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"I know you can't get something for nothing, but we can try!"

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Avalanche

Thu Jun 03 2010, 11:17AM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

Just thought, in case anyone wanted a laugh

This was a quote from our managing director at work, made at a meeting regarding one of our battery charger products. The idea was to power the battery charger from it's charging battery, in order to float charge the same battery.

How do you deal with things like this?! Remember this is a power electronics company who have contracts with the rail and oil industries!!

wtf.

Steve Conner

Thu Jun 03 2010, 11:21AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yeah, sounds like the kind of idea a manager would have :)

Proud Mary

Thu Jun 03 2010, 12:11PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Avalanche wrote ...

The idea was to power the battery charger from it's charging battery, in order to float charge the same battery.

Isn't that what Treasury officials mean by "quantitative easing" - buying their own securities in order to keep the price up?

IntraWinding

Thu Jun 03 2010, 12:26PM

Registered Member #2261 Joined: Mon Aug 03 2009, 01:19AM
Location: London, UK
Posts: 581

Send him some info about perpetual motion machines

radiotech

Thu Jun 03 2010, 02:09PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

"The idea was to power the battery charger from it's charging battery, in order to float charge the same battery.

The float charge is just a maintenance of the a voltage level higher than rated output to prevent some chemical effects from reducing battery life. If a float charge system drew a heavy pulse of current. periodically (like that needed to perform the cells function, ie operate relaying in substation when transmission voltage is cut), to charge smaller cells or supercaps. That interim source could operate the float voltage requirements.
The basis of this lies in the fact that statistically the "equalize charge" power is available most of time, and is used after the power comes up, and raises the battery voltage to the level where cutout occurs, and float float voltage is applied. Obviously any system
eventually runs out of energy, but this never happens practically
when the functionality of the battery/system is viable.
The nature of storage batteries is that after a heavy draw, the voltage creeps back up on its own, but after a few heavy draws, the,
amps *time drops.
The whole reason for the idea is to save energy. Anyone familiar with the big substation battery chargers knows they need power 24/7/365 just hanging there. The auto-equalizer looks good, like
the tritium powered exit lights do.

Avalanche

Thu Jun 03 2010, 05:21PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

The place drives me nuts sometimes, but it can be quite funny!

IntraWinding wrote ...

Send him some info about perpetual motion machines

No! Knowing my place, they'd probably quote and get an order - then in a few months I'd get the sack for not being able to make it work!

Edit: come to think of it, we already make them. Tomorrow, if I remember, I will post a picture of the spec plate on a piece of equipment we manufacture - and see if you guys can see what's wrong.

Conundrum

Fri Jun 04 2010, 06:59PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

bwahaha...

actually this idea might work with a few modifications for situations like mine where i have several series connected batteries of unkknown age/health with different marked capacities.

the idea here is that each battery has its own monitor, and they are networked. If one detects that it is getting close to shutdown it "steals" energy from the good cells to recharge itself on the fly via an isolated SMPS.

sound feasible?

obviously the main annoyance would be the required software and power switching but its not so far fetched to have say, ten windings on a single ferrite core with one going to each battery via a diode and switch with a failsafe voltage monitor to prevent charge voltage exceeding 3.7V so they remain stable.

the total system voltage drives the core with a PWM modulated sinewave proportional to the correction requirements, so if one cell is at 3.0V and the rest at 3.3V the micro calculates how much pulsed power is needed to correct the problem without wasting too much energy.

would work well for LifePO4 packs where often the whole pack dies prematurely because one cell is weaker than the rest or so imbalanced that it is driven into deep discharge causing permanent damage.
interestingly many "dead" drill packs often only have one fried cell or cell pair so can be utilised for outdoor lighting and other applications.

i call it the "reflex battery"...

commercial packs use something similar but only as far as the pack shuts down if it detects an imbalance during charging or discharging. some of the newer ones also "bypass" weak cells with a low value resistor at a certain point to maintain current flow at the expense of a drop in terminal voltage.

The same trick would also be useful for interfacing incompatible solar modules, in this case you set up each so that it detects the current frequency and phase locks its own oscillator to the master.
then each module's output increases the peak circulating power in the core avoiding the need for high voltage rated wiring and isolation between modules as well as removing the "weakest module determines the system current" problem.

think of it as a miniature grid tied inverter for every module :)

-A

"Bother" said Pooh, as his 32 core 3GHz CPU reached the temperature of molten steel...

Avalanche

Fri Jun 04 2010, 08:28PM

Registered Member #103 Joined: Thu Feb 09 2006, 08:16PM
Location: Derby, UK
Posts: 845

That sounds like quite an interesting idea, a bit complex I can think of at least one use for it in one of our products - where a large 72V battery bank is tapped at 24V to power a hydraulic release motor. Over a couple of years, the bank becomes quite noticeably unbalanced, and that is about the time the batteries are replaced anyway. If the cost of the 'reflex' control system didn't outweigh the cost of the 600W DC-DC converter that we use now, then it would be a win.

Hold on, it won't fully charge all the batteries all by itself whilst running the charger's display and cooling fan so it just won't do I'm afraid

I said I was going to post a picture of one of our rating plates, but I forgot. Sorry! Maybe next week...

radiotech

Sat Jun 05 2010, 01:06AM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

"then each module's output increases the peak circulating power in the core "

In a network of power sources composed of different sized generators or batteries, each source can only contribute power
by being of a higher voltage than the grid *where it connects*, and this only happens when that higher voltage drops to the grid voltage
across a series resistance. In the case of parallel connected solar panels, that resistance would be the internal resistance of the individual panel, (same applies to batteries in parallel)

In the case of parallel cells capable of bidirectional flow, energy might be produced resulting in zero current contribution ; this just cancels the loss in the connecting wires.

Bah! they said to the roomfull of mathmeticians. Then they simulated the power network on a big table with resistors and batteries. ammeters and voltmeters. (history of AC distribution before Steinmetz taught them complex arithmetic for electricity.)

Steve Conner

Sat Jun 05 2010, 01:47PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Conundrum, that's what modern battery management systems do anyway. The little ones you find in laptops probably don't, but something like the Tesla Roadster battery pack has exactly what you describe, and the modules are available off the shelf.

The one I saw uses a DC-DC converter to shunt energy between two neighbouring cells. Each module balances two series cells, and in that way the whole string gets balanced.

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