[time-nuts] float chargers for oscillator backup power

EWKehren at aol.com EWKehren at aol.com
Sun Oct 26 19:30:24 EDT 2014

Living in south Florida backup power is short term and long term.  As  lab 
batteries I use T 105 also known as Golf Cart batteries. Costco has those 6  
Volt 200+A batteries for less than $ 80.  Super price. Yes I have Costco  
stock. For charging I use a VIctron Energy Phoenix 12 V 30 A charger. On the  
boat two 28 V 100 A chargers. No afilliation.
These people know what they are doing and if nothing else download info on  
their chargers they know all about how to get maximum life and performance 
out  of batteries. You will find them in top end  boats and RV's but the 
bulk of  their business is off grid power. If you have been on an African 
Safari, the  power most likely was Victron Energy. I think they still have also a 
very good  application note. 
A T 105 like battery if maintained is good for eight years plus. On golf  
carts if used daily they typically last at least 4 years and I talk 70% plus  
Bert Kehren  Miami
In a message dated 10/26/2014 6:03:37 P.M. Eastern Daylight Time,  
phk at phk.freebsd.dk writes:

In  message  
<CAGVVbuFAM=u5Uhdf+wgTUNuoeMZ+=PzWQ4JpugjNJdd60FGp0A at mail.gmail.com>
,  Brian Lloyd writes:

>> There are a variety of inexpensive  wall-wart packaged float chargers for
>> lead acid batteries around.  Might be easier to just get something off 
>>  shelf.
>Some of these things are *extremely* noisy.

Well,  yes and no.

It's amazing how much noise doesn't make it past the  lead-acid battery
due to its low internal resistance.

The real issue  is not the chargers noise in float mode, where it is
barely loaded, but in  bulk-charge mode, where it works full bore.

Unless you want to burn a  LOT of heat charging your batteries with
a linear regulator, you are better  of shaving the noise after the

For OCXO's, the 14.5-11.6V  supply range is going to be much more
important than the noise from the  charger.

In other words, you will need some kind of regulation  between
the battery and the OCXO, and that is where you should cope  with
the noise.

A couple of other concerns should worry you  too:  Isolation and
short-circuit current.

The design I will  suggest, is to find a good charger which takes
care of your battery, and  have it do only that.

Driving the load with the charger almost  invariably means the
battery doesn't get optimal conditions which is why it  will
croak in 5 years instead of the 20 years in the datasheet.

Find  another power supply for your load, at a voltage slightly
above the  chargers bulk-mode (14.5 V) voltage, and use two
solid diodes to "or" the  battery and the "production supply"
onto your "DC-bus".

Don't skimp  on the ATO fuses, put one right next to the battery.

Your OCXO should  be driven using a small isolated DC/DC converter
from this DC  bus.

It is important that the converter is isolated:  It  prevents
ground-loops, but it also allows you to common-mode filter  the
supply to the OCXO to get rid of any noise from the  DC/DC

Yes, it is alot more complex, but the result is  also much better.

Poul-Henning Kamp     | UNIX since Zilog Zeus 3.20
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