Dumb charging idea?

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Dumb charging idea?

Postby SimonR » Tue Apr 14, 2009 7:00 pm

I have a string of 16 Optima batteries (from a scrap yard so they don't matter too much). Would it be entirely dumb to charge them just by rectifying 230v mains - and connecting it straight to the batteries?

That will deliver 14.3v per battery which isn't far off the constant 14.7v they recommend.

OK, it's not a managed, conditioning charge cycle - but I've always charged SLA's on a straight constant voltage source without problems.

'Proper' battery chargers seem to be in the 100's if not 1000's of pounds cost range - whereas this is in the 1's to 10's!

Somewhere I have a 200A bridge rectifier out of a welder - which would do the trick.


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Postby microman » Tue Apr 14, 2009 7:40 pm

The short answer is yes - its pretty dumb! The output of a simple rectifier without any smoothing is not dc - it has a lot of ripple voltage going from 0 to the peak of the input.

The peak voltage of the 230V rms mains is about 325V, so you need to consider what will limit the peak current into the chain at that point. Ultra cheapo 12V chargers often rely on the resistance of the rectifier + transformer and faith. You should be somewhat more careful.

A limiting resistor will do if you can find one with enough power handling. Perhaps one or more electric fire elements?

The current waveform will be a series of short spikes because current will flow only for the portions of the mains cycle when the rectified output exceeds the battery chain voltage. So if you monitor the current with a meter it will indicate (usually) the average. The peak will be a lot higher.

And you will need to continuously monitor the cell voltages to decide when to stop.

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Postby ChrisB » Tue Apr 14, 2009 8:22 pm

Yep have to agree with mircoman.

You might get away with it with floodeds if you watched them but optimas would just get cooked :cry:

Also dont forget you'll be looking at high amounts of DC floating about which if you come into contact with will burn you without you really knowing :shock: or if you get locked onto it you cant let go of the stuff :shock:

I reject reality and substitute my own !!!!!!

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Postby SimonR » Wed Apr 15, 2009 9:51 am

Also dont forget you'll be looking at high amounts of DC floating about which if you come into contact with will burn you without you really knowing Shocked or if you get locked onto it you cant let go of the stuff Shocked

I wondered if somebody would have to get that in. Perhaps I should get a 'grown up' to help me?

I also understand about the peak voltage being higher - so, perhaps the question should have been phrased more along the lines of:

Is there an intrinsic problem with charging SLA's from a source where the RMS voltage is right but the peak to peak is higher (root 2 x in this case). Although, instinctively I might say yes - it's not that different to what appears to be happening in the regenerative braking on my Simotion three phase speed controller.

The controller is using the three half bridges to rectify the back emf generated by the motor. In this case, likewise it is only the RMS which is at an appropriate charging voltage - but the peak seems to be about 380v. Likewise the peak current is high. It only looks at the pack voltage and shuts down the regen when the maximum is exceeded.

Is this only not a problem because the fundamental frequency of the pulses is high (7khz in this case)? With 100Hz pulses, is the situation different?

I agree that limiting the peak current is going to be an issue. Rather than a big resistor, I was actually thinking about an isolating transformer (whose internal resistance limits the peak current) - and ultimately a fuse as the final current limit.

What is the point of this? Why don't I just buy an off the shelf charger like everyone else?

A while back I asked a question about lead acid batteries. The responses in general were along the lines that you would have to be 'hard of thinking' to even consider lead acid and I should forget about it and buy LiFePO's instead. Many forget that not everyone has an unlimited budget.

I have decided that this vehicle is going to be entirely built from scrap. I have also set a budget of £500 for the entire vehicle on the road plus a certain amount of time from me. Although I found the motor and (probably defunct) controller at a scrappy, I'm unlikely to find a suitable charger - and don't have the space for 10 old forklift chargers! :wink:


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Postby gwing » Wed Apr 15, 2009 11:35 am

My (somewhat limited) understanding is that a higher voltage pulsed charge is if anything a good thing. My old battery conditioner worked this way to desuplhate tired L/A batteries and had a 5hz frequency - so your 100hz should be no issue.

One factor might be that pulse chargers (at least those I have seen) supply a square wave such that they are either at high charging voltage or at zero. If your circuit is more sine wave ripple then it will I guess be spending considerable time above zero but below effective charging volts. Whether energy supplied in this range is simply wasted or has some other odd effect I don't know - but it would seem to be at best inefficient.

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Postby microman » Wed Apr 15, 2009 12:54 pm

The difference between your simple charger and regeneration is that the latter has series inductance from the motor/generator windings which smooths the current waveform. You could do the same to provide a nice clean charging current - but the inductance required will be large because the frequency is only 100Hz rather than the 7kHz (reactance is directly proportional to frequency) as you rightly guess. And you will have to make/find/codge it. That could be difficult or expensive or both. Many years ago I would have have trawled Tottenham Court Rd and maybe you could still find something there or on it's modern counterpart eBay?

That could be an elegant and efficient solution. Power resistor(s) (electric fires/heaters) would be cheap, dead easy to design or control, but inefficient.

Using the isolating transformer resistance/reactance to limit the current is possible but it would be very much a trial and error process and I would much prefer the simpler real resistance approach. If you really want to keep costs down car boot sales might be a good source of old heater elements.

Good luck!

PS As somebody who had a direct rectified mains shock many years ago I would echo ChrisB's comments on safety. I survived, probably because I was able to throw the equipment away from me to break my grip. I have been very careful since not to repeat the experiment!!

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Postby GregsGarage » Wed Apr 15, 2009 2:59 pm

Google "bad boy charger" for some links on a simple charger. Another risk with cheap chargers is thermal runaway caused by overcharging. Don't leave the charger unattended or the vehicle parked anywhere that a battery fire could cause damage to other property or put lives at risk if you do have a fire.

Those AGMs need some sort of simple balancer, have a look at this link
About half way down find the link for " Lee Hart's zener diode battery regulators". These shouldn't break the budget.

Greg Fordyce

Daewoo Matiz

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Postby Ecorsa » Sat Jun 27, 2009 5:19 pm

Hi, I am planning the same sort of cheap car, and also plan to make my own charger. The output of a rectifier is 1.414 times the RMS input which is 339V. so i think 24 batteries for 14.1 V each will be about right. The output of the rectifier needs smoothing capacitors the same as any other rectifier.
If it sharges too fast, possibly another battery could be connected in series to drop the volts per battery?

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