digital battery hydrometer [RE-wrenches]

[Dan Fink]

Hi all -- I've been following this 'why I abuse my batteries' thread for
quite some time now. Besides RE system install experience, I have
experience in liquid process control.

Everyone here knows what a nightmare customer battery bank abuse is for
the system designer, seller and installer. Repeated repair calls,
damaged batteries, and blame flying between customer and RE wrench.

The short answer-- you CAN buy a digital battery hydrometer, and also
in-situ acid hydrometer sensors for battery process control. But, the
issues involve can quickly take you WAY past something that's practical
for a home-scale RE system.

--A battery acid refractometer is an option if you must do lots of SG
tests for customers. They range from $80 for automotive grade to $300
for lab grade. You'll still be using a turkey baster to draw the sample,
and must clean it all after every sample. And you have to keep in mind
temperature compensation.

-- a (relatively) inexpensive handheld digital battery hydrometer is
commonly available and might be a good option for an RE Wrench that has
to check battery banks frequently. It will at least prevent acid holes
in your Carrharts, since the pump is usually built in....no turkey
basters. These units also measure temperature and provide a corrected SG
reading. Most can be PC interfaced into a spreadsheet. However,
inexpensive versions generally require that the sensor be cleaned after
each use, just like inexpensive pH meters, etc. They can't be
permanently submersed in electrolyte.

-- in-situ sensors are also available. They are expensive. These STILL
need to be removed from the electrolyte and calibrated at least yearly.
Generally a network of sensors is used, into a PC data acquisition
program. Though i suppose you could just use one. But.....

What I don't understand is WHY go to all this expense? Just monitoring
SG on one battery cell of a large bank is not going to help you or
customer learn much of anything about how the system is performing,
except to detect gross problems. And those you can detect with a $200
Tri_metric. A couple grand spent on digital specific gravity metering of
a couple cells won't give you any clue at all about problems in any of
the other myriad cells in the battery bank.

If you want a fool-proof battery monitoring system, you can buy one.
They use them in nuclear submarines, telecom backup power centers,
utility grid switching stations, etc. The SG and temperature of the
electrolyte in each cell is monitored individually. Control of charge
current can actually be performed on each series and/or parallel string
of cells and in some cases individual cells. If a part of the battery
bank gets too far off the SG of the rest, the charging current to just
the affected parts can be adjusted to compensate, then alarms go off and
make automated cell phone calls to the supervisor, and technicians go
searching for bad connections and loose wires with a thermal imaging
camera!

My advice is:
--never install a system without a Tri-metric or E-meter to track NET
amp hours and kilowatt hours...inputs vs loads, tracked over time.
--research the cost/benefit ratio of a digital handheld hydrometer vs.
your extra time spent with the turkey baster plus cost of new Carrharts
per year.
--Recommend a battery check-up by YOU every year. And check every single
cell with the hydrometer!
--Also check carefully for loose wires and bad connections when you do
your service call. I would place these at #2 in top causes of battery
failure. A SG monitor would not detect this unless it was installed in
the cell(s) that was dying from starvation...it would only detect the
effect of the bad cell spread over the entire bank.

In my opinion, a system monitor that reads Specific Gravity from only
1-3 cells is a false friend. The BEST system monitor in the world is an
experienced RE Wrench armed with historic system data from a Tri-metric,
a turkey baster and good glass float hydrometer, a multimeter....and
Carrharts with lots of holes in them from battery acid. If he/she shows
up in brand new coveralls.....ask for a picture of the last coveralls
before hiring! (grin)

DAN FINK
Technical Director
http://www.otherpower.com/


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