Re: (ET) Trying 29 cells for BB600

[Chris Zach <cz beaker crystel com>]

> 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