# Who wants to help design a simple charger?



## NZero (Jan 30, 2010)

For myself, I have played with batteries and chargers almost all my life. I have built chargers with small ic's that measure voltage and adjust output accordingly. My batteries receive a huge wollop (more then recommended to start with) but then it quickly tapers off so as not to boil the batteries. For example I used to build them for 40Ah batts and start current was 50amps, after 10 minutes it would drop to 5 amps for 10 minutes then 30 amps and so on. When batteries are cold they can take a bigger charge so I tried to put back as much as quickly as possible. My customers have gel and SLA that are 8 years old and still have 90 percent capacity in floor cleaning equipment that is used daily.


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## V96400A (Mar 13, 2011)

Thanks for your reply. I do play with battery chargers as well, I just don't understand some of the more sophisticated ones like the two I have had in my motorcycle. Do you have any suggestions for the full wave rectifier? I am trying to keep the charge current within the specs of a typical household circuit breaker, 10-20 amps should do for small EV chargers.


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## NZero (Jan 30, 2010)

V96400A said:


> Thanks for your reply. I do play with battery chargers as well, I just don't understand some of the more sophisticated ones like the two I have had in my motorcycle. Do you have any suggestions for the full wave rectifier? I am trying to keep the charge current within the specs of a typical household circuit breaker, 10-20 amps should do for small EV chargers.


Can you not use cheaper half wave bridge rectifiers? It'll draw half the current and you could control it with like a 555 timer chip using the voltage to change its own input thus controlling maybe a transistor feeding the bridge rectifier? Thinking how to make this cheap lol


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## NZero (Jan 30, 2010)

Or just 2 diodes instead of a rectifier and a 555 timer and pulse charge with transistors?


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## V96400A (Mar 13, 2011)

I'd rather go with a full wave rectifier for quicker charge time, especially since the prices for 1 full wave rectifier is around $7-10. I do like the 555 timer, it's pretty popular, in that it shows up in all the research that I do. 

The an example of the cost of a motorcycle, given an open source charger is used that costs less than $100.

Open ReVolt Cougar controller: $750 (prebuilt)
K91-4003 Advanced DC motor $800 (KTA Services)
30 LYP 40Ah cells $1600
DIY charger $100
DC/DC $200 (Cloud Electric)
Wiring, connectors, fuses ~$150
Contactor $100 (Cloud Electric)
Total ~ $4000

The above total is meant to be a rough estimate and I rounded up from the total to include the few extras that might be specific to individual builds.
The anticipated range would be up to 60 miles at low speeds. Performance would be pretty good and it would be pretty impressive for the rather small investment.


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## frodus (Apr 12, 2008)

whats your bill of materials?
i.e. what research have you done, and what parts have you priced out?
Schematic?

How are you controlling voltage?
How are you controlling current?
How are you sensing current?
How are you sensing voltage?
Did you add the cost of a circuit breaker/fuse and holder?
How do you intend to shut off the charger if one cell goes too high?


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## V96400A (Mar 13, 2011)

As I mentioned earlier, I intend to build the charger around the voltage coming out of the wall, there should be no need for current sensing or control. The idea is to get a simple, steady DC output voltage from the wall and match the number of batteries and the other components to that. This would work well since LiFePO4 batteries have low cell voltages and there is some wiggle room for their charge current and voltage. The charge current could swing somewhat wildly and the batteries would not mind. There is no need to monitor individual cell voltage as balancing can be a separate issue to be addressed before stringing the pack together (according to Prof. Whitacre at Carnegie Mellon).

Please let me know directly if you think I am asking for too much in this design and why. I literally think it could be as simple as just rectifying the AC, smoothing the output and a voltage sensing circuit as seen at http://pubpages.unh.edu/~aperkins/pdf/Misc-devices/unijunction.pdf to operate a relay to shut off the charger at end of charge. 

I am trying to find the rectifier that I found for 7-10 bucks. 

I already have a charger, but I want to build this one and see how well it works.


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## frodus (Apr 12, 2008)

> As I mentioned earlier, I intend to build the charger around the voltage coming out of the wall, there should be no need for current sensing or control.


Yeah, but you said 32 cells of TS. If you use a rectifier and cap you get 170VDC - the voltage drop of the rectifier, or ~160VDC, which is way over the voltage/cell rating of TS cells (168VDC on 32 cells is 5.25V), which is why I asked. you'll need a transformer to bring that voltage down, or maybe a variac.

Next thing, Not sure how you can get away without current control. When the batteries are discharged, and you put higher voltage on them, they pull whatever they want to get there. The further the voltage the battery is away from the charger voltage, the more it pulls. If you don't limit it somehow, things will start popping (breakers, fuses, rectifiers). These cells need CC then CV.



> The idea is to get a simple, steady DC output voltage from the wall and match the number of batteries and the other components to that. This would work well since LiFePO4 batteries have low cell voltages and there is some wiggle room for their charge current and voltage. The charge current could swing somewhat wildly and the batteries would not mind. There is no need to monitor individual cell voltage as balancing can be a separate issue to be addressed before stringing the pack together (according to Prof. Whitacre at Carnegie Mellon).


The batteries may not mind with the charge current. The problem is, your fuses, breakers, rectifiers and AC line side (1500 watt limit) will care. Batteries pull the amps they need.

If you don't monitor cell voltage, you're risking a lot. Batteries are not IR matched usually, so you get variances. If you don't balance the cells periodically, you could have one cell that gets up to voltage quicker than other cells. The charger won't know that if it's sensing the "pack voltage". User beware, but I'm just throwing that out there. You should have something in place that monitors cell voltages and can turn off the charger, IMHO.



