Building a $175 battery charger for less!!!

Discussion in 'The Projects Forum' started by iONic, Apr 18, 2008.

  1. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
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    I just purchased a pair of 12V 35AH AGM Batteries for my mobility scooter and
    looking to build a charger for them that meets or exceeds the batteries charge requirements and cost much less than buying either the manufacturers charger or a suitable equivalent.

    I'd like any ideas or suggestions you all might have on how I could accomplish this.

    The batteries are connected in series in the scooter, providing a 24V source for mobility. For optimal performance and longevity, however, I would like to charge them separately and thus will have to double the components.
    I will also need to make a switch to "re-wire" the batteries from a series config to a standalone configuration.

    Links to Info/Specs:


    From this information I would like to simulate a charging method similar to the picture below:

    [​IMG]


    There are a few options for building, without micro-processor, with microprocessor, or with some Battery Management Charger, Chip.

    Not sure which way i will go...am hoping to get input and ideas from all of you.

    Thanks

    iONic
     
  2. El3ctroded

    Active Member

    Feb 4, 2008
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    Here's how I might try it:

    Use either a transformer and bridge or brick supply to get about 17-18V and 3-6A output after a bank of caps. Feed that to a lm317 with pass transistor and some output caps. This will give you a 3-5amp power supply that you can regulate the voltage output by using the microcontroller and a digital pot or some other method to alter the voltage at the lm317's adjust pin. The output of all of this goes through a 0.01ohm shunt.

    Use a small microcontroller for the timing and current measurement. Measure the output voltage (voltage after the shunt) using an ADC pin on the microcontroller. Calculate the current by measuring the voltage before the shunt using another ADC pin and compare the two voltage levels. The difference between the two voltages can be used to calculate current.

    The hardware isn't hard, the firmware would be a little work, but not too much. You need a delay routine that can last for 24hrs or 10 min. You need to do the adc conversion and then map the results to actual voltage levels.

    El3ctroded
     
  3. Papabravo

    Expert

    Feb 24, 2006
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    My guess is that you'll spend the bulk of your time looking for components and tools. When you are all done it would not surprise me if the total material and tools cost was right in the ballpark of $175.00 So with your time worth nothing you will have expended all that effort for bupkis.

    I know that's not what you wanted to hear, but when you ask a question in a public forum you are inviting all comers to weigh in.
     
  4. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
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    Absolutely... I have indeed been in these shoes a few times before.
    On this occasion, however I do feel that I probably have already spent the money and have all the parts I need to build it aside from a couple of chips and m friends programming help. It often come down to principle. It is very very hard to see myself paying someone $175.00 for a handful of parts, a box, and an electrical plug! - not to mention that the $175.00 charger price is for a single 12V Battery.... I would need 2 chargers totaling in excess of $350.00!!! This goes beyond "principle", it's a complete fleecing of the consumer, an insult in practicality and intelligence!
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Well, if you've already spent the money and have all the parts you need, why don't you tell us about them and what YOU have designed thus far, instead of asserting your view of consumers being fleeced?

    It's not easy bringing safe, viable, and relatively inexpensive units to the consumer market.

    If you think $350 for a pair of high-tech battery chargers is expensive, why don't you price a commercial grade Lincoln arc welder. It's basically a transformer with multiple taps, and some of them have DC welding options. They're about TWICE that much for ONE unit.
    http://www.mylincolnelectric.com/Catalog/equipmentdatasheet.aspx?p=2494
    And they don't have any sophisticated electronics inside. It's a frame, a cover, a transformer that has multiple taps - and an optional rectifier.

    Now I don't know how closely you've looked at the specifications in the sheets you've posted, but the one where you obtained the graph from informs me that the units start at 500W, and go up to 1300W, or 80A @ 13.6v. That's quite a bit of power.

    Now, you COULD have a pretty neat solution. Get a pair of rebuilt GM 130A alternators. Connect them up to a couple of 220v motors. Build some jumper cables, and connect them directly to your batteries. They'd be charged within a few hours.

    But, the alternators will cost you around $99 each, and the motors about $50 each. Wire and clamps, about $20 each. Oh, you'll need to fabricate some kind of stand. Let's say $5 worth of wood, and some cheap wheels.

    Whoops, we've broken the budget, and there's no warranty.

    So, where's your design?
     
  6. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
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    Well, OK, I guess I should pay some respect for the developers of these products. Sorry All.


    Yes indeed this chargers you mention here is an alternate, not specifically intended for these batteries. It was merely the charge sequence I was interested in mimicking here. The battery Manufacturer's specs call for no more than 10A... The particular charger recommended for the batteries is the PS-124000A-C. It's specs are:

    [​IMG]

    Some test's I performed on a pair of "not so great" SLA 12V 35AH Batteries and the mobility scooter.

    First Set of Batteries:
    Initial Battery Voltage after a 1.5 mile use - 25.75V
    Open Circuit Charger - 27.50V

    Voltage with Load - 29.0V (stabilized)
    Current with Load - 2.70A

    ------------------------------------------------
    Second Set of Batteries...

