Parallel DC Boost Converters?

Sonic01

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Hi Guys!

So, I've built myself a big mobile powerbank for camping with leftover cells from my powerwall project.

Now the problem i have is as follows, the input voltage range for my invert is 11v - 15v, but obv a Li-Ion will suppy 9v-12.6v so as soon as my battery is dropping to 11v, its cutting out.

Now my inverter has a max output of 300w and my BMS goes up to 30A, so the inverter should never be able to overload the BMS and the battery is capable of 30A as well so each component is capable of doing the 300w.

So I figured I just need to bump the voltage up to keep the inverter happy until I reach 9v, quick research shows "DC Boosters" do this however I can't find one with the spec I need, i.e. has to be able to take an input as low as 9v, yet also be able to take an input load of 30A.

The only models i see capable of 25A+ are only accepting 10v+ input, i can see some smaller ones will do 9v but the input is rated at 20A and the output at 12A:

https://www.aliexpress.com/item/327...chweb0_0,searchweb201602_2,searchweb201603_60

So I'm wondering if i can just run a few of these in parallel to be able to take the load? I've tried google around this and all the answers seem to be either "yes its fine" or "no, one will overload and burn out".

my understanding is that if one is slightly stronger then the other, it will just take slightly more load? though it was that simple?

Any advice would be greatly appreciated!
 

Redpacket

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Because that unit apparently has constant current output control your chances of load sharing are better.
You'd set the output voltage to maybe 14V & also set the current limit point as well.
I read this in the notes:
"When using the boost power supply should pay attention to the following points:
1 input power supply voltage must be above 10V."
Ie you still won't get the full voltage range of the cells, but seems likely you'll get the upper 2/3 or so.
They also need extra cooling to do the full spec.

You might also find stability is an issue.

People usually go with either 4s & use an inverter with higher input voltage rating
or go to 7s & use a 24V inverter (better voltage match, but more gear to change...)
 

Sonic01

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so the output control is the max amps it will output right? i.e. I could get 3 of them and set them to output up to 10A each?

and yes I was thinking to put the putput voltage to either 13v or 14v, that way its above the 12.6 max charge of the battery and within the limits of the inverter.

good spot about the input voltage as well, its strange because at the top it says "Input voltage: 8-60V", i looked at all the boosters i couldn find, from 300w to 1800w and they all seem to start at 10v :(

If i do 4s then my max voltage is 16.8v, the roof my my inverter is 15v :(

so changing my battery layout wont help, and i cant find a boost that will do what i need... i could find any more suitable inverters as well, unless maybe i go for something much more powerful which will be too big... I'm not sure what to do :(
 

daromer

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Thats the disadvantage of the battery chemistry you have choosen. Its just wrong from the start :)

What you potentially have that you can do and is most recommended is:
1. Change to LiFePo4 4s setup instead
2. Change inverter that suits the voltage. Either 3s or 4s nominal voltage.
3. go 7s or above and change battery layout and buy a new inverter

Last what you can do is
1. Dont utilize the full range of the battery.
2. Use some kind of boost controller. The more expensive one have wider input-range.

Neither what you choose to go with the only one that isnt costly is the "dont utilize full range as is today" option im afraid. The other cost in some way or another.
 

Korishan

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Agreed with Daromer.

Go with LiFePO4 if you can. If not, go 7s and use a 24V inverter. If that's not an option, go 4s and use an inverter that can handle 16V. If that's not an option, go 4s and use a "buck" converter to drop the voltage down to 14V or so.
Always try to buck down, not boost up. Boosting requires more amps, heavier wire, on the battery->converter side.

Also, if you go 4s, but stay limited to 15 roof, that'd be 3.75V/cell. Which would yield a much longer cycle life of the cells. Though, you do loose capacity. But you gain full current capabilities.

Using common lithium cells for a "12V" application is a very complicated situation, and you will always be compromising, if you want to go inexpensive/simple.
 

daromer

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Why not build a slightrly better battery that doesnt drop that much? Its cheaper and less stressfull to the battery.

If you could spend 70USD on side gear i would recommend just rebuilding the battery to better cope with the stress.
 

Sonic01

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you mean just expand it? more cells in parallel? right now its the perfect size and weight for what i want, it fits perfectly into a 12" stanley tool box :)

any bigger and it wont be so portable, it wouldnt fit into the perfect slot ive made for it in my overlanding vehicle and and i cant afford any more space for that. it would also be harder to just pull out and carry up the ladder into the roof tent :)
 

Korishan

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I would recommend using the buck to drop the voltage to at least 12.5V, not 11.1V. As I touched on earlier, the lower the voltage, the higher the amp draw to meet a certain wattage. This can only be compensated to a degree with larger wire.

Is it possible to configure the cells in a different physical orientation to fit more cells into the Stanley tool box? Can you show us some pictures of your progress?
 

gpn

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Sonic01 said:
you mean just expand it? more cells in parallel? right now its the perfect size and weight for what i want, it fits perfectly into a 12" stanley tool box :)

any bigger and it wont be so portable, it wouldnt fit into the perfect slot ive made for it in my overlanding vehicle and and i cant afford any more space for that. it would also be harder to just pull out and carry up the ladder into the roof tent :)

How big is big? If you have enough cells you could just rearrange what you have to boost the voltage and take advantage of the buck converter. You don't necessarily need to ADD cells, just change the output of the pack.
 

Sonic01

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Korishan said:
I would recommend using the buck to drop the voltage to at least 12.5V, not 11.1V. As I touched on earlier, the lower the voltage, the higher the amp draw to meet a certain wattage. This can only be compensated to a degree with larger wire.

Is it possible to configure the cells in a different physical orientation to fit more cells into the Stanley tool box? Can you show us some pictures of your progress?

yeah sure 12.5v would work, its 0.1v below the minimum that the pack will ever get and i guess that makes less work for the bucks too.

actually now that i think about it, im pretty sure i can fit more cells in here. just checked and i can easily lay a cell down across the top of these and still close it, so i do have just enoughspace to add another 24 cells.

i dont have a spot welder so if i changed the wholeorientation to match the 4 ive laid across the top there then there wouldnt be room for the solder joints etc....

i think i have enough space to just add the 4th cell on top on the existing 3, like ive shown in the picture:

[img=1024x800]
gpn said:
Sonic01 said:
you mean just expand it? more cells in parallel? right now its the perfect size and weight for what i want, it fits perfectly into a 12" stanley tool box :)

any bigger and it wont be so portable, it wouldnt fit into the perfect slot ive made for it in my overlanding vehicle and and i cant afford any more space for that. it would also be harder to just pull out and carry up the ladder into the roof tent :)

How big is big? If you have enough cells you could just rearrange what you have to boost the voltage and take advantage of the buck converter. You don't necessarily need to ADD cells, just change the output of the pack.

yup, thats what im planning. pretty sure i can boost it from 3s24p to 4s24p and still fit in the same box, see above.
 

completelycharged

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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-get-a-cheap-charger-with-over-100A-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).
 

Sonic01

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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.
 

Korishan

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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 ;)
 

Sonic01

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"for the time being".... thats so reassuring :D

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??
 

Korishan

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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.
 

Redpacket

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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.
 

Sonic01

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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...orderId=104992952557449&productId=32946226619

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...
 

Korishan

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completelycharged

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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-get-a-cheap-charger-with-over-100A-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....
 
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