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Charging EI backup battery ?

Larry DeCamp

Well Known Member
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.
 
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/sterling-power-usa12vto12vbatterymaintainer.aspx
 
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I asked Kathy at EarthX about this and she pointed out that their batteries are limited in maximum allowable charging current.

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.

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.

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.
 
I previously ran a diode resistor setup but I never really liked that due to the charge curve.

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?
 
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.

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)
 
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?

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.
 
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)

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.
 
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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 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.

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-lithium-battery-design

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)

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.
 
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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.

Very nice (and the price is certainly right!)
 
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-lithium-battery-design

Dan,

I had missed this; thanks for posting. The graphs are indeed a bit cryptic because of the unexplained time axis.

It would be really interesting to see someone do side-by-side testing of equivalent Odyssey and EarthX batteries through their respective charge-discharge cycles (preferably done by a knowledgeable 3rd party).
 
This is an interesting thread. I went down the same paths...

  • Diode to isolate your E-bus battery
  • DC to DC charger is interesting, one way to go.
  • Aux battery protection device like the Chinese made ones ($3 to $6) for under/over volt protection with relay. I bought two different ones, one with volt display and push button setting and one with high precision pots... Both have 10A relay and can be NC or NO. **
  • Stand alone High-Low volt indicator light is cool mentioned in this thread. Most Eng monitors have warning for low volt. My ND small alternator drives a no output light as well.

** The Boat industry is all over redundant reliable electrical systems, with industrial strength by companies like Blue Sea.... Boats run banks of main and aux batteries. It is critical to protect the Aux batteries from draining. To be mid ocean and not be able to start your engine(s) is bad news. As was said they are well made but heavy in weight.
 
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The CPI2 has a dedicated backup battery charging circuit built in so you don't need to worry about doing this externally.
 
The CPI2 has a dedicated backup battery charging circuit built in so you don't need to worry about doing this externally.

Thanks Ross. I just got up to speed on your CPI2... I am standing by to see that progress. My plan was two CPI's and do the OV protection myself, but depending on price I think I'll switch to the CPI2.

BTW What is the Max/Min operating voltage for the CPI or CPI2.
 
The CPU will operate down to about 7.5V and we've tested to 18V. There are upper and lower voltage thresholds set in the CPI2 software for different battery chemistries.
 
Alan?s point #6

The Chinese boards Dan found sound great. I have a simple switch to isolate the second ignition buss from diode supply And a blue sea volt meter on the panel. First step on startup is check voltage on the little battery with master OFF. Not elegant, but simple and effective.
 
This is an interesting thread. I went down the same paths...

  • Diode to isolate your E-bus battery
  • DC to DC charger is interesting, one way to go.
  • Aux battery protection device like the Chinese made ones ($3 to $6) for under/over volt protection with relay. I bought two different ones, one with volt display and push button setting and one with high precision pots... Both have 10A relay and can be NC or NO. **
  • Stand alone High-Low volt indicator light is cool mentioned in this thread. Most Eng monitors have warning for low volt. My ND small alternator drives a no output light as well.

** The Boat industry is all over redundant reliable electrical systems, with industrial strength by companies like Blue Sea.... Boats run banks of main and aux batteries. It is critical to protect the Aux batteries from draining. To be mid ocean and not be able to start your engine(s) is bad news. As was said they are well made but heavy in weight.

I am setting up a 2 batt system (PC680's) in the 10, with the second batt running a b/u bus. I am putting a 20A diode from the main bus to the b/u batt, but also have a second contactor to put batt 2 on the main bus. The plan is that any significant discharged state on batt #2 will require me to remember to have the #2 contactor closed before the alternator starts charging or my diode will blow. The second contactor also lets me use both batt's for starting, if necessary.

Larry
 
I am setting up a 2 batt system (PC680's) in the 10, with the second batt running a b/u bus. I am putting a 20A diode from the main bus to the b/u batt, but also have a second contactor to put batt 2 on the main bus. The plan is that any significant discharged state on batt #2 will require me to remember to have the #2 contactor closed before the alternator starts charging or my diode will blow. The second contactor also lets me use both batt's for starting, if necessary.

Larry

Sounds like you need to rethink that "plan", or have plenty of spare diodes on hand :eek:
 
Sounds like you need to rethink that "plan", or have plenty of spare diodes on hand :eek:

What would you recommend? Do you see problems / risks, other than letting charge level currents through diode?

I am open to help and suggestions. I had also considered a 30A diode for better margin.

