15S LiPo charger using DC DC converters

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Jeremy Harris   10 GW

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15S LiPo charger using DC DC converters

Post by Jeremy Harris » Nov 13 2010 12:19pm

I've just found these on eBay: http://cgi.ebay.com/SynQor-PQ48050QNA20 ... 588725fb35 They look interesting for building fairly high current (20amp) LiPo balance chargers.

The spec on these SynQor converters is more interesting than most, as the trim down range is 20%, meaning that they'll go from a nominal 5V out to 4V. This looks to make them ideal as LiPo charge modules. They are isolated, so you can hook a load of them up to a big 48V DC supply and charge as many cells in a pack at once as you like.

The crude and simple way to stop them going into current limiting with a near-flat cell is to just fit a low value resistor (something like 0.1 ohm) in the output lead. This will add enough of a volt drop to limit the maximum current to under the 20 amp rating for a cell that's right down at around 2.2V (with the DC DC converter set to 4.2V output voltage).

I've just bought a bunch of these to build a kick-ass charger. I'll keep people posted as to how they work out (as I've had mixed experiences with DC DC converters in the past - some work great, others are not so good).

The data sheet link is here, if anyone's interested: http://synqor.com/datasheets/PQ48050QNA20_Datasheet.pdf

They'll need fan cooling for sure, but that shouldn't be too big a problem. A 350 watt, 48V Meanwell would run maybe three of these, so it looks like I'll have to get some bigger PSUs. Luckily, there's some rather affordable 500W ones here: http://cgi.ebay.co.uk/ws/eBayISAPI.dll? ... K:MEWAX:IT and the chap is only an hours drive away, so I can save on postage. To charge my proposed 15S pack I think I'll need three or four of these, with 15 DC DC converters. A 1500 watt charger should do the job nice and quickly..........................

Jeremy
Last edited by Jeremy Harris on Dec 10 2010 12:57pm, edited 1 time in total.
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Re: DC DC converters for charging LiPo

Post by MrKang » Nov 13 2010 12:54pm

Super product jeremy ! thanx for sharing this wonderfull DC - DC convertor. I wait until your test results are coming up.
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Re: DC DC converters for charging LiPo

Post by vanilla ice » Nov 13 2010 5:02pm

Nice 85-90% efficiency.. reminds me of like xyster's crazy old charge setup except 35-75v DC in to each module instead of AC.

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Re: DC DC converters for charging LiPo

Post by webfootguy » Nov 13 2010 6:38pm

The same seller has 3.3v 25A 48v dc/dc converters as well for the LiFePO4 folks. The prices are great (<10% of typical costs and they are new). Looks like older stock possibly as the datasheet I looked at is from 2003. The same company makes a 50A 3.3v version as well.

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Re: DC DC converters for charging LiPo

Post by Jeremy Harris » Nov 14 2010 6:22am

Here's a quick plot of the expected charge current Vs cell voltage for one of these DC DC converters set to 4.2V and fitted with a 0.1 ohm series resistor to limit the maximum current:
Charge current Vs Cell voltage.jpg
Charge current Vs Cell voltage.jpg (67.85 KiB) Viewed 992 times
This under-runs the converter most of the time, but still gives a respectable charge rate. My experience is that LiPo seem to sit at around 3.85 to 3.95V for most of the charge period, with the voltage rising quite quickly towards the end of the charge. This set up would only charge at 3 to 4A during most of the charge time, so wouldn't make best use of the capability of the converters. Given that they are cheap and would provide inherent simple balancing, with a relatively high balancing current, this may not matter.

