A complicated question about LiFePO4 batteries, - and switching.

On Tuesday, July 24, 2018 at 12:05:18 PM UTC-4, OHM Ω http://aviation.derosaweb.net wrote:
On Tuesday, July 24, 2018 at 9:31:44 AM UTC-5, Dan Marotta wrote:
In my last experimental glider I used two rather stout SPST toggle
switches to control my two LiFePO4 batteries.Â* At some point of the
flight I would switch the second battery on and then switch the first
battery off.Â* I don't think it was really necessary given the longevity
of these batteries compared with the SLAs that I used previously, I
simply switched about half way through the flight so that both batteries
would get some use.Â* I like John's idea with the MOSFETs.

On 7/23/2018 10:43 PM, Andy Blackburn wrote:
On Wednesday, June 20, 2018 at 6:27:33 AM UTC-7, OHM Ω http://aviation.derosaweb.net wrote:
Andy,

I have used the ideal diodes mentioned by Shaun McLaughlin (http://re-voltage.eu/ US$27 each) as I was given a pair to test with. They do what they say with minuscule voltage drops across them. Full disclosure, I am currently using two 1N5821 Schottky diodes in my panel right now (page 74 of my presentation) - small, simple, cheap, plentiful, robust, and easy to mount.

That being said - the availability of inexpensive ideal diodes has exploded in recent years mainly for use in systems like solar panel arrays. See https://www.ebay.com/sch/i.html?_fro...es%29&_sacat=0. I also like the increasing range of mounting styles available allowing for placement options behind our increasingly cramped panels.

John (OHM),

Yup, I have one of Shaun's (on your recommendation) and some of the anti-revere-irrigation ones. I haven't installed either.

My current install uses the Schleicher rotary switch to select: Battery 1, Battery 2, Battery 1+2 (through diodes), Battery 3 (tail). I use a bank of capacitors connected through a power resistor to energize the circuit when the switch is between batteries.

If I had to do it again I would use ideal diodes (though as others have mentioned this is really not nearly a necessary with LiFePO4 batteries since they go through voltages below 11v quite quickly on my battery tester).. I would also separate the batteries to individual switches to eliminate the switch as a single point of failure.

Andy Blackburn

9B

--
Dan, 5J

Dan - Do you have diodes in this circuit to prevent cross-charging of the batteries?

I will repeat something I said above that (without diodes) putting the batteries in parallel can cause a large current to flow from the "high" battery to the "low" battery. Effectively one battery is charging the other.

For dumb SLA batteries this will almost certainly blow the fuse on your batteries (you do have a fuse right at your battery's positive terminal, right??).

On lithium batteries, which probably have intelligent current limiting electronics inside, this should not be an issue. But notice the words "probably" and "should".

YMMV. Be careful out there.

- John OHM Ω

How much current do you project when switching from a low LiFe at 12.4 volts(pretty much done) to high at 13.7? I have been doing this switching with SLA batteries for 25 years. Switch new one on- switch old off a second or two later. Have not blown up anything yet.
UH

On Tuesday, July 24, 2018 at 10:30:11 AM UTC-7, kinsell wrote:
On 07/24/2018 10:05 AM, OHM Ω http://aviation.derosaweb.net wrote:
On Tuesday, July 24, 2018 at 9:31:44 AM UTC-5, Dan Marotta wrote:
In my last experimental glider I used two rather stout SPST toggle
switches to control my two LiFePO4 batteries.Â* At some point of the
flight I would switch the second battery on and then switch the first
battery off.Â* I don't think it was really necessary given the longevity
of these batteries compared with the SLAs that I used previously, I
simply switched about half way through the flight so that both batteries
would get some use.Â* I like John's idea with the MOSFETs.

On 7/23/2018 10:43 PM, Andy Blackburn wrote:
On Wednesday, June 20, 2018 at 6:27:33 AM UTC-7, OHM Ω http://aviation.derosaweb.net wrote:
Andy,

I have used the ideal diodes mentioned by Shaun McLaughlin (http://re-voltage.eu/ US$27 each) as I was given a pair to test with. They do what they say with minuscule voltage drops across them. Full disclosure, I am currently using two 1N5821 Schottky diodes in my panel right now (page 74 of my presentation) - small, simple, cheap, plentiful, robust, and easy to mount.