> Please let me know directly if you think I am asking for too much in this design and why. I literally think it could be as simple as just rectifying the AC, smoothing the output and a voltage sensing circuit as seen at http://pubpages.unh.edu/~aperkins/pdf/Misc-devices/unijunction.pdf to operate a relay to shut off the charger at end of charge.


I think it's doable, but not as simple (or cheap) as you expect. I don't think it's that easy, and having some limited experience in building chargers, BMS and discharching/charging batteries, It's not really that simple.



> I am trying to find the rectifier that I found for 7-10 bucks.
> 
> I already have a charger, but I want to build this one and see how well it works.


It's a fun project, but I think you need at a minimum, a Rectifier, CAP, Large FET to PWM the output, current sensing circuits, voltage sensing circuits and a little microcontroller. 

There's an open source controller on this forum, maybe you can use a slightly simpler version of that.
http://www.diyelectriccar.com/forums/showthread.php?t=59210

just downsize it.

I'd email them and see if they have ideas for a 120VAC only version, simple, that can be pre-programmed and not have an LCD or the "extras".


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## NZero (Jan 30, 2010)

Yes, it would be crude but thats what the 555 is for. The lower the voltage (of the pack) the longer between pulses (output of chip which switches the transistors off) so whilst the current draw may be high it is only for a second then 2 seconds rest, then 1 second pulse then 2 seconds rest......

Then as pack voltage gets higher the current draw will be low (as you said) so the pulses get quicker. 2 seconds on, 1 second off, 2 seconds on 1 second off....

Again crude but offers some control. And I naturally assumed there would be a voltage transformer in there which was changed out depending on the pack voltage you required.

Bill: 
1 box
1 ac cable
1 dc cable set
1 transformer ac/ac
2 power diodes or 1 rectifier assembly
1 555 timer chip
1 fuse 
2 transistors
2 smoothing caps
1 resistor (depending on pack voltage to feed 555 input)
PCB or bread board 

Not looking too cheap when you consider a switch mode power supply will do the same thing and will adjust to multiple voltages.


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## frodus (Apr 12, 2008)

I'd use a full bridge rectifier (4 diodes), not 2 power diodes, the ripple on the DC will be much smoother.

But yeah, basically you're designing a switch mode power supply.....


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## rmay635703 (Oct 23, 2008)

I wonder if a large 1500watt PWM AC dimmer type device you see in stores for cheap would do the trick? The chinese PWM AC dimmers can be had for a lot less than I can realistically build one.

Run the lines through the AC dimmer with some method of controlling it and then through your bridge and cap, a good size inductor coupled with a decent bridge should reduce the inrush to the cap.

If you can figure a way of automatically controlling or shutting off an AC dimmer that is tied to battery behavior I am all ears.


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## spdas (Nov 28, 2009)

IMHO, for $400 or so savings, I would not skimp on a charger. A highly recommended or safe lithium charger is your insurance policy to protect your $5k-$12k worth of batteries.

Francis


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## rmay635703 (Oct 23, 2008)

spdas said:


> IMHO, for $400 or so savings, I would not skimp on a charger. A highly recommended or safe lithium charger is your insurance policy to protect your $5k-$12k worth of batteries.
> 
> Francis


Any charger can be made safe if a simple method of getting the power to the charger killed (be it dumb or smart) is developed.

And I am more interested in lead batteries


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## NZero (Jan 30, 2010)

Maybe a cheap 12v charger but just bang a transformer on the end of it so it chages from 14v to xyz volts?


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## frodus (Apr 12, 2008)

NZero said:


> Maybe a cheap 12v charger but just bang a transformer on the end of it so it chages from 14v to xyz volts?


Transformers only work with AC


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## EVMAN (Jul 26, 2007)

http://evtradinpost.com/index.php?m...ory&fromfrommethod=showhtmllist&fromfromid=28


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## Beemer (Jun 2, 2011)

Why don't they modify the regen on controllers for recharging?

It's a very similar circuit and wouldn't take very many additional components.

As goes the power rectifier route..With or without a timer, you really do need something to sense voltage and current and cell *TEMPERATURE* to cut the power when needed.

Whats more, you must consider say, 3.45V/cell* for safety. There is always a voltage variance where and when you plug it in. Forget depending on BMS's. They simply are not man enough nor reliable enough to stop the unstoppable with bleed resistors.

*When V.out = V.in Then no current passes.

Please, if you burn everything down; even murders your family, don't blame the EV. I want mine to be allowed on the road and insurable.


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## frodus (Apr 12, 2008)

Beemer said:


> Why don't they modify the regen on controllers for recharging?


Series wound motor controllers (like the original poster has) don't have regen.

I think only AC Induction controllers and BLDC/PMAC controllers would work, as they already have the 3-phase regen circuits inside.

I know that some AC EV's have the charger as part of the controller, so they do do it.


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## Beemer (Jun 2, 2011)

frodus said:


> Series wound motor controllers (like the original poster has) don't have regen.
> 
> I think only AC Induction controllers and BLDC/PMAC controllers would work, as they already have the 3-phase regen circuits inside.
> 
> I know that some AC EV's have the charger as part of the controller, so they do do it.


So sad, implementation would be sooo simple and relatively cheap. Series wound motors can act as generators. All they need is a separate current source to the stator windings only.


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## NZero (Jan 30, 2010)

frodus said:


> Transformers only work with AC


Damn I need more coffee. Feel free to riddicule over that very stupid statement.


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## 9852 (Jan 17, 2010)

cheap as it gets

http://www.diyelectriccar.com/forums/showthread.php?t=48691&highlight=variable+voltage+charger

and works to, just need a way to turn it of like a home depot plug in timer, not for lithium though.


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## frodus (Apr 12, 2008)

NZero said:


> Damn I need more coffee. Feel free to riddicule over that very stupid statement.


I've made the same mistake, multiple times, no worries!


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