Can a Switch-mode power supply charge cells inside the battery by which it is being powered?

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Mik
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Can a Switch-mode power supply charge cells inside the battery by which it is being powered?

Could a switch-mode power supply be used to charge individual cells or groups of cells within the string of cells which is supplying the power to the switch-mode power supply?
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If it is possible to connect a switch-mode power supply as shown in the diagram, then it would be a nice on-board backup charging system for unbalanced=unequal capacity cell packs. Very handy for situations when the Vectux would usually end up crawling along or being pushed by me - NO FUN AT ALL! This situation arises whenever the entire 102-cell NiMH string has been drained to the capacity level of the lowest capacity cells. (Usually about 45km)
When extended range is needed the switched connection between switch-mode power supply and the low capacity cells could be closed whilst still riding along, or the scooter left to stand for a while whilst recharging "itself".
The switch-mode power supply could possibly also power the indicators in a "HAZARD WARNING" fashion when needed.

This will lead to a problem later during proper recharging at a power point:

The "good" cells would then have more "rechargeable capacity" than the amount which the weak cells are capable of absorbing. The weak cells might, in extreme cases, even be fully charged already when the other cells are empty. The weak cells might therefore start venting due to pressure build-up when the stock-on-board-charger puts 10A through the string.
This problem could however be controlled more easily than pushing the scooter home!

Have a look at the diagram and tell me if there is something fundamentally wrong with this idea, please!

LinkOfHyrule
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Re: Can a Switch-mode power supply charge cells inside the batte

In theory, I guess that could work, but it would throw the whole thing way, WAY out of balance, as you pointed out. :/

Plus, if one or two cells have only a little less capacity, it's going to do no good. If a cell has enough to be a problem, it really should just be replaced.

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Mik
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Re: Can a Switch-mode power supply charge cells inside the batte

In theory, I guess that could work, but it would throw the whole thing way, WAY out of balance, as you pointed out. :/

Plus, if one or two cells have only a little less capacity, it's going to do no good. If a cell has enough to be a problem, it really should just be replaced.

I don't have the final results of the capacity testing and reconditioning of all 102 cells yet, but I think that I'll be able to combine the weakest cells into a module which has about 20% less capacity than the rest of the pack!

And that would mean about 10km added range at hooning speeds! Sure, it's a hassle balancing the pack afterwards, but I have an idea regarding that, too...;-)

Maybe I can short the string between the last cell before, and the first cell after the "low capacity module", so that the charging current runs through a 30A rated wire instead of through the pre-charged low capacity cells.

I'm planning to install tabs like that anyway. One fat (rocket-launcher type!!!) switch could bypass the module of low capacity cells during charging, which would lead to over-charging of the rest of the pack until the high-temperature cut-off shuts down the stock charger. But usually I'd beat the temp sensors to the job because I would have all that energy left over which I would normally have invested into pushing the empty scooter. LOL

Anyway, need to check what voltage the indicator lights require to work - if it looks like I can combine the "on-the-go-charger" with hazard lights, then I think it's worth giving it a go!

This information may be used entirely at your own risk.

There is always a way if there is no other way!

sparc5
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Re: Can a Switch-mode power supply charge cells inside the batte

This was what my first GreenBMS plan was supposed to do. It works, and even has the added bonus of being able to plug the SMPS to the wall, and charge your scooter with just a few extra parts.

There were two approaches. Single cell chargers on each cell, or less chargers that can be connected to any desired cell using mosfets. In both situations the chargers are powered by the entire pack. I looked at a lot of stand alone NiMH charging ICs. They cost about $1.20 each, but you'd need a way to control when they turn on and off since you obviously wouldn't want to fully charge a single cell or have them all turned on when you're driving. It would serve good way to charge your cells after you're done driving though. The alternative is to build your own charging circuits.

It was scrapped though because the cost got prohibitively high for NiMH and I was nervous of how all the daisy chained modules would perform in a high vibration environment without expensive connectors meant for it.

