www.sterling-power-usa.com/st...aintainer.aspx

Also true of an ordinary AGM battery. Recently a battery manufacturer told me a good rule of thumb was 10% of the Ah rating for normal charge, perhaps 20% for a fast charge. My secondary ignition battery is 3.8 Ah, so we're talking milliamps.

True, in normal operation the charge rate is almost non-existent. The risk is an alternator driven recharge after the little battery is discharged by leaving the switch on while parked. That's when discipline will require some controlled charging time before flight.

An alternate plan might be to add current limiting. The ignition pulls a few amps at most (varies with brand), so a quality 15 amp fuse or breaker in the feed to that ETX12A should ensure it can't be whacked too hard. Mine has a fusible link, a really reliable form of short circuit protection, but not real accurate for current limiting.

Walt, can you tell us a bit more about that charge curve? Something to do with the diode's voltage drop vs the higher charge voltage requirement of the EarthX?

That seems like a reasonable way to deal with the problem. Presumably you also have a way to check the backup battery voltage during preflight to make sure it actually has a charge? (the fusible link won't let you know when its blown)

Too early for number crunching but the bottom line is a resistor/diode combo represents a compromise of max charge current vs charge rate. If you actually run the battery down a bit the "typical" flight will not be sufficient to recharge the battery.

The 3 amp charge rate of the Sterling unit is pretty close to an ideal rate for most small batteries and will top off the battery much quicker than say a 100ma charge rate.

Funny you should mention that. I am right now looking around for some sort of voltage monitor for the IGN#2 battery system. Preferably it would turn on an LED. I already have some nice flashing LEDs for other warnings so I'd just add another to the row.

This one seems interesting. I ordered two and will report in due course:

http://www.tomtop.com/modules-219/p-e1021.html

If the monitored line exceeds a set upper or lower V limit, it closes a relay. I'd probably hide it under the panel, using the built in display for setting only.

Xantrex Echo Charge
 

After playing around with a few different systems I ended up with a Xantrex Echo Charge charging a 12A AGM battery.

It only charges the battery when the bus voltage is above 13V.
I have a hall effect current sensor on that wire, it typically starts charging at 2A and then after about 20 minutes it's down to 1A and after a while it drops to 0.

This thing isn't very small (it's made for boats) and it weighs around 1lb, but it works great.

Lenny

I was poking around on the EarthX website last night. Turns out they recommend using a diode between the main battery and a small backup battery, to drop some voltage on purpose, and thus limit charge current. The graphs don't make much sense to me, but here, read for yourself:

http://earthxbatteries.com/dual-bus-...battery-design

Previously mentioned I was looking at some very inexpensive voltage monitor boards. Ordered two kinds, and I've been playing a bit on the bench tonight. This one seems to be the ticket:

http://www.tomtop.com/modules-219/p-e1021.html

Costs a whopping $4.59. Voltage readout matches my trusty Fluke. Easy to set upper and lower limits in 0.1V increments. Closes an on-the-board relay if voltage reaches either limit, and sets off a buzzer. It will watch voltage on the #2 ignition's battery bus all the time.