[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: (ET) Trying 29 cells for BB600



To get the most out of that battery, you really need a smart charger with
the correct profile.  But I've been saying that for years and obviously I
haven't convinced you.  :-)

The only smart charger I have weighs about 200 pounds and is not going to fit well on the E20 (one of the aircraft chargers). A simple 5a NiCD charger is not going to work as well because these things need to be hit with a lot of current right out of the box.

What *would* work would be something that could go in-line with the Elec-trak's charger to regulate the peak voltage and current coming out of those rectifiers. That *plus* a capacitor to raise the output voltage profile might do it.

You can safely charge NiCd batteries almost indefinitely with a constant
current charge of 0.1C, where C is the C20 amp hour capacity expressed in
amps.   Voltage doesn't matter much, only current, though watching for the
on-charge voltage to fall (negative dv/dt) is a great way for an automatic
NiCd charger to decide when to shut off.

Flooded NiCDs are not the same as sintered plate. According to the mighty BB600 manual the charge profile is (for a 19 cell pack of type BB693 A/U) at under 80f ambient:

  Set charging voltage to 28.7 vdc (1.51vpc)
  Charge until charge current drops below 5a rate.

If over 80f ambient then charge voltage is 27.5 vdc (1.44 vpc)

They're charging with a 1.5kw APU charger so I'm guessing the initial rate is about (1500/28.7) 52 amps which seems a bit high, but these cells appear to become grumpy without a high initial charge rate.

Doing a bit of math for the Elec-trak and assuming it's "summer" you get
30 cell pack: 43.2 volts
29 cell pack: 41.6 volts

So going 29 cells is about between summer and winter. 30 is not quite going to work especially if you expect the E20 charger to be putting 5-10 amps in the pack.

The Elec-trak's charger tends to peak out at 42.5 volts, which is a bit too low to really charge the cells to full but is perfect for lead batteries which comes out to around 2.36 vpc for 18 DC cells. That's a solid 14.1 volts which is about what you want to charge a chunk of lead to.

So if your battery is (say) 34 ah at 20h, you can charge it at a regulated
current of 3.4 amps.

This does not work well in practice on BB600's, especially multiple strings. What worked for those big SAFT 6 volt cells in the old days?

C