BATTERY VALUE

[GordMay]

DEPTH of BATTERY DISCHARGE and VALUE:

The following statements fly in the face of “conventional wisdom”, and may prove controversial. I will appreciate, and will be pleased to incorporate into a revised report, any critical discussion. Please forward your comments to:
Gord@BoatPro.zzn.com

Most authorities recommend “only” discharging deep cycle batteries down to 50% of capacity at most, further suggesting that “less is better”. Not necessarily so !!! The following chart, depicting the performance of Trojan Deep Cycle Lead Acid batteries, suggests that a 40% discharge is optimal, and discharge levels between 50% & 80% are better than 30% or less discharge depths.

At 10% Discharge - 6,200 Life Cycles - Provides 62,000 Lifetime Amp/Hours

At 30 % - 4,400 Cycles - 132,000 Litetime A/H

At 40% - 3,700 Cycles - 148,000 Lifetime A/H

At 80% - 1,700 Cycles - 136,000 Lifetime A/H

~~ I could not format the full chart to this posting - I could fax a copy to anyone requesting ~~

How can this be? Well, the previously cited “authorities” were really commenting on the number of times you could recharge your battery (life cycles), and not the total amount of energy it could deliver over it’s lifetime. Let me suggest that battery “value” is represented by the total Lifetime Amp-Hours it will deliver; not the number of times you can (have to) recharge it. As the above chart indicates, the increased number of life cycles at lower depths of discharge, do not compensate for the increased Amp-Hours available from deeper discharges.

Comments?

Respectfully
Gord May

[Jeff H]

Gordon,

While I see where you are going with this, the reality is that sailboat batteries go through a rhythmic discharge and recharge based on the usuage patterns of the boat. For many of us, there are a lot of a shallow discharge and recharge from starting the running the engine, and then the deeper discharge and recharge cycles that come from using the gear on board when cruising or daysailing. We recharge not by looking at the level of discharge but on a schedule set by the patterns of being aboard a boat, or on boats with wind generators and solar panels, by the cycles foo wind and water. In that senario it is the frequency of comparatively shallow charge cycles that seems to come into play. Also I increasingly come across boats that lack the the capacity to fully recharge a deeply discharged deep cycle in a reasonable period of time. As a result, deep discharging results in batteries sitting in a partially discharged state for longer periods of time, which of course can really reduce the capacity of the battery. While I don't disagree with your analysis perse, in many cases, if not most cases, the number of shallow cycles probably really is a better indicator of the batteries expected lifespan than its overall amp hours over its entire lifespan.

Respectfully,
Jeff

[Pblais]

I think Jeff makes a good point. The ability to recharge is an important issue. A Tojan won't be as fast to charge and will take more input to acctuallyt get fully charged. . An AGM will recharge faster so you'll generally get the battery back up where it needs to be to have the best 'cycle". The last 20% of charge capcity is the hardest to get back. getting it back also has a cost.

My boat came with a bank of T105's and at this point they are too long in the tooth and need to be replaced. I can no longer recharge these batteries fully or even as close as I would like. I'm not totally convinced I'll replace them with new T105's or not. I'm tending to go with AGM's but I need to check out a few things first. I'm not sure the alternator will handle the higher demand from the AGM's, though it appears the AC charger can handle it properly.

The whole process of balancing the charging with the batteries as well as computing the loads are just as complex. With just a superficial look the T105's seem to deliver lots of AH's at a low purchase price. Down the road I'm not so sure it ends up that way. Im also concerned with venting and the maintenace issues as well.

There really is no magic answer to this question.

[GordMay]

Thanks to Jeff & Paul. Check your PM's.
In the near future I'll be posting additional materials on this subject; and will appreciate further comment, critique, and opinion.
Regards,
Gord

[Pblais]

Here is a good discussion with some real details on various battery systems. I think the author does a good job showing that the devil really is in the details.

http://www.vonwentzel.net/Battery/index.html

[29cascadefixer]

Thanks fellas. Batteries, the stuff of discussion ! I will peruse these at a more liesurly pace . Just as an aside, maybe the ultimate answer is a nuclear reactor, sailboat size, towed behind the craft as would be a dinghy. Gad ! Do away with the engine with one a these. One large electrical motor to run the mother ship. do away with sails-----ahhh, nahhhh-sorry. I get carried away sometimes.