That being said - the availability of inexpensive ideal diodes has exploded in recent years mainly for use in systems like solar panel arrays. See https://www.ebay.com/sch/i.html?_fro...es%29&_sacat=0. I also like the increasing range of mounting styles available allowing for placement options behind our increasingly cramped panels.

John (OHM),

Yup, I have one of Shaun's (on your recommendation) and some of the anti-revere-irrigation ones. I haven't installed either.

My current install uses the Schleicher rotary switch to select: Battery 1, Battery 2, Battery 1+2 (through diodes), Battery 3 (tail). I use a bank of capacitors connected through a power resistor to energize the circuit when the switch is between batteries.

If I had to do it again I would use ideal diodes (though as others have mentioned this is really not nearly a necessary with LiFePO4 batteries since they go through voltages below 11v quite quickly on my battery tester).. I would also separate the batteries to individual switches to eliminate the switch as a single point of failure.

Andy Blackburn

9B

--
Dan, 5J

Dan - Do you have diodes in this circuit to prevent cross-charging of the batteries?

I will repeat something I said above that (without diodes) putting the batteries in parallel can cause a large current to flow from the "high" battery to the "low" battery. Effectively one battery is charging the other.

For dumb SLA batteries this will almost certainly blow the fuse on your batteries (you do have a fuse right at your battery's positive terminal, right??).

On lithium batteries, which probably have intelligent current limiting electronics inside, this should not be an issue. But notice the words "probably" and "should".

YMMV. Be careful out there.

- John OHM Ω


In one of the many previous incantations of this discussion, the same
claim was made about huge currents flowing between a charged and
discharged SLA. At my urging, the claimant actually tried doing that
with an ammeter. Didn't see even a flicker of the needle.

You know how you have to apply 14 volts to charge up a 12 volt battery?
A charged battery sitting at probably 12.6V has basically no capability
of charging a moderately discharged battery.

Switching to lithiums, maybe they have current limiting, maybe they
don't. RIchard P shorted one out, the terminals got hot enough to melt
the case in that area. Doesn't sound like much current limiting going
on to me.

Talking to a pilot who just switched to lithiums, he was touting how
great the current limiting feature was. I asked if he ever shorted the
terminals on purpose to test it? Well no. Would he be willing to try
that? Again no. If you really believe you have limiting, then go ahead
and put the wrench across the terminals just to satisfy yourself. Film
it for use on YouTube, just in case.

-Dave

That happens because the internal resistance of the battery performs a balancing function: when the higher voltage donor battery supplies current, its voltage drops due to the IR voltage drop across its internal resistance. The discharged battery voltage, likewise, rises, again due to its internal resistance. These two voltages equalize to some intermediate voltage. If the bad battery has high internal resistance, which is very likely, the actual current flow between the two is correspondingly low.

Most lay people I talk to are unaware of battery internal resistance, and don't know how to analyze a simple dc circuit if they are. This series of lectures will start to get you up to speed on doing this (you will need to watch the whole series):
https://www.khanacademy.org/science/...ircuits-part-1
I consider this time well spent in that it will aid you in troubleshooting elementary electrical problems. Hey we are not talking differential equations here!

Tom

On Tuesday, July 24, 2018 at 10:30:11 AM UTC-7, kinsell wrote:
On 07/24/2018 10:05 AM, OHM Ω http://aviation.derosaweb.net wrote:
On Tuesday, July 24, 2018 at 9:31:44 AM UTC-5, Dan Marotta wrote:
In my last experimental glider I used two rather stout SPST toggle
switches to control my two LiFePO4 batteries.Â* At some point of the
flight I would switch the second battery on and then switch the first
battery off.Â* I don't think it was really necessary given the longevity
of these batteries compared with the SLAs that I used previously, I
simply switched about half way through the flight so that both batteries
would get some use.Â* I like John's idea with the MOSFETs.

