Thanks everyone - clearly I need to go back to Electronics 101 if I'm going to make this work!
WRT #6 below, I *think* if the diode fails in one of my buddy pairs, then the entire string is "open" (motor stops) since I connect neg to battery A and pos to battery B. Since the diode will now disconnect the positive lead between the 2 buddies
[+] -------[batteryA]
| + |- [4 of these are in series to get 48v]
[batteryB]---------- [-]
[failed]
[+] -------[batteryA]
x + |-
[batteryB]---------- [-]
Is current still going to flow through the series string if the positive lead in below diagram is open? (I *think* not...and I don't think it matters which buddy fails)
Is there any reason (other than expense) not to use the pre-wired Schottky isolation diodes sold for marine/motorhome use (to isolate during charging)?
Thanks again!
-Keith
---In electricboats@yahoogroups.com, <matwete@...> wrote :
Schottky diodes are an option, or you can consider power MOSFETs and other silicon options to combine the sources. Perhaps the main concerns here are:
1) Inrush currents exceeding the component ratings when turning ON or imbalanced load currents exceeding ratings during high throttle
2) Power loss --- Although Schottky diodes typically have low 0.15-0.45v drop, a Power Schottky diode (e.g. this dual-Schottky combiner power diode) will see nom. 0.6v drop. With a 48v pack, that's just over 1% loss---not bad. But if you used these for each 12v buddy, that'd be 5% loss. So, used at the pack level, that's not a lot of loss considering the losses thru the prop.
3) Cost of components (diodes themselves ($0.69 for a dual power schottky), circuit board, enclosure, etc.)
4) Failure mode---some components, e.g. Power MOSFETs, can fail SHORTed---it may be better off to use a fuse than risk that
5) Configuration---May need 2nd set of Schottky diodes for charging circuit feeding of batteries
6) "Zipper" failure---Whether imbalance or battery fault, when one of the diodes "opens" and blocks discharge into a low battery, the remaining batteries would be sharing the total current and if under a high cruising load at the time the collective current ratings of the remaining might not be enough. If so, each of the remaining diodes could each Open catastrophically in sequence. At that point, you're dead in the water and will have to either replace all those diodes or put emergency jumpers across one or more of them to get power back. I.E. same result as zippering thru fuses, but with more time required to replace the failed components (assuming they're not soldered in place…).
Assuming Schottky diodes are suitably sized to minimize risk of a zipper effect, diodes do offer an attractive non-destructive blocking capability with minimal (1%) loss if used at pack voltages.
-MT
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com]
Sent: Sunday, May 01, 2016 9:12 AM
To: electricboats@yahoogroups.com
Subject: Re: [Electric Boats] Batteries wired in parallel (and series...)
Thanks James.
What's a little alarming is how rarely this is mentioned in, for example RV discussion threads and 'expert tips' articles (of which there are hundreds). They pretty much all just show them wired directly.
Would a Schottky diode work for isolation? Ive seen that mentioned, but am fuzzy on the particulars.
-Keith
Posted by: aweekdaysailor@yahoo.com
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