60v to 48v buck converter for e-bike

I’m planning to use power tool batteries for my supplying voltage to a new Bafang BBS02 48v kit.

Most of the ones I have are 18vdc and 20vdc. Here are the possible combinations I can use:


3@18v = 54vdc
3@20v = 60vdc

2@18v + 1@20v = 56vdc

2@20v + 1@18v = 58vdc

and finally, 1 battery @ 60v

So I think I’m looking for a buck converter (step-down converter) or a DC-to-DC power converter to step down voltage from 56-60vdc input (supply) to a 48vdc output(load).

I am seeing a DC to DC converter with Input voltage range of 50-72V DC. And Output voltage of 48VDC, Output current 10Amps, Output rated power 480Watts.

Would this be a good fit for the batteries that I want to use with the Bafang BBS02? I want to keep as close to 48vdc on the Bafang so I don't burn up any internal electronics.

Don't know about Bafang voltage requirements but the 18 & 20V packs are both 5 cells in series inside. A 12V pack has 3 cells inside. You could run on 13S, 52V, two 18V or 20V packs plus one 12V pack connected in series.

18v vs 20v Lithium Ion Power Tools - The Truth Uncovered by Tools In Action (length: 3m59s)
https://www.youtube.com/watch?v=dBehZbptnCw
[youtube]dBehZbptnCw[/youtube]

I'd find out how many cells are in the 60v pack and see if my controller could handle that directly. Alternatively, I'd consider running two of the 18/20v packs in series (probably 2s and 2P or 2s and 3P)and then use a boost converter to bring the voltage up to around 54v (the voltage of a fully charged 48v pack). I only suggest the boost converter because I've been running one for a bit and it has been working fine. The main limit is that it will only supply around 1165 total watts in that voltage range.

Sounds like an excellent idea. I may end up getting this one, the Wangdd22 1500W 30A DC Boost Converter Step-up Power Supply, as you suggest. Looks very sturdy, a $40 unit on Amazon.

The Bafang electric motor I have is rated at 750 watts, 48 volts. Motor current Limit:25 amps.

Power tool batteries used for this motor will not exceed 10 Amp Hour, they are lithium-ion. Rated watt/hours of batteries will not be more than 100 watt/hours.

OTOH would it be more efficient to use a step down converter?

IDEALPLUSING on Alibaba is listing a 60v to 48v dc dc converter:

Model IPS-DTD60S48
Input rated voltage 60V DC
Input voltage range 50-72V DC
Efficiency 93% (half load) ; 92% (full load)
Output voltage 48VDC
Output current 10Amps
Output rated power 480Watts
Output peak power 125%

Cost is approx. the same as step up unit above.

BTW can the Wangdd22 1500W 30A DC Boost Converter be used to step down voltage? The spec sheet suggests to me that it would do that:

Input voltage: DC 10V~60V
Output voltage: 12~90V continuously adjustable

If so, that will be top on the list to finish the bike.

My electrical knowledge is generally confined to 24ac irrigation and some DC latching solenoids. Not much more than that.

ngant17 said:

BTW can the Wangdd22 1500W 30A DC Boost Converter be used to step down voltage? The spec sheet suggests to me that it would do that:

Input voltage: DC 10V~60V
Output voltage: 12~90V continuously adjustable

If so, that will be top on the list to finish the bike.

My electrical knowledge is generally confined to 24ac irrigation and some DC latching solenoids. Not much more than that.

Click to expand...

Nope. Output voltage must exceed input voltage. And I'm no electronics expert either.

Just read a recent view on a single-speed ebike. Specs and characteristics below:

Populo Sport ebike ($799.00)
250 watt rear hub motor
36V / 8.7 Ah (313 watt hours) lithium battery pack w/ Panasonic cells
Aluminum frame(35.7 pounds)
Narrow road bike 700c wheels
single speed - 46t chainring / 16t cog
Horizontal drop-outs
max assist (PAS) speed of 20 mph
gear ratio makes it difficult to get up to speed w/o PAS
range: 22.6 miles

This is an example to try and duplicate with my Bafang-powered MTB.

I realize I have a few things working against me.