On 7/23/2018 10:43 PM, Andy Blackburn wrote:
On Wednesday, June 20, 2018 at 6:27:33 AM UTC-7, OHM Ω http://aviation.derosaweb.net wrote:
Andy,

I have used the ideal diodes mentioned by Shaun McLaughlin (http://re-voltage.eu/ US$27 each) as I was given a pair to test with. They do what they say with minuscule voltage drops across them. Full disclosure, I am currently using two 1N5821 Schottky diodes in my panel right now (page 74 of my presentation) - small, simple, cheap, plentiful, robust, and easy to mount.

That being said - the availability of inexpensive ideal diodes has exploded in recent years mainly for use in systems like solar panel arrays. See https://www.ebay.com/sch/i.html?_fro...es%29&_sacat=0. I also like the increasing range of mounting styles available allowing for placement options behind our increasingly cramped panels.

John (OHM),

Yup, I have one of Shaun's (on your recommendation) and some of the anti-revere-irrigation ones. I haven't installed either.

My current install uses the Schleicher rotary switch to select: Battery 1, Battery 2, Battery 1+2 (through diodes), Battery 3 (tail). I use a bank of capacitors connected through a power resistor to energize the circuit when the switch is between batteries.

If I had to do it again I would use ideal diodes (though as others have mentioned this is really not nearly a necessary with LiFePO4 batteries since they go through voltages below 11v quite quickly on my battery tester).. I would also separate the batteries to individual switches to eliminate the switch as a single point of failure.

Andy Blackburn

9B

--
Dan, 5J

Dan - Do you have diodes in this circuit to prevent cross-charging of the batteries?

I will repeat something I said above that (without diodes) putting the batteries in parallel can cause a large current to flow from the "high" battery to the "low" battery. Effectively one battery is charging the other.

For dumb SLA batteries this will almost certainly blow the fuse on your batteries (you do have a fuse right at your battery's positive terminal, right??).

On lithium batteries, which probably have intelligent current limiting electronics inside, this should not be an issue. But notice the words "probably" and "should".

YMMV. Be careful out there.

- John OHM Ω


In one of the many previous incantations of this discussion, the same
claim was made about huge currents flowing between a charged and
discharged SLA. At my urging, the claimant actually tried doing that
with an ammeter. Didn't see even a flicker of the needle.

You know how you have to apply 14 volts to charge up a 12 volt battery?
A charged battery sitting at probably 12.6V has basically no capability
of charging a moderately discharged battery.

Switching to lithiums, maybe they have current limiting, maybe they
don't. RIchard P shorted one out, the terminals got hot enough to melt
the case in that area. Doesn't sound like much current limiting going
on to me.

Talking to a pilot who just switched to lithiums, he was touting how
great the current limiting feature was. I asked if he ever shorted the
terminals on purpose to test it? Well no. Would he be willing to try
that? Again no. If you really believe you have limiting, then go ahead
and put the wrench across the terminals just to satisfy yourself. Film
it for use on YouTube, just in case.

-Dave

All batteries will current limit: that is what their internal resistance does. Again, it is Ohm's law:
I = V / R
where R is the internal resistance. R is about 0.01 ohm on a typical glider SLA battery, so I = 120 amp. Now, you are going to get heating as a result of that energy dissipation, in watts:
W = I^2 * R = 120^2 * .01
or 144 W. This will cause the battery to warm considerably. Given that batteries are poor at radiating heat, the battery can get quite hot, to the point of exploding. This is why batteries should have fuses.

Tom

On Tuesday, July 24, 2018 at 8:56:37 PM UTC-4, 2G wrote:
On Tuesday, July 24, 2018 at 10:30:11 AM UTC-7, kinsell wrote:
On 07/24/2018 10:05 AM, OHM Ω http://aviation.derosaweb.net wrote:
On Tuesday, July 24, 2018 at 9:31:44 AM UTC-5, Dan Marotta wrote:
In my last experimental glider I used two rather stout SPST toggle
switches to control my two LiFePO4 batteries.Â* At some point of the
flight I would switch the second battery on and then switch the first
battery off.Â* I don't think it was really necessary given the longevity
of these batteries compared with the SLAs that I used previously, I
simply switched about half way through the flight so that both batteries
would get some use.Â* I like John's idea with the MOSFETs.

