T&T: battery test and fridge usage

Richard P rpackard43 at gmail.com
Tue May 12 23:56:24 EDT 2020

While killing time in Mexico waiting for a Covid vaccine to be developed, I
decided to make a battery bank test and also test the ambient energy
consumption of our top loading refrigerator/freezer. I post here the method
used and the results, to serve as a guide for any list members not familiar
with such methods. Those more knowledgable should skip this post.

I could not have made these measurements if we were actively living on the
boat. Due to the heat here, we are off the boat now in an air conditioned

As most on this list  know, most battery usage on a boat goes into
refrigeration, especially if you have those front loading poorly insulated
apartment refrigerators.  On Lilliana, our Willard 40 trawler, I tossed out
that fridge when we bought the boat because there was a top loading,
second, fridge/freezer under the pilot seat. That has been adequate for our
usual four month's cruising season in Mexico.

Our house battery bank consists of 8 Trojan 105’s giving nominally 1000Ahr
( for a discharge in 100 hours, when new) at 12V..  They are now five years
old and I wondered how much of that capacity was left. So here is the test.

The boat us unplugged from shore power and the two solar panels are turned
off. After first equalizing the bank I monitored the Ahr consumption over a
period of eight days. The ambient temperature inside the boat was high,
over 95F during the afternoons, dropping to about 80F in the evening. The
freezer section was normally at about 10F and the refrigerator section was
at about 40F. Everything was turned off except the fridge, the propane
sniffer and the link 10 battery monitor. The latter two devices consume a
steady 0.3A.

Atter seven days (actually 197hrs) the total consumption was 325 Ahr.,
about 33% of the bank’s original nominal capacity. Of that, 267Ahr was due
to the fridge alone. That implies the refrigerator , on average uses 1.36A,
Since, when the fridge is running it draws about 4.5A, the duty cycle in
this hot weather, is about 30%.

At the end of the test the at-rest open circuit battery voltage was 12.27V
which corresponds,( according to Trojan specs) to a 63% charge. Since the
consumption of 325 Ahr would correspond to a 67% charge of a new, nominally
1000Ahr. bank, I conclude that I have left about 93% of the original
1000Ahr capacity. Actually since the discharge rate was smaller than the
100hr rate the remaining capacity is not quite 93%. That is not bad for a
five year old battery bank.  I never discharge the bank more than about 20
% and according to the Trojan specs this would give me some thousands of
discharge cycles. They may outlive me even if I escape the Covid!

Our energy consumption when we are on board the boat is of course greater
than the averages mentioned above  because we use lights at night, plus
various electrical gadgets.

My two rather inefficient solar panels produce 50-60  Ahr per day during
our winter cruising season. This then would let us stay at anchor for
several weeks without having to run the engine to recharge the batteries.

Lilliana does also have a massive generator. We have never used it other
than for an annual test of about 15minutes to see if it runs. I keep it
because it is part of a diesel-electric get-home system that came with the
boat. That might be useful to some future owner.

Richard P

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