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

With two SLA batteries that are in good shape it probably won't blow any fuses. The danger is if one battery goes bad, e.g., one of its six cells shorts out, so it effectively becomes a 10 volt battery. This happens fairly often in SLA batteries that are quite old. Then the good battery will keep sending current into the bad one, wasting its energy, and potentially causing the bad one to burst. You can minimize the chances of problems by only using fairly new batteries (say under 4 years old) and only as a matched pair of the same size/brand/age. LFP batteries typically have protection circuits built in that would prevent major issues like that, but why take chances? Schottky diodes are cheap enough.