If you were reasonably sure that you weren't going to charge a cell that was sitting down below about 3.2V, then changing the 0.1 ohm current limiting resistor for a 0.05 ohm one would give this sort of cell voltage vs charge current plot:
Charge current vs cell voltage 0.05 ohm resistor.jpg
Charge current vs cell voltage 0.05 ohm resistor.jpg (79.83 KiB) Viewed 992 times
I'm pretty sure that, even with no current limiting resistor, you couldn't overload these when hooking them up to flat LiPo cells, as the connecting cable resistance would probably be enough to stop the current limit from kicking in - it only takes 30 mohms of cable/connector resistance to drop the voltage enough to keep max current below 20 amps for a typical cell voltage of around 3.9V on charge. LiPo cell voltage seems to rise fairly quickly when they are flat, anyway, which would tend to limit the max current. Ideally we want a zero voltage drop current limiter in series with the DC DC converter, but I don't think it's worth the hassle of making things more complex - the resistor will work well enough and is simple.

The biggest problem is the capability of the balance leads, my guess is that they are only good for around 3 to 5 A maximum. This probably wouldn't be an issue with a paralleled pack though, as long as the connection from the balance lead joining block to the charger was up to the task.

The relatively cheap 28mm diameter Chinese circular military connectors are OK for around 10A per pin for the 14 pin ones, so would seem to be a pretty good choice for hooking a pack up to the charger, at least for up to 12S.

Jeremy
Last edited by Jeremy Harris on Nov 16 2010 3:13am, edited 1 time in total.
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Re: DC DC converters for charging LiPo

Post by olaf-lampe » Nov 14 2010 6:26am

Hi Jeremny,
I tested a few different dc-dc converters a while back, none of them worked as I wished. But I haven't tried the 0.1 ohm shunt to reduce the current.
If you are looking for 30-60V/1.5-1.9kW chargers, check out for benning tebechop 1700 or ascom ( SMPS 40 or ASM 1900)chargers used to charge UPS's in telecom transmission stations. They got replaced frequently and are cheap too. ( i got two devices for 100EUR )
Be sure they come including the D-sub power connector!
Ascom_vs-meanwell.jpg
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Ascom_vs-meanwell2.jpg
Ascom_vs-meanwell2.jpg (46.91 KiB) Viewed 1484 times
-Olaf

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Re: DC DC converters for charging LiPo

Post by Jeremy Harris » Nov 14 2010 7:00am

Thanks for the tip, Olaf. I'm currently looking at a 500W telecom supply with the same connector type, thanks to a tip off from the Mighty Volt. The eBay link above shows it - at just £12.99 (around 15 euro, and I can collect it locally) it looks to be a bargain.

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Re: DC DC converters for charging LiPo

Post by deVries » Nov 15 2010 6:06pm

Hi Jeremy, looks like that .5 ohm resistor you noted below is the way to go. (It is .5 and not .05 that is shown in your plot link below... :?:)
:mrgreen:
If you have used these before and embedded the converters into your battery, then I would be interested in your ideas or pics for ebike use.

Thanks! :cool:
Jeremy Harris wrote:If you were reasonably sure that you weren't going to charge a cell that was sitting down below about 3.2V, then changing the 0.1 ohm current limiting resistor for a 0.5 ohm one would give this sort of cell voltage vs charge current plot:
Charge current vs cell voltage 0.05 ohm resistor.jpg
I'm pretty sure that, even with no current limiting resistor, you couldn't overload these when hooking them up to flat LiPo cells, as the connecting cable resistance would probably be enough to stop the current limit from kicking in - it only takes 30 mohms of cable/connector resistance to drop the voltage enough to keep max current below 20 amps for a typical cell voltage of around 3.9V on charge. LiPo cell voltage seems to rise fairly quickly when they are flat, anyway, which would tend to limit the max current....

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Re: DC DC converters for charging LiPo

Post by Jeremy Harris » Nov 16 2010 3:12am

deVries wrote:Hi Jeremy, looks like that .5 ohm resistor you noted below is the way to go. (It is .5 and not .05 that is shown in your plot link below... :?:)
:mrgreen:
If you have used these before and embedded the converters into your battery, then I would be interested in your ideas or pics for ebike use.

Thanks! :cool:
There's a typo in the text above (now corrected!). The second plot is for a 0.05 ohm resistor, as in the file title. This needs to be rated at around 20 watts for the worst case of a cell that's persistently sitting down at 3.2V, but as this is a pretty unlikely situation I think you could get away with a 10 watt, or maybe even a 5 watt rated resistor. It may well be cheaper and simpler to fit two 0.1 ohm resistors in parallel, as getting hold of resistors that are rated higher than 5 watts gets expensive.