XM-3000...
-DC-DC converter replaced with a Dell D220P-01 power supply.
-72V mod
-Expensive bank charger until I come up with something better... Still trying.
-

Mik
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Re: Can a Switch-mode power supply charge cells inside the batte

This was what my first GreenBMS plan was supposed to do. It works, and even has the added bonus of being able to plug the SMPS to the wall, and charge your scooter with just a few extra parts.

There were two approaches. Single cell chargers on each cell, or less chargers that can be connected to any desired cell using mosfets. In both situations the chargers are powered by the entire pack. I looked at a lot of stand alone NiMH charging ICs. They cost about $1.20 each, but you'd need a way to control when they turn on and off since you obviously wouldn't want to fully charge a single cell or have them all turned on when you're driving. It would serve good way to charge your cells after you're done driving though. The alternative is to build your own charging circuits.

It was scrapped though because the cost got prohibitively high for NiMH and I was nervous of how all the daisy chained modules would perform in a high vibration environment without expensive connectors meant for it.

Thanks, sparc5!

I have been reading many of your posts and might have gotten the idea there, can't remember exactly, I'm now busy on so many forums! Hard to believe I had not written a word on the internet 10 months ago!

Anyway, I am pondering a manual switching system described on ES:
http://www.endless-sphere.com/forums/viewtopic.php?f=14&t=6853

It is seriously flawed if applied to the whole 102-cell string due to the lethal currents involved, but it might well be feasible if I manage to concentrate the cells needing attention in a sub-string with less than, say, 40V.

And sooner or later I'll have to learn what those MOSFETS and oter Gizmos are that can do the work for you.
But for now, I'll be better off switching manually whilst learning the basics. My battery will be better off, too! HAHA.

The connectors, fuses and other pieces needed for 102 cells add up in price very quickly indeed!

I think I will solder most connections to reduce cost and resistance.

This information may be used entirely at your own risk.

There is always a way if there is no other way!

sparc5
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Re: Can a Switch-mode power supply charge cells inside the batte

I read this today before posting: http://www.powerstream.com/NiMH.htm

It confirms you have to monitor temp (cell and ambient) to safely avoid overcharging.

Why don't you make a 10x11 matrix connected to small relays that are connected to a floating voltmeter? Maybe a smaller matrix using a bipolar meter.

XM-3000...
-DC-DC converter replaced with a Dell D220P-01 power supply.
-72V mod
-Expensive bank charger until I come up with something better... Still trying.
-

Mik
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Joined: Tuesday, December 11, 2007 - 15:27
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Re: Can a Switch-mode power supply charge cells inside the batte

I read this today before posting: http://www.powerstream.com/NiMH.htm

It confirms you have to monitor temp (cell and ambient) to safely avoid overcharging.

Why don't you make a 10x11 matrix connected to small relays that are connected to a floating voltmeter? Maybe a smaller matrix using a bipolar meter.

Thanks, very interesting information on that site, consistent with what I have learned so far. The charging efficiency of the Vectrix cells appears a little better than 66.66%; I can frequently detect the beginning of the temperature rise after about 12hrs of C/10 = 3A charging. (Semi-manually with a hand held IR thermometer)

Regarding making a 10x11 matrix etc:
I don't know how to do this, yet!

But I have a strong suspicion that the schematic I made, using 34 rotary switches, is just another way of saying the same thing.

The switches are relays, hand operated relays.

The "Auto range voltmeter" is "floating"....in the sense that it gets to measure the voltage at different points according to the position of the 34 rotary switches = relays.
Here is a link to my initial (and dangerously flawed!) diagram: http://www.endless-sphere.com/forums/download/file.php?id=14586

I am not sure what you are thinking of when you write "small relays" and "bipolar meter".

How small is a small relay? And how will the small relays be told when to switch and how to switch if they have more than two positions?

Activating relays inside the safe battery compartment would be much better than exporting 140V out of there and risking electrocution.

The problem is that the 10x11 matrix contains some contacts with the full voltage of the pack between them. Lethal!

This information may be used entirely at your own risk.

There is always a way if there is no other way!

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