Dc/dc converter recommendation

How-Tos & Esk8 mechanics
41

Unfortunately there’s no way to estimate the “vampire” drain (quescient current). You’d need to request that spec from the seller.

Lots of marketing hyperbole in that listing…
Synchronous rectification has been around for decades and is the standard design for a DC-DC converter operating at moderate to high power levels, above a few watts or when more efficient operation (less heat) is needed.

Non-synchronous operation is still a valid design concept and is commonly used for lower power and lower cost devices, huge numbers every year.

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42

I’v got this buck/ step down converter ordered.
95v max input
500 watt rated, and physically large though.

I intend to use it to step down a 42v 2 amp charger, to 41v and as little as 0.2 amps output

43

You’re only dropping the voltage by 1V? At low current levels?
I’d recommend just using diodes in series (two or three Schottky diodes) to get the voltage drop. There won’t be much heat and the cost will be a lot lower.

What is the highest current level you will use?

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44

Mooch, In the immediate future, I will only have the 2 amp 42vDC power supply, in which to step down the voltage from, into a 10s battery that is not yet even built.

Further in the future, I might be stepping down from 16s lifepo4 off grid setups, without using an inverter, and its double conversion losses.

The supposed 29.4v 1.5 amp supplies which came with my 7S cheapo skates, vary between 29.87 and 30.12v open circuit, and I use a bucker to limit voltage to no more than 29.4v, and when I have 12+ hours to recharge, i will limit voltage to 28.7 and amperage to as low as the CCCV bucker lets me, usually 0.18 amps.

But in general I’ll do a little head math and limit amperage to attain 4.1v per cell when I next expect to be able to make urethane sing its sweet sweet song., and 4.2v if i need to make it sing for a bit longer.

I like spinning potentiometers. I’m new to esk8 but not manipulating voltage into depleted(lead acid) batteries and gauging their health, and performance with repeated observation of voltmeters, ammeters and columb counters as the cycles accumulate.

So I very much respect your testing and knowledge and feel a bit honored to be interacting with you.

My vehicle has dual alternators, modified external voltage regulators whose remote potentiometers are on dashboard, next to voltmeters and ammeters feeding 2 separate banks of healthy older lead acid batteries, and when those become too hard to return to 100% in a timely manner, prisimatic Lifepo4 cells will replace them.

The XL4015 based 5 amp buckers Ive been using, are limited to ~33v input, IIRC, and the specific bucker I linked above is one of the few I found capable of handling higher voltages that I will need for 10s.

But I’m far more.likely to use step up/boost converters from my vehicle, or garage floor based 12vdc nominal Lead acid systems to charge existing and Future Esk8 batteries.

I expect the 42vdc power supply I ordered, to be similarly untrustworthy as to unloaded OCV, so the bucker I linked is basically to step down voltage to 4.2volt s per cell, or less, when I am not near my 12vdc systems, and when I don’t need 2 amps charge rate, so that current potentiometer gets twiddled down to slower amperage rates to reduce stress on battery, and powersupply.

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45

Thank you for that info but I’m a bit confused as what your needs actually are. I’ll just plow ahead with addressing a possible one. My apologies if I am wrong.

You cannot use that DC converter (that you provided a link for) to deliver 42V or 41V to a 10S pack with a 42V input. The input must be at least 2V higher than the output according to its specs. I would go at least 3V higher as these types of low cost units are barely stable in the best of conditions.

You can directly use a 42V CC/CV supply that drops its voltage under load for charging though. As the pack approaches full the charging current will drop and the supply’s voltage will start rising up towards 42V. By the time the pack is fully topped off the supply will be very near its no-load voltage and the pack will be fully charged.

When you want to charge to 41V just use the diode string to drop the voltage or adjust the 42V supply’s output via a pot.

46

Excellent point! I had indeed overlooked that fact.

In my experience with the Xl 4005, Xl4015 based 5 amp buckers, the max output voltage is 0nly 0.29 and 0.19v below input voltage, respectively, and i have no issues diaing in 29.40 from a 29.87v supply. The xl4015 specs say input voltage must be at least 1v higher than expected max output.

