[time-nuts] HP 5065A battery pack

[Bob Camp]

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

> On Nov 21, 2015, at 5:25 AM, Magnus Danielson <magnus at rubidium.dyndns.org> wrote:
> 
> 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:
>> On 11/20/2015 4:54 PM, Bob Camp 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
>>> 
>> 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
>> 
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