On 7/23/2018 10:43 PM, Andy Blackburn wrote:
On Wednesday, June 20, 2018 at 6:27:33 AM UTC-7, OHM Ω http://aviation.derosaweb.net wrote:
Andy,

I have used the ideal diodes mentioned by Shaun McLaughlin (http://re-voltage.eu/ US$27 each) as I was given a pair to test with. They do what they say with minuscule voltage drops across them. Full disclosure, I am currently using two 1N5821 Schottky diodes in my panel right now (page 74 of my presentation) - small, simple, cheap, plentiful, robust, and easy to mount.

That being said - the availability of inexpensive ideal diodes has exploded in recent years mainly for use in systems like solar panel arrays.. See https://www.ebay.com/sch/i.html?_fro...es%29&_sacat=0. I also like the increasing range of mounting styles available allowing for placement options behind our increasingly cramped panels.

John (OHM),

Yup, I have one of Shaun's (on your recommendation) and some of the anti-revere-irrigation ones. I haven't installed either.

My current install uses the Schleicher rotary switch to select: Battery 1, Battery 2, Battery 1+2 (through diodes), Battery 3 (tail). I use a bank of capacitors connected through a power resistor to energize the circuit when the switch is between batteries.

If I had to do it again I would use ideal diodes (though as others have mentioned this is really not nearly a necessary with LiFePO4 batteries since they go through voltages below 11v quite quickly on my battery tester). I would also separate the batteries to individual switches to eliminate the switch as a single point of failure.

Andy Blackburn

9B

--
Dan, 5J

Dan - Do you have diodes in this circuit to prevent cross-charging of the batteries?

I will repeat something I said above that (without diodes) putting the batteries in parallel can cause a large current to flow from the "high" battery to the "low" battery. Effectively one battery is charging the other..

For dumb SLA batteries this will almost certainly blow the fuse on your batteries (you do have a fuse right at your battery's positive terminal, right??).

On lithium batteries, which probably have intelligent current limiting electronics inside, this should not be an issue. But notice the words "probably" and "should".

YMMV. Be careful out there.

- John OHM Ω


In one of the many previous incantations of this discussion, the same
claim was made about huge currents flowing between a charged and
discharged SLA. At my urging, the claimant actually tried doing that
with an ammeter. Didn't see even a flicker of the needle.

You know how you have to apply 14 volts to charge up a 12 volt battery?
A charged battery sitting at probably 12.6V has basically no capability
of charging a moderately discharged battery.

Switching to lithiums, maybe they have current limiting, maybe they
don't. RIchard P shorted one out, the terminals got hot enough to melt
the case in that area. Doesn't sound like much current limiting going
on to me.

Talking to a pilot who just switched to lithiums, he was touting how
great the current limiting feature was. I asked if he ever shorted the
terminals on purpose to test it? Well no. Would he be willing to try
that? Again no. If you really believe you have limiting, then go ahead
and put the wrench across the terminals just to satisfy yourself. Film
it for use on YouTube, just in case.

-Dave

All batteries will current limit: that is what their internal resistance does. Again, it is Ohm's law:
I = V / R
where R is the internal resistance. R is about 0.01 ohm on a typical glider SLA battery, so I = 120 amp. Now, you are going to get heating as a result of that energy dissipation, in watts:
W = I^2 * R = 120^2 * .01
or 144 W. This will cause the battery to warm considerably. Given that batteries are poor at radiating heat, the battery can get quite hot, to the point of exploding. This is why batteries should have fuses.

Tom

For the example 0.01 ohm, make that 1200 amps and 14,400 watts!

There is no issue switching batteries with overlapping on states. I charge my batteries with a smart charger designed for multi chemistry. This shows me everytime I charge, how much current is required by the battery when the supply side is 14.4v. It requires a nearly fully discharged battery to demand over 3 amps, and that is for a voltage differential far greater that you will get from a fully charged battery, as you would get from going from batt A to B. And just to fan the fire so to speak, I did not mention chemistry. As I see similar charge currents regardless of chemistry.

This is simply not a practical matter, as UH reports from years of experience. It is a fiction from the lack of understanding of charge dynamics. These devices are not superconcuctors...