I also have a backup. If the diode blows, I see the b/u bus voltage drop and energize the batt 2 contactor to keep it charged. Then drop the contactor if I lose the main bus or alternator.

Larry
 
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I am setting up a 2 batt system (PC680's) in the 10, with the second batt running a b/u bus. I am putting a 20A diode from the main bus to the b/u batt, but also have a second contactor to put batt 2 on the main bus. The plan is that any significant discharged state on batt #2 will require me to remember to have the #2 contactor closed before the alternator starts charging or my diode will blow. The second contactor also lets me use both batt's for starting, if necessary. Larry
I like to see a diagram to understand what you are doing AND how the diode can blow. Here are my thoughts.

1) Reconsider having a dual large battery starter idea, it may not be necessary, adding weight and complexity, with undesirable failure mode. The #2 battery can be much smaller, with the intent to provide a second independant power source for essential in-flight electrical systems.

2) At no time should you be worried about blowing a diode.


1) Two full size batteries to have extra crank power will add weight and complexity. A good battery, starter and engine in good repair, makes starting on one battery routine, minimizing the value of dual batteries. Typically the #2 battery for the E-bus (essential bus) is a small battery, eg 7.5 Ahr (for weight and size) and good to drive the emergency or essential bus for +45 minutes. The whole idea is to isolate EFIS and or Elect Ignition for min flight power until you can get on the ground.

You might consider battery terminals accessible from cockpit or outside fuselage so you can attach a battery charger, without removing a lot of access panels. If you end up with dead battery you can charge it. Another tried and true design (to add battery to start) is an external AUX jack or receptacle (AN2551, MS3506-1) to charge or jump the airplane with dead battery. You can carry one of those Li-ion jump packs. I'm not a fan of jumping a totally dead battery, puts a heavy strain on the alternator. When jumping of make sure polarity is correct, check and double check. Don't trust FBO ramp'er. https://www.aircraftspruce.com/cate...p/menus/el/jumpercablesplugs_receptacles.html

2) A Schottky diode as you know is a one way valve. In the Fwd mode you get 0.5 volt drop. In the reverse mode you get 100% voltage drop... Schottky come in ratings to handle +100 of amps. Typically (in my mind) the Schottky diode allows the main bus (alternator) to charge the second battery (E-bus), but it keeps the second battery from being drained by main bus (one way flow). The Schottky diode connecting the #2 battery main bus will see little or no current when starting. As you said you will provide second battery contractor (relay) to parallel #2 battery for starting (IMHO not needed). Relay, switch, heavy gauge wires to do this adds weight, complexity and cost. One way you might overload diode fwd current rating is due to battery charging + bus load. I doubt a battery charging will ever take more than 15 amps. You should not have more than 15 amps load on the E-bus (EFIS, EIS, EI, Com). 30 Amp should be enough, but you should run engine until current drops down before turning off your parallel relay.

Another way to go is use the EFIS and Electronic ignition (CPI2) back up batteries integral to their design... A lot of electrical items now have their own back up scheme...
 
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Shottkey diode

I bought one recommended here on VAF. It had huge amperage rating but the attractive part was relatively low voltage drop. If anyone wants details, I will dig up the info.
 
I like to see a diagram to understand what you are doing AND how the diode can blow. Here are my thoughts.

1) Reconsider having a dual large battery starter idea, it may not be necessary, adding weight and complexity, with undesirable failure mode. The #2 battery can be much smaller, with the intent to provide a second independant power source for essential in-flight electrical systems.

2) At no time should you be worried about blowing a diode.


1) Two full size batteries to have extra crank power will add weight and complexity. A good battery, starter and engine in good repair, makes starting on one battery routine, minimizing the value of dual batteries. Typically the #2 battery for the E-bus (essential bus) is a small battery, eg 7.5 Ahr (for weight and size) and good to drive the emergency or essential bus for +45 minutes. The whole idea is to isolate EFIS and or Elect Ignition for min flight power until you can get on the ground.

