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Parallel DC Boost Converters?
#11
The DC Boost converters you have listed will not work in parallel and a lot of non isolated boost converters which have current limiting will also not work.

My post on a poer supply option details the boost converters here :
https://secondlifestorage.com/t-How-to-g...00A-at-12V

The reason is that the return negative line is used to detect current with a small shunt resistance and when the negative lines are in parallel the current is distributed near evenly between all of the units, so one unit is sending out say 10A and only returning 3.3A (in the case of 3 in parallel) while the other units are sending out nothing and have 3.3A current sense active, which then prevents them from turning on.

I had played around with the 1200W and 1800W units a lot (blew FET's and repaired them) and they do work quite well with reasonable efficiency. The 1800W units have a 40A input current limit and think it's a 20A output limit. They also work as step down units, although they are not intended to be and a short failure would pass through the battery voltage (normally FET's will go open circuit).

That said I have only used them to step down from 50V to 48V while charging a pack from a power supply (and then stepping up from 50V to 54V as the pack charges).
If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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#12
actually Korishan i just realised the pack will go down to 12v when drained so I'll set the bucks to something like 11.8v, my wiring is thick enough to handle 40A so I'm not concerned about the extra draw.

@completelycharged, I'm going to raise the pack to 4s/16.8v and use bucks to drop the voltage rather then boost a 3s up, bucks seem to work fine in parallel.
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#13
It's possible that they may work fine for the time being. However, it's also possible that one may overdrive the other. I would recommend adding a diode on each of the Pos legs and measure the voltage "after" the diodes to make sure things are happy.
Not saying they "will" overdrive another, but it is possible.

And diodes are fairly cheap, and good cheap insurance is always a good thing Wink
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#14
"for the time being".... thats so reassuring Big Grin

sorry im not sure what you mean about the diodes... add them to pos between the batt and buck.... or between buck and inverter? what would that achieve? and wouldnt i need to find some super beefy 20a capable ones??
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#15
The diodes would be on the buck->inverter side. It's the output of the bucks that could cause an issue.

The diodes on each Pos leg of each buck will keep another buck from pushing current into another buck.

Use solar panel schottky diodes. They have low forward voltage drop and can handle high currents.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
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#16
I agree with Completelycharged's points re current sensing resistors - seems likely this will cause issues.
It might be offset by cable resistance in the -ve lines.

re diodes & power going into another buck, I don't think you'll need to worry about this.
I would not expect the output stages of "the other buck" to absorb power. It might have a couple of resistors but no serious load or harm.
Running off solar, DIY & electronics fan :-)
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#17
Completelycharged was talking about using boosts though, not bucks.

The bucks I have ordered state specifically that they can be used in parallel:

https://www.aliexpress.com/snapshot/0.ht...2946226619

Thanks for the diode explanation Korishan, I'm debating whether i need them since it already says they are suitable for parallel use...

My bucks and new BMS arrived today so hopefully I can start working on this soon...
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#18
(08-14-2019, 01:45 PM)Sonic01 Wrote: The bucks I have ordered state specifically that they can be used in parallel:

https://www.aliexpress.com/snapshot/0.ht...2946226619

Provide a different link, please. This one is asking me to create an account/login before seeing the item
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
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#19
oops, try this:

https://www.aliexpress.com/item/32946226...4c4dQ0kKv8
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#20
They are technically the same units.... is you have a look at one of my previous posts there are several different designs made, some with flaws.
https://secondlifestorage.com/t-How-to-g...00A-at-12V

The seller says they work in parallel, good luck with trusting what the seller says....

That said they may (have not tested to prove) work in step down mode in parallel as long as the current limit is not reached.... step up, fail.
Switching from boost to buck to boost in parallel, no idea what will happen....
If you can't quantify how much they cost, it's a deal, I'll buy 5 of them for 3 lumps of rocking horse ......
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