A complicated question about LiFePO4 batteries, - and switching.

Another question regarding the above battery type! There are variety of techniques used when switching between batteries - on/off/on switches, good switches with a cut out between the movement, two batteries with a diode to prevent flow between both batteries, etc.
The question I have concerns an older glider with limited battery space (Libelle), previously using SLA batteries and and a simple switch between the two. I will be upgrading with a new panel, likely LiFe batteries and want a back-up as well. While I understand the function of the diodes in the switching circuit, is there any reason to be concerned about their use with the LiPo battery and the internal Battery Management System? Imagine two LiFePO4 batteries that have equal charges, only one is discharging at a time until some threshold is reached and they switch over as determined by diodes outside the internal battery system. Is there a reason to question this type of circuit with the use of this new battery technology and the Battery Management System within?
While I love these new batteries, I don't want to bring on any new problem in the air, being quite aware that the wiring is only to contain smoke.

Thanks ahead, Mark

On Monday, June 18, 2018 at 9:22:24 PM UTC-4, wrote:
Another question regarding the above battery type! There are variety of techniques used when switching between batteries - on/off/on switches, good switches with a cut out between the movement, two batteries with a diode to prevent flow between both batteries, etc.
The question I have concerns an older glider with limited battery space (Libelle), previously using SLA batteries and and a simple switch between the two. I will be upgrading with a new panel, likely LiFe batteries and want a back-up as well. While I understand the function of the diodes in the switching circuit, is there any reason to be concerned about their use with the LiPo battery and the internal Battery Management System? Imagine two LiFePO4 batteries that have equal charges, only one is discharging at a time until some threshold is reached and they switch over as determined by diodes outside the internal battery system. Is there a reason to question this type of circuit with the use of this new battery technology and the Battery Management System within?
While I love these new batteries, I don't want to bring on any new problem in the air, being quite aware that the wiring is only to contain smoke.

Thanks ahead, Mark

There shouldn't be an issue, as long as both batteries have diodes to ensure that current can only come out of the batteries, not into them. The BMS will still serve as a second line of defense to limit the current (in either direction) if needed.

The only downside of the diodes is that you lose a bit of the battery voltage, since the diode has some voltage drop between its terminals. You can use Schottkey type diodes to minimize that voltage drop, about 0.3V as compared with 0.7V with a standard silicon diode. This is less of a problem with LiFe batteries, since their voltage is somewhat higher than that of SLAs, especially after they're partially discharged.

An arrangement that can be used to eliminate the voltage drop, especially if using SLAs, is to have both the two diodes and also an SPDT or on/off/on switch, from the junction of the two diodes to the other end of either diode. (Or you can think of it as the normal switch arrangement, with the diodes added in parallel with the two arms of the switch.) That way you get the full voltage of whichever battery is chosen by the switch at the time, but if you forget to switch over, or if the switch fails, you still get the other battery to kick in if the first one gets weak. E.g., with normal silicon diodes, if the first battery is down to 12.0V and the second one is sitting at 12.7V then the diode in series with the second one will start conducting, keeping the voltage supplied to your gizmos from falling lower than 12.0V at first, decreasing slowly from there as both batteries work effectively in parallel albeit at different voltage levels. If you then remember and flip the switch you'd get the full voltage of the second battery. Moreover, with this arrangement, if the switch momentarily disconnects both batteries while you flip it, there is no supply gap to your gizmos since the current can keep flowing through the diode. More precisely, the momentary dip in voltage is limited to the small voltage drop in the diode.

On 06/18/2018 07:22 PM, wrote:
Another question regarding the above battery type! There are variety of techniques used when switching between batteries - on/off/on switches, good switches with a cut out between the movement, two batteries with a diode to prevent flow between both batteries, etc.
The question I have concerns an older glider with limited battery space (Libelle), previously using SLA batteries and and a simple switch between the two. I will be upgrading with a new panel, likely LiFe batteries and want a back-up as well. While I understand the function of the diodes in the switching circuit, is there any reason to be concerned about their use with the LiPo battery and the internal Battery Management System? Imagine two LiFePO4 batteries that have equal charges, only one is discharging at a time until some threshold is reached and they switch over as determined by diodes outside the internal battery system. Is there a reason to question this type of circuit with the use of this new battery technology and the Battery Management System within?
While I love these new batteries, I don't want to bring on any new problem in the air, being quite aware that the wiring is only to contain smoke.

Thanks ahead, Mark


Do you not have the space required to install one big battery, instead
of multiple small ones? Never understood the fascination with switching
batteries, it just a distraction during flying. I think there's some
airline captain wannabees who just want more switches to flip.

-Dave

Redundancy, that's the answer.

On Mon, 18 Jun 2018 21:24:27 -0600, kinsell wrote:

Do you not have the space required to install one big battery, instead
of multiple small ones? Never understood the fascination with switching
batteries, it just a distraction during flying. I think there's some
airline captain wannabees who just want more switches to flip.

