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
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