I will report back on what I find regarding this delta, once the new one I linked arrives, and if it can’t achieve 41v , from a 42.xx supply, I can deploy it elsewhere on other projects, or buy a 60vdc supply.

I had some large diodes from a 40amp transformer based marine charger, but they, and far too much other stuff i wish i still had, got abandoned in California when I had to move to Florida.

I’m far more likely to use a Dc to dc boost converter from my 12vdc systems for my future 10s battery charging. If the grid goes down, I can still be esk8ing.

I am tempted to go open my 29.87v supply and see if it has a pot inside to adjust voltage.

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47

I’m guessing that the dropout voltage spec (min input-output voltage difference) is at the max rated output current level and gets smaller as the current drops. Hopefully that’s the case with that bulk converter.

But you have that boost converter idea so you’re set otherwise. :slightly_smiling_face:

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48

I just rechecked my XL4015 bucker unloaded, on 7s power supply 29.92v in, and 29.83v out was as high as I could dial the output. Loaded with a 3.5amp 12v fan, the delta was far higher, but nowhere near 2 volts. Been a long while since i tested.

I have CV only Xl4005’s in use as 12v brushkess fan speed controllers, but would have to pry off some potting to read voltage on input and output tabs. I will have to check on a new xl4005 bucker and powering a 3 to 5 amp load.

I pried apart the plastic casing of a different, so far unused, 7s power supply. ( I have 3).
No pot inside. I guess some resistor on pcb could be replaced with a potentiometer, and allow for voltage adjustment, but that is beyond my abilities to figure out which one.
I’ll check for a pot on 2a 42v supply too, when it arrives.

I did check the 7s power supply unloaded voltage on barrel plug… 30.13v.
Even with a functioning bms, this seems dangerous to me.

49

Yea, different supplies can give you verrrrry different dropout specs. :slightly_smiling_face:

50

I got around to testing that bucker/step down converter I linked, and unloaded, nothing on the output at all, input 41.97v, output 38.87v, so it is nearly worthless in the particular task for which I bought it.

My new ACDC POS 42vdc 2.0 amp charger/supply measured 41.97 output unloaded, but maxed out at 0.83 amps chargng my 10s, 35.92v battery, with or without bucker inline, making it nearly worthless too.

I was not able to adjust current up or down on the new step down converter from the POS 42v supply.

I then powered 500 watt Bucker/step down converter from my 36v battery, lowered its output voltage to 29.36v, and plugged it in to charge my 7s depleted battery. My wattmeter registered 8.5 amps briefly, then 0. The 7.5 amp fuse on output lead blew.

Now Disconnected , I cranked current potentiometer CCW multiple times, until it clicked, then replaced 7.5 amp fuse, reconnected it to 36v battery, output wattmeter read 29.36v, then plugged it into 7s battery, and 0.74 amps were flowing.

I was able to dial amps well up past 6 before reversing screwdriver, but could not dial them down below 0.74.

So this 500 watt bucker is more limited in function than my lower voltage xl4005 xl4015 5 amp max buckers, which will allow me to throttle amperage to below 0.1 amps, and I can easily get a 7s battery upto 29.4v output from 29.83v input with enough time.

If I had a 12s AcDc charger I could limit the linked buckers voltage to 41.0v and dial amperage down to as low as 0.74, and as high as the 12s supply can deliver.

I have 3 1.5 amp 29.83 to 29.13v power supply/chargers, but have been instead using my 400 watt CCCV Booster, from my 12v nominal lead acid batteries to charge my 7s batteries. The voltage pot is back wards, Turn ccw to raise voltage, CW to lower, but the Amperge potentiometer is normal.

I can throttle current to as low as 0.09amps, and dial amps way up, though input amperage above 8 is to be avoided, and it will turn 5 watts into heat with 12.4v input, at 2 amps output, and 3.8 watts is dissipated as heat when input voltage is 13.8v.

So this 400 watt DC booster powered by my 2S 6v Lead Acid AGMs, is my favorite 7s charging source, as I can charge at 6 amps, or as little as 0.1 amp and those Agms can be fed by the sun, or the grid.

I’ve not yet tried this booster on my new 10s battery but Im sure the higher the V delta the less efficient it is.