He's talking about a Libelle, which has a battery compartment that was
apparently designed to hold one larger, flattish battery which now seems
to be an obsolete form factor. However, it will hold a pair or 7.2 AH 12v
SLAs if they are stood on one end and the top of the battery compartment
is raised 50mm by extending the hold-down bolts.

FWIW, My Std Libelle is set up that way, though with the hold-down bolts
running through brass tubes to keep them aligned with the screw holes
they fit into. I mount my EW Microrecorder on the battery lid with two
diodes so it always draws from the highest voltage battery and run power
to the panel separately from each battery via an XLR socket on the lid,
which also holds the fuses: one battery powers the radio and T&B while
the other runs both varios (SDI C4 and Borgelt B.40 with 9v backup
battery), PNA (running LK8000) and FLARM. This more or less balances the
load on both batteries - their recharge times after a day's flying are
fairly similar.

--
Martin | martin at
Gregorie | gregorie dot org

On Monday, June 18, 2018 at 9:22:24 PM UTC-4, wrote:
Another question regarding the above battery type! There are variety of techniques used when switching between batteries - on/off/on switches, good switches with a cut out between the movement, two batteries with a diode to prevent flow between both batteries, etc.
The question I have concerns an older glider with limited battery space (Libelle), previously using SLA batteries and and a simple switch between the two. I will be upgrading with a new panel, likely LiFe batteries and want a back-up as well. While I understand the function of the diodes in the switching circuit, is there any reason to be concerned about their use with the LiPo battery and the internal Battery Management System? Imagine two LiFePO4 batteries that have equal charges, only one is discharging at a time until some threshold is reached and they switch over as determined by diodes outside the internal battery system. Is there a reason to question this type of circuit with the use of this new battery technology and the Battery Management System within?
While I love these new batteries, I don't want to bring on any new problem in the air, being quite aware that the wiring is only to contain smoke.

Thanks ahead, Mark

I use good quality switches(DPST with both contacts wired in parallel fot added reliability). Each is wired from the battery to my main bus. To change over I switch the new one on and other off a second or two later. I have had no issues with doing this. The LiFePo4 batteries are pretty much out of power at 12.3 volts or so. Fully charged they show about 13.7 or so. The resulting voltage differential doesn't generate enough current to cause a problem in my experience. I do not suggest leaving both on in parallel.
FWIW
UH

A beginning comment: With modern avionics drawing less power than their predecessors and LiFePo4 with greatly increased energy capacity, maybe only one battery is needed these days for most glider pilots. Just sayin'.

I agree that our batteries should not be run in parallel (without diodes). If one is of a significantly different voltage than the other, then the higher one will charge the lower one possible at a high rate (current) that you don't want. This can potentially blow one or both battery fuses (you do have a fuse on each battery, right???), or might cause one battery to fail in a bad way, either of which can leave you in a poor situation especially during flight.

I have a presentation on my thoughts on the subject of two battery systems at http://aviation.derosaweb.net/presentations#wiring.

Here is my setup on my current glider.

- Two batteries (lithium)
- Each battery has a fuse.
- Two separate power buses with one battery powering each.
- One bus is for "communications" (radio, transponder, FLARM) and the other bus is for "navigation" (GPS, Vario, etc.).
- Each bus has a separate breaker (5A) which is also a master switch (a true breaker/switch by Tyco http://www.aircraftspruce.com/catalo...clickkey=5950).
- But I do have Schottkey diodes in each bus. Why, you might ask, do I have diodes if I have two separate electrical buses? Because I can bridge the two buses with another switch for that "just in case situation" (belts and braces engineering over design). I have never have to use this switch but then again you never know ...

Thanks, John OHM Ω

We just turn them on and leave them on...or tell the first officer to do it.

On Monday, June 18, 2018 at 8:28:44 PM UTC-7, Emir Sherbi wrote:
Redundancy, that's the answer.

As with SLAs, prefer switches for each battery. The locking toggle switches made by Alcoswitch are great, for example their SPDT part # MTL106D.
Switch battery 2 on before switching battery 1 off. There is no need for capacitors or diodes.
Believe two standard 10Ah LiFePo4 will fit in the Libelle's battery compartment.
Jim

John,

Have you tried any of these?

https://www.ebay.com/itm/15A-Solar-P...sAAOSw-0RbApaz

They claim almost zero voltage drop compared to Schottkey diodes. Of course with the rapid voltage fallow of LiFePO4 batteries, it may only make a tiny difference in effective capacity.

Andy Blackburn
9B


On Tuesday, June 19, 2018 at 6:23:07 AM UTC-7, OHM Ω http://aviation.derosaweb.net wrote:

- But I do have Schottkey diodes in each bus. Why, you might ask, do I have diodes if I have two separate electrical buses? Because I can bridge the two buses with another switch for that "just in case situation" (belts and braces engineering over design). I have never have to use this switch but then again you never know ...

Thanks, John OHM Ω