    Initial Battery Voltage - 24.00V
    Open Circuit Charger - 27.50V

    Voltage with Load - 29.0V (stabilized)
    Current with Load - 2.80A
    ------------------------------------------------

    From an analog perspective I had drawn this up.

    [​IMG]

    This was mearly a 2 step setup without a procesor.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    Well, a linear solution is obviously (or perhaps not) waste a lot of power in heat dissipation. While it would work, it would cost a pretty penny over time in the amount of energy wasted.

    Tell us about the transformer you have - what is it's secondary voltage @ current rating?

    You will need at least ONE transformer, from the standpoint of safety alone. If just one, it will have to be able to output around 30vac @ 4A to 8A.

    If you wish to build dual chargers, you'll need a pair that can output around 16vac @ 4A to 8A.

    Having two chargers would be better than one single charger. Batteries seem to age differently; if a cell in one battery shorts (which frequently happens) a single charger would wind up damaging the remaining good battery.

    You would not necessarily need a switch to isolate the batteries during charging. The transformers already perform the isolation from any line or ground reference, and as such are "floating". You might even use banana-type jacks/plugs to connect your charger to your batteries, if you stay within their rated current limits. You would need high quality components though, as the nickle-plated variety you might find at your local electronics store will have high resistance, and won't stand up over time. Silver plated will have the lowest resistance. Gold plated will have higher resistance, but will not corrode as quickly as silver.

    So, tell us about the transformers you have?
     
  8. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
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    I had an opportunity to work for a battery charger designer for the summer, but had turned it down after a few days. I was researching as much as I could and came across the "Battery technology handbook By H A Kiehne". I thought charging batteries would be a joke, which was until I had read the many pages of technical information on how to charge a battery properly and safely. I suggest you take a look at it if you have some time.

    Steve
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
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    Nice book, but it's about $200. :eek:

    However, a preview (of selected pages) is available on Google Books:
    http://books.google.com/books?id=1H...oi=print&ct=title&cad=one-book-with-thumbnail
    Don't try printing these pages, unless you like to simply transfer a lot of blank paper from the paper tray to your output tray.

    Sorry you turned that job down, Steve. I know it's not your primary interest, but storage cells have had an extraordinary impact on our culture in the past, and with the prices of oil escalating will have a greater impact upon our future.
     
  10. El3ctroded

    Active Member

    Feb 4, 2008
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    If you want to want to make the whole thing into a switching supply, then I've got a project I need to start that we could combine work on somewhat (meaning when I have some spare time). The supply would be right, with minor changes, for the charger as well as what I want if for. Then a cheap 14pin PIC and some code and we've got a nice little charger.

    What's it going to cost? Depends upon if I can do it in a 2-layer PCB or not, at the size I want. If 4-layer, just to get the first prototypes will cost over $250. If I can get it on a 2-layer board, we're still looking at over $100. None of that counts time spent on it... But it's less if I have someone chipping in because I get 4 boards (minimum order) and usually enough parts to build 10 samples. Usually I make at least 2 prototype PCB orders. The first usually has a couple errors, and hopefully I find them all before fixing them and submitting the next order. Of course this would take quite a while, especially with my schedule.

    El3ctroded
     
  11. scubasteve_911

    Senior Member

    Dec 27, 2007
    1,202
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    A 4-layer board for a battery charger? I highly doubt that. I've put FPGAs on 4-layer PCBs :p With switching power supplies, you want to keep all the really noisy switching on one layer if you can. The rest is on the other layer. I go with advanced circuits for my stuff, www.4pcb.com , since they seem pretty cheap and do a decent job.

    Hi Wookie,

    That book is really good, but $$$. I just found a place to download it :) About the job, I found a really great consulting job instead. Consulting exposes you to a variety of sub-disciplines and eventually makes you a very well rounded engineer. Plus they paid a bit more :)

    Steve
     
  12. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
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    El3ctroded,

    The manufacturer of my Batteries, PowerSonic, makes both a transformer and switching supply version. I was more inclined to reach for the Switching Supply. I've actually found a much better price for the charger(PS-124000A-C) from Mouser.... $98.00, for two $196.00. This is much more reasonable than $175.00 for 1 charger.

    Are you interested in building a dual charger to charge 2 batteries or a single charger for 1 battery?
     
  13. El3ctroded

    Active Member

    Feb 4, 2008
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    scubasteve: I said "at the size I want". I would be looking to make the whole PCB at no more than 1.5 x 3 inches. I recently made a 15A buck-boost using the LTC3780 at a similar size PCB and due to current levels a 4-layer board was preferred to give plenty of copper planes for the high currents.

    iONic, I was looking at single charger units. But at the price you found them at from Mouser, it's not worthwhile... I recommend you just buy them.
     
  14. iONic

    Thread Starter AAC Fanatic!

    Nov 16, 2007
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    Yes, I think that the price is more reasonable and as I have the batteries in my possession now I do not want to wait too long before I give them good charging. Mouser it is...

    iONic
     
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