20 hour vs. 50 hour battery rating

[Glenn Ashmore]

Interesting question came up this evening perusing the Surrette L16H battery
specs. The specs say they have a 20 hour rating of 400AH and a 50 hour
rating of 476AH. Almost 20% more. That is to be expected as total amp
hours increase with slower draw rates. The 20 hour rate is used to compare
one battery against another but if in normal good practice a bank is only
discharged from90% to 40% over 24 hours, wouldn't it be more appropriate to
use the 50 hour rather than the 20 hour rating when estimating the actual
usable amp hours?.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com

 

[Steve Lusardi]

Of course it would, but in practise you should not consider that low of a
discharge. In practise, it should never get below 60%, if you want
reasonable battery life. It is much better to install extra capacity, as it
greatly extends battery life.
Steve

 

[Larry]

wouldn't it be more appropriate to
use the 50 hour rather than the 20 hour rating when estimating the
actual usable amp hours?.

Not on our boat.....(c;



Larry


No, you STILL can't run the electric heater off the inverter.....

 

[Matt Colie]

Glenn,
At one time (either very long ago or recently depending on your age) I
did a great deal of battery work.

Batteries are now rated by:
Reserve capacity (minutes at 25a to 10.5v(12vnom)
20 hr (current * 20hrs to discharge to 10.5v)
50 hr
Cold Cranking Amperes (30sec current to 7.2v at 32F)
0CA (0F Cranking current (same but 0F)

None of these numbers can be used for anything except comparison. If
you run most any battery to its limit by these numbers, it will be
damaged.

If you are going to use the rating to size the bank, go looking for the
for a set of discharge tables (I'll keep looking for the set that I have
- too). You might be able to get a set from a real battery supplier,
but autoparts people will have no idea what you are talking about. This
set of tables will tell you what capacity you can expect (expressed as
percent of 20 or 50hr rate) that you might get at another rate and
temperature to a give terminal voltage.

My actual advice that seems to work best - put in all the battery that
you can afford (weight-space then cost).

How soon to launch?

Fair wind and Smooth Sea
Matt Colie

 

[Glenn Ashmore]

OK, here is one for you. Surrette doesn't seem to have the Peukert exponent
for the L16H on their site but the spec sheet does have the capacity for
draw times from 1 hour to 100 hours. Using that and the formula on the site
you referenced solved for n (Peukert) gives a Peukert exponent of 1.22 which
when plugged into the original formula tracks almost exactly to the Surrette
specs until the draw gets extremely large or extremely small so it appears
that my math was correct.

However, using the formula in the back of the Link 20 manual working from a
reserve minutes at 25A of 870 I get 1.44. If you plug 1.44 for Peukert into
the original formula you do get the stated 870 minutes @ 25amps but the
calculated capacities do not track the Surrette spec at all. .

It appears that the Peukert formulas must be taken with a grain of salt.

Incidentally, I got a price on a set of four Surrette S-530 (L16H) batteries
delivered to Macon from DC Battery Specialist in Miami for only $20 more
than the local price for a set of Trojan L16Hs. Actually less with sales
tax and way less than any internet source with shipping.

--
Glenn Ashmore

I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com