RR

On Tuesday, July 24, 2018 at 2:22:09 PM UTC-7, wrote:
On Tuesday, July 24, 2018 at 12:05:18 PM UTC-4, OHM Ω http://aviation.derosaweb.net wrote:
On Tuesday, July 24, 2018 at 9:31:44 AM UTC-5, Dan Marotta wrote:
In my last experimental glider I used two rather stout SPST toggle
switches to control my two LiFePO4 batteries.Â* At some point of the
flight I would switch the second battery on and then switch the first
battery off.Â* I don't think it was really necessary given the longevity
of these batteries compared with the SLAs that I used previously, I
simply switched about half way through the flight so that both batteries
would get some use.Â* I like John's idea with the MOSFETs.

On 7/23/2018 10:43 PM, Andy Blackburn wrote:
On Wednesday, June 20, 2018 at 6:27:33 AM UTC-7, OHM Ω http://aviation.derosaweb.net wrote:
Andy,

I have used the ideal diodes mentioned by Shaun McLaughlin (http://re-voltage.eu/ US$27 each) as I was given a pair to test with. They do what they say with minuscule voltage drops across them. Full disclosure, I am currently using two 1N5821 Schottky diodes in my panel right now (page 74 of my presentation) - small, simple, cheap, plentiful, robust, and easy to mount.

That being said - the availability of inexpensive ideal diodes has exploded in recent years mainly for use in systems like solar panel arrays.. See https://www.ebay.com/sch/i.html?_fro...es%29&_sacat=0. I also like the increasing range of mounting styles available allowing for placement options behind our increasingly cramped panels.

John (OHM),

Yup, I have one of Shaun's (on your recommendation) and some of the anti-revere-irrigation ones. I haven't installed either.

My current install uses the Schleicher rotary switch to select: Battery 1, Battery 2, Battery 1+2 (through diodes), Battery 3 (tail). I use a bank of capacitors connected through a power resistor to energize the circuit when the switch is between batteries.

If I had to do it again I would use ideal diodes (though as others have mentioned this is really not nearly a necessary with LiFePO4 batteries since they go through voltages below 11v quite quickly on my battery tester). I would also separate the batteries to individual switches to eliminate the switch as a single point of failure.

Andy Blackburn

9B

--
Dan, 5J

Dan - Do you have diodes in this circuit to prevent cross-charging of the batteries?

I will repeat something I said above that (without diodes) putting the batteries in parallel can cause a large current to flow from the "high" battery to the "low" battery. Effectively one battery is charging the other.

For dumb SLA batteries this will almost certainly blow the fuse on your batteries (you do have a fuse right at your battery's positive terminal, right??).

On lithium batteries, which probably have intelligent current limiting electronics inside, this should not be an issue. But notice the words "probably" and "should".

YMMV. Be careful out there.

- John OHM Ω

How much current do you project when switching from a low LiFe at 12.4 volts(pretty much done) to high at 13.7? I have been doing this switching with SLA batteries for 25 years. Switch new one on- switch old off a second or two later. Have not blown up anything yet.
UH

Plus one. KISS principle.

On 7/24/2018 7:22 PM, wrote:
On Tuesday, July 24, 2018 at 8:56:37 PM UTC-4, 2G wrote:
On Tuesday, July 24, 2018 at 10:30:11 AM UTC-7, kinsell wrote:
On 07/24/2018 10:05 AM, OHM Ω http://aviation.derosaweb.net wrote:
On Tuesday, July 24, 2018 at 9:31:44 AM UTC-5, Dan Marotta wrote:
In my last experimental glider I used two rather stout SPST toggle
switches to control my two LiFePO4 batteries.Â* At some point of the
flight I would switch the second battery on and then switch the first
battery off.Â* I don't think it was really necessary given the longevity
of these batteries compared with the SLAs that I used previously, I
simply switched about half way through the flight so that both batteries
would get some use.Â* I like John's idea with the MOSFETs.

On 7/23/2018 10:43 PM, Andy Blackburn wrote:
On Wednesday, June 20, 2018 at 6:27:33 AM UTC-7, OHM Ω http://aviation.derosaweb.net wrote:
Andy,

I have used the ideal diodes mentioned by Shaun McLaughlin (http://re-voltage.eu/ US$27 each) as I was given a pair to test with. They do what they say with minuscule voltage drops across them. Full disclosure, I am currently using two 1N5821 Schottky diodes in my panel right now (page 74 of my presentation) - small, simple, cheap, plentiful, robust, and easy to mount.

