Friday, December 11, 2009

DIY EV Battery Charger

The last major item I need to find for my conversion is a battery charger.  Amazingly, I'm staying within my budget so far (detailed in my wired post), but I'll probably go over if I pay for a new charger.

Russco and Quick Charge make some decent chargers that are relatively affordable compared to nice but more expensive Zivans and Manzanitas.  But if I buy new, I'm still looking at at least $500.  I have heard of people making their own chargers and got curious about how difficult and expensive it might be.  All a charger really does is apply a dc current to a battery right?  How hard can it be?

I did a little digging and found some reasonable plans.  Building your own charger is most tempting when your batter pack's voltage is close to household voltage, as mine will be at 120V.  Household voltage is around 110V, and it is a relatively simple matter to convert AC to DC.  Small voltage modification can be achieved with an inductor to bump the 110V household to 120V or so that is the optimum charging voltage.

I was pleasantly surprised to see how easy and cheap this would be to do, and I have to say I am very tempted to do it.  I figure I don't have too much to lose, since I got my batteries for $5 each.  If I had spent thousands on the batteries, I might be more leery of subjecting such an investment to an experimental charger.

There are some downsides, however.  The main benefit of manufactured chargers is the managed charge.  Not only do they shut off by themselves when the batteries are full, but most of them also have a multi-phase charge profile which is better for battery health and maximum charge.

It wouldn't be too difficult to install a timer switch on the unit, and with a little time I could learn how much time to give the charger based on a given discharge. But the charger would have to be watched closely to make sure the batteries would never be overcharged.

I also don't think it would be terribly difficult to integrate some control circuitry and automated voltage monitoring, but with all that the project would grow to be much more complicated.  I think it would probably still be cheaper than the manufactured chargers, but I need cheap AND simple.

I've also heard of "Capacitive Chargers" that will charge any voltage pack (more of a trickle charge), and can be made from very cheap parts.  Wilderness EV says they work well and sells them on their site.  I can't find much more information on them though, and it almost seems too good to be true.  Anyone have information?

4 comments:

  1. The hard part with charging batteries is charging them quickly. If you are comfortable with the charging taking forever, you can build a small current charger and leave it on indefinately without harm. Its like the sears charger for my car, it only charges at 2amps and I can leave it on my car overnight.

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  2. I read about the Capacity Charger at http://e-volks.com/capacith_charger.html
    I don't know anything about capacitive chargers, but I know how lead acid batteries works. and this senectence "It is important to remember that batteries don't care about voltage when being charges". This is a total lie, if you charge a 12 volt battery with more than 14.4 V at room temperature the battery will be damaged! You can read more about it at www.batteryuniversity.com/partone-13.htm

    If you would like to build you own charger, there are two diy intelligent charger projects that I know about. You can find them at diyelectriccar.com search for "$200 Build you own intelligent charger" and at ecomodder.com search for DIY Open Source EV Charger

    Good luck!
    Lars

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  3. You may want to check out forklift battery chargers as an affordable alternative. We run a fleet of electric forklifts and the charge times are quite short. A quick check of Ebay shows a range of forklift chargers ranging from 24 to 48 volts from a hundred dollars on up.

    You would have to charge up your batteries in banks rather then as a whole but you might be able to pick up a really nice industrial grade charger for a good price. Just be sure to check the input power requirements, you may have issues finding one that isn't three phase power.

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  4. Check out the Lee Hart Battery Management System.
    http://homepage.ntlworld.com/electric.mini/bat_bal/relay.html

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