I currently have a LiFePO4 charger that uses DC DC converters and it has worked well for over a year. I used 3.3V converters trimmed up to 3.63V (their maximum trim-up voltage). They can deliver 15A or so into a near flat cell, but average around 6 to 10A.

I'd be wary of fitting a charging system like this on to a bike, for two reasons. Firstly, the converters will draw a small current from the cells all the time they are connected and not powered, as they have a voltage sense circuit connected to the output. Secondly, they get pretty hot when delivering high current - I have fan cooling on my present charger to keep them at an acceptable temperature and plan to do the same with these.

Jeremy
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Re: DC DC converters for charging LiPo

Post by Jeremy Harris » Dec 09 2010 8:33am

An update.

The 5V 20A SynQor DC DC converters arrived (from Israel) and I've tested one and made up some boards for my mega LiPo charger. The data sheet seems to have an error, as when I followed the formula for calculating the value of trim resistor needed it gave me (as near as dammit) a value of 22k for 4.2V. When I tested a converter with this value I ended up with an output voltage of 4.79V............ After a bit of experimenting, I've found that a 3k9 resistor gives a nice repeatable 4.14V, which I think will be fine.

I've made up three boards, each about 6" wide and hold 5 converters. These boards have been designed to stack, so that I can have a three layer arrangement with a fan blowing air through the stack from side to side. Here's a pic:
SynQor DC DC converter boards.JPG
SynQor DC DC converter boards.JPG (107.13 KiB) Viewed 8403 times
The boards have places for low value (probably around 0.1R) resistors that will act as crude current limiters. As the maximum current from each converter is about 20A, the stack of boards should be able to deliver around 1200 watts of charge into low cells. I will probably limit this to maybe half that, maybe a bit less, to make life easier on the multipin connector I need to hook this up to a 15S pack.

If anyone is interested, the seller still has some for sale, at $4.95 each plus shipping. The ebay number is 380224535349.

Jeremy
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Re: 15S LiPo charger using DC DC converters

Post by Jeremy Harris » Dec 10 2010 1:04pm

I finished building and testing these boards today and it all works fine, just as expected. The 15 isolated outputs are all between 4.14 and 4.15V, probably ideal for charging LiPo and maintaining a fairly long life. The charge current on a pack that was fairly well run down was around 8 to 9 amps initially (I've used two 0.1 ohm resistors in parallel to limit current) dropping rapidly to around an amp or so right at the end of charge, then near enough zero as the pack came up to the charger output voltage.

I just need a big power supply to drive it now, around 48V at 16A or so should just about do it. I'll either look at the cheap supplies from ebay I mentioned previously, or I may go for a couple of 500W Meanwells. I'd like to get the whole lot in one box if I can. It needs forced air cooling, but the board stack has been arranged to make this fairly easy.
Mega LiPo charger 1.JPG
Mega LiPo charger 1.JPG (127.81 KiB) Viewed 8354 times
Mega LiPo charger 2.JPG
Mega LiPo charger 2.JPG (113.53 KiB) Viewed 8364 times
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Re: 15S LiPo charger using DC DC converters

Post by texaspyro » Dec 10 2010 1:23pm

Very nice... I do the same thing using some Lucent converters. They are a little bit smaller and totally potted.

It looks like a little more space between the boards may make the cooling airflow a bit better. And I hope you picked up a couple of spare modules. It really sucks to build up a nifty device with surplus parts, have one fry, and then have no way to replace it (or pay retail for a replacement).

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Re: 15S LiPo charger using DC DC converters

Post by deVries » Dec 10 2010 4:09pm

Jeremy Harris wrote:I finished building and testing these boards today and it all works fine, just as expected. The 15 isolated outputs are all between 4.14 and 4.15V, probably ideal for charging LiPo and maintaining a fairly long life.
Nice work & neat finish. 8) Is it easy to make those circuit boards that you attached the converters & resistors/jumper to? It would be interesting to learn how to do this too, or could one buy these boards to solder the parts onto from you? (If you could make any extras. This is just a question, as I don't know 'now' that I have an application for this yet.)