That being said - the availability of inexpensive ideal diodes has exploded in recent years mainly for use in systems like solar panel arrays. See https://www.ebay.com/sch/i.html?_fro...es%29&_sacat=0. I also like the increasing range of mounting styles available allowing for placement options behind our increasingly cramped panels.

John (OHM),

Yup, I have one of Shaun's (on your recommendation) and some of the anti-revere-irrigation ones. I haven't installed either.

My current install uses the Schleicher rotary switch to select: Battery 1, Battery 2, Battery 1+2 (through diodes), Battery 3 (tail). I use a bank of capacitors connected through a power resistor to energize the circuit when the switch is between batteries.

If I had to do it again I would use ideal diodes (though as others have mentioned this is really not nearly a necessary with LiFePO4 batteries since they go through voltages below 11v quite quickly on my battery tester). I would also separate the batteries to individual switches to eliminate the switch as a single point of failure.

Andy Blackburn

9B
--
Dan, 5J
Dan - Do you have diodes in this circuit to prevent cross-charging of the batteries?

I will repeat something I said above that (without diodes) putting the batteries in parallel can cause a large current to flow from the "high" battery to the "low" battery. Effectively one battery is charging the other.

For dumb SLA batteries this will almost certainly blow the fuse on your batteries (you do have a fuse right at your battery's positive terminal, right??).

On lithium batteries, which probably have intelligent current limiting electronics inside, this should not be an issue. But notice the words "probably" and "should".

YMMV. Be careful out there.

- John OHM Ω

In one of the many previous incantations of this discussion, the same
claim was made about huge currents flowing between a charged and
discharged SLA. At my urging, the claimant actually tried doing that
with an ammeter. Didn't see even a flicker of the needle.

You know how you have to apply 14 volts to charge up a 12 volt battery?
A charged battery sitting at probably 12.6V has basically no capability
of charging a moderately discharged battery.

Switching to lithiums, maybe they have current limiting, maybe they
don't. RIchard P shorted one out, the terminals got hot enough to melt
the case in that area. Doesn't sound like much current limiting going
on to me.

Talking to a pilot who just switched to lithiums, he was touting how
great the current limiting feature was. I asked if he ever shorted the
terminals on purpose to test it? Well no. Would he be willing to try
that? Again no. If you really believe you have limiting, then go ahead
and put the wrench across the terminals just to satisfy yourself. Film
it for use on YouTube, just in case.

-Dave
All batteries will current limit: that is what their internal resistance does. Again, it is Ohm's law:
I = V / R
where R is the internal resistance. R is about 0.01 ohm on a typical glider SLA battery, so I = 120 amp. Now, you are going to get heating as a result of that energy dissipation, in watts:
W = I^2 * R = 120^2 * .01
or 144 W. This will cause the battery to warm considerably. Given that batteries are poor at radiating heat, the battery can get quite hot, to the point of exploding. This is why batteries should have fuses.

Tom
For the example 0.01 ohm, make that 1200 amps and 14,400 watts!
Wow!Â* That's a lot!Â* Let's see, now...Â* 14,400 watts / 3,600
seconds/hour * 1/4 sec (as in 1 quarter second to switch one on and then
the other off) =Â* 1 watt hour or 0.001 KWH.

I won't be losing any sleep over it.Â* 2G's math is correct but, in the
absence of a dead short, I see no problems with doing without diodes.Â*
Still, I keep both batteries fused very close to the positive terminals.
--
Dan, 5J

On 07/24/2018 08:16 PM, Craig Funston wrote:
On Tuesday, July 24, 2018 at 2:22:09 PM UTC-7, wrote:
On Tuesday, July 24, 2018 at 12:05:18 PM UTC-4, OHM Ω http://aviation.derosaweb.net wrote:
On Tuesday, July 24, 2018 at 9:31:44 AM UTC-5, Dan Marotta wrote:
In my last experimental glider I used two rather stout SPST toggle
switches to control my two LiFePO4 batteries.Â* At some point of the
flight I would switch the second battery on and then switch the first
battery off.Â* I don't think it was really necessary given the longevity
of these batteries compared with the SLAs that I used previously, I
simply switched about half way through the flight so that both batteries
would get some use.Â* I like John's idea with the MOSFETs.

