Based on my experience with some fast-charge systems, I'd like to respectfully disagree with a few things which have been said here. First of all, as long as you meet certain safety conditions (like keeping the battery below the gassing voltage, compensated by temperature), it should not be harmed (or have its life cut short) by fast charging. Secondly, with an intelligent charger, it is not necessary to charge each 12V unit separately. The problem of imbalance can be dealt with by equalization charges. Thirdly, while NiMH are MUCH better in terms of capacity per mass and capacity per volume, they are MUCH more expensive than lead acid for a given capacity (rough figures are 20 cents per watt-hour for Pb Acid, 70 cents per watt-hour for NiMH). For aircraft, you often cannot use Pb Acid because of the weight, but for a scooter or electric bike, Pb Acid may be acceptable. If you want to buy an off-the-shelf item to do what you want, Rikard, there are several 36V Pb Acid chargers available. I would highly recommend the one by Soneil. It is about $150 USD, though, but has great performance and reliability. Are you concerned about charging time? If not, the suggestion of using a simple trickle-then-float charger would be easiest. That could be as simple as a wall-wart. 60Hz ripple should be OK. As a rough capacity estimate, assume 7 milliamp-hours per cubic centimeter for sealed Pb Acid batteries. Sean On Mon, Mar 24, 2008 at 1:18 AM, Bob Axtell wrote: > Rikard Bosnjakovic wrote: > > My father-in-law has aquired an electrical bike of some kind. It's > > battery driven and runs on three 12V batteries (in series, thus using > > 36V). He never got a batterycharger and there seems to exist none for > > this particular model, so he asked me if I could peek around and build > > a solution. > > > > I found this problem interesting, so I accepted it. Yet I have not > > read up to anything so I'm just out for guessings (it's 04:06 in the > > morning, so bear with me) before I sober up and dig down to the gory > > details of battery charging tomorrow. > > > > What I've grasped so far is this, that I need to output 36V > > (preferably some volt higher) to the batteries to be charged. But what > > about charging currents? Must it be supplied at a rate defined by the > > batteries, or can I supply whatever current I want to (given that > > lower currents == longer charging time)? > > > > The batteries are chinese and contains no labeling whatsoever, so I > > have no clue about them more that they are all 12V in series and they > > go through a 25Amp fuse. They were pretty heavy, so I would guess some > > lead kind of batteries. > > > > More guessings: For a full bridge rectifier giving the correct > > voltage, is a ripple capacitor(?) required? That is, do batteries > > require a steady current or will a pulsing voltage do? > > > > Now I will stop guessing and go sobering up, had a bit too much of the > > scottish grouse-bottle while at my father-in-law's... > > > > > > > I have designed a 48V battery system using NiMH. You must isolate each > 12V battery so that it > can be charged separately. Do this with dual diodes and two separate 15V > power supplies (15V is > the charge point for lead-acid batteries with a diode drop). The > amperage rating needed to charge > is dependent on what size the Chinese batteries are. > > The reason that you need to separate and charge each battery is because > if one battery has weak cells, the other > batteries can be damaged or improperly charged. In this way, each pack > gets seperate treatment. > > Having said all that, I strongly suggest you use NiMH packs instead of > lead-acid, which will halve the > weight and deliver the same power. The packs are more expensive but the > performance is well worth > it. You will be required to monitor the battery temperature carefully in > order to know when the packs > are fully charged. > > --Bob Axtell > > > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist