On 11/20/2015 10:52 AM, cdelect@juno.com wrote:

21 Corby, in three rows of seven, from my notes when I re-celled mine.

Dan

Hi

Well, the charger brings the stack up to about 33V (34.8V CR7 minus
1.4V in Q5 and Q4).

Since they are NiCad’s that sort of limits the choices in stack size.
Best bet (and a nice round sounding number) is 24 cells. That would
1.375V per cell which is just below the “standard” 1.41 V float voltage
commonly used.  As Q4 and Q5 cut off, the voltage might rise a bit
above the 33V number, but at pretty low current.

The other choices would be 23 cells at 1.43V, (that’s a bit much for float
charging) and 25 cells at 1.32V (you will loose capacity at that point).

On the other side, the battery disconnect drops out (CR11 and CR15)
at 23V. That would be 0.96V per cell on 24 which is still rational for a NiCad.

Bob

Hi Corby:

You could guess if you know the nominal pack voltage (divide that by 1.2 Volts/cell) or if you know the interior
dimensions I might be able to see what fits.

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cdelect@juno.com wrote:

Hi

I don’t in any way doubt your count. If indeed they are putting 21 cells into that circuit,
the NiCads likely don’t last as long as they might with a different charging circuit ….

Bob

On 11/20/2015 4:54 PM, Bob Camp wrote:

Bob, I was answering Corby's query; mine had an original -hp- battery
pack which I assumed was designed by someone who knew what they were
doing, so I just put new Chinese made 2.2 Ah Nicad cells in it, exactly
the same number as the original, however I don't know the capacity of
the originals, they were unmarked.  Five or six years later it still
will hold the thing up for at least twenty minutes, which I would think
is what matters.

It would in any case be difficult to fit any more than 21 in the space
available.

Dan

Hi,

Lacking the charging controller in mine, what would be a good approach
to go about and build one?

Need to replace the battery setup in my XSRM setup too, the packs I
installed is now dead.

Tempted to think in terms of LiFePO.

Cheers,
Magnus

On 11/21/2015 03:45 AM, Dan Rae wrote:

Hi

The key issue is that there has been a lot learned about charging batteries
since the 5065A was designed. Just as other parts of the 5065 can be improved
with some work, so can the battery charger. Given that NiCad’s aren’t cheap,
it’s worth noting that the circuit could be worked on before somebody buys
a bunch of batteries to re-cell one.

Moving a bit further off topic and back to our original conversation when you re-did
yours (2 years ago?) - you don’t get a lot of standby time even with the full load of
NiCads in there.

Bob

In message 5650469B.1040106@rubidium.dyndns.org, Magnus Danielson writes:

A Mascot lead-acid charger and two VRLAs :-)

Really... You should build a Lab-wide 24V battery-backed DC supply instead.

Lithium is a bad choice for standby applications.

Normally we overcharge lithium batteries to get max capacity, but that
is also what kills them short of 1000 cycles.  If you leave them in the
charger "forever" they die "a lot sooner than forever".

When used in standby applications you can only use 70-80% of their
capacity because you have to stay out of the "overcharge" domain.

Unless it is kit which you lug around, VRLA is the way to go for standby.

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Hi

The 5065A has a nice connector on the back for an external DC supply Rather than doing
anything inside the box, I’d do a battery pack outside the unit. If you have multiple devices
you want to run on battery - do they also have DC inputs? If not can a DC input be added
with minimal damage to the unit?

LiFeP04 is the darling of the battery industry these days. They do give you a lot of energy
in a small volume. They really do need charge balancing controllers on each cell. Price
is in the “not cheap” range. Keeping them running a long time is a common goal.

Lead Acid batteries stolen from a golf cart are the other approach. They are at least
worth looking at as a cost comparison. They have been around long enough that there is a lot
of data on them.

For a backup system in a normal urban environment (=outages are rare), I probably would
design a charger from scratch or modify an existing device. Most of what I have seen at a
price you can afford is not doing the sort of job that will keep the batteries running for a long
time. All of the information on charging voltages etc is out on the web. There aren’t a lot of
secrets in this area. You probably want to optimize for long battery life rather than maximum
charge or fast recharge. The “charge it until it gets so hot it almost explodes” approach is
not what you want here. (Yes indeed … err… some chargers are designed that way by …errr..
some designers).

About all a charger is:

Power source (DC supply) with adjustable voltage and (maybe adjustable) current limit
Temperature sensor(s)  … (LM73’s do nicely)
Volt meter  (ADC channel works ok)
Current meter (ADC channel and an isolation amp)
Smarts to run it (an MCU)

Assuming that you have a basement full of junk, the power supply probably is a “zero cost”
item. The rest of it is a project using any of the multitude of cheap MCU boards out there. Is
it $20 or $40 .. who knows. Either way it’s pretty cheap. With the LiFePO4’s the balancing
stuff is a bit of a pain on large stacks. I’d probably use the off the shelf modules for that part.

One thing that was not commonly done in the 1960’s, but that is pretty easy today is to put
a switcher on the battery output. That allows you to drive the gizmo with just the right voltage
and not waste a lot of power in the linear regulators. It also makes the whole process of running
multiple devices off of a single battery bus a lot easier. If this one needs 21V and that one needs 14.5V,
you can make them both happy. You can also do it with just about any stack of batteries that makes
mechanical sense.

No, that’s not a one size fits all solution like a UPS. It does take some planning and some fuses. It
is a full blown project. Given how cheaply they sell stuff when it goes on sale, you probably will not
save much money on the first pass. The LiFePO4 system likely will run forever (if the current data
is correct … only time will tell). The lead acid systems will eventually need to be re-celled.

Lots to think about. Way to many variables with each of our local power companies and inventories of
equipment. No single correct answer. That’s even without getting into the

Bob