Would like to see a pic showing the balance leads/harness with connector too, and what you finally connect it to. :mrgreen:

Thanks for posting this project. It's one I will probably do at some point thanks to posts such as yours to show us how to do it so well with at least a touch of perfection. :wink:

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Re: 15S LiPo charger using DC DC converters

Post by Jeremy Harris » Dec 10 2010 4:55pm

The boards are fairly easy to make, but take a bit of time and fiddling about. The process is pretty well described on the "simple BLDC controller" thread, with a few good ideas from others thrown in. It is definitely a reasonably easy DIY process, especially for boards like this with no narrow tracks and only fairly big holes

There are quite a few printed circuit prototyping companies around who'll make a small batch of boards for you if you don't want the hassle of a DIY job. I've never used them, but I've heard good reports from those that have.

The leads I used today for a test were a lash up, but I've got some 10 way connectors coming in the post. The plan is to have three charging leads coming from the charger, each with a 10 way connector on the end. This way I can charge my bike pack (which is 10S) by just using two of the leads and the 15S pack I'm planning for the resurrected Yamaha RD50 project. I could also make up adapter leads to charge any other pack from 1 to 15S easily enough. Finding high current multipole connectors is a struggle, which is why I've opted to use three 10 way ones, rather than one 30 way.

Jeremy
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Re: 15S LiPo charger using DC DC converters

Post by magudaman » Dec 11 2010 4:36pm

Very Very nice build I was originally hoping to build something similar but the DC-DC units I bought were non isolated :cry: Should have done my research. This seem like the way to go verse bleeding the final currents.
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Re: 15S LiPo charger using DC DC converters

Post by johnnythefox » Dec 11 2010 4:38pm

Great work here Jeremy I like the simple solution to cell balancing and was hoping you would post on this so we could all benefit from your knowledge.
You said finding high current multipole connectors is a struggle, so could you share your link?

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Re: 15S LiPo charger using DC DC converters

Post by magudaman » Dec 12 2010 12:09am

I meant to asking in my other post but how do you go about creating your boards? They look great!
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Re: 15S LiPo charger using DC DC converters

Post by Jeremy Harris » Dec 12 2010 4:58am

johnnythefox wrote:Great work here Jeremy I like the simple solution to cell balancing and was hoping you would post on this so we could all benefit from your knowledge.
You said finding high current multipole connectors is a struggle, so could you share your link?
I've ended up buying some Chinese "military" connectors. I did some hunting around and the 10 pin ones are rated at 20A per contact (as are some of the ones with less pins). These are available from ebay (search for "military connector" and you should find them) from several suppliers. I used a seller called "partspipe", but have no idea how reliable or otherwise they are. They may be with me before Christmas, with luck. As soon as I get them I'll let you know what they are like.

As far as I can tell, these Chinese connectors seem to be very similar to the JAE aerospace type connectors, but made to a Chinese military specification. I know that at least one other ebike manufacturer/supplier uses the same connector type and so far I've not heard of any problems with them. I'm familiar with standard NATO type military connectors (Amphenol, Cannon etc) and have a few lying around (they are megabucks to buy though) so I'll be able to compare the Chinese ones with something I know to be good quality when they arrive.

Jeremy
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Re: 15S LiPo charger using DC DC converters

Post by Jeremy Harris » Dec 12 2010 5:03am

magudaman wrote:I meant to asking in my other post but how do you go about creating your boards? They look great!
There's a bit of a discussion on the way to make these boards at home on the Simple BLDC Controller thread, here: http://endless-sphere.com/forums/viewto ... =2&t=23350 Pretty much all you need to know about doing it's in that thread.

It's not a very precise method, I've just made a small board this morning with 25 thou wide tracks and that's probably close to the thinnest you can go, but it is cheap and fairly easy to do with no special tools needed, except a way to drill the board, if you're using through hole parts.