1) My tires are typical knobby MTB tires, 24in. diameter
2) Heavier steel frame vrs aluminum
3) Watt-hours of my single 60v 4Ah Greenworks battery is pathetic(240), about 1/3 the rated 750 watts of Bafang BBS02. OTOH the Populo battery exceeds its motor wattage by 125%.
4) No horizontal dropouts for me
5) Non-PAS mode only works for me at level 1, maybe level 2 when battery power starts to go down.

So I'll be keeping the Popolo specs in mind as I slowly try to improve efficiency in my Bafang MTB.

My newest idea is the use a single 60v battery and a reserve of 2 fully-charged 60v batteries (assuming 64v at full charge). And a switch to automatically connect all three batteries in parallel after the first 60v battery drops below 56.5v.

I got this number from a simple math solution for one unknown voltage X1 when all are in parallel. (X1+ X2 + X3) / 3 <= 61.5v. That would give me 12Ahrs x 61.5 = 738 watts. Pretty close to optimum 750w with BBS02.

This eliminates the work of having to drain down two 60v batteries to avoid over-voltages going into Bafang controller.

Normally you want to start with all batteries at equal charge levels. Otherwise, the higher voltage batteries will discharge themselves into the lower voltage battery. OTOH, it seems like you are simply looking for simple ways to "waste" some stored energy. So maybe it isn't such a bad idea? I dunno. Others more expert than me can weigh in on that.

IMO, what you really need is a charger that will charge the batteries to 61v max. That way you not only don't have to invent ways to bleed the batteries down, but you can extend their life by not fully charging them. Win - win.

Also, you seem to be confusing watt hours (batteries) with watts (motor). The watts your are calculating for the batteries is watt hours. That's an energy capacity - like how many "gallons" of power that you have. The watts for the motor is simply a recommended maximum amount of power that you should send to the motor at any given moment. Your batteries might be able to deliver far more than 750 watts at any given time - especially if you run more in parallel.

wturber said:

Normally you want to start with all batteries at equal charge levels. Otherwise, the higher voltage batteries will discharge themselves into the lower voltage battery.

Click to expand...

And they're going to try to do it as instantly as possible, so depending on cell resistance / etc it can be some very high currents--usually bad for the cells to be charged that way, but it depends on the cell specs, and the BMSes that are in there and what protections they have.


I'd recommend what others here have--just don't charge them all the way up in the first place, so you don't have to drain them down, or risk damaging them by too-rapid charging (when connecting unequal-voltage packs together in parallel).

It's possible your charger already has a voltage adjustment inside it that you can use to turn it down far enough.

ngant17 said:

That would give me 12Ahrs x 61.5 = 738 watts. Pretty close to optimum 750w with BBS02.

Click to expand...

As Wturber notes, Watts (W) are not what you get in that calculation--you get Watt-hours (Wh). The "750w" of the motor systme is the typical power it might be able to run at.
The "738Wh" of the battery packs is the capacity (runtime) of the the batteries, and doesn't directly have anything to do with how many watts it can output. (though the "C-rate" of teh batteries, multiplied by their Ah, *does* tell you how many Amps (A) it can output, either peak or contuously depending on which one the given C-rate is for.

Also note that the fully-charged voltage of the pack is not used to calculate Wh; rather the average voltage of the pack is used for that.

So if the pack is typically around, say, 54v (you'd have to check what it actually is) at half-charge, then it's 12Ah * 54v = 648Wh.

I guess I'm following your advice on using partially-charged 60v batteries. My routine is to take a fully charged 60v battery (~64v) and run it for a few minutes in my Greenworks lawnmower. That usually shows up as 61.5v or less on my VOM. Then it's plug 'n play with the e-bike.

I would prefer to streamline my process: just take a fully-charged 60v battery off my charger and plug it into bike.

For such a convenience, IMHO a buck converter is needed on the 60v battery, to protect an over-voltage going into Bafang controller. This should be able handle another a fully-charged second or third 60v battery that would be in parallel to first battery. As Wturber mentioned in one of these threads, I probably need a third battery because they are all rated individually at a measly 4.0Ah. The reconditioned one I got off ebay for $95 was a great deal, especially considering the going price for 60v ebike batteries. Greenworks brand can be bought locally which is helpful for returns.

I need to have a full charge on at least one of the batteries for business with my 60v lawnmower. At this time my smart money is investing into a buck converter for running the Bafang motor . Otherwise you may be right, I would need to use a separate battery charger rated for something less than 60v.