On 7/23/2018 10:43 PM, Andy Blackburn wrote:
On Wednesday, June 20, 2018 at 6:27:33 AM UTC-7, OHM Ω http://aviation.derosaweb.net wrote:
Andy,

I have used the ideal diodes mentioned by Shaun McLaughlin (http://re-voltage.eu/ US$27 each) as I was given a pair to test with. They do what they say with minuscule voltage drops across them. Full disclosure, I am currently using two 1N5821 Schottky diodes in my panel right now (page 74 of my presentation) - small, simple, cheap, plentiful, robust, and easy to mount.

That being said - the availability of inexpensive ideal diodes has exploded in recent years mainly for use in systems like solar panel arrays. See https://www.ebay.com/sch/i.html?_fro...es%29&_sacat=0. I also like the increasing range of mounting styles available allowing for placement options behind our increasingly cramped panels.

John (OHM),

Yup, I have one of Shaun's (on your recommendation) and some of the anti-revere-irrigation ones. I haven't installed either.

My current install uses the Schleicher rotary switch to select: Battery 1, Battery 2, Battery 1+2 (through diodes), Battery 3 (tail). I use a bank of capacitors connected through a power resistor to energize the circuit when the switch is between batteries.

If I had to do it again I would use ideal diodes (though as others have mentioned this is really not nearly a necessary with LiFePO4 batteries since they go through voltages below 11v quite quickly on my battery tester). I would also separate the batteries to individual switches to eliminate the switch as a single point of failure.

Andy Blackburn

9B

--
Dan, 5J

Dan - Do you have diodes in this circuit to prevent cross-charging of the batteries?

I will repeat something I said above that (without diodes) putting the batteries in parallel can cause a large current to flow from the "high" battery to the "low" battery. Effectively one battery is charging the other.

For dumb SLA batteries this will almost certainly blow the fuse on your batteries (you do have a fuse right at your battery's positive terminal, right??).

On lithium batteries, which probably have intelligent current limiting electronics inside, this should not be an issue. But notice the words "probably" and "should".

YMMV. Be careful out there.

- John OHM Ω

How much current do you project when switching from a low LiFe at 12.4 volts(pretty much done) to high at 13.7? I have been doing this switching with SLA batteries for 25 years. Switch new one on- switch old off a second or two later. Have not blown up anything yet.
UH

Plus one. KISS principle.


Two switches and two fuses should work fine, certainly an improvement
over some of the Rube Goldberg solutions proposed here.

But two batteries, two fuses, and two Schottky diodes would be an
improvement. Perfect for everyone except jet jockeys who want more
switches to flip.

Finally, one properly sized battery with a fuse is probably the best
solution, half the number of BMS boards to blow up. Can't get much
simpler than that. There's a lot to be said for reducing component count.

There's a lot to be said for reducing component count.

Yes, there is a definite charm to single point-of-failure systems.

But charm don't count when the failure occurs and you are out of backups and ideas.

Right on, Mark.

As a former jet jockey, myself, I prefer simplicity and fewer switches.Â*
And, whereas one big battery would be nice (forgetting single point
failures), space quickly becomes an issue.Â* It's much easier to locate
two or more smaller batteries around the ship than finding a place for a
big one.Â* One of my friends removed the nose ballast from his glider and
replaced it with an SLA battery.Â* A much better use of lead, don'tcha think?

....And if you want more switches, go to an air museum and take a look in
the cockpit of a MiG-21
https://www.google.com/search?q=mig-21+cockpit&rlz=1C1CHBF_enUS697US697&source=lnms&tb m=isch&sa=X&ved=0ahUKEwjjsanSoL3cAhUF-6wKHfHSA_EQ_AUICigB&biw=1365&bih=662#imgrc=kTdKHIU LN9Ro9M:...

On 7/25/2018 8:55 PM, wrote:
There's a lot to be said for reducing component count.

Yes, there is a definite charm to single point-of-failure systems.

But charm don't count when the failure occurs and you are out of backups and ideas.


--
Dan, 5J