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Re: 15S LiPo charger using DC DC converters

Post by olaf-lampe » Dec 12 2010 6:08am

Hi Jeremy,
how do you know all cells get charged? Do you use a cell log for it? Would be nice to have a two-colored LED on each converter output to show the status.
I'd like to see a charge graph to see how fast the current drops.
-Olaf

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Re: 15S LiPo charger using DC DC converters

Post by Jeremy Harris » Dec 12 2010 6:22am

olaf-lampe wrote:Hi Jeremy,
how do you know all cells get charged? Do you use a cell log for it? Would be nice to have a two-colored LED on each converter output to show the status.
I'd like to see a charge graph to see how fast the current drops.
-Olaf
The simple answer is that, at the moment, I have no built in way of knowing when all cells are charged, short of putting a meter on each output. I am thinking of putting a current detector on the input to the DC DC converters, to detect when the charge current drops to a low level. The 15 converters draw around 1 amp with no load from 48V, so just fitting an ammeter into the feed would be a reasonably good way of seeing how close to end of charge the pack is.

One thing I'm not sure about is the effect of keeping cells at around 4.15V for a time once they are fully charged. My guess is that the effect of doing this for a few hours is probably negligible, but I may look at building a simple current threshold detector that turns the charger off a set period of time after the supply current drops to something like 1.2A or thereabouts and turns on an "end of charge" light. This should be easy enough to do, I might even be able to just modify the unit that Gary is selling, this one: http://www.tppacks.com/proddetail.asp?p ... CHG-CNTRLR

Jeremy
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Re: DC DC converters for charging LiPo

Post by otas » Jan 08 2011 9:38pm

Jeremy Harris wrote:
I'd be wary of fitting a charging system like this on to a bike, for two reasons. Firstly, the converters will draw a small current from the cells all the time they are connected and not powered, as they have a voltage sense circuit connected to the output. Secondly, they get pretty hot when delivering high current - I have fan cooling on my present charger to keep them at an acceptable temperature and plan to do the same with these.

Jeremy
Jeremy, what is the current draw from the cells when they are connected and not powered, as I am considering building it in the bike and leaving it connected??
thanks
otto

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Re: 15S LiPo charger using DC DC converters

Post by Jeremy Harris » Jan 09 2011 11:10am

I've just measured it and it's a bit too high to leave connected for long periods of time, around 5mA per cell. It would be OK for short periods though, as it's only going to drain the pack down by around 0.12Ah per day.

I think it's be best connected via a multipin connector. If you don't mind the size, then a European trailer connector would do the job, they have 13 pins, are weatherproof and are OK for up to around 20A per pin. You need 11 pins for charging, so could double up on two pins and use the same connector to plug the pack in to the bike.

Jeremy
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Re: 15S LiPo charger using DC DC converters

Post by otas » Jan 09 2011 7:29pm

I am reluctant to use connector, as I want to leave it permanently connected. How about a diode in each DC-DC convertor feeding the Lipo batt. with the voltage sense before the diode. The DC-DC convertor output would have to be increased due to the forward voltage drop, and we would have to compensate for forward voltage change in relation to the current not to exceed the 4.2V in case of Lipo's. It will have to be a compromise, whether to live with the drain, or isolate it with diode, or reduce the sense current?
otto

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Re: 15S LiPo charger using DC DC converters

Post by jag » Feb 11 2011 4:04pm

Hi Jeremy,

Any updates on how your DC-DC converter chargers performs?

I was also curious if in practice one could make a universal LiPo, LiFePO4 charger, by switching voltage adjust resistors. This would mean going below the -20% down adjustment rated in the spec. I was curious if you tried it in practice, and at what the lowest output voltage the converter would work (Fig 17 in the spec on the other hand could be interpreted that the low voltage shut off should be as low as 1V, but that is probably optimistic...)

I also noticed that you limit current with resistors, instead of relying on the internal CC current limitation. Is that because you wanted a tapering off charge current or that the CC mode didn't work well?

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