You might consider battery terminals accessible from cockpit or outside fuselage so you can attach a battery charger, without removing a lot of access panels. If you end up with dead battery you can charge it. Another tried and true design (to add battery to start) is an external AUX jack or receptacle (AN2551, MS3506-1) to charge or jump the airplane with dead battery. You can carry one of those Li-ion jump packs. I'm not a fan of jumping a totally dead battery, puts a heavy strain on the alternator. When jumping of make sure polarity is correct, check and double check. Don't trust FBO ramp'er. https://www.aircraftspruce.com/cate...p/menus/el/jumpercablesplugs_receptacles.html

2) A Schottky diode as you know is a one way valve. In the Fwd mode you get 0.5 volt drop. In the reverse mode you get 100% voltage drop... Schottky come in ratings to handle +100 of amps. Typically (in my mind) the Schottky diode allows the main bus (alternator) to charge the second battery (E-bus), but it keeps the second battery from being drained by main bus (one way flow). The Schottky diode connecting the #2 battery main bus will see little or no current when starting. As you said you will provide second battery contractor (relay) to parallel #2 battery for starting (IMHO not needed). Relay, switch, heavy gauge wires to do this adds weight, complexity and cost. One way you might overload diode fwd current rating is due to battery charging + bus load. I doubt a battery charging will ever take more than 15 amps. You should not have more than 15 amps load on the E-bus (EFIS, EIS, EI, Com). 30 Amp should be enough, but you should run engine until current drops down before turning off your parallel relay.

Another way to go is use the EFIS and Electronic ignition (CPI2) back up batteries integral to their design... A lot of electrical items now have their own back up scheme...

The 10 probably needs the weight of two 680's. My EFIS and two EI's will draw around 5 amps and I would like the ability to run the GPS and possibly the transponder (I fly a lot of IFR). So my desired emergency load is 8+ amps. A 680 discharging at that rate will only give me 1.5 hours in perfect condition and will get closer to an hour with some age.

The second battery has three connections. 1) The bridge to the main bus with the diode for trickle charge. 2) A contactor to bridge it to the main bus when needed/desired. 3) A feed through a breaker switch (20A) for the backup bus. Each leg of this bus bridges with the main bus feeds just before each breaker that is backed up. The main bus branch feeds to these CB's have diodes to prevent back draining.

My concern with the diode is that if I don't energize the batt 2 contactor and batt 2 is low, it can easily draw more than 20 amps once the alternator starts pumping out juice. I have seen my 680 on the 6 take 40+ amps for a short period after an extended starting effort. I believe that with the contactor closed, it should take the bulk of the load.

I have also considered two diodes in parallel to increase throughput and possible redundancy. However I don't know the downside if any. I also believe that if I blow a diode, I will see the volts drop on the b/u buss and can close the contactor to keep the battery charged.

I have two charging jacks cut into the baggage bulkhead for charging.

Larry
 
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The 10 probably needs the weight of two 680's. [snip]...

My concern with the diode is that if I don't energize the batt 2 contactor and batt 2 is low, it can easily draw more than 20 amps once the alternator starts pumping out juice. [snip]

Larry
Sounds like you have a plan... Good luck. Have fun. :) As far as 40 amp draw to charge the #2, that is a lot, even if transient. I believe you, but I'm also of the opinion for a PC680 (18 Ahr) battery, the initial draw will drop off fairly quickly.

I have a spare PC680 that is 50% charged to test. My charger is micro processor controlled so it won't allow current spike. I think attaching to a large fully charged automotive lead acid battery with car running might be best. If the transient is too fast I might miss it. However if it is that fast I wounder if the diode would notice. I think those diodes are good for transient (suppressing spike)? Interesting & fun to think about. The previously mentioned charger would take care of this and moderate charging current.

Go to a bigger diode? That might increase forward voltage drop, which you don't want. Anyway I understand what you are doing now.
 
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Sounds like you have a plan... Good luck. Have fun. :) As far as 40 amp draw to charge the #2, that is a lot, even if transient. I believe you, but I'm also of the opinion for a PC680 (18 Ahr) battery, the initial draw will drop off fairly quickly.

I have a spare PC680 that is 50% charged to test. My charger is micro processor controlled so it won't allow current spike. I think attaching to a large fully charged automotive lead acid battery with car running might be best. If the transient is too fast I might miss it. However if it is that fast I wounder if the diode would notice. I think those diodes are good for transient (suppressing spike)? Interesting & fun to think about. The previously mentioned charger would take care of this and moderate charging current.

Go to a bigger diode? That might increase forward voltage drop, which you don't want. Anyway I understand what you are doing now.

I have seen 40+ amps on the screen for maybe 30-60 seconds. In fairness, my steady state draw is around 15 amps, so the battery was only taking around 30 amps.

Larry
 
The second battery has three connections. 1) The bridge to the main bus with the diode for trickle charge. 2) A contactor to bridge it to the main bus when needed/desired. 3) A feed through a breaker switch (20A) for the backup bus. Each leg of this bus bridges with the main bus feeds just before each breaker that is backed up. The main bus branch feeds to these CB's have diodes to prevent back draining.

My concern with the diode is that if I don't energize the batt 2 contactor and batt 2 is low, it can easily draw more than 20 amps once the alternator starts pumping out juice. I have seen my 680 on the 6 take 40+ amps for a short period after an extended starting effort. I believe that with the contactor closed, it should take the bulk of the load.

I have also considered two diodes in parallel to increase throughput and possible redundancy. However I don't know the downside if any. I also believe that if I blow a diode, I will see the volts drop on the b/u buss and can close the contactor to keep the battery charged.

I have two charging jacks cut into the baggage bulkhead for charging.

Larry

This back up battery charging challenge has been around for a long time, a search of the archives will provide much reading material.

Implementing a system with a known failure point if "procedures" are not followed is a bad idea, you're just asking for problems (or anyone else who may operate the aircraft).

Options are switch to a dual battery system, examples from the 'Z' diagrams will help you there, or limit the charge current on the "backup" battery.
 
This back up battery charging challenge has been around for a long time, a search of the archives will provide much reading material.

Implementing a system with a known failure point if "procedures" are not followed is a bad idea, you're just asking for problems (or anyone else who may operate the aircraft).

Options are switch to a dual battery system, examples from the 'Z' diagrams will help you there, or limit the charge current on the "backup" battery.

Thanks for your input. I'll do more searching.

Larry
 
Reviving this thread a bit since I am planning to switch to dual EI in the future and need to come up with a simple system. Battery charging is on issue. I did some searching after reading through this thread and found this charger:

Tekonsha Two Stage 12 V DC Battery Charger. It is 5.5x3.75x1.5". A little larger than what I would like, but more compact than some other options. Listed as 0.25 pounds.

2024-07.png


The plan would be to connect this from the main buss. I assume it has internal diodes, so it doesn't back feed from the aux battery.

The ignition system would be fed from the battery buss and Aux battery via diodes. The system would then draw power from whichever has the higher voltage. In a resistor trickle charge system, this would work since the Aux battery voltage would always be lower than the main voltage (with the alternator working). However, I couldn't find a spec on the output voltage for this charger. For all I know it could have a step-up regulator. If it just uses system voltage it will likely be lower. I don't want a manual switch of any kind to change power source.
 
Colin,

What battery are you planning to charge with this?

I’m confused, why not just charge the batteries using the alternator? For that matter why a “resistor trickle charge”?

Carl
 
Colin,

What battery are you planning to charge with this?

I?m confused, why not just charge the batteries using the alternator? For that matter why a ?resistor trickle charge??

Carl

Not using a full-size battery for backup. It would be a small 4ah AGM or similar. Connecting directly into the system with the PC680 would result in overcharging that little battery.
 
Not using a full-size battery for backup. It would be a small 4ah AGM or similar. Connecting directly into the system with the PC680 would result in overcharging that little battery.

Sorry, no. Healthy batteries in parallel do not act that way. There is no issue with having different size batteries (assuming same chemistry) in parallel on an alternator. Each battery only cares about terminal voltage. From that it will draw whatever charging current it gets based on battery internal resistance.

I also suggest that a 4ah battery is far too small for an electrically dependent engine backup battery.

PM me if you want details.
Carl
 
Sorry, no. Healthy batteries in parallel do not act that way. There is no issue with having different size batteries (assuming same chemistry) in parallel on an alternator. Each battery only cares about terminal voltage. From that it will draw whatever charging current it gets based on battery internal resistance.

I also suggest that a 4ah battery is far too small for an electrically dependent engine backup battery.

PM me if you want details.
Carl

I'm no battery expert, that is just based off what I have read here. If that is the case, why do TWC and companies like that get such a premium for their backup batteries?

If I can get away with just the diodes, that is one less component to fail, so sounds good to me.

The backup is only for the ignition. Average current draw per side should be less than 1.5amps. So the 4ah battery should give me up to an hour assuming a total electrical failure. SDS backup uses a 2.9ah battery, but I'm not sure if that is for a single or dual system.
 
If you use AGM battery, which is a lead/ acid chemistry, internal resistance is high enough to self limit charge current to an acceptable level. You can connect directly to the bus for charging.

If you are talking about the TCW battery, the cells are LiFeO2, which have very low internal resistance, and will not self limit without a control circuit.
 
If you use AGM battery, which is a lead/ acid chemistry, internal resistance is high enough to self limit charge current to an acceptable level. You can connect directly to the bus for charging.

If you are talking about the TCW battery, the cells are LiFeO2, which have very low internal resistance, and will not self limit without a control circuit.


Ok, that makes sense. This is why the forum is so great, get to learn something new every day.
 
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